Non-contact transfer printing

Abstract

A transfer printing process that exploits the mismatch in mechanical or thermo-mechanical response at the interface of a printable micro- or nano-device and a transfer stamp to drive the release of the device from the stamp and its non-contact transfer to a receiving substrate are provided. The resulting facile, pick-and-place process is demonstrated with the assembling of 3-D microdevices and the printing of GAN light-emitting diodes onto silicon and glass substrates. High speed photography is used to provide experimental evidence of thermo-mechanically driven release.

Claims

1. A method of transferring ink from a donor substrate to a receiving substrate, said method comprising: providing a non-ablative transfer device having a transfer surface; providing said donor substrate having a donor surface, said donor surface having ink thereon, wherein said ink is a micro-sized or nano-sized prefabricated electronic, optical, or electro-optical device or device component thereof; contacting at least a portion of said transfer surface with at least a portion of said ink; separating said transfer surface from said donor surface, wherein said ink is transferred from said donor surface to said transfer surface; positioning said transfer surface having said ink disposed thereon into alignment with a receiving surface of said receiving substrate; and actuating said transfer device, said ink, or both of said transfer device and said ink by generating a non-ablative force that releases at least a portion of said ink from said transfer surface, thereby transferring said ink to said receiving surface, wherein said step of actuating comprises mechanically stressing an interface between said transfer surface and said ink so as to cause delamination, thereby resulting in release of said ink.

2. The method of claim 1, wherein a gap remains between said ink disposed on said transfer surface and said receiving surface during the actuation.

3. The method of claim 1, wherein the non-ablative actuation force is generated while maintaining at least a portion of said gap.

4. The method of claim 1, wherein said ink is in contact with the receiving surface during the actuation.

5. The method of claim 1, wherein the actuation is electrostatic.

6. The method of claim 1, wherein said step of actuating said transfer device uses a laser, a piezoelectric actuator, a gas source, a vacuum source, an electromagnetic source, an electrostatic source, an electronic source, a heat or thermal source, or a combination thereof.

7. The method of claim 6, wherein said electrostatic source generates an applied electric field on said transfer surface, said ink disposed on said transfer surface, or both.

8. The method of claim 6, wherein the actuation is thermal.

9. The method of claim 8, wherein the thermal actuation is enabled by providing electromagnetic radiation.

10. The method of claim 9, wherein the electromagnetic radiation is infrared radiation.

11. The method of claim 6, wherein said heat source heats said transfer device, said ink, or both of said transfer device and said ink, thereby thermally actuating said transfer device, said ink, or both of said transfer device and said ink.

12. The method of claim 11, wherein said heat source produces a temperature of said transfer surface selected from the range of 275 degrees C. to 325 degrees C.

13. The method of claim 6, wherein said heat source produces a temperature gradient in said transfer device selected from the range of 10.sup.4 degrees C. per cm to 10.sup.5 degrees C. per cm.

14. The method of claim 1, wherein the magnitude and spatial distribution of said force is selected so as to generate a separation energy between said ink and said transfer surface equal to or greater than 1 J/meter.sup.2.

15. The method of claim 1, wherein the prefabricated device or device component is a semiconductor element.

16. The method of claim 1, wherein the prefabricated device or device component is a light-emitting diode.

17. The method of claim 1, wherein the prefabricated device or device component has a lateral dimension in the range of 100 nm to 100 microns.

18. The method of claim 1, wherein the transfer device is an elastomeric stamp.

19. The method of claim 1, wherein at least a portion of said transfer surface directly contacts at least a portion of said ink.

20. A method of transferring ink from a donor substrate to a receiving substrate, said method comprising: providing a non-ablative transfer device having a transfer surface; providing said donor substrate having a donor surface, said donor surface having ink thereon, wherein said ink is a micro-sized or nano-sized prefabricated electronic, optical, or electro-optical device or device component thereof; contacting at least a portion of said transfer surface with at least a portion of said ink; separating said transfer surface from said donor surface, wherein said ink is transferred from said donor surface to said transfer surface; positioning said transfer surface having said ink disposed thereon into alignment with a receiving surface of said receiving substrate; and actuating said transfer device, said ink, or both of said transfer device and said ink by generating a non-ablative force that releases at least a portion of said ink from said transfer surface, thereby transferring said ink to said receiving surface, wherein said step of actuating comprises electrostatic actuation.

21. The method of claim 20, wherein a gap remains between said ink disposed on said transfer surface and said receiving surface during the actuation.

22. The method of claim 20, wherein the non-ablative actuation force is generated while maintaining at least a portion of said gap.

23. The method of claim 20, wherein said ink is in contact with the receiving surface during the actuation.

24. The method of claim 20, wherein said electrostatic source generates an applied electric field on said transfer surface, said ink disposed on said transfer surface, or both.

25. The method of claim 20, wherein the magnitude and spatial distribution of said force is selected so as to generate a separation energy between said ink and said transfer surface equal to or greater than 1 J/meter.sup.2.

26. The method of claim 20, wherein the prefabricated device or device component is a semiconductor element.

27. The method of claim 20, wherein the prefabricated device or device component is a light-emitting diode.

28. The method of claim 20, wherein the prefabricated device or device component has a lateral dimension in the range of 100 nm to 100 microns.

29. The method of claim 20, wherein the transfer device is an elastomeric stamp.

30. The method of claim 20, wherein at least a portion of said transfer surface directly contacts at least a portion of said ink.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(A), 1(B), 1(C), 1(D): Schematic of the laser transfer printing steps: FIG. 1(A), 1the PDMS stamp is aligned with the donor substrate to pick up the ink; FIG. 1(B), 2the ink is transferred to the stamp; FIG. 1(C), 3the stamp is aligned to a receiving substrate and a laser pulse is used to heat up the ink-stamp interface; and FIG. 1(D), 4the ink is transferred to the receiving substrate and the stamp is withdrawn for the next printing cycle.

(2) FIG. 2: A schematic depiction and photograph of the laser-driven non-contact transfer printing (LNTP) print head. The laser beam is brought into the print head by an optical fiber, bent and focused on the ink-stamp interface. A dichroic mirror allows for monitoring of the process with a high-speed camera positioned above the stamp.

(3) FIGS. 3(A), 3(B), 3(C), 3(D): Micrographs of examples of printing using the LNTP process. FIG. 3(A) 1001003 micron silicon squares printed between metallic traces on a silicon wafer, FIG. 3(B) 3-D pyramid printed with the same silicon squares, FIG. 3(C) A silicon square printed on a silicon cantilever, and FIG. 3(D) 1001000.32 micron ultrathin Si square printed onto a structured substrate.

(4) FIGS. 4(A), 4(B), 4(C): Printing InGaN-based -LEDs. FIG. 4(A) InGaN-based -LED printed onto a structured silicon substrate, FIG. 4(B) Schematic stacks of the InGaN-based -LED, FIG. 4(C) Functioning -LED printed onto a CVD-grown polycrystalline diamond on silicon substrate.

(5) FIG. 5: Frames from a high-speed film showing (a) the delamination process that starts at the corners (frame 2) and progresses towards the center resulting in the chip leaving the stamp and (b) a partial delamination event in which the delamination front begins moving towards the center from the corners before reversing directions. The chip remains adhered to the stamp.

(6) FIG. 6: Schematic of apparatus for measuring laser energy incident on the ink by the difference in energy arriving at a calibrated photodiode with and without the ink present on the stamp.

(7) FIG. 7: Power meter measurements with the ink on the stamp for a single 4 millisecond long laser pulse.

(8) FIG. 8: Power meter measurement with no ink on the stamp for a single 4 ms long laser pulse.

(9) FIGS. 9(A), 9(B), 9(C), 9(D): FIG. 9(A) Finite element model of the transfer printing system, FIG. 9(B) Temperature distribution in the post and attached chip at 1.8 milliseconds, FIG. 9(C) Energy release rate distribution with time, and FIG. 9(D) Temperature gradient through the stamp-ink interfaces.

(10) FIG. 10: Analytic model for delamination of stamp-ink interface.

(11) FIG. 11: Scaling law for delamination of stamp-ink interface.

(12) FIGS. 12(A), 12(B): A schematic depiction FIG. 12(A) and photograph FIG. 12(B) of the laser-driven non-contact transfer printing (LNTP) of a silicon square onto a water droplet.

(13) FIG. 13: (top) A patterned stamp with 4 posts retrieves ink from a donor substrate and transfers it to a receiving substrate, (middle) results of 3 printing cycles displaying ink from a dense donor substrate, which is expanded on a receiving substrate, and (bottom) SEM images of representative micro-LED, shown in sequence, (left) donor substrate before retrieval, (center) after retrieval from the Si substrate, and (right) after transfer-printing onto a receiving substrate.

(14) FIG. 14: Automated Transfer Printing Machine showing the four axes of motion and integrated optics.

(15) FIGS. 15(A), 15(B), 15(C), 15(D): Schematic of the thermal mismatch strains resulting in bending induced delamination of the silicon printing chip from the PDMS stamp. FIG. 15(A) Geometry of the initial setup. FIG. 15(B) Resulting forces and moments on the system as a result of the thermal mismatch strains. FIG. 15(C) To relieve strain energy, the system deforms in bending. The PDMS stamp is more compliant and as a result its curvature is more pronounced. FIG. 15(D) Deformation due to bending in the system produces delamination of the printing chip from the stamp. The delamination front at the interface moves from the corners of the chip towards its center.

(16) FIG. 16: The energy release rate of the PDMS-1001003 mm silicon ink-stamp system as a function of chip temperature is calculated by the finite-thickness correction to Stoney's formulation [16] by Freund [17].

(17) FIG. 17: Finite element model of the post and ink showing (top) temperature gradient in the post and attached ink and (bottom) a slice of the post showing the temperature gradients and the deformation.

(18) FIG. 18: Photograph of the laser micro-transfer print head.

(19) FIG. 19: Beam power at the stamp-ink interface plane as a function of the laser current.

(20) FIGS. 20(A), 20(B), 20(C): Examples of structures constructed by laser micro-transfer printing. FIG. 20(A) Optical micrograph of silicon squares printed on a silicon substrate with gold traces; FIG. 20(B) A 3-D pyramidal structure built of silicon squares; and FIG. 20(C) A bridge structure built by printing a silicon plate on two bars patterned on a silicon substrate. (Scale: Silicon squares in micrographs have sides of 100 m).

