Transient electronic devices comprising inorganic or hybrid inorganic and organic substrates and encapsulates

Abstract

The invention provides transient devices, including active and passive devices that physically, chemically and/or electrically transform upon application of at least one internal and/or external stimulus. Incorporation of degradable device components, degradable substrates and/or degradable encapsulating materials each having a programmable, controllable and/or selectable degradation rate provides a means of transforming the device. In some embodiments, for example, transient devices of the invention combine degradable high performance single crystalline inorganic materials with selectively removable substrates and/or encapsulants.

Claims

1. A method of using a transient electronic device, said method comprising the steps of: providing the transient electronic device comprising: a substrate; one or more active or passive electronic device components supported by said substrate; wherein said active or passive electronic device components independently comprise a selectively transformable material; and an encapsulant layer at least partially encapsulating said one or more active or passive electronic device components; wherein said substrate, said encapsulant layer or both independently comprise a selectively removable inorganic material responsive to an external or internal stimulus; wherein at least partial removal of said substrate, said encapsulant layer or both in response to said external or internal stimulus initiates at least partial transformation of said one or more active or passive electronic device components providing a programmable transformation of the transient electronic device in response to said external or internal stimulus at a pre-selected time or at a pre-selected rate, wherein said programmable transformation provides a change in function of the transient electronic device from a first condition to a second condition; and exposing said transient electronic device to said external or internal stimulus resulting in said at least partial removal of said substrate or encapsulant layer to expose said one or more active or passive electronic device components to said external or internal stimulus, thereby providing said programmable transformation of the transient electronic device; wherein said removal of said substrate, said encapsulant layer or both in response to said internal or external stimulus occurs via a phase change, dissolution, hydrolysis, bioresorption, etching, corrosion, a photochemical reaction, an electrochemical reaction or any combination of these processes.

2. The method of claim 1, wherein said one or more active or passive electronic device components comprise one or more inorganic semiconductor components, one or more metallic conductor components or one or more inorganic semiconductor components and one or more metallic conductor components.

3. The method of claim 1, wherein said step of exposing said transient electronic device to said external or internal stimulus results in the entire removal of said substrate, said encapsulant layer or both.

4. The method of claim 1, wherein said step of exposing said transient electronic device to said external or internal stimulus results in less than the entire removal of said substrate, said encapsulant layer or both.

5. The method of claim 1, wherein said step of exposing said transient electronic device to said external or internal stimulus exposes at least 1% of an outer surface of said one or more active or passive electronic device components.

6. The method of claim 1, wherein said step of exposing said transient electronic device to said external or internal stimulus exposes 1% to 100% of an outer surface of said one or more active or passive electronic device components.

7. The method of claim 1, wherein said substrate, said encapsulant layer or both independently comprise an entirely inorganic structure or a composite inorganic and organic structure.

8. The method of claim 7, wherein said entirely inorganic structure comprises one or more of SiO.sub.2, spin-on-glass, Mg, Mg alloys, Fe, W, Zn, Mo, Si, SiGe, Si.sub.3N.sub.4 and MgO.

9. The method of claim 7, wherein said composite inorganic and organic structure comprises an inorganic layer having a first surface adjacent said active or passive electronic device components and a second surface adjacent to an organic layer or an organic layer having a first surface adjacent said active or passive electronic device components and a second surface adjacent said inorganic layer.

10. The method of claim 9, wherein said inorganic layer comprises one or more of SiO.sub.2, spin-on-glass, Mg, Mg alloys, Fe, W, Zn, Mo, Si, SiGe, Si.sub.3N.sub.4 and MgO and said organic layer comprises one or more of a polyanhydride and poly(dimethyl siloxane) (PDMS).

11. The method of claim 1, wherein said device is an entirely inorganic device, wherein said active or passive electronic device components, said substrate and said encapsulant layer each are independently entirely composed of one or more inorganic materials.

12. The method of claim 1, wherein said substrate, said encapsulant layer or both independently have a preselected transience profile in response to said external or internal stimulus, wherein said preselected transience profile is characterized by a removal of 0.01% to 100% of said substrate or said encapsulant layer over a time interval selected from the range of 1 ms to 5 years or a decrease in average thickness of said substrate or said encapsulant layer at a rate selected over the range of 0.01 nm/day to 100 microns s.sup.1.

13. The method of claim 1, wherein said selectively removable inorganic material of said substrate, said encapsulant layer or both independently comprises a metal, a metal oxide, a ceramic or a combination of these, a crystalline material, an amorphous material or a combination thereof, a single crystalline material, polycrystalline material or doped crystalline material, a glass, a thin film, a coating, a foil or any combination of these, or a nanostructured layer or a microstructured layer.

14. The method of claim 1, wherein said selectively removable inorganic material of said substrate, said encapsulant layer or both independently comprises Mg, W, Mo, Fe, Zn, or an alloy thereof, SiO.sub.2, MgO, N.sub.4Si.sub.3, SiC, a spin-on-glass, a solution processable glass, a biocompatible material, a bioinert material or a combination of biocompatible and bioinert materials.

15. The method of claim 1, wherein said substrate, said encapsulant layer or both independently comprise a multilayer structure comprising one or more thin films, coatings, or foils comprising said selectively removable inorganic material.

16. The method of claim 1, wherein said substrate, said encapsulant layer or both independently comprises a composite inorganic and organic structure having a multilayer geometry.

17. The method of claim 15, wherein said multilayer structure further comprises one or more electrically insulating layers, barrier layers or any combinations thereof; wherein said one or more electrically insulating layers or barrier layers is provided in physical contact, electrical contact or both with said one or more thin films, coatings, or foils; wherein said one or more electrically insulating layers or barrier layers comprises an exterior layer of said multilayer structure or wherein said one or more electrically insulating layers or barrier layers comprises an interior layer of said multilayer structure in physical contact or electrical contact with said one or more active or passive electronic device components; or wherein said one or more electrically insulating layers or barrier layers comprises a polymer, an insulating ceramic, a glass, SiO.sub.2, spin-on glass, MgO or any combination of these.

18. The method of claim 17, wherein said multilayer structure comprises a metal foil or thin metal film having a first side in physical contact with a first electronically insulating layer or barrier layer; wherein said first electronically insulating layer or barrier layer is an exterior layer of said multilayer structure or wherein said first electronically insulating layer or barrier layer is an interior layer of said multilayer structure in physical contact or electrical contact with said active or passive electronic device components; wherein said first electronically insulating layer or barrier layer comprises a polymer layer or coating, a metal oxide layer or coating, a glass layer or coating or any combination of these; wherein said multilayer structure comprises said metal foil or thin metal film having a second side coated in contact with a second electronically insulating layer or barrier layer; or wherein said metal foil or thin metal film is provided between said first electronically insulating layer or barrier layer and said second electronically insulating layer or barrier layer.

19. The method of claim 1, wherein said substrate, said encapsulant layer or both independently have a porosity selected from the range of 0.01% to 99.9%, an extent of crystallinity selected from the range of 0.01% to 100%, or a density selected from the range of 0.01% to 100% compared to bulk prior to said at least partial removal of said substrate, said encapsulant layer or both in response to said external or internal stimulus.

20. The method of claim 1, wherein a time for a thickness of said selectively removable inorganic material to reach zero is provided by the expression: $t_c=\frac{4{}_{m}M\left(H_{2}O\right)}{{\mathrm{kw}}_{0}M\left(m\right)}\frac{\sqrt{\frac{{\mathrm{kh}}_0^2}{D}}}{\tanh \sqrt{\frac{{\mathrm{kh}}_0^2}{D}}};$ where t.sub.c is the critical time, .sub.m is the mass density of the material, M(H.sub.2O) is the molar mass of water, M(m) is the molar mass of the material, h.sub.0 is the initial thickness of the material, D is the diffusivity of water, k is the reaction constant for the dissolution reaction, and w.sub.0 is the initial concentration of water; wherein k has a value selected from the range of 110.sup.5 s.sup.1 to 110.sup.10 s.sup.1.

21. The method of claim 1, wherein said substrate, said encapsulant layer or both are substantially impermeable to water, limit a net leakage current to the surroundings to 0.1 A/cm.sup.2 or less, or undergo an increase in volume equal to or less than 10% upon exposure to an aqueous or nonaqueous solvent prior to said at least partial removal of said substrate, said encapsulant layer or both in response to said external or internal stimulus.

22. The method of claim 15, wherein said thin film, coating, or foil has an average thickness over or underneath of said one or more active or passive electronic device components less than or equal to 1000 m prior to said at least partial removal of said substrate, said encapsulant layer or both in response to said external or internal stimulus.

23. The method of claim 1, wherein said substrate, said encapsulant layer or both independently have a thickness selected from the range of 0.1 m to 1000 m, an average modulus selected over the range of 0.5 KPa to 10 TPa, a net flexural rigidity less than or equal to 110.sup.4 Nm, or a net bending stiffness less than or equal to 110.sup.8 GPa m.sup.4 prior to said at least partial removal of said substrate, said encapsulant layer or both in response to said external or internal stimulus.

24. The method of claim 1, wherein said substrate, said encapsulant layer or both are at least partially optically transparent in the visible or infrared regions of the electromagnetic spectrum.

25. The method of claim 1, wherein said substrate, said encapsulant layer or both are generated via physical vapor deposition, chemical vapor deposition, sputtering, atomic layer deposition, electrochemical deposition, spin casting, electrohydrodynamic jet printing, screen printing or any combination of these.

26. The method of claim 1, wherein said substrate, said encapsulant layer or both cover or support a percentage of an exterior area or an interior area of said one or more active or passive electronic device components selected from the range of 1% to 100% or wherein said substrate, said encapsulant layer or both cover or support 10% or more of an exterior area or an interior area of said one or more active or passive electronic device components.

27. The method of claim 1, wherein said one or more inorganic semiconductor components comprise a polycrystalline semiconductor material, single crystalline semiconductor material or a doped polycrystalline or single crystalline semiconductor material, Si, Ga, GaAs, ZnO or any combination of these.

28. The method of claim 2, wherein said one or more metallic conductor components comprise Mg, W, Mo, Fe, Zn or an alloy thereof.

29. The method of claim 2, wherein said one or more inorganic semiconductor components, one or more metallic conductor components or both comprise a component of an electronic device selected from the group consisting of a transistor, a diode, an amplifier, a multiplexer, a light emitting diode, a laser, a photodiode, an integrated circuit, a sensor, a temperature sensor, an electrochemical cell, a thermistor, a heater, a resistive heater, an antenna, a battery, an energy storage system, an actuator, a nanoelectromechanical system or a microelectromechanical system, and an actuator and arrays thereof.

30. The method of claim 1, wherein said device is a communication system, a photonic device, a sensor, an optoelectronic device, a biomedical device, a temperature sensor, a photodetector, a photovoltaic device, a strain gauge, an imaging system, a wireless transmitter, an electrochemical cell, an antenna, a battery, an energy storage system, an actuator, a nanoelectromechanical system or a microelectromechanical system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A-1D. FIGS. 1A-1D provide schematic diagrams illustrating side views of transient electronic devices of the invention.

(2) FIG. 2. Time dependent change in the thicknesses of thin layers of PECVD SiO.sub.2 in PBS at room temperature and 37 C.

(3) FIG. 3. Time dependent change in the thicknesses of thin layers of thermally grown SiO.sub.2 (wet and dry oxidation) at different pH in PBS solutions at room temperature, and 37 C.

(4) FIG. 4. Change in resistance of test structures of various metals in (a) DI water and (b) pH 7.4 Hank's solution.

(5) FIG. 5. The schematic illustration of inorganic layered substrate structure, using metal foils coated with spin-on-glass (SOG).

(6) FIG. 6A-6C. Dissolution kinetics of SiO.sub.2 in aqueous solution at different pH and temperature (1) FIG. 6A SiO.sub.2 thermally grown by dry or wet oxidation, (2) FIG. 6B SiO.sub.2 deposited by plasma enhanced chemical vapor deposition and (3) FIG. 6CSiO.sub.2 deposited by electron beam evaporation. Calculated (lines) and experimental (symbols) dissolution rates of silicon oxides in buffer solution at different pH (black, pH 7.4; red, pH 8; blue, pH 10; magenta, pH 12) at room (left) and physiological (right, 37 C.) temperature. The thickness was measured by spectroscopic ellipsometry.

(7) FIG. 7A-7B. FIG. 7(A) Measured data (symbols) and numerical fits (lines) for pH-dependent dissolution kinetics of oxides (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide) at room temperature and 37 C. FIG. 7(B) SiO.sub.2 film property dependency exhibited as film density versus dissolution rate.

(8) FIG. 8. Experimental results for dissolution study of SiN.sub.X. The plots provide measurements of thickness (nm) as a function of time (days).

(9) FIG. 9A-9B. FIG. 9(A) pH-dependent dissolution kinetics of nitrides (black, LPCVD nitride; red, PECVD nitride with LF mode; blue, PE-CVD nitride with HF mode) at room temperature and 37 C. FIG. 9(B) SiN.sub.X film property dependency exhibited as film density versus dissolution rate.

(10) FIG. 10. Dissolution kinetics for different oxides immersed in various aqueous solutions. A) Bovine serum (pH 7.4) at 37 C. (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide), B) sea water (pH 7.8) at RT (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide), C) Bovine serum (pH 7.4) at 37 C. (black, LPCVD nitride; red, PECVD nitride with LF mode; blue, PECVD nitride with HF mode), D) sea water (pH 7.8) at RT (black, LPCVD nitride; red, PECVD nitride with LF mode; blue, PECVD nitride with HF mode).

(11) FIG. 11. Curing mechanism and dissolution study of spin-on-glass encapsulation layers cured at various temperatures and times.

(12) FIG. 12. pH dependent dissolution studies of amorphous silicon (a-Si), polycrystalline silicon (p-Si) and silicon germanide (SiGe).

(13) FIG. 13. Electrical dissolution rates and thicknesses of sputter deposited Mg, Mg alloy (AZ31 B, Al 3%, Zn 1%), Zn, Mo, W, CVD deposited W, and E-beam evaporated Fe in DI water and Hanks' solution with pH 5-8.

(14) FIG. 14. Current versus voltage plots for devices containing transient metal components.

(15) FIG. 15. Photographs showing dissolution of a transient transistor array on a biodegradable metal foil.

(16) FIG. 16. Dissolution kinetics of various metal foils (Mo, Zn, Fe, W) under physiological conditions (PBS, pH 7.4, 37 C.).

(17) FIG. 17. Schematic of a fabrication strategy using metal foils. 1) Lamination of metal foil on a carrier substrate (e.g., PDMS coated on glass), 2) fabrication of a transistor array directly on the metal foil, and 3) peeling of the device from the carrier substrate.

(18) FIG. 18. Demonstrations of inorganic substrates.

(19) FIG. 19. Schematic illustrations of encapsulation methods for transient electronic devices, showing defects (e.g. pinholes) covered by a bilayer of SiO.sub.2/Si.sub.3N.sub.4; ALD provides a defect-free layer.

