High-temperature nanocomposite emitting film, method for fabricating the same and its application

Abstract

An inventive thin-film radiative structure is provided that includes a thin nanocomposite radiative film deposited on a substrate, the thin-film including a mix of finely dispersed phases formed by elements Mo, Si, C, O in the following atomic percentage terms: Mo from 10 to 20%, Si from 15 to 30%, C from 15 to 60%, O from 0 to 20%, and one or a combination of elements Ti, Zr, Hf, Cr, Si, Al, and B in percentage terms of 0-30%. The thin-film radiative structure has an emissivity of more than 0.7 for wavelengths 2-20 m at temperatures above 500 C., and a sheet resistance of between 10 and 150 Ohm/sq. The radiative film may be used as a thermoresistive element in thin-film infra-red thermal emitters and infra-red heaters, and in nondispersive infrared sensors (NDIR) and photo-acoustic gas sensors, and as the radiative element in IR signaling devices.

Claims

1. A thin-film radiative structure, comprising: a substrate; and a radiative film deposited on and supported by the substrate, the radiative film comprising a mixture of at least first and second nanocrystals sputtered from an initial target comprising MoSi.sub.2 and SiC; the at least first and second nanocrystals being finely mixed and different from each other in phase or elemental composition, each having an average crystal grain size of 100 nm or smaller; and the elemental composition of each of the at least first and second nanocrystals independently comprising: Mo in an amount of 10 to 20 atomic %, Si in an amount of 15 to 30 atomic %, C in an amount of 15 to 60 atomic %, and O in an amount of 0 to 20 atomic %, and at least one element selected from the group consisting of: Ti, Zr, Hf, Cr, Al, and B in an amount of 0 to 30 atomic %.

2. The thin-film radiative structure of claim 1, wherein the at least first and second nanocrystals each have an average crystal grain size of 2 to 100 nm.

3. The thin-film radiative structure of claim 1, wherein the at least first and second nanocrystals each have an average crystal grain size of 2 to 10 nm.

4. The thin-film radiative structure of claim 1, wherein the radiative film has an emissivity of at least 0.7 for wavelengths 2 to 20 m at a temperature above 500 C.

5. The thin-film radiative structure of claim 1, wherein the radiative film has a sheet resistance of 10 to 150 Ohm/sq.

6. The thin-film radiative structure of claim 1, wherein the substrate is a low stress membrane with stress in a range of 50 to 150 MPa.

7. The thin-film radiative structure of claim 1, wherein the substrate is a membrane with low tensile or compressive stress of 50 to 150 MPa.

8. The thin-film radiative structure of claim 1, wherein the substrate is made of silicon nitride.

9. The thin-film radiative structure of claim 1, wherein the substrate is a multilayer structure consisting of alternating layers of silicon nitride and silicon oxide.

10. The thin-film radiative structure of claim 9, wherein the silicon nitride and silicon oxide layers each have an individual thickness of 0.05 to 1.0 m.

11. The thin-film radiative structure of claim 1, wherein the substrate is ceramic.

12. The thin-film radiative structure of claim 1, wherein the radiative film has a thickness of 0.05 to 1.0 m.

13. An infra-red thermal emitter, comprising: the thin-film radiative structure of claim 1; and an electric current source configured to heat the radiative film by passing an electric current through the radiative film.

14. An infra-red heater, comprising: the thin-film radiative structure of claim 12; and an electric current source configured to heat the radiative film by passing an electric current through the radiative film.

15. A method of fabricating a thin film, the method comprising: sputtering from an initial target comprising MoSi.sub.2 and SiC under an atmosphere of argon (Ar) or Ar mixed with an oxygen (O.sub.2) partial pressure of 0 Pa to 110.sup.2 Pa to thereby form a radiative film on a substrate, the SiC being included in the initial target in an amount of 10 to 85 mol %, with the remainder being MoSi.sub.2; the radiative film comprising a mixture of at least first and second nanocrystals; the at least first and second nanocrystals being finely mixed and different from each other in phase or elemental composition, each having an average crystal grain size of 100 nm or smaller; and the elemental composition of each of the at least first and second nanocrystals independently comprising: Mo in an amount of 10 to 20 atomic %, Si in an amount of 15 to 30 atomic %, C in an amount of 15 to 60 atomic %, and O in an amount of 0 to 20 atomic %, and at least one element selected from the group consisting of: Ti, Zr, Hf, Cr, Al, and B in an amount of 0 to 30 atomic %.