(21) FIG. 21: Examples of printing on curved surfaces, (left) printing on a single 1 mm ceramic sphere, (middle) printing on a non-uniform array of 500 m silica beads, and (right) printing onto a liquid NOA droplet. (Scale: in all the micrographs, the printed squares have sides of 100 m).

(22) FIG. 22: Examples of printing on partial and recessed surfaces. (Left) A silicon square printed onto an AFM cantilever, demonstrating assembly on an active structure, (Middle) Printing on a ledge, and (right) printing into recessed spaces. (Scale: in all the micrographs, the printed squares have sides of 100 microns).

(23) FIG. 23: Lateral transfer errors as a function of stand-off height.

(24) FIGS. 24(A), 24(B): Schematic of laser power measurement set up and a typical measurement for a pulse FIG. 24(A) without the ink and FIG. 24(B) with the ink on the stamp.

(25) FIGS. 25(A), 25(B), 25(C): Schematic showing the amount of energy required for delamination as a function of FIG. 25(A) pulse width FIG. 25(B) ink thickness and FIG. 25(C) ink size.

(26) FIG. 26: A flowchart showing steps for transferring ink from a donor substrate to a receiving substrate, according to exemplary embodiments of the present invention.

(27) FIGS. 27(A), 27(B): Exemplary means for actuating a FIG. 27(A) transfer device, ink, or FIG. 27(B) both of a transfer device and ink, according to the present invention.

(28) FIGS. 28(A), 28(B): FIG. 28(A) Electromagnetic radiation passes through a substantially transparent transfer device and is absorbed by ink adhered to the transfer surface of transfer device and FIG. 28(B) A transfer device contains embedded absorbing material that absorbs electromagnetic radiation to prevent excessive heating of the ink.

(29) FIGS. 29(A), 29(B), 29(C): Schematics of illumination geometries suitable for use with the present invention: FIG. 29(A) Transmission through a substantially transparent transfer device, FIG. 29(B) Transmission through a substantially transparent receiving substrate, and FIG. 29(C) Illumination of the interface between the transfer device and ink from the side.

DETAILED DESCRIPTION OF THE INVENTION

(30) 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.

(31) Delamination refers to separation at an interface between substantially parallel, contacting layers when energy at the interface becomes greater than the energy of adhesion holding the layers in contact with one another.

(32) Ink refers to a discrete unit of material capable of being transferred from a donor substrate to a receiving substrate. Ink may be solid, liquid or a combination thereof. Ink may, for example, be an atomic or molecular precursor to a device component, a device component, or a prefabricated device.

(33) A device is a combination of components operably connected to produce one or more desired functions. A prefabricated device is a device that is fabricated on a donor substrate, but destined for a receiving substrate that is less capable than the donor substrate of supporting the fabrication process or incapable of supporting the fabrication process.

(34) 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. Other components include, but are not limited to, thin film transistors (TFTs), transistors, electrodes, integrated circuits, circuit elements, control elements, microprocessors, transducers, islands, bridges and combinations thereof.

(35) Actuating broadly refers to a process wherein a device, device component, structure, or material is acted upon, for example, so as to cause a change in one or more physical, chemical, optical or electronic properties. In an embodiment, for example, actuating comprises one or more of mechanically actuating, optically actuating, electrically actuating, electrostatically actuating, magnetically actuating, and thermally actuating. In some methods and systems of the invention, actuating involves a process in which energy is provided to, or taken away from, a device, device component, structure, or material, such as a transfer device and/or ink. In some embodiments, for example, the energy provided, or taken away, is thermal energy, mechanical energy, optical energy, electronic energy, electrostatic energy or any combination of these. In some methods and systems of the invention, actuating involves activating a transfer device and/or ink so as to generate a force that releases at least a portion of the ink from the transfer surface. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to electromagnetic radiation, such as laser radiation, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to thermal energy, such as heat, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to an electromagnetic field, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to a magnetic field, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves physically contacting and/or moving a transfer device and/or ink so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device, for example, using a piezoelectric actuator, source of a fluid (e.g., gas source) or a vacuum source. In an embodiment, for example, actuating involves a process wherein a transfer device or ink disposed on the surface of the transfer device does not physically contact the receiving surface of a substrate.

(36) Alignment is used herein to refer to the relative arrangement or position of surfaces or objects. For example, the transfer surface of the transfer device and receiving surface of the receiving substrate are in alignment when a gap between the surfaces is a consistent, predetermined separation distance along a vertical axis perpendicular to the planes of the surfaces.

(37) Registration is used in accordance with its meaning in the art of microfabrication. Registration refers to the precise positioning of ink, components and/or devices on a selected region of a substrate or relative to ink, components and/or devices that preexist on a substrate. For example, alignment of the transfer surface and receiving surface brings ink disposed on the transfer surface into registration with selected regions of the receiving surface. In some embodiments, the selected regions correspond to ink, devices or device components prepositioned on the receiving surface of the receiving substrate.

(38) 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 Al.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, ZnSiP.sub.2, CuCl, PbS, PbSe, PbTe, FeO, FeS.sub.2, NiO, EuO, EuS, PtSi, TIBr, 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.a, 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, Tl.sub.2SnTe.sub.5, Tl.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.

(39) 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.

(40) 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 electronic 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.

(41) Functional layer refers to a layer that imparts some functionality to a device. For example, a functional layer may contain semiconductor components. Alternatively, the functional layer may comprise multiple layers, such as multiple semiconductor layers separated by support layers. The functional layer may comprise a plurality of patterned elements, such as interconnects running between electrodes or islands.

(42) Structural layer refers to a layer that imparts structural functionality, for example by supporting and/or encapsulating device components.

(43) 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 described herein 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.

(44) 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 elastomeric transfer devices include stamps, molds and masks. The transfer device affects and/or facilitates material transfer from a donor material to a receiver material. The methods of the present invention do not substantially degrade the elastomeric transfer device. As used herein, substantial degradation refers to chemical/physical decomposition or material removal occurring within at least 50 nm or within at least 100 nm of the transfer surface of the elastomeric transfer device.

(45) 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.

(46) 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.

(47) Conformal contact refers to contact established between two or more surfaces. In one aspect, conformal contact involves a macroscopic adaptation of one or more surfaces (e.g., contact surfaces) to the overall shape of another surface. In another aspect, conformal contact involves a microscopic adaptation of one or more surfaces (e.g., contact surfaces) to another surface resulting in an intimate contact substantially free of voids. In an embodiment, conformal contact involves adaptation of an ink surface(s) to a receiving surface(s) such that intimate contact is achieved, for example, wherein less than 20% of the surface area of an ink surface of the device does not physically contact the receiving surface, or optionally less than 10% of an ink surface of the device does not physically contact the receiving surface, or optionally less than 5% of an ink surface of the device does not physically contact the receiving surface.

(48) 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:

(49) $\begin{array}{cc}E=\frac{\left(\mathrm{stress}\right)}{\left(\mathrm{strain}\right)}=\left(\frac{L_0}{L}\right)\left(\frac{F}{A}\right),& \left(I\right)\end{array}$
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:

(50) $\begin{array}{cc}E=\frac{\left(3+2\right)}{+},& \left(\mathrm{II}\right)\end{array}$
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.

(51) 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.

(52) 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.

(53) 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.

(54) Thermomechanically driven, non-contact transfer printing devices and methods will now be described with reference to the figures and the following non-limiting examples.

(55) FIG. 26 provides a flowchart 2800 showing steps for transferring ink from a donor substrate to a receiving substrate. In step 2802, a transfer device having a transfer surface is provided. Next, in step 2804, a donor substrate having a donor surface with ink thereon is provided. In step 2806, at least a portion of the transfer surface is contacted with at least a portion of the ink. When the transfer surface is separated from the donor surface, in step 2808, at least a portion of the ink is transferred from the donor surface to the transfer surface. The transfer surface having the ink disposed thereon is then positioned into alignment with a receiving surface of the receiving substrate, wherein a gap remains between the ink disposed on the transfer surface and the receiving surface, in step 2810. Finally, in step 2812, the transfer device, the ink, or both of the transfer device and the ink are actuated by generating a force that releases at least a portion of the ink from the transfer surface while maintaining at least a portion of said gap, thereby transferring at least a portion of the ink to the receiving surface.

(56) FIG. 27 shows several exemplary means for actuating the transfer device, the ink, or both of the transfer device and the ink in step 2812. FIG. 27A shows a stamp 2900(1) having a conductive coil 2902 embedded in the stamp. A power source 2904 supplies a current within coil 2902 to create resistive heating or a magnetic field.

(57) FIG. 27B shows a stamp 2900(2) having a channel 2906 formed therethrough. Ink 2910 is disposed at a distal end of channel 2906, while a vacuum or fluid source 2908 at a proximal end of channel 2906 is in fluidic communication with channel 2906. Using this system, for example, vacuum 2908 may be applied to hold ink 2910 onto the transfer surface until registration is complete. Stopping vacuum 2908 allows ink 2910 to be released from the transfer surface. Alternatively, ink 2910 may be released from the transfer surface upon application of a positive gas pressure, e.g., a short burst of gas. When positive pressure is used to release ink 2910, the gas may replace either a vacuum or neutral pressure. For example, ink 2910 may adhere to the transfer surface in the absence of a vacuum (i.e., under conditions of ambient/neutral pressure).

(58) FIGS. 28A and 28B show two exemplary embodiments of the present invention. In FIG. 28A, electromagnetic radiation (shown as a dashed line) passes through a substantially transparent transfer device 3000(1) onto ink 3002(1) adhered to the transfer surface of transfer device 3000(1). The electromagnetic radiation is at least partially absorbed by ink 3002(1) to generate heat within the ink and areas of the transfer surface in contact with ink 3002(1). Alternatively, FIG. 28B shows a transfer device 3000(2) containing embedded, coated, or laminated absorbing material 3004. The absorbing material 3004 may form a contiguous or non-contiguous layer or may be randomly dispersed within or on the transfer device material. Electromagnetic energy (shown as a dashed arrow) is absorbed by absorbing material 3004. Heat created by absorbing material 3004 is transferred to transfer device 3000(2) and ink 3002(2). In one embodiment, absorbing material 3004 is a thermal adhesive or a photoactivated adhesive. In an embodiment, absorbing material 3004 has a coefficient of thermal expansion selected from the range of 300 ppm C..sup.1 to 1 ppm C..sup.1, a Young's modulus selected from the range of 100 MPa to 500 GPa, a thickness selected from the range of 2 microns to 10 microns, and/or is selected from the group consisting of materials that absorb at the wavelength of irradiation, such as silicon, graphite, carbon black, metals with nanostructured surfaces, and combinations thereof.