(20) FIG. 20. (A) Measurements of changes in resistance of Mg traces (300 nm thick) encapsulated with different materials and thicknesses while immersed in deionized (DI) water at room temperature. A single layer of ALD SiO.sub.2 (orange, 20 nm), PECVD SiO.sub.2 (black, 1 m) and PECVD-LF Si.sub.3N.sub.4 (red, 1 m), a double layer of PECVD SiO.sub.2/PECVD-LF Si.sub.3N.sub.4 (blue, 500/500 nm), PECVD SiO.sub.2/ALD SiO.sub.2 (magenta, 500/20 nm), PECVD-LF Si.sub.3N.sub.4/ALD SiO.sub.2 (purple, 500/20 nm), and a triple layer of PECVD SiO.sub.2/PECVD-LF Si.sub.3N.sub.4 (Cyan, 200/200/200/200/100/100 nm) were used for the encapsulation.

(21) FIG. 21. A series of micrographs of a serpentine trace of Mg (initially 300 nm thick) during dissolution in DI water at room temperature. Dissolution begins from local defects, then rapidly propagates outward.

(22) FIG. 22A-22D. Demonstration of the electrical properties of electronic devices fabricated on metal foils. FIG. 22(A) Transistor array on Fe foil (10 m thick), FIG. 22(B) Diode array on Zn foil (10 m thick), FIG. 22(C) Capacitor array on Mo foil (10 m thick), FIG. 22(D) Inductor array on Mo foil (10 m thick).

(23) FIG. 23A-23D. Transience of transient devices on inorganic substrates with inorganic encapsulation. FIG. 22(A) Transistor on Mo foil with MgO encapsulation (800 nm), FIG. 22(B) Diode on Mo foil with MgO encapsulation (800 nm), FIG. 22(C) Capacitor on Mo foil with MgO encapsulation (800 nm), FIG. 22(D) Inductor on Mo foil with MgO encapsulation.

(24) FIG. 24. Sample structure for a dissolution test of single wafer SiGe (Ge) with atomic force microscopy (AFM). (a) Schematic illustration of test structure: array of square holes (3 m3 m20 nm) of PECVD SiO.sub.2 mask on SiGe single wafer. (b) AFM topographical images and c) profiles of SiGe, at different stages of dissolution in buffer solution (pH 10) at 37 C.

(25) FIG. 25. Dissolution kinetics of various semi-conductors in various buffer solutions, with different pH at room and physiological temperatures. (a) polycrystalline silicon, (b) amorphous silicon, (c) silicon-germanium and (d) germanium.

(26) FIG. 26. Dissolution kinetics of different types of silicon in various aqueous solutions. (a) Tap water (pH 7.8), deionized water (DI, pH 8.1) and spring water (pH 7.4), (b) Coke (pH 2.6), (c) milk (pH 6.4) at room temperature, (d) bovine serum (pH 7.4) at room and 37 C., and (e) sea water (pH 7.8) at room temperature. (f) Changes in resistance of a meander trace formed from a phosphorous doped polycrystalline and amorphous Si NM (35 nm) in phosphate buffer solution (pH 10) at 37 C.

(27) FIG. 27. Thin film solar cell with fully transient materials. (a), (b) Image and structure of amorphous Si based photovoltaic cell array on degradable substrate. (c) Performance of unit cell of solar cells. (d) Electrical transience behavior of a-Si diode and (e) transience of performance of solar cell.

(28) FIG. 28. Schematic illustration, images and data from a structure for testing the dissolution of thin (100 nm thick) square pads of SiO.sub.2 formed by plasma-enhanced chemical vapor deposition (PECVD). a) Schematic illustration of the test structure, which consists of an array of square pads (3 m3 m100 nm) of PECVD SiO.sub.2 deposited at 350 C. on a thermally grown oxide (tg-oxide) on a silicon (100) wafer, with inset optical micrograph. b) AFM topographical images and c) profiles of a representative pad at different stages of hydrolysis in buffer solution (pH 12) at physiological temperature (37 C.).

(29) FIG. 29. AFM surface topography and thickness profile for PECVD oxide at various stages of dissolution in (a), (b) buffer solution (pH 7.4) at 37 C., and (c), (d) buffer solution (pH 10) at 37 C.

(30) FIG. 30. AFM surface topography and thickness profile for E-beam oxide at various stages of dissolution in buffer solution with (a), (b) pH 7.4, and (c), (d) pH 8, and (e), (f) pH 10, at 37 C.

(31) FIG. 31. Dissolution kinetics, as defined by the rate of change of film thicknesses, of different silicon oxides in various aqueous solutions, with different values of pH at room and physiological temperatures. a) Calculated (lines) and measured (symbols) values for the time-dependent dissolution of thermally grown SiO.sub.2 (dry oxidation) in buffer solutions (black, pH 7.4; red, pH 8; blue, pH 10; purple, pH 12) at room (left) and physiological (right, 37 C.) temperature. b) Calculated (lines) and measured (symbols) dissolution behaviors of PECVD SiO.sub.2 in diverse aqueous solutions with different pH at room (left) and physiological (right, 37 C.) temperature. c) Calculated (lines) and experimental (symbols) results of dissolution studies on E-beam SiO.sub.2 in aqueous solutions at different pH and temperature. d) Dependence of dissolution kinetics of silicon oxide films on pH (black, tg-oxide (dry); red, tg-oxide (wet); blue, PECVD SiO.sub.2; purple, E-beam SiO.sub.2) at physiological temperature (37 C.) corresponding to experimental data (symbol) and numerical fits (line). e) Measurements of dissolution rates of silicon oxides as a function of film density in buffer solution (pH 7.4) at room (black) and physiological (red, 37 C.) temperature.

(32) FIG. 32. Bonding energy of Si 2p for tg-oxide (black, dry oxidation), PECVD oxide (red), and E-beam oxide (blue) measured by XPS.

(33) FIG. 33. Measured density of several oxides (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide) determined by XRR. The triangles indicate the critical angles.

(34) FIG. 34. TEM images and diffraction patterns (inset) of a) PECVD oxide and b) E-beam oxide.

(35) FIG. 35. AFM measurements of surface roughness of PECVD SiO.sub.2 while immersed in different pH solutions at 37 C. a) Average surface roughness (Ra) at various stages of dissolution in buffer solution (black, pH 7.4; red, pH 8; blue, pH 10; magenta, pH 12), b) surface topographic images after 6 days in buffer solution (top right, pH 7.4; bottom left, pH 10; bottom right, pH 12).

(36) FIG. 36. Dissolution kinetics via hydrolysis of various silicon nitrides in aqueous solutions at different pH and temperature. a) Calculated (lines) and measured (symbols) values for the dissolution of Si.sub.3N.sub.4 formed by low-pressure chemical-vapor deposition (LPCVD) in buffer solutions (black, pH 7.4; red, pH 8; blue, pH 10; purple, pH 12) at room (left) and physiological (right, 37 C.) temperature. b) Calculated (lines) and measured (symbols) dissolution behaviors of PECVD Si.sub.3N.sub.4 (low-frequency mode) in diverse aqueous solutions with different pH at room (left) and physiological (right, 37 C.) temperature. c) Calculated (lines) and experimental (symbols) results of dissolution study on PECVD Si.sub.3N.sub.4 (high-frequency mode) in aqueous solutions at different pH and temperature. d) Calculated (lines) and experimental (symbols) results of the dependence of dissolution kinetics of silicon nitride films on pH (black, LP-CVD Si.sub.3N.sub.4; red, PE-CVD Si.sub.3N.sub.4 (low frequency); blue, PE-CVD Si.sub.3N.sub.4 (high frequency)) at physiological temperature (37 C.). e) Measured dissolution rate of silicon nitrides as a function of film density in buffer solution (pH 7.4) at room (black) and physiological (red, 37 C.) temperature.

(37) FIG. 37. Densities of nitrides (black, LPCVD nitride; red, PECVD nitride with LF mode; blue, PECVD nitride with HF mode) measured by XRR. The triangles indicate the critical angles.

(38) FIG. 38. Measurements of changes in thickness of ALD SiO.sub.2 during immersion in buffer solution (pH 7.4) at 37 C.

(39) FIG. 39A-39E. FIG. 39(A) Structures of the components of mixture 7A of Table 3 showing reactive groups. FIG. 39(B) Component ratios and indication that degradation rate increases as hydrophobicity decreases. FIG. 39(C) Reaction scheme for forming a polyanhydride encapsulant material comprising a phosphodiester group within the polymeric chain. FIG. 39(D) Reaction scheme for forming a polyanhydride encapsulant material comprising a silyl ether group within the polymeric chain. FIG. 39(E) Reaction scheme for forming a polyanhydride encapsulant material comprising an ether group within the polymeric chain.

(40) FIG. 40. Schematic of water permeability test set-up.

(41) FIG. 41. Performance of the organic encapsulants compared to materials of other classes, such as inorganic encapsulants, as change in conductor resistance over time.

(42) FIG. 42A-C. Dissolution rates of three polyanhydrides (FIG. 42A: A1 T1, FIG. 42B: A1T2, FIG. 42C: A1T4) in buffer solutions with pH 5.7 (squares), pH 7.4 (circles) and pH 8 (triangles).

DETAILED DESCRIPTION OF THE INVENTION

(43) In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.

(44) Functional layer refers to a layer that imparts some functionality to the device. For example, the functional layer may contain semiconductor components, metallic components, dielectric components, optical components, piezoelectric components, etc. Alternatively, the functional layer may comprise multiple layers, such as multiple semiconductor layers, metallic layers or dielectric layers separated by support layers. The functional layer may comprise a plurality of patterned elements, such as interconnects running between electrodes or islands. The functional layer may be heterogeneous or may have one or more properties that are inhomogeneous. Inhomogeneous property refers to a physical parameter that can spatially vary, thereby effecting the position of the neutral mechanical plane within a multilayer device.

(45) Structural layer refers to a layer that imparts structural functionality, for example by supporting, securing and/or encapsulating device components. The invention includes transient devices having one or more structural layers, such as encapsulating layers, embedding layers, adhesive layers and/or substrate layers.

(46) Semiconductor refers to any material that is an insulator at a very low temperature, but which has an appreciable electrical conductivity at a temperature of about 300 Kelvin. In the present description, use of the term semiconductor is intended to be consistent with use of this term in the art of microelectronics and electronic devices. Useful semiconductors include those comprising elemental semiconductors, such as silicon, germanium and diamond, and compound semiconductors, such as group IV compound semiconductors such as SiC and SiGe, group III-V semiconductors such as AlSb, AlAs, AlN, AlP, BN, BP, BAs, GaSb, GaAs, GaN, GaP, InSb, InAs, InN, and InP, group III-V ternary semiconductors alloys such as AI.sub.xGa.sub.1-xAs, group II-VI semiconductors such as CsSe, CdS, CdTe, ZnO, ZnSe, ZnS, and ZnTe, group I-VII semiconductors such as CuCl, group IV-VI semiconductors such as PbS, PbTe, and SnS, layer semiconductors such as PbI.sub.2, MoS.sub.2, and GaSe, oxide semiconductors such as CuO and Cu.sub.2O. The term semiconductor includes intrinsic semiconductors and extrinsic semiconductors that are doped with one or more selected materials, including semiconductors having p-type doping materials and n-type doping materials, to provide beneficial electronic properties useful for a given application or device. The term semiconductor includes composite materials comprising a mixture of semiconductors and/or dopants. Specific semiconductor materials useful for some embodiments include, but are not limited to, Si, Ge, Se, diamond, fullerenes, SiC, SiGe, SiO, SiO.sub.2, SiN, AlSb, AlAs, AlIn, AlN, AlP, AlS, BN, BP, BAs, As.sub.2S.sub.3, GaSb, GaAs, GaN, GaP, GaSe, InSb, InAs, InN, InP, CsSe, CdS, CdSe, CdTe, Cd.sub.3P.sub.2, Cd.sub.3As.sub.2, Cd.sub.3Sb.sub.2, ZnO, ZnSe, ZnS, ZnTe, Zn.sub.3P.sub.2, Zn.sub.3As.sub.2, Zn.sub.3Sb.sub.2, ZnSi P.sub.2, CuCl, PbS, PbSe, PbTe, FeO, FeS.sub.2, NiO, EuO, EuS, PtSi, TlBr, CrBr.sub.3, SnS, SnTe, PbI.sub.2, MoS.sub.2, GaSe, CuO, Cu.sub.2O, HgS, HgSe, HgTe, HgI.sub.2, MgS, MgSe, MgTe, CaS, CaSe, SrS, SrTe, BaS, BaSe, BaTe, SnO.sub.2, TiO, TiO.sub.2, Bi.sub.2S.sub.3, Bi.sub.2O.sub.3, Bi.sub.2Te.sub.3, BiI.sub.3, UO.sub.2, UO.sub.3, AgGaS.sub.2, PbMnTe, BaTiO.sub.3, SrTiO.sub.3, LiNbO.sub.3, La.sub.2CuO.sub.4, La.sub.0.7Ca.sub.0.3MnO.sub.3, CdZnTe, CdMnTe, CuInSe.sub.2, copper indium gallium selenide (CIGS), HgCdTe, HgZnTe, HgZnSe, PbSnTe, TI.sub.2SnTe.sub.5, TI.sub.2GeTe.sub.5, AlGaAs, AlGaN, AlGaP, AlInAs, AlInSb, AlInP, AlInAsP, AlGaAsN, GaAsP, GaAsN, GaMnAs, GaAsSbN, GaInAs, GaInP, AlGaAsSb, AlGaAsP, AlGaInP, GaInAsP, InGaAs, InGaP, InGaN, InAsSb, InGaSb, InMnAs, InGaAsP, InGaAsN, InAlAsN, GaInNAsSb, GaInAsSbP, and any combination of these. Porous silicon semiconductor materials are useful for aspects described herein. Impurities of semiconductor materials are atoms, elements, ions and/or molecules other than the semiconductor material(s) themselves or any dopants provided to the semiconductor material. Impurities are undesirable materials present in semiconductor materials which may negatively impact the electronic properties of semiconductor materials, and include but are not limited to oxygen, carbon, and metals including heavy metals. Heavy metal impurities include, but are not limited to, the group of elements between copper and lead on the periodic table, calcium, sodium, and all ions, compounds and/or complexes thereof.

(47) A semiconductor component broadly refers to any semiconductor material, composition or structure, and expressly includes high quality single crystalline and polycrystalline semiconductors, semiconductor materials fabricated via high temperature processing, doped semiconductor materials, inorganic semiconductors, and composite semiconductor materials.

(48) A component is used broadly to refer to an individual part of a device. An interconnect is one example of a component, and refers to an electrically conducting structure capable of establishing an electrical connection with another component or between components. In particular, an interconnect may establish electrical contact between components that are separate. Depending on the desired device specifications, operation, and application, an interconnect is made from a suitable material. Suitable conductive materials include semiconductors and metallic conductors.