(59) In one embodiment, absorbing material 3004 forms a contiguous or non-contiguous coating or laminated layer on the surface of transfer device 3000(2), such that ink 3002(2) is in direct contact with absorbing material 3004. The absorbing material may be applied to the ink or the transfer surface prior to the step of contacting at least a portion of the transfer surface with at least a portion of the ink, and the absorbing material may be removed after the step of applying a force to the transfer surface.

(60) In another embodiment, absorbing material 3004 is embedded within transfer device 3000(2) and disposed within 10 micrometers from the transfer surface upon which ink 3002(2) is adhered. In this embodiment, ink 3002(2) may be protected from excessive heating because the relative heating of transfer device 3000(2) and ink 3002(2) may be preselected by determining the placement, concentration and composition of absorbing material 3004. For example, to minimize heating of ink 3002(2), absorbing material 3004 may be positioned farther from the transfer surface than when greater heating of ink 3002(2) is desired.

(61) FIGS. 29A-29C provide schematics of illumination geometries suitable for use with the present invention. In FIG. 29A, electromagnetic radiation (shown as a dashed line) passes through a substantially transparent transfer device and is absorbed by ink adhered to the transfer surface of the transfer device. In FIG. 29B, electromagnetic radiation (shown as a dashed line) passes through a substantially transparent receiving substrate and is absorbed by ink adhered to the transfer surface of a transfer device. In FIG. 29C, electromagnetic radiation is applied from the side and at least partially focused onto the interface between the transfer device and ink adhered thereon.

Example 1: Laser-Driven Non-Contact Transfer Printing (LNTP)

(62) Mietl [10] describes a transfer printing process involving both the pick-up of microstructures from a donor substrate and their deposition or printing onto a receiving substrate using an elastomeric stamp. The present invention also starts with an elastomeric stamp made of PDMS and optionally patterned with posts, to selectively engage the desired nano- or micro-devices on the donor or inking substrate. The mechanism for inking the stamp is similar to previously described mechanisms [4-8], relying on the strong adhesive forces between PDMS and the nano- or micro-devices to extract the ink from the donor or inking substrate. For deposition, however, the inked stamp is brought close (between 3 to 10 microns) to the receiving substrate onto which the devices are to be deposited. A pulsed laser beam is focused on the interface between the stamp and the devices to release and drive the device to the receiving substrate. The wavelength of the laser is chosen so that the stamp material is transparent, while the ink is more absorbing. FIG. 1 shows a schematic of the Laser-driven Non-contact Transfer Printing (LNTP) process.

(63) To realize this process, a LNTP print head is created by using an electronically pulsed 30 W 805 nm laser diode with a minimum pulse width of 1 ms. The laser is coupled into the system through a 250 m core optical fiber. At the end of the fiber are a 4 mm diameter collimator and a focusing lens with a 30 mm focal distance to focus the laser beam on a circular area with a diameter of approximately 400-800 m. FIG. 2 shows a schematic and photograph of the LNTP print head. The laser beam is brought in through the side of the print head, bent through 90 degrees by a dichroic mirror and focused onto the surface of a (typically, 200200 m, 100 m tall) post patterned on the PDMS stamp. An objective directly above the stamp along with a CCD camera and suitable optics allows the observation of the process with pixel resolution of 1 m.

(64) The laser print head is tested by using a 22 mm, 1 mm thick PDMS stamp with a 200200 m, 100 m tall post patterned on it. The stamp is affixed to a glass backing. For the ink, a donor substrate is fabricated using conventional fabrication processes to obtain anchored, but undercut, 1001003 m square single crystal silicon chips. An automated printer is constructed by integrating a programmable, computer-controlled xyz positioning stage, with the print head, high-resolution optics and vacuum chucks for the donor and receiving substrates. As depicted in the process schematic of FIG. 1, the printer moves and locates the stamp enabling the pick up of a single chip. The stage is then moved to locate the chip directly above a receiving substrate (for example in FIG. 3(a), an RC1 cleaned, patterned silicon substrate with 50 micron gold traces) at a distance of 10 microns from it. The laser pulse width was set to 2 ms and the laser power was gradually increased until delamination was observed. FIG. 3(a) shows the results of this printing protocol.

(65) A second feasibility test is conducted to demonstrate the construction of 3-dimensional assemblies using such a process. Here a 3-layer pyramid, shown in FIG. 3(b), is constructed of the same 1001003 m silicon squares. In a third test, simulating the printing of microstructures into other functional structures, the same square silicon chip is printed onto an AFM cantilever, something that would be difficult to achieve with other processes. (See FIG. 3(c).) Finally, FIG. 3(d) shows a 320 nm thick silicon chip printed onto a structured surface. This verifies the claim that the process is independent of the properties of the receiving substrate and demonstrates the ability of the process to print ultrathin microstructures.

(66) Transfer printing of an InGaN-based -LED onto a CVD-grown polycrystalline diamond on silicon substrate is demonstrated in FIG. 4. These InGaN-based -LEDs comprise epitaxial layers on a (111) silicon wafer. The active device layers comprise a p-type GaN layer (110 nm of GaN:Mg), multiple quantum well (MQW) (5InGaN/GaN:Si of 3 nm/10 nm), and an n-type layer (1700 nm of GaN:Si). Metal layers of Ti/Al/Mo/Au (15 nm/60 nm/20 nm/100 nm) and Ni/Au (10 nm/10 nm) are deposited and annealed in optimized conditions to form ohmic contacts to n-GaN and p-GaN, respectively. These LEDs are printed utilizing a single 1 ms laser pulse. FIG. 4(a) shows an InGaN-based -LED printed onto a structured silicon substrate while FIG. 4(b) shows a schematic of the stacks of the InGaN-based -LED. FIG. 4(c) shows that the -LED is functional after having been printed onto a silicon substrate coated with a CVD-grown polycrystalline diamond film.

(67) LNTP Mechanism and Experimental Observations.

(68) The primary phenomenon driving the LNTP process is not ablation but, instead, the mismatched thermo-mechanical responses of the stamp and the ink which cause the delamination of the ink from the stamp and its transfer to the receiving substrate. The mechanism by which the microstructure is delaminated from the stamp and transferred to the receiving substrate is described herein and high-speed photography evidence in support of this mechanism is provided.

(69) Since a PDMS stamp is transparent in the near IR range, the laser radiation is transmitted through the stamp and is incident on the ink which absorbs some fraction of the incident laser energy and, as a result, heats up. The ink, in turn, acts as a heat source for the PDMS stamp, conducting heat across the stamp-ink interface to raise the temperature of the PDMS stamp in the vicinity of the interface. The rise of temperature in the stamp and ink leads to thermal expansions in both. This, due to the considerable difference in the coefficients of thermal expansion for the two materials (.sub.s=310 ppm/ C. for PDMS [11] and .sub.c=2.6 ppm/ C. for Silicon [12]) and the restriction placed on their free expansion by the contact interface between them, must be accommodated by bending (or the formation of a curvature) in the stamp-ink composite. This stresses the interface and, when the energy release rate due to delamination at the interface exceeds the work of adhesion of the interface, the ink is released from the stamp. The increase in bending strain (and hence bending strain energy difference between the stamp and the ink) from the center of the ink to its boundaries and the stress concentration at the discontinuity caused by the boundary of the ink suggest that the delamination by this proposed mechanism will start at the outside boundary/corner of the ink and progress inwards towards its center. This predicted inward propagation of the delamination front is in remarkable contrast to the outward propagation that is observed when ablation of a sacrificial layer or the stamp materials is the mechanism driving the delamination and ejection of the microstructure (See [13]).

(70) To observe the delamination mechanism, the printer's high-resolution camera was replaced with a high-speed camera (Phantom v7.3). Preliminary tests indicated that the illumination produced by the laser pulse was sufficient to produce adequate contrast in the image frames of the camera at speeds up to around 2500 fps. FIG. 5(a) shows four frames recoded when working with the laser set to produce a flux of 10 watts for an interval of 0.004 seconds at the stamp. In the frame taken at 2.5 ms after the start of the laser pulse, the delamination process can be clearly observed to have started at the corners of the chip and progressed some distance inwards. By 3 ms the chip has released from the stamp and moved out of focus of the camera (i.e., transferred onto the substrate by 3.5 ms). To better observe the progress of the delamination front, the laser power was gradually decreased to a point where there is not enough strain energy to drive the delamination to completely separate the chip and the stamp. FIG. 5(b) shows a situation, observed at a laser power flux of 8 watts for 0.004 seconds, where the delamination front is seen to develop at the corners and propagate inwards towards the center of the chip, but then retract back to the edges and corners of the chip, suggesting insufficient strain energy release to complete the delamination of the chip from the stamp. These observations of the initiation of the delamination front at the outside edges of the chip and its propagation towards the center, along with the fact that the stamp is not damaged and can be used repeatedly for pick up and printing, suggest a thermo-mechanical phenomenon rather than the ablation of the polymer stamp material at the interface.

(71) A Thermo-Mechanical Fracture Mechanics Model for LNTP.

(72) To verify the plausibility of the mechanism proposed, the amount of radiation absorbed by the ink during a typical laser pulse used for printing was measured. This information was then used as the input for analytic and numerical models to determine the temperature of the ink and the stamp at and around the stamp-ink interface. This leads to a high enough energy release rate at the stamp-ink interface that exceeds the work of adhesion such that the ink delaminates from the stamp. Finally, a scaling law for delamination of the stamp-ink interface is established, which governs the critical time for delamination.

(73) To measure the heat flux available in a laser pulse used for delamination, the receiving substrate is replaced with a photodiode power meter (Thorlabs S142C) as depicted in FIG. 6. The rest of the setup is maintained exactly the same as originally shown in FIG. 2. The laser beam travels through the optical fiber, collimator and focusing lens, and the dichroic mirror reflects the focused laser beam to the ink (1001003 m silicon chip). Part of the laser beam energy that is incident on the ink is absorbed by it and the rest reflected away by its surface. The remaining energy in the beam passes around the ink (with a negligible amount transmitted through the 3 m thickness of the chip) and is captured by the photodiode power meter. This power meter is chosen to have a very fast response time (<200 ns) compared to the laser pulse width (4 ms), high optical power range (5 W-5 W) to withstand the intensity of the beam, high resolution (1 nW) and big laser beam inlet (12 mm) to be able to easily capture the entire laser pulse energy precisely. The photodiode power signal is then translated to laser power utilizing a pre-calibrated reader (Thorlabs PM100D). A data acquisition card captures the analog output of the calibrated reader at a sampling rate of 40 kHz and stores it on a PC for subsequent analysis.