(49) Other components include, but are not limited to, thin film transistors (TFTs), transistors, diodes, electrodes, integrated circuits, circuit elements, control elements, photovoltaic elements, photovoltaic elements (e.g. solar cell), sensors, light emitting elements, actuators, piezoelectric elements, receivers, transmitters, microprocessors, transducers, islands, bridges and combinations thereof. Components may be connected to one or more contact pads as known in the art, such as by metal evaporation, wire bonding, and application of solids or conductive pastes, for example. Electronic devices of the invention may comprise one or more components, optionally provided in an interconnected configuration.

(50) Neutral mechanical plane (NMP) refers to an imaginary plane existing in the lateral, b, and longitudinal, I, directions of a device. The NMP is less susceptible to bending stress than other planes of the device that lie at more extreme positions along the vertical, h, axis of the device and/or within more bendable layers of the device. Thus, the position of the NMP is determined by both the thickness of the device and the materials forming the layer(s) of the device. In an embodiment, a device of the invention includes one or more inorganic semiconductor components, one or more metallic conductor components or one or more inorganic semiconductor components and one or more metallic conductor components provided coincident with, or proximate to, the neutral mechanical plane of the device.

(51) Coincident refers to the relative position of two or more objects, planes or surfaces, for example a surface such as a neutral mechanical plane that is positioned within or adjacent to a layer, such as a functional layer, substrate layer, or other layer. In an embodiment, a neutral mechanical plane is positioned to correspond to the most strain-sensitive layer or material within the layer.

(52) Proximate refers to the relative position of two or more objects, planes or surfaces, for example a neutral mechanical plane that closely follows the position of a layer, such as a functional layer, substrate layer, or other layer while still providing desired conformability without an adverse impact on the strain-sensitive material physical properties. Strain-sensitive refers to a material that fractures or is otherwise impaired in response to a relatively low level of strain. In general, a layer having a high strain sensitivity, and consequently being prone to being the first layer to fracture, is located in the functional layer, such as a functional layer containing a relatively brittle semiconductor or other strain-sensitive device element. A neutral mechanical plane that is proximate to a layer need not be constrained within that layer, but may be positioned proximate or sufficiently near to provide a functional benefit of reducing the strain on the strain-sensitive device element when the device is conformed to a tissue surface. In some embodiments, proximate to refers to a position of a first element within 100 microns of a second element, or optionally within 10 microns for some embodiments, or optionally within 1 micron for some embodiments.

(53) Electronic device generally refers to a device incorporating a plurality of components, and includes large area electronics, printed wire boards, integrated circuits, component arrays, biological and/or chemical sensors, physical sensors (e.g., temperature, strain, etc.), nanoelectromechanical systems, microelectromechanical systems, photovoltaic devices, communication systems, medical devices, optical devices, energy storage systems, actuators and electro-optic devices.

(54) Sensing refers to detecting the presence, absence, amount, magnitude or intensity of a physical and/or chemical property. Useful electronic device components for sensing include, but are not limited to electrode elements, chemical or biological sensor elements, pH sensors, temperature sensors, strain sensors, mechanical sensors, position sensors, optical sensors and capacitive sensors.

(55) Actuating refers to stimulating, controlling, or otherwise affecting a structure, material or device component, such as one or more inorganic semiconductor components, one or more metallic conductor components or an encapsulating material or layer. In an embodiment, actuating refers to a process in which a structure or material is selectively transformed, for example, so as to undergo a chemical or physical change such as removal, loss or displacement of a material or structure. Useful electronic device components for actuating include, but are not limited to, electrode elements, electromagnetic radiation emitting elements, light emitting diodes, lasers, magnetic elements, acoustic elements, piezoelectric elements, chemical elements, biological elements, and heating elements.

(56) An actuator is a device component that directly or indirectly initiates at least partial transformation of a transient electronic device in response to a user initiated external trigger signal, for example by initiating an at least partial transformation of a selectively transformable material of a transient electronic device. For example, an actuator may initiate at least partial transformation of a transient device by absorbing energy supplied to the device and utilizing or converting that energy to affect the at least partial transformation. For example, an actuator may initiate at least partial transformation of a transient device by exposing a device component comprising selectively transformable material to an internal or external stimulus resulting in an at least partial transformation. For example, an actuator may initiate at least partial transformation of a transient device by supplying energy (e.g., thermal, electromagnetic radiation, acoustic, RF energy, etc.) to an intermediate material or device component which affects the transformation, such as supplying energy to an encapsulating material, inorganic semiconductor components, or metallic conductor components. Thus, the actuator may comprise a single component or multiple components that alone or in combination facilitate transformation of the transient electronic device. In some embodiments, an actuator of the invention is directly or indirectly provided in communication with a transmitter, for example, via one or more receiver device components.

(57) A user initiated trigger signal includes any action, other than the mere placement of a transient device in a particular environment, by which a person may start or initiate a programmable transformation of a transient device. Exemplary user initiated trigger signals include providing real-time user input data to the device or a transmitter in communication with the device (e.g., pressing a button, flipping a switch, setting a timer, etc.), providing at least one non-ambient external source of energy directly or indirectly to the device (e.g., an electric field, a magnetic field, acoustic energy, pressure, strain, heat, light, mechanical energy, etc.), and/or programming software to execute computer-readable instructions, which may be based on data received from the device, for example data from a feedback loop. In an embodiment, the user initiated external trigger signal is an electronic signal, an optical signal, a thermal signal, a magnetic signal, a mechanical signal, a chemical signal, an acoustic signal or an electrochemical signal. In an embodiment, the invention provides a transient electronic device configured to receive a user initiated trigger signal, for example, a user initiated trigger signal provided by a transmitter and received by a receiver component of the device.

(58) A non-ambient external source of energy includes energy having a magnitude at least 10% greater, or at least 25% greater, or at least 50% greater than the magnitude of ubiquitous energy of the same form found in the environment in which a transient device is located.

(59) The terms directly and indirectly describe the actions or physical positions of one component relative to another component. For example, a component that directly acts upon or touches another component does so without intervention from an intermediary. Contrarily, a component that indirectly acts upon or touches another component does so through an intermediary (e.g., a third component).

(60) Island refers to a relatively rigid component of an electronic device comprising a plurality of semiconductor components. Bridge refers to structures interconnecting two or more islands or one island to another component. Specific bridge structures include semiconductor and metallic interconnects. In an embodiment, a transient device of the invention comprises one or more semiconductor-containing island structures, such as transistors, electrical circuits or integrated circuits, electrically connected via one or more bridge structures comprising electrical interconnects.

(61) Encapsulate refers to the orientation of one structure such that it is at least partially, and in some cases completely, surrounded by one or more other structures, such as a substrate, adhesive layer or encapsulating layer. Partially encapsulated refers to the orientation of one structure such that it is partially surrounded by one or more other structures, for example, wherein 30%, or optionally 50%, or optionally 90%, of the external surface of the structure is surrounded by one or more structures. Completely encapsulated refers to the orientation of one structure such that it is completely surrounded by one or more other structures. The invention includes transient devices having partially or completely encapsulated inorganic semiconductor components, metallic conductor components and/or dielectric components, for example, via incorporation of a polymer encapsulant, such as a biopolymer, silk, a silk composite, or an elastomer encapsulant.

(62) Barrier layer refers to a component spatially separating two or more other components or spatially separating a component from a structure, material, fluid or environment external to the device. In one embodiment, a barrier layer encapsulates one or more components. In some embodiments, a barrier layer separates one or more components from an aqueous solution, a biological tissue or both. The invention includes devices having one or more barrier layers, for example, one or more barrier layers positioned at the interface of the device with an external environment.

(63) A barrier layer(s), and optionally a sacrificial layer on a substrate, may be etched to produce a mesh structure, where at least a portion of the barrier layer(s), and optionally the sacrificial layer on a substrate, is removed. For example a portion of the barrier layer(s) disposed approximately 10 nanometers or more from an inorganic semiconductor component or additional component is removed. Removal of at least a portion of the barrier layer(s), and optionally the sacrificial layer on the substrate, may produce (i) one or more holes within the barrier layer(s) and/or (ii) electrical components, which are physically joined by a barrier layer(s) at a proximal end and physically separated at a distal end. In one embodiment, a mesh structure may be disposed upon a contiguous substrate, which provides structural support for the device during deployment into an environment.

(64) Contiguous refers to materials or layers that are touching or connected throughout in an unbroken sequence. In one embodiment, a contiguous layer of an implantable biomedical device has not been etched to remove a substantial portion (e.g., 10% or more) of the originally provided material or layer.

(65) Active circuit and active circuitry refer to one or more components configured for performing a specific function. Useful active circuits include, but are not limited to, amplifier circuits, multiplexing circuits, current limiting circuits, integrated circuits, transistors and transistor arrays. The present invention includes devices wherein the one or more inorganic semiconductor components, one or more metallic conductor components and/or one or more dielectric components comprise an active circuit or plurality of active circuits.

(66) Substrate refers to a material, layer or other structure having a surface, such as a receiving surface, that is capable of supporting one or more components or devices. A component that is bonded to the substrate refers to a component that is in physical contact with the substrate and unable to substantially move relative to the substrate surface to which it is bonded. Unbonded components or portions of a component, in contrast, are capable of substantial movement relative to the substrate. In an embodiment, the invention provides devices wherein one or more inorganic semiconductor components, one or more metallic conductor components and/or one or more dielectric components are directly or indirectly bonded to the substrate, for example, via an adhesive layer or via an adhesion layer.

(67) A selectively transformable material is a material that undergoes a physical change and/or a chemical change under pre-selected and/or predetermined conditions, such as conditions of time, pressure, temperature, chemical or biological composition, and/or electromagnetic radiation. Selectively transformable materials useful for some device applications undergo a physical transformation, such as a phase change including melting, sublimation, etc., optionally at a preselected time or at a preselected rate or in response to a preselected set of conditions or change in conditions. Selectively transformable materials useful for some device applications undergo a chemical transformation, such as decomposition, disintegration, dissolution, hydrolysis, resorption, bioresporption, photodecomposition, depolymerization, etching, or corrosion, optionally at a preselected time or at a preselected rate or in response to a preselected set of conditions or change in conditions. The pre-selected condition(s) may occur naturally, for example, provided by conditions of a device environment (e.g., ambient temperature, pressure, chemical or biological environment, natural electromagnetic radiation, etc.) or may occur via artificial condition(s) provided to, or within, a transient electronic device, such as a user or device initiated temperature, pressure, chemical or biological environment, electromagnetic radiation, magnetic conditions, mechanical strain, or electronic conditions. When the selectively transformable material of a transient electronic device is exposed to the condition(s) that initiate transformation of the material, the selectively transformable material may be substantially completely or completely transformed at a pre-selected time or a pre-selected rate. Devices of the invention include selectively transformable materials that undergo a complete transformation, substantially complete transformation or an incomplete transformation. A selectively transformable material that is substantially completely transformed is 95% transformed, or 98% transformed, or 99% transformed, or 99.9% transformed, or 99.99% transformed, but not completely (i.e., 100%) transformed. In some embodiments, a selectively transformable material undergoes a chemical change resulting in a change in a physical, chemical, electronic or optoelectronic property, optionally at a pre-selected time or at a pre-selected rate. In an embodiment, for example, a selectively transformable material undergoes a chemical or physical change resulting in a change of a first composition characterized by a conducting or semiconducting material to a second composition characterized as an insulator. In some embodiments, a selectively transformable material is a selectively removable material.

(68) A selectively removable material is a material that is physically and/or chemically removed under pre-selected or predetermined conditions such as conditions of time, pressure, temperature, chemical or biological composition, and/or electromagnetic radiation. In an embodiment, for example, a selectively removable material is removed via a process selected from the group consisting of decomposition, disintegration, dissolution, hydrolysis, resorption, bioresporption, photodecomposition, and depolymerization, optionally at a preselected time or at a preselected rate or in response to a preselected set of conditions or change in conditions. In an embodiment, for example, a selectively removable material is removed by undergoing a phase change, such as melting or sublimation, resulting in loss or relocation of the material, optionally at a preselected time or at a preselected rate or in response to a preselected set of conditions or change in conditions. The pre-selected condition(s) may occur naturally, for example, provided by conditions of a device environment (e.g., ambient temperature, pressure, chemical or biological environment, natural electromagnetic radiation, etc.) or may occur via artificial condition(s) provided to, or within, a transient electronic device, such as a user or device initiated temperature, pressure, chemical or biological environment, electromagnetic radiation, electronic conditions. When the selectively removable material of a transient electronic device is exposed to the condition(s) that initiate removal of the material, the selectively removable material may be substantially completely removed, completely removed or incompletely removed at a pre-selected time or a pre-selected rate. A selectively removable material that is substantially completely removed is 95% removed, or 98% removed, or 99% removed, or 99.9% removed, or 99.99% removed, but not completely (i.e., 100%) removed.

(69) A pre-selected time refers to an elapsed time from an initial time, t.sub.0. For example, a pre-selected time may refer to an elapsed time from a component/device fabrication or deployment, to a critical time, t.sub.c, for example, when the thickness of a selectively removable material exposed to a pre-selected condition(s) reaches zero, or substantially zero (10% or less of initial thickness, 5% or less of initial thickness, 1% or less of initial thickness) or when a property (e.g. conductance or resistivity) of a selectively removable material reaches a threshold value; e.g., a decrease in conductivity equal to 50%, optionally for some applications 80%, and optionally for some applications 95% or alternatively when conductivity equals 0. In an embodiment, the preselected time may be calculated according to:

(70) $t_c=\frac{4{}_{m}M\left(H_{2}O\right)}{{\mathrm{kw}}_{0}M\left(m\right)}\frac{\sqrt{\frac{{\mathrm{kh}}_0^2}{D}}}{\tanh \sqrt{\frac{{\mathrm{kh}}_0^2}{D}}};$
where t.sub.c is the critical time, .sub.m is the mass density of the material, M(H.sub.2O) is the molar mass of water, M(m) is the molar mass of the material, h.sub.0 is the initial thickness of the material, D is the diffusivity of water, k is the reaction constant for the dissolution reaction, and w.sub.0 is the initial concentration of water.

(71) A pre-selected rate refers to an amount of selectively removable material removed from a device or component per unit time. The pre-selected rate may be reported as an average rate (over the lifetime of the device or component) or an instantaneous rate. When a rate type is not specified, an average rate is assumed.

(72) A programmable transformation refers to a pre-selected or predetermined physical, chemical and/or electrical change within a transient electronic device that provides a change of the function of the device from a first condition to a second condition. A programmable transformation may be pre-set at the time of component/device fabrication or deployment or a real-time triggered programmable transformation controlled by a transmitter that provides a signal received by the device.

(73) A transience profile describes a change in physical parameters or properties (e.g., thickness, conductivity, resistance, mass, porosity, etc.) of a material as a function of time, e.g., thickness gained/lost over time. A transience profile may be characterized by a rate, for example, the rate of change of the physical dimensions (e.g., thickness) or physical properties (e.g., mass, conductivity, porosity, resistance, etc.) of a selectively transformable material. The invention includes selectively transformable materials having a transience profile characterized by a rate of change of the physical dimensions (e.g., thickness) or physical properties (e.g., mass, conductivity, etc.) that is constant or varies as a function of time.

(74) Degradable refers to material that is susceptible to being chemically and/or physically broken down into smaller segments. Degradable materials may, for example, be decomposed, resorbed, dissolved, absorbed, corroded, de-polymerized and/or disintegrated. In some embodiments, the invention provides degradable devices.

(75) Bioresorbable refers to a material that is susceptible to being chemically broken down into lower molecular weight chemical moieties by reagents that are naturally present in a biological environment. In an in-vivo application, the chemical moieties may be assimilated into human or animal tissue. A bioresorbable material that is substantially completely resorbed is highly resorbed (e.g., 95% resorbed, or 98% resorbed, or 99% resorbed, or 99.9% resorbed, or 99.99% resorbed), but not completely (i.e., 100%) resorbed. In some embodiments, the invention provides bioresorbable devices.

(76) Biocompatible refers to a material that does not elicit an immunological rejection or detrimental effect when it is disposed within an in-vivo biological environment. For example, a biological marker indicative of an immune response changes less than 10%, or less than 20%, or less than 25%, or less than 40%, or less than 50% from a baseline value when a biocompatible material is implanted into a human or animal. In some embodiments, the invention provides biocompatible devices.

(77) Bioinert refers to a material that does not elicit an immune response from a human or animal when it is disposed within an in-vivo biological environment. For example, a biological marker indicative of an immune response remains substantially constant (plus or minus 5% of a baseline value) when a bioinert material is implanted into a human or animal. In some embodiments, the invention provides bioinert devices.

(78) Ecocompatible refers to a material that is environmentally benign in that it may be degraded or decomposed into one or more compounds that occur naturally in the environment. In some embodiments, the invention provides ecocompatible devices.

(79) Nanostructured material and microstructured material refer to materials having one or more nanometer-sized and micrometer-sized, respectively, physical dimensions (e.g., thickness) or features such as recessed or relief features, such as one or more nanometer-sized and micrometer-sized channels, voids, pores, pillars, etc. The relief features or recessed features of a nanostructured material have at least one physical dimension selected from the range of 1-1000 nm, while the relief features or recessed features of a microstructured material have at least one physical dimension selected from the range of 1-1000 m. Nanostructured and microstructured materials include, for example, thin films (e.g., microfilms and nanofilms), porous materials, patterns of recessed features, patterns of relief features, materials having abrasive or rough surfaces, and the like. A nanofilm structure is also an example of a nanostructured material and a microfilm structure is an example of a microstructured material. In an embodiment, the invention provides devices comprising one or more nanostructured or microstructured inorganic semiconductor components, one or more nanostructured or microstructured metallic conductor components, one or more nanostructured or microstructured dielectric components, one or more nanostructured or microstructured encapsulating layers and/or one or more nanostructured or microstructured substrate layers.

(80) A nanomembrane is a structure having a thickness selected from the range of 1-1000 nm or alternatively for some applications a thickness selected from the range of 1-100 nm, for example provided in the form of a ribbon, cylinder or platelet. In some embodiments, a nanoribbon is a semiconductor, dielectric or metallic conductor structure of an electronic device. In some embodiments, a nanoribbon has a thickness less than 1000 nm and optionally less than 100 nm. In some embodiments, a nanoribbon has a ratio of thickness to a lateral dimension (e.g., length or width) selected from the range of 0.1 to 0.0001.

(81) Dielectric refers to a non-conducting or insulating material. In an embodiment, an inorganic dielectric comprises a dielectric material substantially free of carbon. Specific examples of inorganic dielectric materials include, but are not limited to, silicon nitride, silicon dioxide, silk, silk composite, elastomers and polymers.

(82) Polymer refers to a macromolecule composed of repeating structural units connected by covalent chemical bonds or the polymerization product of one or more monomers, often characterized by a high molecular weight. The term polymer includes homopolymers, or polymers consisting essentially of a single repeating monomer subunit. The term polymer also includes copolymers, or polymers consisting essentially of two or more monomer subunits, such as random, block, alternating, segmented, grafted, tapered and other copolymers. Useful polymers include organic polymers or inorganic polymers that may be in amorphous, semi-amorphous, crystalline or partially crystalline states. Crosslinked polymers having linked monomer chains are particularly useful for some applications. Polymers useable in the methods, devices and components include, but are not limited to, plastics, elastomers, thermoplastic elastomers, elastoplastics, thermoplastics and acrylates. Exemplary polymers include, but are not limited to, acetal polymers, biodegradable polymers, cellulosic polymers, fluoropolymers, nylons, polyacrylonitrile polymers, polyamide-imide polymers, polyimides, polyarylates, polybenzimidazole, polybutylene, polycarbonate, polyesters, polyetherimide, polyethylene, polyethylene copolymers and modified polyethylenes, polyketones, poly(methyl methacrylate), polymethylpentene, polyphenylene oxides and polyphenylene sulfides, polyphthalamide, polypropylene, polyurethanes, styrenic resins, sulfone-based resins, vinyl-based resins, rubber (including natural rubber, styrene-butadiene, polybutadiene, neoprene, ethylene-propylene, butyl, nitrile, silicones), acrylic, nylon, polycarbonate, polyester, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyolefin or any combinations of these.

(83) Elastomeric stamp and elastomeric transfer device are used interchangeably and refer to an elastomeric material having a surface that can receive as well as transfer a material. Exemplary conformal transfer devices useful in some methods of the invention include elastomeric transfer devices such as elastomeric stamps, molds and masks. The transfer device affects and/or facilitates material transfer from a donor material to a receiver material. In an embodiment, a method of the invention uses a conformal transfer device, such as an elastomeric transfer device (e.g. elastomeric stamp) in a microtransfer printing process, for example, to transfer one or more single crystalline inorganic semiconductor structures, one or more dielectric structures and/or one or more metallic conductor structures from a fabrication substrate to a device substrate.

(84) Elastomer refers to a polymeric material which can be stretched or deformed and returned to its original shape without substantial permanent deformation. Elastomers commonly undergo substantially elastic deformations. Useful elastomers include those comprising polymers, copolymers, composite materials or mixtures of polymers and copolymers. Elastomeric layer refers to a layer comprising at least one elastomer. Elastomeric layers may also include dopants and other non-elastomeric materials. Useful elastomers include, but are not limited to, thermoplastic elastomers, styrenic materials, olefinic materials, polyolefin, polyurethane thermoplastic elastomers, polyamides, synthetic rubbers, PDMS, polybutadiene, polyisobutylene, poly(styrene-butadiene-styrene), polyurethanes, polychloroprene and silicones. In some embodiments, an elastomeric stamp comprises an elastomer. Exemplary elastomers include, but are not limited to silicon containing polymers such as polysiloxanes including poly(dimethyl siloxane) (i.e. PDMS and h-PDMS), poly(methyl siloxane), partially alkylated poly(methyl siloxane), poly(alkyl methyl siloxane) and poly(phenyl methyl siloxane), silicon modified elastomers, thermoplastic elastomers, styrenic materials, olefinic materials, polyolefin, polyurethane thermoplastic elastomers, polyamides, synthetic rubbers, polyisobutylene, poly(styrene-butadiene-styrene), polyurethanes, polychloroprene and silicones. In an embodiment, a polymer is an elastomer.

(85) Conformable refers to a device, material or substrate which has a bending stiffness that is sufficiently low to allow the device, material or substrate to adopt any desired contour profile, for example a contour profile allowing for conformal contact with a surface having a pattern of relief features. In certain embodiments, a desired contour profile is that of a tissue in a biological environment.

(86) Conformal contact refers to contact established between a device and a receiving surface. In one aspect, conformal contact involves a macroscopic adaptation of one or more surfaces (e.g., contact surfaces) of a device to the overall shape of a surface. In another aspect, conformal contact involves a microscopic adaptation of one or more surfaces (e.g., contact surfaces) of a device to a surface resulting in an intimate contact substantially free of voids. In an embodiment, conformal contact involves adaptation of a contact surface(s) of the device to a receiving surface(s) such that intimate contact is achieved, for example, wherein less than 20% of the surface area of a contact surface of the device does not physically contact the receiving surface, or optionally less than 10% of a contact surface of the device does not physically contact the receiving surface, or optionally less than 5% of a contact surface of the device does not physically contact the receiving surface. In an embodiment, a method of the invention comprises establishing conformal contact between a conformal transfer device and one or more single crystalline inorganic semiconductor structures, one or more dielectric structures and/or one or more metallic conductor structures, for example, in a microtransfer printing process, such as dry transfer contact printing.

(87) Young's modulus is a mechanical property of a material, device or layer which refers to the ratio of stress to strain for a given substance. Young's modulus may be provided by the expression:

(88) $\begin{array}{cc}E=\frac{\left(\mathrm{stress}\right)}{\left(\mathrm{strain}\right)}=\left(\frac{L_0}{L}\right)\left(\frac{F}{A}\right),& \left(I\right)\end{array}$
where E is Young's modulus, L.sub.0 is the equilibrium length, L is the length change under the applied stress, F is the force applied, and A is the area over which the force is applied. Young's modulus may also be expressed in terms of Lame constants via the equation:

(89) $\begin{array}{cc}E=\frac{\left(3+2\right)}{+},& \left(\mathrm{II}\right)\end{array}$
where and are Lame constants. High Young's modulus (or high modulus) and low Young's modulus (or low modulus) are relative descriptors of the magnitude of Young's modulus in a given material, layer or device. In some embodiments, a high Young's modulus is larger than a low Young's modulus, preferably about 10 times larger for some applications, more preferably about 100 times larger for other applications, and even more preferably about 1000 times larger for yet other applications. In an embodiment, a low modulus layer has a Young's modulus less than 100 MPa, optionally less than 10 MPa, and optionally a Young's modulus selected from the range of 0.1 MPa to 50 MPa. In an embodiment, a high modulus layer has a Young's modulus greater than 100 MPa, optionally greater than 10 GPa, and optionally a Young's modulus selected from the range of 1 GPa to 100 GPa. In an embodiment, a device of the invention has one or more components, such as substrate, encapsulating layer, inorganic semiconductor structures, dielectric structures and/or metallic conductor structures, having a low Young's modulus. In an embodiment, a device of the invention has an overall low Young's modulus.

(90) Inhomogeneous Young's modulus refers to a material having a Young's modulus that spatially varies (e.g., changes with surface location). A material having an inhomogeneous Young's modulus may optionally be described in terms of a bulk or average Young's modulus for the entire material.

(91) Low modulus refers to materials having a Young's modulus less than or equal to 10 MPa, less than or equal to 5 MPa or less than or equal to 1 MPa.

(92) Bending stiffness is a mechanical property of a material, device or layer describing the resistance of the material, device or layer to an applied bending moment. Generally, bending stiffness is defined as the product of the modulus and area moment of inertia of the material, device or layer. A material having an inhomogeneous bending stiffness may optionally be described in terms of a bulk or average bending stiffness for the entire layer of material.

(93) Transient devices and methods of making and using the devices will now be described with reference to the figures. For clarity, multiple items within a figure may not be labeled and the figures may not be drawn to scale.

(94) FIGS. 1A-1D provide schematic diagrams illustrating side views of transient electronic devices of the invention. FIGS. 1A and 1B are directed to transient electronic devices having a multilayer encapsulant layer comprising a selectively removable inorganic material in combination with a barrier layer or electrically insulating layer. FIGS. 1C and 1D are directed to a transient electronic device having a multilayer substrate comprising a selectively removable inorganic material in combination with a barrier layer or electrically insulating layer.

(95) In FIG. 1A, transient electronic device 100A comprises substrate 105 supporting, directly or indirectly, one or more inorganic semiconductor components, one or more metallic conductor components or one or more inorganic semiconductor components and one or more metallic conductor components 110, which in some embodiments comprise semiconductor devices or semiconductor device components. Encapsulant layer 0115A is provided so as to completely or partially encapsulate the inorganic semiconductor components and/or metallic conductor components 110, for example by encapsulating at least 20%, 50%, 70% or 90% of the area or volume of the inorganic semiconductor components and/or metallic conductor components 110. As shown in FIG. 1A, encapsulant layer 115A is a multilayer structure comprising an interior layer 120, comprising a barrier layer or electrically insulating layer, and an exterior layer 125 comprising a selectively removable inorganic material, such as an inorganic thin film, foil or coating. In an embodiment, for example, interior layer 120 is an electrically insulating layer preventing electrical contact between exterior layer comprising a selectively removable inorganic material 125 and the underlying inorganic semiconductor components and/or metallic conductor components 110. In an embodiment, for example, exterior layer 125 comprises a metal foil or metal oxide layer having a preselected transience profile, wherein at least partial removal of the exterior layer 125 in response to an external or internal stimulus at least partially exposes the inorganic semiconductor components and/or metallic conductor components 110, for example, to the internal or external stimulus and/or to an external environment.

(96) In FIG. 1B, transient electronic device 100B comprises substrate 105 supporting, directly or indirectly, one or more inorganic semiconductor components, one or more metallic conductor components or one or more inorganic semiconductor components and one or more metallic conductor components 110, which in some embodiments comprise semiconductor devices or semiconductor device components. Encapsulant layer 115B is provided so as to completely or partially encapsulate the inorganic semiconductor components and/or metallic conductor components 110, for example by encapsulating at least 20%, 50%, 70% or 90% of the area or volume of the inorganic semiconductor components and/or metallic conductor components 110. As shown in FIG. 1B, encapsulant layer 115B is a multilayer structure comprising an interior layer 120 comprising a barrier layer or electrically insulating layer, and an intermediate layer 125 comprising a selectively removable inorganic material, such as an inorganic thin film, foil or coating and an exterior layer 130 comprising a barrier layer or electrically insulating layer. In an embodiment, for example, interior layer 120 is an electrically insulating layer preventing electrical contact between intermediate layer comprising a selectively removable inorganic material 125 and the underlying inorganic semiconductor components and/or metallic conductor components 110. In an embodiment, for example, exterior layer 130 is a barrier layer substantially impermeable to an external composition, such as an external solvent (e.g., water, biofluid, etc.). In an embodiment, for example, intermediate layer 125 comprises a metal foil or metal oxide layer having a preselected transience profile, wherein at least partial removal of the intermediate layer 125 and interior layer 120 in response to an external or internal stimulus at least partially exposes the inorganic semiconductor components and/or metallic conductor components 110, for example, to the internal or external stimulus and/or to an external environment.