(74) This experiment is performed in two steps: in the first stage the ink is loaded on the stamp and subjected to a 4 ms long laser pulse with intensity just below that needed to produce delamination. The photodiode power meter measures the energy in the laser pulse that passes around the chip. In the second step of this measurement, the ink is removed from the stamp and the same 4 ms laser pulse is sent to the stamp with the photodiode power meter measuring the energy in the laser pulse that emerges out from the stamp. The difference between these two measurements is the energy in the pulse that is absorbed by the ink.

(75) FIGS. 7 and 8 show the power meter measurements with and without the ink on the stamp, respectively. As shown in FIG. 7, the photodiode power meter receives 0.00895 Joules during a 4 ms laser pulse with the ink loaded on the stamp and, as shown in FIG. 8, it receives 0.00917 Joules for the identical laser pulse when there is no ink loaded on the stamp. Therefore, the incident energy to the silicon ink during a 4 ms laser pulse is 0.224 mJ, the difference between these two values. For the absorptivity 0.672 of the silicon chip [14], the energy absorbed by the silicon chip is 0.151 mJ. This energy heats up the ink and the PDMS stamp across the stamp-ink contact interface to drive the delamination.

(76) Finite element method [15] is used in the transient heat transfer analysis. The top surface of the glass backing layer is fixed, and the top surface of the silicon chip is constrained to move with the bottom surface of the post on the PDMS stamp. Other surfaces in this model are free to move. As explained earlier, the silicon chip absorbs part of the incident laser energy and behaves as a heat source. As indicated by the experimental measurements, the heat source here is the silicon chip or ink surface at the stamp-ink interface that inputs 0.151 mJ of energy over a 4 ms interval, that is, 0.0376 W of power. Finite element analysis is performed for a 4 ms interval of time. An axisymmetric model is used and hence the equivalent radius of the silicon chip is 56 m with a same in-plane area as the 100100 m square chip.

(77) FIGS. 9(a) and 9(b) show the temperature distribution in the cross section cut along the center line of the ink, at 1.8 ms. This is approximately the time when delamination starts because the analysis gives the energy release rate 0.15 J/m.sup.2 (FIG. 9(c)) at 1.8 ms, which just reaches the work of adhesion 0.15 J/m.sup.2 for the stamp-ink interface reported in the literature [16], suggesting the start of delamination. This distribution of temperature is expected, considering the high thermal conduction coefficient of silicon and low thermal conduction coefficient of PDMS and the fact that most of the laser energy is absorbed in the silicon chip and PDMS is almost transparent at the laser wavelength utilized. The analysis suggests that most of the deformation occurs in the PDMS close to the silicon chip while the chip itself undergoes a trivial deformation. This is expected considering the mismatch in thermal expansion coefficients and the stiffnesses of silicon and PDMS. Also, the PDMS bulges to accommodate the difference in thermal strains between the ink and the stamp. This provides the driving force for the delamination process. FIG. 9(d) shows an almost uniform temperature in the ink but a sharp drop to room temperature immediately outside the ink (because of the low thermal conductivity of PDMS).

(78) An analytical model is developed to establish a scaling law governing the delamination of the silicon chip from the PDMS post. For simplicity, an axisymmetric model is adopted for the system of the PDMS post and silicon chip (FIG. 10), where r.sub.silicon=56 m is the equivalent radius of the square silicon chip by enforcing the same in-plane area, h.sub.silicon=3 m is the thickness of the silicon chip. The temperature rise T.sub.PDMS in PDMS (from the ambient temperature) is determined from the transient heat conduction equation

(79) $\frac{{}^2{T}_{\mathrm{PDMS}}}{r^2}+\frac{1}{r}\frac{T_{\mathrm{PDMS}}}{r}+\frac{{}^2{T}_{\mathrm{PDMS}}}{z^2}=\frac{c_{\mathrm{PDMS}}{}_{\mathrm{PDMS}}}{{}_{\mathrm{PDMS}}}\frac{T_{\mathrm{PDMS}}}{t}$
with the initial condition T.sub.PDMS|.sub.t=0=0, where c.sub.PDMS=1460 J.Math.kg.sup.1.Math.K.sup.1, .sub.PDMS=970 kg.Math.m.sup.3, and .sub.PDMS=0.15 W.Math.m.sup.1.Math.K.sup.1 are respectively the specific heat, mass density, and heat conductivity of PDMS [11]. The temperature distribution then induces a thermal strain in PDMS, which gives analytically the energy release rate G for the delamination of the stamp-ink interface [17]. For the work of adhesion of the stamp-ink interface, the criterion for interface delamination G= gives the absorbed laser power P by the silicon chip as a function of critical time t for delamination

(80) $\begin{array}{cc}\frac{P{}_{\mathrm{PDMS}}}{{}_{\mathrm{PDMS}}}\sqrt{\frac{{}_{\mathrm{PDMS}}}{r_{\mathrm{silicon}}}}=f\left(\frac{{}_{\mathrm{PDMS}}t}{c_{\mathrm{PDMS}}{}_{\mathrm{PDMS}}{r}_{\mathrm{silicon}}^2},\frac{c_{\mathrm{silicon}}{}_{\mathrm{silicon}}{h}_{\mathrm{silicon}}}{c_{\mathrm{PDMS}}{}_{\mathrm{PDMS}}{r}_{\mathrm{silicon}}}\right)& \left(1\right)\end{array}$
where .sub.PDMS=3.110.sup.4K.sup.1 and .sub.PDMS=0.67 MPa are respectively the coefficient of thermal expansion and shear modulus of PDMS, c.sub.silicon=708 J.Math.kg.sup.1.Math.K.sup.1 and .sub.silicon=2300 kg.Math.m.sup.3[11,18] are respectively the specific heat and mass density of the silicon chip. This suggests that the normalized absorbed laser power

(81) $\frac{P{}_{\mathrm{PDMS}}}{{}_{\mathrm{PDMS}}}\sqrt{\frac{{}_{\mathrm{PDMS}}}{r_{\mathrm{silicon}}}}$
depends on the normalized critical time for delamination

(82) $\frac{{}_{\mathrm{PDMS}}t}{c_{\mathrm{PDMS}}{}_{\mathrm{PDMS}}{r}_{\mathrm{silicon}}^2}$
via a single non-dimensional combination of the specific heat and mass density of silicon and PDMS, and aspect ratio of silicon chip,

(83) $\frac{c_{\mathrm{silicon}}{}_{\mathrm{silicon}}{h}_{\mathrm{silicon}}}{c_{\mathrm{PDMS}}{}_{\mathrm{PDMS}}{r}_{\mathrm{silicon}}}.$

(84) The function, , involves a number of integrals and is evaluated numerically to produce the curve shown in FIG. 11 with

(85) $\frac{c_{\mathrm{silicon}}{}_{\mathrm{silicon}}{h}_{\mathrm{silicon}}}{c_{\mathrm{PDMS}}{}_{\mathrm{PDMS}}{r}_{\mathrm{silicon}}}=0.0616$
for the situation being modeled. For the situation reported in the experiment and used in the FEA model, P=0.0376, gave the critical time for delamination to be 1.8 ms. This is indicated by the circular red dot on the graph, agreeing well with the analytical model's prediction.

(86) To further verify the scaling law, an experiment was conducted in which the pulse time was kept constant and the laser power was gradually increased until delamination occurred. The incident power of the silicon chip corresponding to these conditions was measured as previously described at the beginning of this section (see FIG. 6). In this manner, the incident power necessary for complete delamination was obtained for pulse widths ranging from 1 to 4 ms. Taking the pulse width as a rough approximation of the start of delamination (in fact, this would be a slight overestimation of delamination time, because when complete delamination occurred, it typically occurred within a 0.5 ms interval), the black squares are plotted on the graph of FIG. 11. For pulse widths of 1, 2, 3 and 4 ms, the corresponding absorbed laser power by the silicon chip in experiments was 0.0672, 0.0403, 0.0269 and 0.0222 W, respectively. These suggest that the experimentally observed delamination times agree well with the scaling law obtained from the analytical model.

(87) Conclusions and Discussions.

(88) A millisecond laser pulse from a near infrared diode laser with power in the tens of watts was focused at the interface between a transparent stamp (of PDMS) and absorbing microdevices (of SCS, GAAS and GAN) ink, that have about a 2 orders of magnitude difference in the coefficient of thermal expansion. The strain energy release rate generated at the stamp-ink interface is sufficient to overcome the work of adhesion at the interface, and therefore results in the release and transfer of the microdevice from the stamp to a nearby receiving substrate. High-speed photography evidence clearly shows the delamination process is resulting from the elastic mismatch strain when the temperature of the stamp-ink system is raised. Measurements of IR flux incident on the chip, coupled with analytical and numerical models further validate the approach.

(89) Because the stamp is not damaged during this process, it is possible to use this as the basis of a simple, pick-and-place assembly process for assembling 3-D microdevices that cannot easily be fabricated by other processes, as well as for printing functional microdevices into or onto different substrates to enable emerging technologies such as flexible and stretchable electronics. This ability to transfer microdevices from a PDMS stamp to different receiving substrates has been integrated into printer by creating a laser print head and installing it into a computer controlled positioning stage. The full printing cycle, i.e. extracting microdevices from the growth/fabrication substrate and assembling them on a receiving substrate has been successfully implemented and successfully demonstrated for a number of cases where such transfer would be difficult, if not impossible.

(90) One challenge in laser-driven transfer printing is to reduce the temperatures at which delamination and transfer occur. Increasing the laser power increases strain energy release rate and facilitates delamination at the stamp-ink interface. But, it also increases the temperatures of the microdevice and the stamp. The analytical and numerical models presented above suggest that effective methods to reduce the stamp temperature include increasing the elastic modulus, coefficients of thermal expansion and thermal conductivity, the specific heat, mass density, and thickness of the ink. Decreasing the specific heat and mass density of the stamp also help to reduce the temperatures reached during the process.