(97) In FIG. 1C, transient electronic device 200A comprises substrate 215A supporting, directly or indirectly, one or more inorganic semiconductor components, one or more metallic conductor components or one or more inorganic semiconductor components and one or more metallic conductor components 210, which in some embodiments comprise semiconductor devices or semiconductor device components. Encapsulant layer 220 is provided so as to completely or partially encapsulate the inorganic semiconductor components and/or metallic conductor components 210, for example by encapsulating at least 20%, 50%, 70% or 90% of the area or volume of the inorganic semiconductor components and/or metallic conductor components 210. As shown in FIG. 1C, substrate 215A is a multilayer structure comprising an interior layer 230, comprising a barrier layer or electrically insulating layer, and an exterior layer 205 comprising a selectively removable inorganic material, such as an inorganic thin film, foil or coating. In an embodiment, for example, interior layer 230 is an electrically insulating layer preventing electrical contact between exterior layer comprising a selectively removable inorganic material 205 and the inorganic semiconductor components and/or metallic conductor components 210. In an embodiment, for example, exterior layer 205 comprises a metal foil or metal oxide layer having a preselected transience profile, wherein at least partial removal of the exterior layer 205 in response to an external or internal stimulus at least partially exposes the inorganic semiconductor components and/or metallic conductor components 210, for example, to the internal or external stimulus and/or to an external environment.

(98) In FIG. 1D, transient electronic device 200B comprises substrate 215B supporting, directly or indirectly, one or more inorganic semiconductor components, one or more metallic conductor components or one or more inorganic semiconductor components and one or more metallic conductor components 210, which in some embodiments comprise semiconductor devices or semiconductor device components. Encapsulant layer 220 is provided so as to completely or partially encapsulate the inorganic semiconductor components and/or metallic conductor components 210, for example by encapsulating at least 20%, 50%, 70% or 90% of the area or volume of the inorganic semiconductor components and/or metallic conductor components 210. As shown in FIG. 1D, substrate 215B is a multilayer structure comprising an interior layer 230A comprising a barrier layer or electrically insulating layer, and an intermediate layer 205 comprising a selectively removable inorganic material, such as an inorganic thin film, foil or coating and an exterior layer 230B comprising a barrier layer or electrically insulating layer. In an embodiment, for example, interior layer 230A is an electrically insulating layer preventing electrical contact between intermediate layer comprising a selectively removable inorganic material 205 and the inorganic semiconductor components and/or metallic conductor components 210. In an embodiment, for example, exterior layer 230B is a barrier layer substantially impermeable to an external composition, such as an external solvent (e.g., water, biofluid, etc.). In an embodiment, for example, intermediate layer 205 comprises a metal foil or metal oxide layer having a preselected transience profile, wherein at least partial removal of the intermediate layer 205 and interior layer 230A in response to an external or internal stimulus at least partially exposes the inorganic semiconductor components and/or metallic conductor components 210, for example, to the internal or external stimulus and/or to an external environment.

EXAMPLE 1

Inorganic Substrates and Encapsulation Layers for Transient Electronics

(99) Background and Motivation

(100) This example demonstrates a new class silicon-based electronic devices that are physically transient, for example, in the sense that they dissolve or otherwise transform at controlled rates when exposed to water in the environment or the body[1]. In some embodiments, these systems comprise transient materials, such as magnesium for metal electrodes and interconnects, MgO and/or SiO.sub.2 for gate and interlayer dielectrics, and single crystal silicon nanomembranes (Si NMs) for semiconductors. In all cases, extensive engineering studies of the key properties, including dissolution mechanisms[2], for each material are important for device engineering. On-going research indicates the ability to enhance/control the dissolution rates of transient components via control over the morphology and chemical compositions of the various functional layers. Additionally, the properties of the aqueous environments can have a strong influence. Initial experiments on Si NMs, for example, reveal the dependence of dissolution rates on various aqueous environments, such as blood, sea water, serum, tap water, and simulated body solution with different pH levels. Similar studies on various candidates for the conductive layers, for instance, Mg alloy (AZ31 B), iron (Fe), molybdenum (Mo), tungsten (W) and zinc (Zn), establish their utility and range of uses.

(101) Besides these functional layers, encapsulation and substrate materials play important roles in these systems because their transient characteristics may also define the operational lifetimes. Silk, other biomaterials and synthetic polymers are of strong interest, but their inability to serve as completely impermeable water barriers, without dimensional change associated with swelling, represent daunting technical challenges. Inorganic materials may provide compelling alternatives. A natural, attractive candidate as an encapsulant and/or component of a layered substrate construct is SiO.sub.2 deposited using various conditions in PECVD to control density and, therefore, dissolution rate. Another promising related possibility is spin-on-glass (SOG), due to its solution processability and excellent planarization properties. Metal foils have potential as mechanically tough, rugged but flexible substrates. This example describes use of these materials as substrates and encapsulation layers.

(102) The present example highlights the significance of establishing (a) dissolution rates for various inorganic materials relevant to encapsulation layers and substrates for transient electronic systems, and (b) demonstration vehicles for the use of these materials in electronic devices with practical ranges of transience times. The present example describes (1) the study of the kinetics of dissolution for silicon, silicon oxide, spin-on-glass, and metals in various solutions, (2) design and demonstration of an inorganic-based substrate system and encapsulation strategy using these materials, and (3) integration of simple transient electronic components (resistors, inductors, transistors) onto these substrates, with encapsulation, to demonstrate function.

(103) Dissolution Rates for Inorganic Materials as Substrates and Encapsulation Layers

(104) Deposited and Grown layers of Silicon Dioxide (SiO.sub.2)

(105) Silicon dioxide (SiO.sub.2) serves as an example material option for the gate and interlayer dielectrics of previously reported transient electronic devices [1]. An ability to control the rate of dissolution in this material, and to exploit it in substrates and encapsulation layers forms a focus of this example. FIG. 2 shows that a thin film of SiO.sub.2 deposited by plasma enhanced chemical vapor deposition (PECVD) dissolves in PBS solution at room temperature and physiological temperature, respectively, over the course of weeks. The chemical mechanisms here are related to those of single crystalline silicon, i.e. silicon oxide reacts water via hydrolysis to form silicic acid (Si(OH).sub.4). A key recent finding is that the dissolution rates depend strongly on deposition conditions. This example explores the dissolution of PECVD SiO.sub.2 formed at various temperatures between 200 and 500 C., as well as wet and dry thermally grown oxides (1100 C.; data in FIG. 3). Preliminary findings show that thermal oxide has extremely slow dissolution compared to low temperature PECVD material. These two examples bracket a broad range of transience times that can be accessed in this single material system, thereby highlighting its potential value as an encapsulation layer, or as a component of a layered substrate construct (see, below).

(106) Bulk Foils and Thin Film Coatings of Metals (Mg, Mg alloy, Fe, W and Zn)

(107) Conductive materials are essential for electrodes and interconnects in transient electronics. This example shows, however, that conductive materials, for example in the form of foils or coatings, also provide an important class of materials for substrates and/or encapsulation layers when combined with insulating layers to avoid unwanted electrical effects. Systematic fundamental studies of dissolution kinetics of various metals, both as bulk foils and thin film coatings, in biological environments provide important guidelines on selecting metals for these purposes. Six metals are of interest: magnesium (Mg), magnesium alloy (AZ31 B with 3 wt % aluminum and 1wt % zinc), tungsten (W), molybdenum (Mo), zinc (Zn) and iron (Fe). Preliminary dissolution measurements in de-ionized (DI) water and simulated bio-fluid (Hank's solution, pH 7.4) at room temperature appear in FIG. 4. Here, films with thicknesses of 150 nm or 300 nm were formed by electron beam (E-beam) evaporation (Fe, Mo) or magnetron sputtering (Mg, Mg alloys, Zn and W). In general, W and Fe exhibit much slower degradation rates ( few days) compared to Mg, Mg alloys and Zn (<few hours) due to the non-protective nature of their hydrolysis products, namely magnesium oxide and zinc oxide respectively. Resistance changes in Fe appear very gradually, due to the formation of a relatively protective iron oxide layer. Compared to sputtered W films, CVD W possesses a much lower resistivity and slower dissolution rate in DI water, making it ideal for long term transience. A reactive diffusion model can capture the trends for Mg, Mg alloys, Zn and W (solid lines in FIG. 4), thereby providing the ability to predict transience times as a function of thin film thickness, pH, and other parameters. Based on these results, metals combined with suitably processed SiO.sub.2 enable the degradation times that span a desired range, from minutes to months, and possibly longer. In the form of foils and layered assemblies, these materials are attractive as flexible substrates, as described next.

(108) Inorganic Substrate Structures for Transient Electronics

(109) Inorganic substrates based on coated metal foils could represent an attractive alternative to silk-based biopolymers, or synthetic polymers. A schematic illustration of one possibility appears in FIG. 5. The structure simply utilizes a metal foil (e.g. Mg foil, 5-10 m thick) as a supporting material with films of spin cast SOG as electrically insulating layers and water barriers on top and bottom. The foil imparts a level of mechanical robustness that is not present in the SOG films alone. Additional layers of PECVD SiO.sub.2 and/or metals can be added to the structure to enhance the barrier properties and to minimize probability for electrical leakage pathways, thereby offering the possibility to extend the transience times. A variety of combinations of layered structures based on the materials described in the previous section creates a rich design space.

(110) Demonstration Vehicles for Transient Electronics Based on Inorganic Substrates and Encapsulants

(111) To evaluate practical performance of encapsulants and substrate structures outlined in the previous sections, test vehicles ranging from serpentine resistors and inductors to Si NM transistors may be built. Electrical evaluation of these components may be performed as a function of time for complete immersion in DI water and PBS at physiological conditions. The kinetics associated with transience in function may be studied, and correlated to the choice of materials and layered structures.

REFERENCE

(112) 1. S.-W. Hwang, H. Tao, D.-H. Kim, H. Cheng, J.-K. Song, E. Rill, M. A. Brenckle, B. Panilaitis, S. M. Won, Y.-S. Kim, Y. M. Song, K. J. Yu, A. Ameen, R. Li, Y. Su, M. Yang, D. L. Kaplan, M. R. Zakin, M. J. Slepian, Y. Huang, F. G. Omenetto and J. A. Rogers, A Physically Transient Form of Silicon Electronics, Science 337, 1640-1644 (2012). 2. R. Li, H. Cheng, Y. Su, S.-W. Hwang, L. Yin, H. Tao, M. A. Brenckle, D.-H. Kim, F. G. Omenetto, J. A. Rogers and Y. Huang, An Analytical Model of Reactive Diffusion for Transient Electronics, Advanced Functional Materials, ASAP (2013). DOI: 10.1002/adfm.201203088

EXAMPLE 2

Inorganic Substrates and Encapsulation Layers for Transient Electronics

(113) FIG. 6 shows dissolution kinetics of SiO.sub.2 in aqueous solution at different pH and temperature (1) SiO.sub.2 thermally grown by dry or wet oxidation, (2) SiO.sub.2 deposited by plasma enhanced chemical vapor deposition and (3) SiO.sub.2 deposited by electron beam evaporation. Calculated (lines) and experimental (symbols) dissolution rates of silicon oxides in buffer solution at different pH (black, pH 7.4; red, pH 8; blue, pH 10; magenta, pH 12) at room (left) and physiological (right, 37 C.) temperature. The thickness was measured by spectroscopic ellipsometry. There was no difference is dissolution rates between thermally grown SiO.sub.2 grown by the dry or wet oxidation method.

(114) FIG. 7A shows measured data (symbols) and numerical fits (lines) for pH-dependent dissolution kinetics of oxides (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide) at room temperature and 37 C. Higher pH concentration increases the possibility of reaction between SiO and OH.sup., and therefore increases the dissolution rate. Quartz and amorphous silica showed the same trend in the literature.[1]

(115) FIG. 7B shows SiO.sub.2 film property dependency exhibited as film density versus dissolution rate. Low density films have more chances to meet reaction species, which increases dissolution rate. Low density films also have fewer atoms per layer, which causes the dissolution rate in terms of thickness to decrease faster.

(116) FIG. 8 shows a dissolution study of SiN.sub.X. SiN.sub.X is oxidized to SiO.sub.2 in a first step and converted to silicic acid in a second step.[2] Final products include NH.sub.3, which the human body produces about 4 g of daily. The same estimated NH.sub.3 amount as a device less than or equal to 1 mg.

(117) FIG. 9A shows measured data (symbols) and numerical fits (lines) for pH-dependent dissolution kinetics of nitrides (black, low pressure CVD; red, PECVD-LF; blue, PECVD-HF) at room temperature and 37 C. FIG. 9B shows SiN.sub.x film property dependency exhibited as film density versus dissolution rate.

(118) FIG. 10 shows dissolution kinetics for different oxides immersed in various aqueous solutions. A) Bovine serum (pH 7.4) at 37 C. (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide), B) sea water (pH 7.8) at RT (black, tg-oxide by dry oxidation; red, tg-oxide by wet oxidation; blue, PECVD oxide; magenta, E-beam oxide), C) Bovine serum (pH 7.4) at 37 C. (black, LPCVD nitride; red, PECVD nitride with LF mode; blue, PECVD nitride with HF mode), D) sea water (pH 7.8) at RT (black, LPCVD nitride; red, PECVD nitride with LF mode; blue, PECVD nitride with HF mode). Dissolution rates of SiO.sub.2/SiN.sub.x in bovine serum and sea water are 10 and 4 times faster than buffer solution under the same conditions. Likely, the cations in bovine serum and sea water (K.sup.+, Na.sup.+, Ca.sup.2+and Mg.sup.2+) accelerate the dissolution rate of SiO.sub.2.

(119) FIG. 11 shows the curing mechanism for spin-on-glass and dissolution studies of spin-on-glass encapsulation layers cured at various temperatures and times. Silicate based spin-on glass (SOG) is dissolvable, and dissolution rate can be controlled by adjusting curing conditions and thickness. Increasing curing temperature and time gives slower dissolution rates due to fewer OH bonds in the cured SOG. Combining SOG with other oxide layers improves inorganic encapsulation properties.

(120) FIG. 12 shows pH dependent dissolution studies of amorphous silicon (a-Si), polycrystalline silicon (p-Si) and SiGe. The dissolution rate of a-Si/p-Si is similar to single crystalline Si (100), which means the dissolution of Si does not depend on type of Si. Dissolution of SiGe (8:2 wt %) is slightly slower than single crystalline Si because SiGe comprises mostly SiSi bonds with relatively few SiGe bonds (<10%).

(121) FIG. 13 shows electrical dissolution rates and thicknesses of sputter deposited Mg, Mg alloy (AZ31 B, Al 3%, Zn 1%), Zn, Mo, W, CVD deposited W, and E-beam evaporated Fe in DI water and Hanks' solution with pH 5-8.

(122) FIG. 14 shows current versus voltage plots for devices containing transient metal components. N-type MOSFETs built with transient metal thin films exhibit on/off ratios >10.sup.4 and mobilities 250 cm.sup.2/Vs. Functionality degradation in DI water was measured without encapsulation at V.sub.d=0.2 V.

(123) FIG. 15 shows photographs of the dissolution of a transient transistor array on a biodegradable metal foil.

(124) FIG. 16 provides plots of dissolution kinetics of various metal foils (Mo, Zn, Fe, W) under physiological conditions (PBS, pH 7.4, 37 C.).

(125) FIG. 17 provides a schematic of a fabrication strategy using metal foils involving: 1) Lamination of metal foil on a carrier substrate (e.g., PDMS coated on glass), 2) fabrication of a transistor array directly on the metal foil, and 3) peeling of the device from the carrier substrate.

(126) FIG. 18 demonstrate the use of inorganic substrates. For example, metal foils, such as Fe with a thickness of 5 m or Mo with a thickness of 10 m, may be used as substrates to support device fabrication. Devices may also be inorganic, e.g. Mg-based transitors. The entire array may be flexible, as shown.

(127) FIG. 19 provides schematic illustrations of encapsulation methods for transient electronic devices, showing defects (e.g. pinholes) covered by a bilayer of SiO.sub.2/Si.sub.3N.sub.4; ALD provides a defect-free layer. Defects, such as pinholes, are the primary cause of leakage of vapors or fluids in encapsulation with PE-CVD SiO.sub.2 and SiN.sub.x. Multilayer structures of both silicon oxides and silicon nitrides can reduce such defects. An ALD layer has fewer defects than a PE-CVD layer.

(128) FIG. 20 shows measurements of changes in resistance of Mg traces (300 nm thick) encapsulated with different materials and thicknesses while immersed in deionized (DI) water at room temperature. A single layer of ALD SiO.sub.2 (orange, 20 nm), PECVD SiO.sub.2 (black, 1 m) and PECVD-LF Si.sub.3N.sub.4 (red, 1 m), a double layer of PECVD SiO.sub.2/PECVD-LF Si.sub.3N.sub.4 (blue, 500/500 nm), PECVD SiO.sub.2/ALD SiO.sub.2 (magenta, 500/20 nm), PECVD-LF Si.sub.3N.sub.4/ALD SiO.sub.2 (purple, 500/20 nm), and a triple layer of PECVD SiO.sub.2/PECVD-LF Si.sub.3N.sub.4 (Cyan, 200/200/200/200/100/100 nm) were used for the encapsulation.

(129) FIG. 21 shows a series of micrographs of a serpentine trace of Mg (initially 300 nm thick) during dissolution in DI water at room temperature. Dissolution begins from local defects, then rapidly propagates outward.

(130) FIG. 22 shows a demonstration of the electrical properties of electronic devices fabricated on metal foils. (A) Transistor array on Fe foil (10 m thick), (B) Diode array on Zn foil (10 m thick), (C) Capacitor array on Mo foil (10 m thick), (D) Inductor array on Mo foil (10 m thick). Electronic devices were successfully fabricated on biodegradable metal substrates (Fe, W, Mo, Zn, Mg). The performance of transient devices on metal substrates is comparable to devices fabricated on non-transient substrates.

(131) FIG. 23 shows transience of transient devices on inorganic substrates with inorganic encapsulation. (A) Transistor on Mo foil with MgO encapsulation (800 nm), (B) Diode on Mo foil with MgO encapsulation (800 nm), (C) Capacitor on Mo foil with MgO encapsulation (800 nm), (D) Inductor on Mo foil with MgO encapsulation.

(132) FIG. 24 shows a sample structure for a dissolution test of single wafer SiGe (Ge) with atomic force microscopy (AFM). (a) Schematic illustration of test structure: array of square holes (3 m3 m20 nm) of PECVDSiO.sub.2 mask on SiGe single wafer. (b) AFM topographical images and c) profiles of SiGe, at different stages of dissolution in buffer solution (pH 10) at 37 C.

(133) FIG. 25 shows dissolution kinetics of various semi-conductors in various buffer solutions, with different pH at room and physiological temperatures. (a) polycrystalline silicon, (b) amorphous silicon, (c) silicon-germanium and (d) germanium.

(134) FIG. 26 shows dissolution kinetics of different types of silicon in various aqueous solutions. (a) Tap water (pH 7.8), deionized water (DI, pH 8.1) and spring water (pH 7.4), (b) Coke (pH 2.6), (c) milk (pH 6.4) at room temperature, (d) bovine serum (pH 7.4) at room and 37 C., and (e) sea water (pH 7.8) at room temperature. (f) Changes in resistance of a meander trace formed from a phosphorous doped polycrystalline and amorphous Si NM (35 nm) in phosphate buffer solution (pH 10) at 37 C.

(135) FIG. 27 shows a thin film solar cell with fully transient materials. (a), (b) Image and structure of amorphous Si based photovoltaic cell array on degradable substrate. (c) Performance of unit cell of solar cells. (d) Electrical transience behavior of a-Si diode and (e) transience of performance of solar cell.

REFERENCES

(136) [1] K. G. Knauss & T. J. Wolery, Geochim. Cosmochim. Acta 52, 43-53 (1998). W. A. House & L. A. Hickinbotham, J. Chem. Soc. Faraday, Trans. 88, 2021-2026 (1992). [2] E. Laarz et al., J. Am. Ceram. Soc. 83, 2394-2400 (2000). Toxicological Profile for Ammonia, published by the U.S. Department of Health and Human Services, ATSDR (2004).

EXAMPLE 3

Dissolution Behaviors and Applications of Silicon Oxides and Nitrides in Transient Electronics

(137) Background and Motivation

(138) Silicon oxides and nitrides are key materials for dielectrics and encapsulations in a class of silicon-based high performance electronics that has the ability to completely dissolve in a controlled fashion with programmable rates, when submerged in bio-fluids and/or relevant solutions. This type of technology, referred to as transient electronics, has potential applications in biomedical implants, environmental sensors and other envisioned areas. The results presented here provide comprehensive studies of transient behaviors of thin films of silicon oxides and nitrides in diverse aqueous solutions at different pH scales and temperatures. The kinetics of hydrolysis of these materials primarily depends on not only pH levels/ion concentrations of solutions and temperatures, but also morphology and chemistry of films determined by the deposition methods and conditions. Encapsulation strategies with a combination of layers demonstrate enhancement of the lifetime of transient electronic devices, by reducing water/vapor permeation through the defects.

(139) Introduction

(140) Materials for insulation, passivation and encapsulation in microelectronics are critically important for proper operation of the devices. Silicon oxides and nitrides are in widespread use not only for digital and analog circuits but also for thin film display electronics and others, due to their excellent properties as gate and interlayer dielectrics, passivation coatings,.sup.[1-3] and barriers against water penetration..sup.[4-5] This paper explores the materials aspects of use of these films in a different, emerging class of electronics, whose defining characteristic is solubility in water, with environmentally and biologically benign end products. This type of technology, sometimes referred to as one type of transient electronics, could be important for temporary biomedical implants, resorbable sensors and monitors for the environment, green disposable consumer devices and other systems that are not well served by conventional electronics, which last for decades and involve biologically and environmentally harmful materials. Initial demonstrations relied either on miniaturized, non-degradable inorganic components integrated with resorbable silk substrates and encapsulating layers,.sup.[6,7] or on synthetic and/or nature-inspired organic active and passive materials..sup.[8-10] An important advance followed from the observation that monocrystalline, device-grade silicon in ultrathin forms (i.e. nanomembranes), can dissolve, at various rates, in different types of biofluids as well as in seawater and other naturally occurring forms of water, all of relevance to envisioned applications. The end product, silicic acid, is biocompatible and environmentally benign at the low levels of concentration that are associated with small nanomembranes of silicon. Representative examples of demonstration devices include high performance complementary metal-oxide-semiconductor (CMOS) transistors and simple circuits, solar cells, strain/temperature sensors, digital imaging devices, wireless power scavenging systems and others..sup.[11-13] Additional inorganic semiconductor options include ZnO, of interest in part due to its piezoelectric properties, for transient mechanical energy harvesters, actuators and others..sup.[14] In most of these examples, MgO, which undergoes hydrolysis to Mg(OH).sub.2, serves as the dielectric and encapsulation layer. Initial observations suggested that SiO.sub.2 might provide another option. Here, we study this material in detail, and also provide evidence that SiN.sub.x represents another alternative.

(141) Dissolution Studies of Different Types of Oxides

(142) Previous work on bulk materials establishes that the mechanism for hydrolysis of silicon oxides is SiO.sub.2+2H.sub.2O.fwdarw.Si(OH).sub.4..sup.[15-17] Because OH.sup.31 initiates this reaction, the concentration of OH.sup. (pH of solution) strongly influences the dissolution rate, as observed in studies of the dissolution kinetics of quartz and amorphous silica..sup.[15-18] Here, we examine materials in forms and with chemistries widely utilized in the semiconductor industry, as thin films grown/deposited using standard or slightly modified techniques. The results reveal essential aspects of hydrolysis in such cases, including the influence of morphology and chemistry, as defined by the conditions and methods for deposition. To examine the dependence of the dissolution rate on pH and type of oxide, systematic studies were performed in buffer solutions with pH between 7.4 to 12, and at different temperatures. Three different classes of materials were examinedthin films of oxides formed by 1) growth using dry (O.sub.2 gas) and wet (H.sub.2O vapor) thermal oxidation (tg-oxide), 2) plasma enhanced chemical vapor deposition (PECVD oxide) and 3) electron-beam (E-beam oxide) evaporation.

(143) Spectroscopic ellipsometry (J. A. Wooldman Co. Inc., USA) revealed the dissolution rate as a time dependent change in thickness. Atomic force microscopy (AFM, Asylum Research MFP-3D, USA) provided information on the surface topography as well as independent measurements of thickness. Test structures of PECVD and E-beam materials for AFM measurements consisted of arrays of isolated square films (3 m3m100 nm) patterned on tg-oxide, whose dissolution rate is much slower than that of other materials, as shown subsequently. FIG. 28a presents a schematic illustration of a test structure, and an optical micrograph in the inset. FIGS. 28b and 28c provide AFM images and thickness profiles at several stages of immersion in aqueous buffer solution (pH 12) at 37 C. (Additional AFM images appear in FIGS. 29 and 30). These results indicate that the oxides dissolve in a uniform fashion, without any significant change in surface topography, formation of flakes or other non-ideal behaviors like those observed, for example, in transient metals under similar conditions..sup.[19] In all cases, samples were immersed in 50 mL of aqueous solutions, removed, rinsed and dried, and then measured (spectroscopic ellipsometry; AFM). After measurements (total times of several hours), samples were placed back into fresh solutions. The solutions were replaced every other day. (The dissolution rates, for all cases examined, did not change substantially for various time intervals for solution replacement (e.g. for every 1, 2, 4 or 7 days).

(144) FIG. 31a-c and FIG. 6(1) provide the dissolution kinetics of tg-oxide (dry and wet oxidation), PECVD oxide and E-beam oxide in terms of the change in thickness as a function of time in buffer solutions (pH 7.4 to 12) at room temperature (RT) and 37 C. The tg-oxides and E-beam oxide exhibit the slowest and fastest rates, respectively, under the same conditions. Four main factors affect the rate: temperature, pH and ionic content of the solutions, and chemical/morphological properties of the films. The dissolution rate of each oxide increases with temperature, with an expected Arrhenius dependence, consistent with previous studies..sup.[17, 20]

(145) FIG. 31d shows a linear dependence of the dissolution rate for each type of oxide in buffer solutions with different pH at (see more details at RT in FIG. 7), similar to related observations..sup.[15-17] The relationship can be written log r=a+n[pH], where r is the dissolution rate, and a and n are constants (n=0.33 for quartz when r is in mol/m.sup.2s)..sup.[16] The values of n for the data in FIG. 31d and FIG. 7 are between 0.31 to 0.44 (at 37 C.) and 0.22 to 0.62 (at RT), respectively. The kinetics can also be influenced by the concentration of ions in the solution..sup.[21,22] As an example, bovine serum (pH 7.4) and sea water (pH 7.8) show rates that are 9 and 4 times higher than those observed at similar pH in buffer solution, respectively, likely due to the presence of additional ions (ex. K.sup.+, Na.sup.+, Ca.sup.2+ and Mg.sup.2+) in these liquids..sup.[21,22]

(146) The dissolution rate can also be affected, of course, by the physical and chemical properties of the films, which in turn depend on growing/deposition methods and conditions. Thermal oxide is known to be uniformly dense..sup.[23] Oxide created by PECVD can show different stoichiometries and densities, due to by-products from the Si H.sub.4 source gas as it reacts with Si to form SiH. Such effects can be particularly important for low temperature deposition..sup.[24] E-beam oxide formed from a pure source of SiO.sub.2 (i.e. pellets) can involve nanoscale fragmentation during evaporation, which can potentially lead to alterations in the stoichiometry and reductions in density..sup.[25]

(147) X-ray photoelectron spectroscopy (XPS) and X-ray reflectometry (XRR) reveal the stoichiometries, atomic bond configurations and densities. The tg-oxide (dry oxidation) and PECVD oxide have chemistries close to SiO.sub.2 (i.e., Si:O=1:2), while the E-beam oxide is oxygen rich, at SiO.sub.2.2 (Si:O=1:2.2), as shown in Table 1.

(148) TABLE-US-00001 TABLE 1 Atomic concentration of different silicon oxides measured by XPS. Carbon (C) and fluorine (F) are considered surface contamination. (%) Si O C F x (SiO.sub.x) tg-SiO.sub.2 (dry) 31.6 64.0 4.3 0.1 2.0 PECVD SiO.sub.2 31.5 63.5 3.6 1.4 2.0 E-beam SiO.sub.2 27.5 61.3 10.8 0.4 2.2

(149) The Si 2P spectra (FIG. 32) indicate that the SiO bond energies are almost identical for the three oxides. FIG. 31e and FIG. 33 show the dependence of the dissolution rate on the film density (2.3 g/cm.sup.3 for tg-oxides, 2.1 g/cm.sup.3 for PECVD oxide, 1.9 g/cm.sup.3 for E-beam oxide). Reduced density can enhance the ability of aqueous solutions to diffuse into the material, thereby accelerating the hydrolysis reaction by increasing the reactive surface area..sup.[11,19,26] Previous research.sup.[11,19,26] suggests that a reactive diffusion model can capture some of the behaviors. A modified version of this model, assuming applicability of continuum physics, provides a simple, approximate means to incorporate the effect of density variations associated with porosity. Here, the concentration of water in the porous material is first determined from the partial differential equation for reactive diffusion D.sub..sup.2w/z.sup.2kw=w/t , where k and D.sub.e are the reaction constant and the diffusivity in the porous media, respectively. Since the mass of the air pore is negligible compared with that of the porous material, the effective density .sub.eff of the porous material is related to the density .sub.s of the fully dense material as

(150) $\begin{array}{cc}{}_{\mathrm{eff}}=\frac{V_s}{V_{\mathrm{air}}+V_s}{}_s,& \left(1\right)\end{array}$
where V.sub.s and V.sub.air are the volumes of material and air pore, respectively. At time t=0, the air pores are filled with water, or w|.sub.t=0=w.sub.0(.sub.s.sub.eff)/, (0z<h.sub.0). The water concentration is constant at the top surface of the material w|.sub.z=h0=w.sub.0(w.sub.0=1 g/cm .sup.3) and the water flux is zero at the bottom surface w/z|.sub.z=0=0. By the method of separation of variables, the water concentration field can be written