REFERENCES

(91) [1] R. Wartena, A. E. Curtright, C. B. Arnold, A. Piqu, and K. E. Swider-Lyons, Li-ion Microbatteries Generated by a Laser Direct-Write Method, Journal of Power Sources, 126 (1-2), 193-202 (2004). [2] J. Bohandy, B. F. Kim, and F. J. Adrian, Metal deposition from a supported metal film using an excimer laser, Journal of Applied Physics, 60, 1538 (1986). [3] A. S. Holmes, S. M. Saidam, Sacrificial layer process with laser-driven release for batch assembly operations, Journal of Microelectromechanical Systems, 7 (4) (1998). [4] Y-L. Loo, D. V. Lang, J. A. Rogers and J. W. P. Hsu, Electrical Contacts to Molecular Layers by Nanotransfer Printing, Nano Letters, 3(7), 913-917 (2003). [5] J. Zaumseil, M. A. Meitl, J. W. P. Hsu, B. Acharya, K. W. Baldwin, Y-L. Loo and J. A. Rogers, Three-dimensional and Multilayer Nanostructures Formed by Nanotransfer Printing, Nano Letters, 3(9), 1223-1227 (2003). [6] E. Menard, L. Bilhaut, J. Zaumseil, and J. A. Rogers, Improved Chemistries, Thin Film Deposition Techniques and Stamp Designs for Nanotransfer Printing, Langmuir, 20(16), 6871-6878 (2004). [7] M. A. Meitl, Y. Zhou, A. Gaur, S. Jeon, M. L. Usrey, M. S. Strano and J. A. Rogers, Solution Casting and Transfer Printing Single-Walled Carbon Nanotube Films, Nano Letters, 4(9), 1643-1647 (2004). [8] Y. Sun and J. A. Rogers, Fabricating Semiconductor Nano/Microwires and Transfer Printing Ordered Arrays of Them onto Plastic Substrates, Nano Letters, 4(10), 1953-1959 (2004). [9] A. Pique, S. Mathews R. Auyeung, and B. Pratap Sood, Laser-based technique for the transfer and embedding of electronic components and devices, United States Patent Application 20090217517. [10] M. S. Meitl, Z. T. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y. Huang, I. Adesida, R. G. Nuzzo, and J. A. Rogers, Transfer printing by kinetic control of adhesion to an elastomer stamp, Nature Mat. 5, 33-38 (2006). [11] J. E. Mark (ed.), Polymer Data Handbook, Oxford University Press, New York (1999). [12] Y. Okada, and Y. Tokumaru, Precise determination of lattice parameter and thermal expansion coefficient of silicon between 300 and 1500K, J. Appl. Phys., 56 (2), 314-320 (1984). [13] A. S. Holmes, S. M. Saidam, Sacrificial layer process with laser-driven release for batch assembly operations, Journal of Microelectromechanical Systems, 7 (4) (1998). [14] M. A. Green, and M. J. Keevers, Optical Properties of Intrinsic Silicon at 300 K, Progress in Photovoltaics, 3 (3), 189-192 (1995). [15] ABAQUS Analysis User's Manual V6.9 (Dassault Systmes, Pawtucket, R I, 2009). [16] S. Kim, J. Wu, A. Carlson, S. H. Jin, A. Kovalsky, P. Glass, Z. Liu, N. Ahmed, S. L. Elgan, W. Chen, P. M. Ferreira, M. Sitti, Y. Huang and J. A. Rogers, Microstructured Elastomeric Surfaces with Reversible Adhesion and Examples of Their Use in Deterministic Assembly by Transfer Printing, Proceedings of the National Academy of Sciences USA 107 (40), 17095-17100 (2010). [17] Z. Suo, Singularities interacting with interfaces and cracks, International Journal of Solids and Structures, 25(10), 1133-1142 (1989). [18] S. A. Campbell, The Science and Engineering of Microelectronic Fabrication, Oxford University Press, New York (2001).

Example 2: Laser-Driven Non-Contact Transfer Printing (LNTP) onto Liquid Substrates

(92) The LNTP process of the present invention can be used to transfer micro- or nano-devices (ink) to receiving substrates having various surface characteristics because the LNTP process is independent of receiving surface characteristics. For example, the receiving surface may be planar, rough, charged, neutral, non-planar, and/or contoured.

(93) The present example demonstrates the applicability of the LNTP methods to liquids, biological cells, and the like. In the present example, a glass-backed transfer stamp having a 100 m PDMS post was used to transfer a 3 m thick100 m100 m silicon chip onto a water droplet disposed on a hydrophobic gold coating. The hydrophobicity of the gold coating causes the water droplet to present a highly spherical surface for receiving the silicon chip. A schematic of the technique is shown in FIG. 12(a) and a photograph of the silicon chip after transfer to the surface of the water droplet is shown in FIG. 12(b).

Example 3: A Prototype Printer for Laser Driven Micro-Transfer Printing

(94) This Example demonstrates a new mode of automated micro transfer printing called laser micro transfer printing (LTP). As a process, micro-transfer printing provides a unique and critical manufacturing route to extracting active microstructures from growth substrates and deterministically assembling them into or onto a variety of functional substrates ranging from polymers to glasses and ceramics and metallic foils to support applications such as flexible, large-area electronics, concentrating photovoltaics and displays. Laser transfer printing extends micro-transfer printing technology by providing a non-contact approach that is insensitive to the preparation and properties of the receiving substrate. It does so by exploiting the difference in the thermo-mechanical responses of the microstructure and transfer printing stamp materials to drive the release of the microstructure or ink from the stamp and its transfer to substrate. This Example describes the process and the physical phenomena that drive it. It focuses on the use of this knowledge to design and test a print head for the process. The print head is used to demonstrate the new printing capabilities that LTP enables.

Introduction

(95) In Micro-Transfer Printing (TP), a patterned viscoelastic stamp is used to pick up and transfer functional microstructures made by conventional microfabrication techniques in dense arrays on typical growth/handle substrates (such as silicon, germanium, sapphire or quartz) to a broad range of receiving substrates such as transparent, flexible and stretchable polymers, glass, ceramics and metallic foils. This provides an efficient pathway to the manufacture of flexible electronics and photovoltaics, transparent displays, wearable electronics, conformal bio-compatible sensors and many more [1, 2].

(96) FIG. 13 shows a schematic of the process along with photographs of the donor substrate with microstructures (also referred to as ink) and a receiving substrate with printed microstructures. The transfer printing stamp is typically made of molded polydimethylsiloxane (PDMS) and patterned with posts to selectively engage microstructures on the donor substrate. The ink is picked up by adhesion to the PDMS posts. Printing occurs when the inked stamp is subsequently brought into contact with a receiving substrate, followed by a slow withdrawal of the stamp. Adhesiveless transfer printing exploits the viscoelastic rate-dependent adhesion at the stamp-ink interface to enable either retrieval or printing via control of the separation velocity [3,4]. This approach to printing fabricated microstructures without adhesives simplifies downstream processing and is easily automatable by integrating onto a programmable, computer controlled positioning stage. FIG. 14 shows an automated micro-transfer printing machine. The major components of the system include (a) an automated XY-stage for positioning, (b) a Z-stage for moving the stamp up and down and controlling the separation speed and force, (c) an orientation stage that assists in obtaining parallel alignment between stamp and the receiving and donor substrates and (d) an imaging system used for alignment and monitoring of the printing process. The typical size of the printed inks ranges from 10's of microns up to the millimeter scale. The microstructure donor substrate is usually densely packed and can be of centimeter scale. The receiving substrate's dimensions are, in general, several times larger, especially when the ink is sparsely distributed on it. The stamp surfaces are typically patterned with posts with substantially the same lateral dimensions as the microstructures being printed.

(97) While the process is simple and easy to implement, its robustness is dependent on the properties and preparation of the surface of the receiving substrate. For successful printing, the adhesion between the ink and receiving surface must be sufficient to extract the ink from the stamp and, when these conditions are satisfied, the surface must be clean and flat so that good contact is developed with the ink. Thus, printing on low-adhesion surfaces, patterned surfaces or soft gels can be challenging.

(98) The process depicted in FIG. 13 can be scaled into a high transfer-rate, parallel printing process by increasing the number of posts on the stamp. As this parallelism increases, additional challenges accrue. Small misalignments between the substrate and the stamp get magnified as the size of the stamp increases causing substantial variations in the printing conditions at posts in different areas of the stamps leading to printing failures. Failure to print a microstructure in one cycle can result in repeated failures at that post in subsequent cycles, until the residual micro-structure is removed. When large receiving substrates are involved, waviness of the substrates gives rise to non-repeatable variability in printing conditions across the stamp. Finally, when large area expansions are involved, i.e., the printed microstructures have a high pitch or low areal density on the receiving substrates, the stamps used have posts that are spaced far apart and are therefore susceptible to stamp collapse [9,10], especially when larger printing forces are used to compensate for misalignments (wedge errors) between the stamp and the substrate. Such collapses result in the peeling out of microstructures by the stamp wherever contact occurs, and can damage both the donor and receiver substrates.

(99) In this Example, a new, non-contact mode for this process is developed that uses a laser to supply the energy required to drive the release of the ink from the stamp and its transfer to the receiving substrate. Since it does not rely on the strength of ink-substrate interface, created by mechanically pressing the ink onto the receiving substrate, to achieve its release from the stamp, the process does not depend on properties or the preparation of the receiving substrate for successful printing. Further, by using a scanned laser beam to address different inks or microstructures on the stamp, high-throughput modes of printing, not susceptible to small wedge errors between the stamp and the substrate, are possible. Thus, this new process mode, called Laser-Driven Micro-Transfer Printing (LTP), is a highly scalable, robust and versatile printing process.

(100) The next section describes the laser transfer printing process and the phenomena it exploits. It also provides a detailed design of the laser print head for a prototype laser transfer printing tool along with its calibration and testing. The third section demonstrates successful LTP for situations that would be difficult to achieve with conventional transfer printing. It also explores one important parameter, separation distance of the stamp and receiving substrate on the accuracy of the transfer. Finally, conclusions are discussed.

(101) Laser-Driven Micro-Transfer Printing

(102) Process Description

(103) LTP builds on micro-transfer printing technology [3, 4]. It uses the same well-developed semiconductor processing technologies for creating donor substrates with dense arrays of printable microstructures, the same materials and techniques for fabricating the transfer stamps, and the stamps are inked with microstructures using the same strategies [3,4]. The critical point of departure is the printing or transfer of the ink from the stamp to the receiving substrate. Instead of using contact-based mechanical means, LTP uses a pulsed laser beam focused on the interface between the stamp and the microstructure to release and drive the microstructure to the receiving substrate. The wavelength of the laser is chosen so that the stamp material is transparent to the laser while the ink is absorbing, e.g., an IR laser with wavelength 805 nm. Additionally, the stamp material is chosen so as to have a large mismatch in the coefficient of thermal expansion (CTE). For example, in the prototype reported here, single crystal silicon is used as the ink and PDMS as the stamp with CTEs of 2.6 ppm/ C. and 310 ppm/ C. respectively, to produce a CTE mismatch of two orders of magnitude.

(104) FIG. 1 shows a schematic of the LTP process. For the printing step, the inked stamp is positioned so that the ink is close (about 6-10 microns) to the receiving substrate. A pulsed laser beam is then focused on the interface between the stamp and the ink to cause the transfer of the ink to the substrate. Since a PDMS stamp is transparent in the near IR range, the laser radiation is transmitted through the stamp and is absorbed by the microstructure ink. As a result, the ink heats up and acts as a heat source for the PDMS stamp, conducting heat across the stamp-ink interface to raise the temperature of the PDMS stamp in the vicinity of the interface. The rise of temperature in the stamp and ink leads to thermal expansions in both. Due to the large CTE mismatch for the two materials (.sub.s=310 ppm/ C. [11] for PDMS and .sub.c=2.6 ppm/ C. for silicon [12]) and their free expansion being restricted by the contact interface between them, the thermal strain must be accommodated by bending (or the formation of a curvature) in the stamp-ink composite. This stresses the interface and, when the energy release rate due to delamination at the interface exceeds the work of adhesion of the interface, the ink is released from the stamp.

(105) Bohandy [13] was the first to report a laser-driven deposition process. Holmes and Saidam [14] reported a process called Laser-Driven Release and used it for printing prefabricated metal microstructures from a glass fabrication substrate onto a receiving substrate. Arnold and Pique [15] have reported widely on what they call the Laser-Induced Forward Transfer (LIFT) process. In all these approaches, the driving mechanism is laser ablation at the interface. Much of the reported research uses pico- or femtosecond lasers and sacrificial layers at the microstructure-support structure (stamp) interface with a low vaporization temperature and a high absorptivity at the laser wavelength to enhance the delamination forces produced by ablation. The unique aspects, then, of LTP, include but are not limited to: Use of microsecond scale pulses and reliance on a thermo-mechanical phenomenon based on thermal strain mismatch to drive the transfer printing process; Use of lower temperatures (250 to 300 C. instead of temperatures reaching 1000 C.), which leads to less damage to active microstructures. the stamp properties are tuned to achieve both extraction of ink from the donor substrate and deposition onto the receiving substrate the stamp remains substantially undamaged (because the process is driven by a reversible physical strain in the stamp rather than an irreversible chemical change in it), thus enabling a repeated pick-and-place process mode.

(106) Detailed modeling and analysis of the process are described in [23]. This Example concentrates on the design of the printing tool for the process.

(107) Prototype Laser Micro-Transfer Printer Design

(108) A prototype LTP was developed by designing a printhead and integrating it with an xyz-positioning stage. A schematic of the print head is shown in FIG. 6. The print head was developed so that printing could be observed through the stamp. The laser radiation is brought into the system via an optical cable from one side of the print head. A dichroic mirror is used to direct the laser beam towards the stamp below it. A GRIN lens at the end of the optical cable is used to focus the laser beam on the ink.

(109) One of the first steps in the realization of the schematic of the prototype print head of FIG. 6 was to estimate the power requirements (i.e., size the laser for the print head) and perform an analysis of whether a thermo-mechanical delamination process was possible without damaging the PDMS stamp. For this analysis (and for experimental verification) a single crystal silicon square with a lateral dimension of 100 microns and a thickness of 3 microns was used as the model or representative ink. First, temperatures at which thermal mismatch strains in the Si-PDMS system give rise to energy release rates sufficient to overcome the work of adhesion at the Si-PDMS interface were calculated. The power of the laser system required to drive the steady state temperature of this system past the delamination temperature was then computed.

(110) To compute the delamination temperature, the approach originally proposed by Stoney [16] for an infinitely thin film as modified by Freund [17] for finite film thickness was used. Silicon was used as the thin film (thickness, h.sub.c=3 m) and PDMS as the substrate (thickness, h.sub.s=100 m) to model film delamination. As previously mentioned, the PDMS stamp has a higher coefficient of thermal expansion; thus, when heated, the PDMS expands more than the Si ink, although the expansion is constrained due to a common interface shared by the two materials. As a result, strains accrue in both materials. To estimate this strain, a constant, uniform temperature distribution throughout the ink and the immediate vicinity of the post on the stamp was assumed. The strain energy exists solely because of an incompatible elastic mismatch strain that arises when the temperature is increased by an amount T above room temperature (the conditions at which the interface was created) due to heating by laser pulse, as no external applied tractions or stresses exist in the system. Consequently, the Si chip undergoes a biaxial tensile stress; assuming the printing chip is an isotropic, elastic, homogenous material; its strain energy density at the interface is given by, U(z=h.sub.s):

(111) $\begin{array}{cc}{U.Math.}_{z=\frac{h_s}{2}}=\frac{E_c}{1-{}_c}{\left({\mathrm{.Math.}}_o-\frac{h_s}{2}+{\mathrm{.Math.}}_m\right)}^2& \left(1\right)\end{array}$
where the elastic modulus (E.sub.c=179.4 GPa) and Poisson ratio (.sub.c=0.28) denote the elastic constants of silicon [3]. Hence, the strain energy density is composed of the midplane extensional strain, .sub.0, the strain arising from the mismatch in thermal expansion coefficients between the chip and substrate, .sub.m, and the curvature, , of the chip about a center of curvature equivalent to half of the substrate's thickness, h.sub.s/2. The mismatch in thermal expansion coefficients of the stamp and chip produces a strain, .sub.m=(.sub.s.sub.c)T.

(112) The potential energy, V, is found by integrating Equation 1 with respect to the height of the system. By taking the variants of the potential energy and checking for stability of the system (i.e. V/.sub.0=0 and V/=0), two equations and two unknowns are obtained, the midplane extensional strain (.sub.0) and the curvature (), that can be solved to yield:

(113) 0$\begin{array}{cc}=\frac{{}_{\mathrm{st}}\left(1+h\right)}{\left[1+4\mathrm{hm}+6h^{2}m+4h^{3}m+h^4{m}^2\right]},& \left(2a\right)\\ {\mathrm{.Math.}}_o=\frac{{\mathrm{.Math.}}_{\mathrm{st}}\left(1+h^{3}m\right)}{\left[1+4\mathrm{hm}+6h^{2}m+4h^{3}m+h^4{m}^2\right]},\mathrm{where}{}_{\mathrm{st}}=\frac{6{\mathrm{.Math.}}_m}{h_s}\mathrm{hm}\mathrm{and}{\mathrm{.Math.}}_{\mathrm{st}}=-{\mathrm{.Math.}}_m\mathrm{hm}.& \left(2b\right)\end{array}$

(114) In these equations, shorthand notation is used where h (=h.sub.c/h.sub.s) and m (=E.sub.c*(1.sub.s)/E.sub.s(1.sub.c)) refer to the ratios of the thicknesses and biaxial moduli of the chip to the substrate, respectively. Also, .sub.st and .sub.st refer to the solution of the Stoney equation, where the chip is infinitely thin. From this analysis, the stress in the chip at the interface is given by:

(115) $\begin{array}{cc}{}_c=\frac{E_c}{1-{}_c}\left({\mathrm{.Math.}}_o-\frac{h_s}{2}+{\mathrm{.Math.}}_m\right).& \left(3\right)\end{array}$

(116) The strain energy accumulation in the system is relieved by deformation, giving rise to a curvature of the microstructure/stamp system, as shown in FIG. 15. The bending strain energy associated with this curvature produces the driving force for delamination at the ink-stamp interface. The energy release rate associated with such delamination due to relaxation of bending strain is given by:

(117) $\begin{array}{cc}G=\frac{1-{}_c^2}{2E_c}{\left({}_c-{}_a\right)}^2{h}_c& \left(4\right)\end{array}$
where .sub.a is the applied external stress [26], which is zero in this case. When this energy release rate is greater than the adhesion energy of the Si-PDMS interface, one can expect delamination to occur and the ink to be released from the stamp. The above analysis was used to arrive at a relationship between the energy release rate, G (J/m.sup.2), and the temperature to which the system is raised above room temperature, T ( C.). This is shown in FIG. 16.

(118) A number of investigators have reported values in the range of 0.05 to 0.4 J/m.sup.2 for the adhesion energy of Si-PDMS interfaces [4, 10, 18-20]. From FIG. 16, choosing a conservative value of 0.5 J/m.sup.2 for G, produces a corresponding delamination temperature between 275-300 C. This value is well within the range that PDMS can withstand without decomposing, especially for short, millisecond, durations [21].

(119) As stated in the description of the process, the laser heats up the Si ink that, in turn, heats up the interface and the PDMS in the vicinity. To achieve this, a COMSOL finite element model was used with the Si ink acting as the heat source. The strength of the heat source was varied and the corresponding steady state temperatures were computed. FIG. 17 shows the schematic of the model with a 1001003 m thick silicon chip attached to a 200200100 m high PDMS post. The bottom surface of the PDMS stamp (in FIG. 17) is fixed and the bottom surface of the silicon ink is constrained to move with the top surface of the post on the PDMS stamp. Other surfaces in this model are free to move. The heat source in the model is the square-shaped area at the stamp-ink interface. The exposed surfaces of the silicon and PDMS lose heat to the surroundings by convection. The model uses 75000 nodes to perform a transient heat transfer analysis in COMSOL 3.5 for run intervals up to 5 milliseconds (typical laser pulse times range from 1 to 5 ms) with the silicon ink, PDMS and surroundings initially at 27 C. FIG. 17 shows the results of one run, in which 135 mJ of heat is input into the system over a 3.4 millisecond interval. From this simulation, one can see that the temperatures reached in the system are about 584 K, slightly higher than 300 C., sufficient to cause delamination without damaging the stamp.