(151) $\begin{array}{cc}w\left(y,t\right)=w_0\left\{\frac{\cosh \left(\sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}\frac{y}{h_0}\right)}{\cosh \sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}+2\underset{n=1}{\overset{}{.Math.}}{B_n\left(-1\right)}^n\left(n-\frac{1}{2}\right)e^{-\left[\frac{{\mathrm{kh}}_0^2}{D_e}+{\left(n-\frac{1}{2}\right)}^2{}^2\right]\frac{D_{e}t}{h_0^2}}\cos \left[\left(n-\frac{1}{2}\right)\frac{y}{h_0}\right]\right\},& \left(2\right)\end{array}$
where B.sub.n is

(152) $\begin{array}{cc}{B}_n=\frac{1}{\frac{{\mathrm{kh}}_0^2}{D_e}+{\left(n-\frac{1}{2}\right)}^2{}^2}+\frac{\frac{{}_{\mathrm{eff}}}{{}_s}-1}{{\left(n-\frac{1}{2}\right)}^2{}^2}.& \left(3\right)\end{array}$

(153) When one mole of material reacts with q moles of water, then integration of materials dissolved at each location through the thickness and over time leads to an expression for the remaining thickness h, normalized by its initial thickness h.sub.0 as

(154) $\begin{array}{cc}\frac{h}{h_0}=1-\frac{w_{0}M}{q{}_{\mathrm{eff}}{M}_{H_{2}O}}\frac{{\mathrm{kh}}_0^2}{D_e}\left\{\frac{D_{e}t}{h_0^2}\frac{\tanh \sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}{\sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}-2\underset{n=1}{\overset{}{.Math.}}{B}_n\frac{1-e^{-\left[\frac{{\mathrm{kh}}_0^2}{D_e}+{\left(n-\frac{1}{2}\right)}^2{}^2\right]\frac{D_{e}t}{h_0^2}}}{\left[\frac{{\mathrm{kh}}_0^2}{D_e}+{\left(n-\frac{1}{2}\right)}^2{}^2\right]}\right\}.& \left(4\right)\end{array}$

(155) The effective diffusivity of water in a porous medium is linearly proportional to the pores available for the transport, which is equivalent to the air fraction in the porous medium

(156) $\begin{array}{cc}{D}_e\frac{V_{\mathrm{air}}}{V_{\mathrm{air}}+V_s}=\frac{{}_s-{}_{\mathrm{eff}}}{{}_s}.& \left(5\right)\end{array}$

(157) The density of SiO.sub.2 is 2.33 g/cm.sup.3, 2.10 g/cm.sup.3 and 1.90 g/cm.sup.3 for the case of thermally grown, PECVD and E-beam oxides, respectively. If SiO.sub.2 with a density of 2.34 g/cm.sup.3 has a diffusivity of 810.sup.16 cm.sup.2/S at body temperature, then the diffusivities for PECVD SiO.sub.2 and E-beam SiO.sub.2 can be calculated from Equation (5) as 1.610.sup.14 cm.sup.2/s and 2.9210.sup.14 cm.sup.2/s. Reaction constants are fitted to the experimental data and the dissolution rate, dh/dt, is then estimated from

(158) 0$\begin{array}{cc}-\frac{\mathrm{dh}}{\mathrm{dt}}=\frac{w_{0}M}{q{}_{\mathrm{eff}}{M}_{H_{2}O}}{\mathrm{kh}}_0\left\{\frac{\tanh \sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}{\sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}-2\underset{n=1}{\overset{}{.Math.}}{B}_n{e}^{-\left[\frac{{\mathrm{kh}}_0^2}{D_e}+{\left(n-\frac{1}{2}\right)}^2{}^2\right]\frac{D_{e}t}{h_0^2}}\right\},& \left(6\right)\end{array}$
which can be simplified to

(159) $\begin{array}{cc}-\frac{\mathrm{dh}}{\mathrm{dt}}=\frac{w_{0}M}{q{}_{\mathrm{eff}}{M}_{H_{2}O}}{\mathrm{kh}}_0\frac{\tanh \sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}{\sqrt{\frac{{\mathrm{kh}}_0^2}{D_e}}}.& \left(7\right)\end{array}$

(160) In FIG. 31a-c, the reaction constants (k) are 1.710.sup.9 (tg-oxide with dry oxidation), 1.610.sup.8 (PECVD oxide), and 1.310.sup.9 (E-beam oxide) s.sup.1 in buffer solution with pH 7.4 at 37 C. The results suggest that density influences the dissolution rate not only through changes in rates for diffusion into the material, but also through differences in reactivity. One possibility is that dissolution can occur not just at a molecular level, but also through removal of nanoscale pieces of material that might be released from the film as narrow regions of the porous matrix disappear by hydrolysis. Careful transmission electron microscopy (TEM, JEOL 2010F, USA) studies (FIG. 34) suggest, however, that the porous structures in the PECVD and E-beam oxides do not involve voids with dimensions larger than one or two nanometers. AFM observation of surfaces with sub-nanometer roughness (average roughness <0.4 nm, FIG. 35) throughout the course of the dissolution process also supports the notion that the film disappears uniformly and gradually, at the molecular level, without the release of pieces of material. Additional work is necessary to uncover an atomic level understanding of the dependence of reactivity on density.

(161) Dissolution Studies of Various Classes of Nitrides

(162) Studies of the dissolution kinetics of silicon nitrides were performed in procedures and under conditions similar to those for the silicon oxides. Silicon nitride hydrolyzes in aqueous solution in two steps: (1) oxidation into silicon oxide (Si.sub.3N.sub.4+6H.sub.2O.fwdarw.3SiO.sub.2+4NH.sub.3) and (2) hydrolysis of silicon oxide (SiO.sub.2+2H.sub.2O.fwdarw.Si(OH).sub.4), where the overall reaction is Si.sub.3N.sub.4+12H.sub.2O.fwdarw.3Si(OH).sub.4+4NH.sub.3..sup.[27-29] Because silicon dioxide serves as an intermediate product in these reactions, the dependence of rate on pH might be expected to be similar to that observed in the oxides. Low pressure chemical vapor deposition (LPCVD) and PECVD techniques were used to form the silicon nitrides studied here. For PECVD nitrides, two different frequency modes were employed to vary the properties of the films, including residual stress. Spectroscopic ellipsometry revealed the changes in thickness of films deposited on silicon substrates.

(163) FIG. 36a-c shows the dissolution behavior of LPCVD nitride, PECVD-LF nitride (low frequency, LF) and PECVD-HF nitride (high frequency, HF) in buffer solutions (pH 7.4 to 12) at RT and 37 C. Here, three factors (temperature, pH and film characteristics) were considered. The dissolution rate increases with temperature, as expected. FIG. 36d and FIG. 9 show the pH dependence, which is similar to that observed in the oxides. The kinetics suggests a linear relationship between dissolution rate and pH according to log r=a+n [pH], where n ranges from 0.11 to 0.28 for 37 C. and 0.26 to 0.31 for RT. As with the oxides, the nitrides were studied in bovine serum at 37 C. and sea water at RT; the rates are 8 times and 4 times higher than those at similar pH in buffer solution, likely due to chemical substances in the serum and sea water (FIG. 10).

(164) Effects of stoichiometry and density were also investigated. Table 2 shows that the stoichiometry of the LPCVD film is Si.sub.3N.sub.3.9, while that of the PECVD films is Si.sub.3N.sub.4.3(LF) and Si.sub.3N.sub.3.3 (HF).

(165) TABLE-US-00002 TABLE 2 Atomic concentration of different silicon nitrides measured by XPS. Carbon (C) and fluorine and (F) are considered surface contamination (%) Si N F C x (Si.sub.3N.sub.x) LPCVD Si.sub.3N.sub.4 36.3 47.8 5.3 10.7 3.9 PECVD-LF Si.sub.3N.sub.4 35.2 50.6 6.2 8.1 4.3 PECVD-HF Si.sub.3N.sub.4 36.9 40.1 11.8 11.2 3.3

(166) FIG. 36e and FIG. 37 show the dependence of the dissolution rate on average film density. The densities are 3.1 g/cm.sup.3 for LPCVD, 3.0 g/cm.sup.3 for PECVD-LF and 2.5 g/cm.sup.3 for PECVD-HF. The results suggest that LPCVD nitride exhibits the lowest dissolution rate, at least partly due to its favorable stoichiometry and high density. PECVD-HF nitride shows the fastest dissolution rate due to its non-stoichiometric chemistry and its low density. The modified reactive diffusion model described previously can provide some utility in capturing the effects of porosity, subject to limitations associated with its approximations. From the results of FIG. 36a-c, the reaction constants (k) were found to be 8.010.sup.8, 4.510.sup.7 , and 4.010.sup.7 s.sup.1 for LPCVD nitride, PECVD-LF nitride and PECVD-HF nitride, respectively, in buffer solution (pH 7.4) at 37 C. by modified reactive diffusion model where the density of closely packed amorphous nitrides was 3.16 g/cm.sup.3.

(167) Encapsulation Strategy with Inorganic Layers

(168) In addition to their use as gate and interlayer dielectrics, silicon oxides and nitrides can be considered as transient passivation/encapsulation layers. These materials are well known to be good barrier materials for permeation of water vapor in conventional electronics..sup.[4,5,30,31] Previous research.sup.[4,31] on encapsulation with PECVD oxide and nitride in organic light-emitting diode (OLED) devices indicates that defects, such as pinholes, are a primary cause of leakage of vapors or fluids. We show here that multilayer structures of both silicon oxides and nitrides can reduce such defects and that these materials can be used in transient electronics.

(169) As shown in FIG. 19, a combination of multiple different layers, i.e. SiO.sub.2 and Si.sub.3N.sub.4, improves the performance of the encapsulation. Multiple layers with different materials can reduce water/vapor permeation through an underlying layer, by cooperative elimination of defects..sup.[4,31] Atomic layer deposition (ALD) provides a complementary strategy to reduce effects arising from defects..sup.[32,33] A double layer of PECVD SiO.sub.2 (or PECVD-LF Si.sub.3N.sub.4) and ALD SiO.sub.2 represents effective means of encapsulation, even with thin layers (FIG. 19). The dissolution rate of a single layer of ALD SiO.sub.2 is 0.08 nm/day in buffer solution (0.1 M, pH 7.4) at 37 C. (FIG. 38), similar to that of PECVD SiO.sub.2 in the same conditions.

(170) FIG. 20 presents measured changes in resistance of a serpentine-shaped Mg trace (300 nm), with several encapsulation approaches at various times for immersion in deionized water at room temperature. Samples with a single layer of ALD SiO.sub.2 (20 nm), PECVD SiO.sub.2 or Si.sub.3N.sub.4 (1 m) show increases in resistance after just a few hours of immersion. A combination of PECVD SiO.sub.2 (500 nm) and Si.sub.3N.sub.4 (500 nm) extends this time to 1 day. Triple layers of PECVD SiO.sub.2 and Si.sub.3N.sub.4 (200 nm/200 nm/200 nm/200 nm/100 nm/100 nm, total thickness 1 m) extend to 10 days. Combinations of PECVD SiO.sub.2/ALD SiO.sub.2 (500/20 nm) and PECVD Si.sub.3N.sub.4/ALD SiO.sub.2 (500/20 nm) show characteristic times of 5 and 7 days, respectively. These results suggest that the ALD SiO.sub.2 layer has much fewer defects than PECVD SiO.sub.2 or Si.sub.3N.sub.4. A single layer of ALD (20 nm) provides similar timescale as a single layer of SiO.sub.2 or Si.sub.3N.sub.4 (1 m). Although combined used of PECVD SiO.sub.2 and ALD SiO.sub.2 shows extended lifetimes, a single layer of ALD SiO.sub.2 itself is not sufficiently thick to cover uniformly the sorts of structures found in transient electronics, with the Mg resistor (300 nm) as a simple example. These dissolution behaviors lead to two-stage kinetics in the functional transience of this test structure: i) encapsulation layers define the first time period, i.e. stable operation with negligible changes in electrical properties, ii) the Mg defines the second, i.e. rapid degradation in function. The optical microscope images in FIG. 21 clearly reveal that the dissolution of Mg begins with leakage of water from local defects, which then quickly propagate laterally. These results suggest that an efficient encapsulation strategy is critically important in removing these leakage pathways, to increase the time for stable operation. Also, encapsulation with these inorganic materials can be improved by combined use of biodegradable polymers as suggested in previous encapsulation studies in OLED devices..sup.[31,34]

(171) Conclusion

(172) The results reported here provide a foundation of understanding of hydrolysis in silicon oxides and nitrides for applications in transient electronics, and their dependence on temperature, pH and film properties. An appealing aspect of these materials for these applications is that they are already well developed and widely used in conventional electronics. Opportunities range not only from gate and interlayer dielectrics to passivation and encapsulation layers but also to window layers and antireflection coatings in photovoltaics or optoelectronics systems.

(173) Studies of the kinetics of hydrolysis of thin films of silicon oxides and nitrides and the use of these materials as encapsulants were presented for applications. Dissolution rates of various types of silicon oxides and nitrides were examined for their dependence on the pH and ionic concentration, temperature of the solution and the morphology and chemistry of films. The encapsulation approaches based on multiple, different thin layers of oxides and nitrides prevent water permeation for up to 10 days in simple transient electronic test structures.

(174) Experimental Section

(175) Test structures for silicon oxides and nitrides: Thin layers of silicon oxides (SiO.sub.2) were prepared in three different ways, all on silicon wafers (University Wafer): (1) Thermally grown (tg-oxides) (dry and wet oxidation), (2) plasma-enhanced chemical vapor deposited from precursor gases (PECVD, Trion Technology, USA) at 350 C., and (3) electron beam (E-beam) evaporated from SiO.sub.2 pellets (99.99%, Kurt J. Lesker Company, USA). The nitrides were deposited onto similar wafers. The films were formed by low-pressure chemical vapor deposition (LPCVD) and by PECVD (Surface Technology Systems, Newport, UK) at 300 C. using low frequency (LF, 380 kHz) and high frequency (HF, 13.56 MHz). In all cases, the thickness was controlled at 100 nm. The test structures for measurement by atomic force microscope (AFM) consisted of arrays of square pads (3 m3 m100 nm), fabricated by photolithography and reactive ion etching (RIE).

(176) Dissolution experiments: Samples were placed into 50 mL of aqueous buffer solutions with different pH (pH 7.4-12, Sigma-Aldrich, USA) at either room temperature (RT) or physiological temperature (37 C.). Studies also involved bovine serum (pH 7.4, Sigma-Aldrich, USA) at 37 C. and sea water (pH 7.8) at room temperature. In all cases, the samples were removed from the solutions, rinsed with DI water, and measured by spectroscopic ellipsometry (J. A. Wooldman Co. Inc., USA) to determine thickness and/or atomic force microscopy (AFM, Asylum Research MFP-3D, USA) to determine both the thickness and surface morphology. After such measurements, each of which lasted a few hours, the samples were returned to the solutions. The solutions were replaced every other day.

(177) Characterization of film properties: Film density was measured using X-ray reflectometry (XRR, Xpert MRD System, Netherlands). X-ray photoelectron spectroscopy was performed with a system from Axis ULTRA, UK. To avoid surface oxidation of the nitrides, the measurements were conducted shortly after oxide removal in buffered oxide etchant (BOE, 6:1, Transene Company Inc., USA) for a few seconds. Transmission electron microscopy (TEM, JEOL 2010F (S)TEM, USA) was used to study the porous microstructure of the PECVD and E-beam oxides.