(120) From this value of heat input rate, it is possible to approximate to 150 mJ over 4 ms or 0.0375 W and to calculate the power required in the laser pulse, but one must account for reflective and transmission losses as well as for the intensity distribution in the beam. For 800 nm radiation, the coefficient of absorption for silicon, .sub.c=10.sup.3 cm.sup.1 or its absorption depth is about 10 m. The intensity of the radiation emerging from a 3 m thick sheet of silicon as a fraction of the intensity of the incident radiation, I.sub.0, is given by:

(121) $\begin{array}{cc}\frac{I}{I_0}=\exp \left(-{}_{c}h\right)& \left(5\right)\end{array}$
which for h=3 m becomes approximately 0.75. With 75% of the radiation lost to transmission, only 25% of the radiation that enters the silicon is available for heating the ink. Dealing next with the fraction of the beam area that is incident on the silicon ink, one major consideration is to uniformly heat the ink across its lateral dimension. If one considers a Gaussian beam, then too small of a beam diameter will result in a hot spot at the center of the ink. The power, P(r), contained within a radius r of the beam is given by (see, for example, [22]):

(122) $\begin{array}{cc}P\left(r\right)=P\left(\right)\left[1-\exp \left(\frac{-2r^2}{{}_0^2}\right)\right]& \left(6\right)\end{array}$
where P() is the total power in the beam and .sub.0 is the beam radius. For r=0.23 .sub.0, the intensity drop from the beam center to the perimeter of the circle is 0.1 or 10%. This will provide relatively uniform heating, but only 10% of the beam energy is contained in the circle. Finally, one must deal with the reflectivity of polished silicon, which at 800 nm is 0.328. Thus only 67.2% of the radiation incident on the ink is absorbed by, or transmitted through, it.

(123) In summary, to provide the required 0.0375 W of heating, the beam power in the plane of the ink-stamp interface must be:

(124) $\begin{array}{cc}P=\frac{0.0375}{0.25*0.1*0.672}2.25W& \left(7\right)\end{array}$

(125) Thus, it is not only feasible to thermo-mechanically delaminate the model silicon ink from the PDMS stamp by exploiting the mismatch in CTEs, it is possible to do so with a moderately powered diode laser.

(126) FIG. 18 shows a photograph of the print head. A Jenoptik continuous wave, fiber-coupled (fiber core diameter of 0.2 mm), passively-cooled, 808 nm 30 W laser diode with electronic pulse control is used. A higher power rating was chosen to be able to account for losses in the coupling and cable, and to accommodate different materials and thinner and larger lateral dimension inks. The pulse resolution for the laser is 1 millisecond. The print head is integrated onto a custom-assembled, gantry-type XYZ positioning stage. The stage has 1 micron resolution, 150 mm of travel in the X and Y directions and 100 mm of travel in the Z direction. It is fitted with high (1 mm) resolution optics, capable of observing the process through the stamp. Except for the difference in the print head, the structure of the printer is very much like that shown in FIG. 14.

(127) Calibration and Testing

(128) The prototype printer along with the laser printing head is calibrated to relate the beam power available at the ink-stamp interface for different current settings of the laser. Also, the validity numbers used in the analysis and design of the printer are verified.

(129) To relate the current settings on the laser and the beam energy as it arrives at the stamp-ink interface, a photodiode power meter with a pre-calibrated reader (Thorlabs PM100D) is used, as shown in the schematic of FIG. 19. This power meter is chosen to have a very fast response time (<200 ns) compared to the laser pulse width (typically >1 ms), high optical power range (5 W-5 W) to withstand the intensity of the beam, high resolution (1 nW) and large inlet aperture (12 mm) to be able to easily capture the entire laser beam during a pulse. A data acquisition card captures the analog output of the calibrated reader at a sampling rate of 40 kHz and stores it on a PC for subsequent analysis. The laser pulse time is set to 10 ms and the laser is pulsed with different current settings. The readings taken are averaged after those corresponding to the first and last milliseconds of the pulse are deleted to get rid of transients. This is repeated three times for each current setting. As can be seen in FIG. 19, the relationship between beam-power at the ink-stamp interface and the current setting for the laser is linear, with a threshold current of 5 amps. The calibration is done in the current range of 5 amps to 13 amps, with the beam power ranging from 0 to 5.25 watts (sufficient for laser printing, with the model inks)

(130) To verify the delamination conditions previously stated, a two-step experiment is performed. The model ink (1001003 mm silicon square) is loaded onto the stamp using the standard transfer printing pick-up step [3, 4]. Next the printing step is attempted. Here the pulse duration is set to 4 ms and pulses of increasing power (obtained by gradually increasing the current) are used until the power level at which transfer occurs is reached. This gives the minimum energy input settings for a 4 ms pulse at which transfer of the ink takes place. After this, the receiving substrate is replaced with the photodiode power meter and two laser power recordings are made with the same pulse times but a current setting just a little bit lower that that needed to achieve transfer. The first measurement is made with the beam passing through an empty stamp and the second is made with the ink on the stamp. Integrating the power measured across the duration of the pulse gives the total energy arriving at the power meter due to the pulse. The difference between the total energy arriving at the photometer with and without the ink gives the sum of the energy reflected and absorbed by the ink. Knowing the reflectivity, it is possible to obtain the energy absorbed by the ink and available for heating the ink. Also, Equation 7 gives the beam power at the plane of the ink-stamp interface required for delamination and transfer to be around 2.25 W. Examining the power recording allows for verification of the design.

(131) FIGS. 7 and 8 show the power recordings by the photodiode power meter. Integrating the areas under the curves, it can be seen that the difference in energy reaching the power meter is 0.224 mJ. Accounting for the reflectance of the silicon inks, energy available for heating the ink is 0.134 mJ, a value very close to that predicted by the thermo-mechanical delamination analysis. Additionally, from this recording, it can be see that the beam power required for delamination is around 2.5 W, while 2.25 W was the computed power requirement. Thus, the approach to designing the print head can be considered to be reasonably accurate.

(132) Demonstrating LTP

(133) LTP provides new capabilities for transfer printing technology. As previously stated, it is substantially independent of the properties and topography of the receiving surface. Hence, it should be possible to print on surfaces with low adhesion energy, structured surfaces where contact area is a small fraction of the surface, and non-flat surfaces. Each of these cases was tested and demonstrated to be feasible. Additionally, the possibility of printing on liquids and gels is also demonstrated. Finally, positional errors for printing on low adhesion energy surfaces are experimentally characterized. The model ink, 1001003 micron Si squares, was used for these demonstrations. Further, the printing for these demonstrations was conducted with the pulse time set to 4 ms, and the power level set to 2.5 W.

(134) Printing silicon inks on silicon surfaces is generally difficult with flat PDMS stamps because of the low adhesion at the SiSi interfaces. It is easily accomplished by the LPT process. FIG. 20(a) shows a small array of silicon chips printed onto a silicon substrate to bridge gold traces that were pre-patterned on the surface. FIG. 20(b) shows a multi layered structure of silicon squares which would be extremely challenging to achieve with conventional transfer printing as contact is made only at the corners of the squares. FIG. 20(c) demonstrates the printing of a silicon chip between two pedestals.

(135) Printing of inks on non-flat (e.g. spherical) surfaces, including the surface of a liquid droplet, was performed. FIG. 21 shows some results where silicon squares are successfully printed on individual spheres, a non-uniform array of beads and on the surface of a NOA droplet.

(136) Finally, to demonstrate printing on partial and recessed surfaces, a number of substrates with different features were prepared. FIG. 22 shows examples of printing on ledges, beams and inside concave features. Some of these printing demonstrations exhibit the kind of precise placement that the process is capable of producing. This precision in placement is dependent on a number of set-up factors such as precise centering of the beam on the ink. It is also dependent on process variables, the key variable being the stand-off or distance of the stamp from the receiving substrate. To characterize this dependence, printing was performed at the lowest energy for reliable delamination (4 ms pulses with the power setting at 2.5 W and the same model ink) with different stand-off heights onto a substrate patterned with fiducials. First the stamp is brought in close to the substrate and aligned to the fiducial on the substrate using the optics on the printer (about 1 m resolution) and the positioning stages (also 1 m resolution). It is then withdrawn to the appropriate height and transfer printed. The error in the transfer process is obtained through image analysis of frames taken after alignment (with the ink still on the stamp) and after printing. This experiment is conducted for different stand-off heights ranging from 5 m to 300 m, with 5 repetitions at each stand-off height. FIG. 23 shows the observed dependence of transfer errors on printing stand-off height. Within the resolution of experimental observations, the transfer errors become insignificant at stand-off heights of about 20 m.

CONCLUSIONS

(137) In this Example a new mode of transfer printing has been demonstrated and an automated transfer printing machine to implement the new mode was prototyped. In this mode of micro-transfer printing, a laser supplies the energy to drive a thermo-mechanical delamination process that releases the ink from the stamp and transfers it to the receiving substrate. A procedure for designing the print head is developed and verified. This new printing mode, called Laser Micro-Transfer Printing (LTP), extends the versatility of micro transfer printing by making the process virtually independent of the properties and preparation of the receiving substrate. Thus, printing on low adhesion surfaces, curved, partial and recessed surfacesoperations that are typically difficult in more conventional modesare easily performed, as demonstrated on a prototype laser micro-transfer printer.