(178) Encapsulation tests: Serpentine traces of Mg (300 nm thick) were defined by e-beam evaporation and liftoff using a patterned layer of photoresist (AZ 2070, MicroChem, USA) on a glass substrate. Each trace was then encapsulated with various overcoats of PECVD SiO.sub.2, PECVD-LF Si.sub.3N.sub.4, and ALD SiO.sub.2 (Savannah, Cambridge Nanotech, USA). Encapsulation layers at both ends of the trace were removed by RIE, to allow continuous measurement of changes in resistance while immersed in aqueous solutions.

REFERENCES

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EXAMPLE 4

Polyanhydrides for Transient Encapsulation

(180) Several polyanhydrides were prepared from various combinations of anhydrides, linkers and thiols, which were combined and irradiated with UV light to initiate a polymerization or curing reaction. The reagent combinations are shown in Table 3. The polyanhydrides were screened to identify candidates for transient encapsulation. Mixtures were tested for UV curability, film integrity, stability toward delamination and stability for 1 day toward dissolution. Passing results are indicated with Os and negative results are indicated with Xs. From these results, mixture 7A was selected as the candidate with the highest potential for a transient organic encapsulant characterized by UV curability, good film integrity, stability toward delamination and stability toward dissolution for at least 1 day.

(181) TABLE-US-00003 TABLE 3 Combinations of anhydrides, linkers and thiols to yield polyanhydrides. Stable for 1 Stable day UV Film for dissol'n Anhydride Linker Thiol Cure Integrity delam. test 1A X 1B X X 2A O O O .box-tangle-solidup. 2B X 0 O X (Cracks) O .box-tangle-solidup. 3A O O X X 3B X X 4A O O O .box-tangle-solidup. 4B X 0 O O O .box-tangle-solidup. 5A X 5B X X 7A O O O O 7B X 0 O O O O

(182) The ratio of anhydride and hydrophobic chain compound (i.e., thiol) in mixture 7A was tuned to optimize transient encapsulation properties. As shown in FIG. 39A, 4-pentanoic anhydride contains a degradable chain and two alkene end groups capable of reacting with 1,4-butane dithiol. 1,3,5-triallyI-1,3,5-triazene-2,4,6(1H, 3H, 5H)-trione contains three alkene groups capable of reacting with 1,4-butane dithiol. FIG. 39B provides the ratios of each component and shows that degradation rate increases as hydrophobicity decreases (hydrophilicity increases). This result is consistent with water permeating a hydrophilic encapsulant more rapidly than a hydrophobic encapsulant.

(183) FIGS. 39C-E show additional polyanhydride encapsulant materials formed by photocuring mixtures of one or more anhydride monomers and one or more thiol monomers. FIG. 39C shows a polyanhydride encapsulant material incorporating a phosphodiester group within the polymeric chain. The phosphodiester group increases susceptibility of the polymer to degradation by base and/or enzymes. FIG. 39D shows a polyanhydride encapsulant material incorporating a silyl ether group within the polymeric chain. The silyl ether group increases susceptibility of the polymer material to degradation by acid. FIG. 39E shows a polyanhydride encapsulant material incorporating an ether group within the polymeric chain. The ether group increases susceptibility of the polymer material to degradation by acid. As shown by these examples, suitable polymeric encapsulants may be synthesized and/or selected to achieve a selected programmable transience under a specific environment.

(184) FIG. 40 shows a schematic of a water permeability test set-up. A magnesium conductor is applied to a glass substrate and two extended electrodes are soldered to ends of the conductor to monitor resistance as a function of time. An organic encapsulant, such as 7A, is applied over the entire device, which is located in a petri dish. DI water is added over the organic encapsulant.

(185) FIG. 41 shows performance of organic encapsulants compared to materials of other classes, such as inorganic encapsulants, as a function of change in conductor resistance over time. A1TX is the polyanhydride synthesized by combining pentanoic anhydride and 1,3,5 triallyI-1,3,5 triazene-2,4,6 (1H, 3H, 5H) trione with a 1:X molar concentration. (SiO.sub.2/SiN)X3 describes a multilayer stack of triple layers of silicon oxide and silicon nitride (SiO.sub.2/SiN/SiO.sub.2/SiN/SiO.sub.2/SiN). PDMS is poly(dimethyl siloxane). BCB is bisbenzocyclobutene, a non-degradable polymer for comparison with degradable organic and inorganic encapsulants. A1T4 had the lowest water permeation within the polyanhydride class. Combining inorganic and organic encapsulation ((SiO.sub.2/SiN)X3+A1T4) gave the best results (27 days). Multiple layers helped to cover defect of previous layers and had better performance against water permeation. In this example, the step of depositing the inorganic layer occurred at 200350 C., which the organic layer could not withstand without undergoing a chemical and/or physical transformation. Thus, the inorganic layer was applied first, with a top organic layer serving as a mechanical buffer layer because the organic layer is less brittle than the inorganic layer.

(186) In an embodiment, multilayer inorganic-organic encapsulation may include alternating organic and inorganic layers. For example, a first organic layer may cover an electronic device. The organic layer may be conformal and/or may be an electrical insulator. A second encapsulation layer, applied on top of the first organic layer may be an inorganic layer for reducing water permeability. A third encapsulation layer, applied on top of the second encapsulation layer, may be an organic layer, e.g., for filling pinholes or defects in the underlying inorganic layer.

(187) In an embodiment, multilayer inorganic-organic encapsulation may include alternating organic and inorganic devices. For example, an inorganic layer may be applied over an electronic device. The inorganic layer may be conformal and/or may be an electrical insulator. A second encapsulation layer, applied on top of the first inorganic layer, may be an organic layer, e.g., for filling pinholes or defects in the first inorganic layer. A third encapsulation layer, applied on top of the second encapsulation layer, may be an inorganic layer, e.g., for reducing water permeability.

(188) In alternative embodiments, multilayer inorganic-organic encapsulation stacks may include organic layers in direct contact with neighboring organic layers and/or inorganic layers in direct contact with neighboring inorganic layers.

(189) In an embodiment, a multilayer encapsulation stack comprises two, three, four, five, six, seven, eight, nine, ten, twelve, fifteen, twenty or more layers.

(190) FIGS. 42A-C show the dissolution rates of three polyanhydrides (A1T1, A1T2, A1T4) in buffer solutions with pH 5.7 (squares), pH 7.4 (circles) and pH 8 (triangles). The dissolution rates of the polyanhydrides was highest in acid and lowest in base. A1T4 has low water permeability (FIG. 41) but a slow dissolution rate due to a low concentration of anhydride.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

(191) All references cited throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

(192) The following references relate generally to flexible and/or stretchable semiconductor materials and devices. Each is hereby incorporated by reference in its entirety: U.S. patent application Ser. No. 12/778,588, filed on May 12, 2010, PCT International Application No. PCT/US05/19354, filed Jun. 2, 2005 and published under No. WO2005/122285 on Dec. 22, 2005, U.S. Provisional Patent Application No. 61/313,397, filed Mar. 12, 2010, U.S. patent application Ser. No. 11/851,182, filed Sep. 6, 2007 and published under No. 2008/0157235 on Jul. 3, 2008, and PCT International Application No. PCT/US07/77759, filed Sep. 6, 2007 and published under No. WO2008/030960 on Mar. 13, 2008.

(193) The following references relate generally to bioresorbable substrates and methods of making bioresorbable substrates. Each is hereby incorporated by reference in its entirety: PCT Patent Application PCT/US03/19968 filed Jun. 24, 2003, PCT Patent Application PCT/US04/000255 filed Jan. 7, 2004, PCT Patent Application PCT/US04/11199 filed Apr. 12, 2004, PCT Patent Application PCT/US05/20844 filed Jun. 13, 2005, and PCT Patent Application PCT/US06/029826 filed Jul. 28, 2006.

(194) The following references relate generally to transient electronic devices. Each is hereby incorporated by reference in its entirety: U.S. patent application Ser. No. 13/624,096 and PCT International Application No. PCT/US2012/056538, each filed Sep. 21, 2012.

(195) The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, and method steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.

(196) When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individually or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.

(197) The following references relate generally to fabrication methods, structures and systems for making electronic devices, and are hereby incorporated by reference to the extent not inconsistent with the disclosure in this application.

(198) It must be noted that as used herein and in the appended claims, the singular forms a, an, and the include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to a cell includes a plurality of such cells and equivalents thereof known to those skilled in the art, and so forth. As well, the terms a (or an), one or more and at least one can be used interchangeably herein. It is also to be noted that the terms comprising, including, and having can be used interchangeably. The expression of any of claims XX-YY (wherein XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form, and in some embodiments is interchangeable with the expression as in any one of claims XX-YY.

(199) TABLE-US-00004 Attorney Publication Publication Docket No. Application No. Filing Date No. Date Pat. No. Issue Date 145-03 US 11/001,689 Dec. 1, 2004 2006/0286488 Dec. 21, 2006 7,704,684 Apr. 27, 2010 18-04 US 11/115,954 Apr. 27, 2005 2005/0238967 Oct. 27, 2005 7,195,733 Mar. 27, 2007 38-04A US 11/145,574 Jun. 2, 2005 2009/0294803 Dec. 3, 2009 7,622,367 Nov. 24, 2009 38-04B US 11/145,542 Jun. 2, 2005 2006/0038182 Feb. 23, 2006 7,557,367 Jul. 7, 2009 43-06 US 11/421,654 Jun. 1, 2006 2007/0032089 Feb. 8, 2007 7,799,699 Sep. 21, 2010 38-04C US 11/423,287 Jun. 9, 2006 2006/0286785 Dec. 21, 2006 7,521,292 Apr. 21, 2009 41-06 US 11/423,192 Jun. 9, 2006 2009/0199960 Aug. 13, 2009 7,943,491 May 17, 2011 25-06 US 11/465,317 Aug. 17, 2006 137-05 US 11/675,659 Feb. 16, 2007 2008/0055581 Mar. 6, 2008 90-06 US 11/782,799 Jul. 25, 2007 2008/0212102 Sep. 4, 2008 7,705,280 Apr. 27, 2010 134-06 US 11/851,182 Sep. 6, 2007 2008/0157235 Jul. 3, 2008 8,217,381 Jul. 10, 2012 151-06 US 11/585,788 Sep. 20, 2007 2008/0108171 May 8, 2008 7,932,123 Apr. 26, 2011 216-06 US 11/981,380 Oct. 31, 2007 2010/0283069 Nov. 11, 2010 7,972,875 Jul. 5, 2011 116-07 US 12/372,605 Feb. 17, 2009 213-07 US 12/398,811 Mar. 5, 2009 2010/0002402 Jan. 7, 2010 8,552,299 Oct. 8, 2013 38-04D US 12/405,475 Mar. 17, 2009 2010/0059863 Mar. 11, 2010 8,198,621 Jun. 12, 2012 170-07 US 12/418,071 Apr. 3, 2009 2010/0052112 Mar. 4, 2010 8,470,701 Jun. 25, 2013 38-04A1 US 12/564,566 Sep. 22, 2009 2010/0072577 Mar. 25, 2010 7,982,296 Jul. 19, 2011 71-07 US 12/669,287 Jan. 15, 2010 2011/0187798 Aug. 4, 2011 60-09 US 12/778,588 May 12, 2010 2010/0317132 Dec. 16, 2010 43-06A US 12/844,492 Jul. 27, 2010 2010/0289124 Nov. 18, 2010 8,039,847 Oct. 18, 2011 15-10 US 12/892,001 Sep. 28, 2010 2011/0230747 Sep. 22, 2011 8,666,471 Mar. 4, 2014 15-10A US 14/140,299 Dec. 24, 2013 19-10 US 12/916,934 Nov. 1, 2010 2012/0105528 May 3, 2012 8,562,095 Oct. 22, 2013 3-10 US 12/947,120 Nov. 16, 2010 2011/0170225 Jul. 14, 2011 118-08 US 12/996,924 Dec. 8, 2010 2011/0147715 Jun. 23, 2011 126-09 US 12/968,637 Dec. 15, 2010 2012/0157804 Jun. 21, 2012 50-10 US 13/046,191 Mar. 11, 2011 2012/0165759 Jun. 28, 2012 151-06A US 13/071,027 Mar. 24, 2011 2011/0171813 Jul. 14, 2011 137-05A US 13/095,502 Apr. 27, 2011 216-06B US 13/100,774 May 4, 2011 2011/0266561 Nov. 3, 2011 38-04A2 US 13/113,504 May 23, 2011 2011/0220890 Sep. 15, 2011 8,440,546 May 14, 2013 136-08 US 13/120,486 Aug. 4, 2011 2011/0277813 Nov. 17, 2011 151-06B US 13/228,041 Sep. 8, 2011 2011/0316120 Dec. 29, 2011 43-06B US 13/270,954 Oct. 11, 2011 2012/0083099 Apr. 5, 2012 8,394,706 Mar. 12, 2013 3-11 US 13/349,336 Jan. 12, 2012 2012/0261551 Oct. 18, 2012 38-04E US 13/441,618 Apr. 6, 2012 2013/0100618 Apr. 25, 2013 134-06B US 13/441,598 Apr. 6, 2012 2012/0327608 Dec. 27, 2012 28-11 US 13/472,165 May 15, 2012 2012/0320581 Dec. 20, 2012 7-11 US 13/486,726 Jun. 1, 2012 2013/0072775 Mar. 21, 2013 29-11 US 13/492,636 Jun. 8, 2012 2013/0041235 Feb. 14, 2013 84-11 US 13/549,291 Jul. 13, 2012 2013/0036928 Feb. 14, 2013 25-06A US 13/596,343 Aug. 28, 2012 2012/0321785 Dec. 20, 2012 8,367,035 Feb. 5, 2013 150-11 US 13/624,096 Sep. 21, 2012 2013/0140649 Jun. 6, 2013 38-04A3 US 13/801,868 Mar. 13, 2013 2013/0320503 Dec. 5, 2013 8,664,699 Mar. 4, 2014 38-04A4 US 14/155,010 Jan. 14, 2014 125-12 US 13/835,284 Mar. 15, 2013 30-13 US 13/853,770 Mar. 29, 2013

(200) Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention.

(201) Whenever a range is given in the specification, for example, a range of integers, a temperature range, a time range, a composition range, or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. As used herein, ranges specifically include the values provided as endpoint values of the range. As used herein, ranges specifically include all the integer values of the range. For example, a range of 1 to 100 specifically includes the end point values of 1 and 100. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.

(202) As used herein, comprising is synonymous and can be used interchangeably with including, containing, or characterized by, and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, consisting of excludes any element, step, or ingredient not specified in the claim element. As used herein, consisting essentially of does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms comprising, consisting essentially of and consisting of can be replaced with either of the other two terms. The invention illustratively described herein suitably can be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

(203) One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed can be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.