REFERENCES

(138) [1] Kim R, Kim D, Xiao J., Kim B, Park S, Panilaitis B, Ghaffari R, Yao J, Li M, Liu Z., Malyarchuk V, Kim D, Le, A, Nuzzo R G, Kaplan D, Omenetto F, Huang Y, Kang Z, & Rogers J A. (2010) Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics. Nature Materials 9, 929-937. [2] Yoon J, A J Baca, A I Park, P Elvikis, J B Geddes, L Li, R H Kim, J Xiao, S Wang, T H Kim, M J Motala, B Y Ahn, E B Duoss, J A Lewis, R G Nuzzo, P M Ferreira, Y Huang, A Rockett and J A Rogers (2008) Ultrathin Silicon Solar Microcells for Semitransparent, Mechanically Flexible and Microconcentrator Module Designs. Nature Materials 7, 907-915. [3] Meitl M A, Zhu Z T, Kumar V, Lee K J, Feng X, Huang Y Y, Adesida I, Nuzzo R G and Rogers J A (2006) Transfer Printing by Kinetic Control of Adhesion to an Elastomeric Stamp. Nature Materials 5, 33-38. [4] Kim S, Wu J, Carlson A, Jin S H, Kovalsky A, Glass P, Liu Z, Ahmed N, Elgan S L, Chen W, Ferreira P M, Sitti M, Huang Y and Rogers J A (2010) Microstructured Elastomeric Surfaces with Reversible Adhesion and Examples of Their Use in Deterministic Assembly by Transfer Printing. Proceedings of the National Academy of Sciences USA 107(40), 17095-17100. [5] Ishikawa F N, Chang H K., Ryu K., Chen P C, Badmaev A, De Arco L G, Shen G, Zhou C (2009) Transparent Electronics Based on Transfer Printed Aligned Carbon Nanotubes on Rigid and Flexible Substrates. ACS Nano 3, 73-79. [6] Bower C A, Menard E, Bonafede E (2010) Active-Matrix OLED Display Backplanes Using Transfer-Printed Microscale Integrated Circuits. Proceeding of the 59.sup.th Electronic Component and Technology Conference, San Diego, Calif., USA. [7] Lee K J, Meitl M A, Ahn J H, Rogers J A, Nuzzo R G, Kumar V and Adesida I (2006) Bendable GaN High Electron Mobility Transistors on Plastic Substrates. Journal of Applied Physics 100(12), 124507-124507-4. [8] Ko H C, Stoykovich M P, Song J, Malyarchuk V, Choi W M, Yu C J, Geddes J B, Xiao J, Wang S, Huang Y and Rogers J A (2008) A Hemispherical Electronic Eye Camera Based on Compressible Silicon Optoelectronics. Nature 454, 748-753. [9] Hsia K J, Huang Y, Menard E, Park J U, Zhou W, Rogers J A and Fulton J M (2005) Collapse of stamps for soft lithography due to interfacial adhesion. Applied Physics Letters 86(15), 1900303. [10] Huang Y Y, Zhou W X, Hsia K J, Menard W, Park J U, Rogers J A and Alleyne A G (2005) Stamp collapse in soft lithography. Langmuir 21(17), 8058-8068. [11] Mark J E (ed.) (1984), Polymer Data Handbook, Oxford University Press, New York. [12] Okada Y, and Y Tokumaru (1984) Precise determination of lattice parameter and thermal expansion coefficient of silicon between 300 and 1500K. J. Appl. Phys., 56 (2), 314-320. [13] Bohandy J, B F Kim, and F J Adrian (1986) Metal deposition from a supported metal film using an excimer laser. Journal of Applied Physics, 60, 1538. [14] Holmes A S and S M Saidam (1998) Sacrificial layer process with laser-driven release for batch assembly operations. Journal of Microelectromechanical Systems, 7. 4, 416-422. [15] Wartena R, A E Curtright, C B Arnold, A Piqu, and K E Swider-Lyons (2004) Li-ion Microbatteries Generated by a Laser Direct-Write Method. Journal of Power Sources, 126 (1-2), 193-202. [16] G G Stoney G G (1909) The tension of metallic films deposited by electrolysis. Proc. R Soc. Lond A 82, 553, 172-175. [17] Freund L B and S Suresh (2003) Thin Film MaterialsStress, Defect Formation, and Surface Evolution. Cambridge University Press, Cambridge. [18] Chaudhury, M K and G M Whitesides (1991) Direct measurement of interfacial interactions between semispherical lenses and flat sheets of poly(dimethylsiloxane) and their chemical derivatives. Langmuir, 7 (5), pp. 1013-1025. [19] Armani D, C Liu and N Aluru (1999) Re-configurable fluid circuits by PDMS elastomer micromachining. MEMS '99 Twelfth IEEE International Conference, Orlando, Fla., 222-227. [20] Deruelle M, L Leger and M Tirrell (1995) Adhesion at the solid-elastomer interface: influence of the interfacial chains. Macromolecules 28, 7419-7428. [21] Camino G, S M Lomakin, M Lazzari (2001) Polydimethylsiloxane thermal degradation Part 1. Kinetic aspects, Polymer, 42, 2395-2402. [22] http://www.rpgroup.caltech.edu/courses/aph162/2007/Protocols/Optics/e3872_Gaussian-Beam-Optics.pdf [23] Saeidpourazar R, R Li, Y Li, M D Sangid, C L, Y Huang, J A Rogers and P M Ferreira (2011) Laser-driven Non-contact Transfer Printing of Prefabricated Microstructures. Submitted to IEEE/ASME J MEMS.

Example 4: Laser Driven Micro-Transfer Printing Parameters

(139) This Example explores parameters related to laser micro-transfer printing. The setup used for this parametric study directs the beam from the optical cable through the stamp and makes it incident on a photodiode to obtain the incident power/energy. A typical photodiode has two limitations. First, the precalibrated board is slow and cannot be integrated with the set up to be synchronized with the laser pulse. Second, the power range for measurements is limited to about 2.5 W. To overcome these limitations, faster but uncalibrated data-acquisition was used and a 5% optical filter was used to reduce the power. Overlapping measurements were made to relate the pre-calibrated power measurements without the filter to those made with the high-speed data acquisition system with the filter.

(140) Power Required for Delamination

(141) To compute the power incident on the chip (ink), for each experiment reported, power measurements were made with and without the ink on the stamp. The difference provides the energy incident on the ink. Knowing the emissivity, the absorbed energy can be estimated. FIGS. 24(a) and 24(b) show schematically how the measurements were made. The incident energy is the difference in the area under the power curves of FIGS. 24(a) and 24(b). Measurements were made by fixing the pulse width and gradually increasing the power level until delamination was achieved. For each of these experiments, 100 micron silicon squares were used as the ink. Pulse widths ranging from 1 ms to 7 ms were tested. Incident energy was calculated using the difference in areas under the power curves of the pulse.

(142) The power required for delamination decreases with pulse width up to a point and then stays constant. After about 4 ms pulses, the minimum power to delaminate stayed the same. This is possibly because the steady state temperature reached for lower power settings was not high enough to produce the energy release rate to overcome the adhesion energy at the interface.

(143) FIG. 25 provides a schematic showing the amount of energy required for delamination as a function of (a) pulse width, (b) ink thickness and (c) ink size.

(144) Effect of Ink Thickness

(145) For these experiments all other factors were kept constant, only the chip (ink) thickness was varied. 100100 micron chips were subjected to 4 ms laser pulses, where pulse width was shown to be substantially constant. The pulse power was gradually increased until delamination was achieved.

(146) Power measurements were made with and without the chip on the stamp to obtain the energy input into the process (by taking the difference in the area under the power curve). Incident energy may be a misnomer here because transmission losses could be quite high for the thinner chips. Transmitted energy would be captured by the power sensor. Therefore the trend seen must be due to factors other than transmission losses.

(147) The strain energy due to bending that is stored in the chip decreases as the cube of the chip thickness. Therefore the system must be deformed much more to produce the energy release rate needed to overcome the adhesion energy at the interface. Therefore more energy must be input into the system for thinner chips.

(148) Effect of Ink Size

(149) For these experiments all other factors were kept constant, only the chip (ink) size was varied. As shown in FIG. 25(c), square chips with varying lateral dimensions and a thickness of 3 microns were subjected to 4 ms laser pulses, where pulse width was shown to be substantially constant. As shown in FIG. 25(b), square chips with varying thicknesses were subjected to 4 ms laser pulses.

(150) As shown in FIG. 25(a), the pulse power was gradually increased until delamination was achieved. Power measurements were made with and without the chip on the stamp to obtain the energy input into the process (by taking the difference in the area under the power curve). The increase in energy required for delamination rises more sharply than the power in the laser beam. This is because larger chips use a larger fraction of the energy in the beam. A much sharper increase is seen in the incident energy for delamination. This takes into consideration the actual laser flux incident on the chip and channeled into the delamination process. There might be a quadratic relationship between chip dimensions and energy required for delamination.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

(151) 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).

(152) 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 invention has been specifically disclosed by preferred embodiments, exemplary 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. The specific embodiments provided herein are examples of useful embodiments of the invention and it will be apparent to one skilled in the art that the invention can 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 apparent 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.

(153) 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 isomer 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.

(154) 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.

(155) TABLE-US-00001 Application Publication Publication No. Filing Date No. Date Patent No. Issue Date 11/001,689 Dec. 1, 2004 2006/0286488 Dec. 21, 2006 7,704,684 Apr. 27, 2010 11/115,954 Apr. 27, 2005 2005/0238967 Oct. 27, 2005 7,195,733 Mar. 27, 2007 11/145,574 Jun. 2, 2005 2009/0294803 Dec. 3, 2009 7,622,367 Nov. 24, 2009 12/564,566 Sep. 22, 2009 2010/0072577 Mar. 25, 2010 7,982,296 Jul. 19, 2011 13/113,504 May 23, 2011 2011/0220890 Sep. 15, 2011 11/145,542 Jun. 2, 2005 2006/0038182 Feb. 23, 2006 7,557,367 Jul. 7, 2009 11/423,287 Jun. 9, 2006 2006/0286785 Dec. 21, 2006 7,521,292 Apr. 21, 2009 12/405,475 Mar. 17, 2009 2010/0059863 Mar. 11, 2010 11/675,659 Feb. 16, 2007 2008/0055581 Mar. 6, 2008 11/465,317 Aug. 17, 2006 11/423,192 Jun. 9, 2006 2009/0199960 Aug. 13, 2009 7,943,491 May 17, 2011 11/421,654 Jun. 1, 2006 2007/0032089 Feb. 8, 2007 7,799,699 Sep. 21, 2010 12/844,492 Jul. 27, 2010 2010/0289124 Nov. 18, 2010 8,039,847 Oct. 18, 2011 11/851,182 Sep. 6, 2007 2008/0157235 Jul. 3, 2008 11/585,788 Sep. 20, 2007 2008/0108171 May 8, 2008 7,932,123 Apr. 26, 2011 13/071,027 Mar. 24, 2011 2011/0171813 Jul. 14, 2011 13/228,041 Sep. 8, 2011 11/981,380 Oct. 31, 2007 2010/0283069 Nov. 11, 2010 7,972,875 Jul. 5, 2011 13/100,774 May 4, 2011 12/669,287 Jan. 15, 2010 2011/0187798 Aug. 4, 2011 12/418,071 Apr. 3, 2009 2010/0052112 Mar. 4, 2010 12/398,811 Mar. 5, 2009 2010/0002402 Jan. 7, 2010 12/996,924 Dec. 8, 2010 2011/0147715 Jun. 23, 2011 13/120,486 Aug. 4, 2011 12/778,588 May 12, 2010 2010/0317132 Dec. 16, 2010 12/968,637 Dec. 15, 2010 12/947,120 Nov. 16, 2010 2011/0170225 Jul. 14, 2011 12/916,934 Nov. 1, 2010 13/046,191 Mar. 11, 2011

(156) 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.

(157) 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.

(158) 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.

(159) 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.

(160) 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.