16. The method of claim 15, wherein: the initial target further comprises at least one of an oxide, a metal carbide, or a boride, the oxide being included in the initial target in an amount of 0 to 30 mol %, the metal carbide being included in the initial target in an amount of 0 to 60 mol %, and the boride being included in the initial target in an amount of 0 to 60 mol %.

17. The method of claim 16, wherein the oxide is selected from the group consisting of SiO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3, and combinations thereof.

18. The method of claim 16, wherein the boride is selected from the group consisting of TiB.sub.2, ZrB.sub.2, Hf B.sub.2, CrB.sub.2, SiB.sub.4, SiB.sub.6, CB.sub.2, and combinations thereof.

19. The method of claim 16, wherein the metal carbide is selected from the group consisting of TiC, HfC, ZrC, and combinations thereof.

20. The method of claim 15, further comprising applying an electrical bias to the substrate, the electrical bias being 0 to 400 V.

21. The method of claim 20, wherein the electrical bias is applied at a radio frequency of 100 KHz to 13.6 MHz.

22. The method of claim 15, further comprising thermally annealing the radiative film on the substrate at a temperature of 800 to 950 C. under an air, argon, or vacuum atmosphere.

23. The method of claim 22, wherein the thermal annealing is performed for 1 to 10 hours.

24. The thin-film radiative structure of claim 1, wherein the initial target further comprises one or more of SiO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3, TiB.sub.2, ZrB.sub.2, HfB.sub.2, CrB.sub.2, SiB.sub.4, SiB.sub.6, CB.sub.2, TiC, ZrC, and/or HfC.

25. A device, comprising the thin-film radiative structure of claim 1 configured as a thermo-resistor that is heatable by an electric current.

26. The device according to claim 25, wherein the device comprises a nondispersive infrared sensor (NDIR), a photo-acoustic gas sensor, or an infrared signaling device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is further detailed with respect to the following non-limiting specific embodiments of the present invention. The appended claims should not be construed as being limited to the specific devices so detailed.

(2) FIGS. 1A-1B are cross-sectional side and top views that depict the basic thin-film structure of an infrared emitter (IRE) chip;

(3) FIG. 2 is a graph of resistance as a function of background oxygen pressure inside a vacuum chamber for film deposition, and illustrates the dependence of sheet resistance of the films received by sputtering of a target: MoSi.sub.2+35 mole % SiC in a partial pressure of oxygen P.sub.oxygen.

(4) FIG. 3 is a graph of microanalysis results from an energy-dispersive X-ray spectroscopy (EDAX) of the films produced by sputtering of a target with a (MoSi.sub.2+35 mole % SiC) composition.

DESCRIPTION OF THE INVENTION

(5) The present invention has utility as a method for forming devices with inventive forms of inorganic nanocomposite thin films that are capable of emitting large amounts of thermal energy in the infrared wavelength region as a result of resistive heating. In certain embodiments, the fabrication of inventive thin films is by sputtering in a high vacuum of a mix of various refractory materials consisting of MoSi.sub.2, SiC and/or oxides of various metals, e.g., Si, Ti, Zr, Al, B and/or boride metals, e.g., TiB.sub.2, ZrB.sub.2, HfB.sub.2, CrB.sub.2, SiB.sub.4, SiB.sub.6, CB.sub.2 and/or carbide metals, e.g., TiC, ZrC, HfC.

(6) It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

(7) As used herein, high-temperature stability is defined as compositional retention to at least 500 C., and up to 900 C., and an emissivity of at least 0.7 for wavelengths 2-20 m.

(8) Unless otherwise stated herein, compositional percentages are provided in atomic percentages of a total composition for separate chemical elements and in molecular percent (mole %) for chemical compounds.

(9) Embodiments of the present invention relate to a new class of nanocomposite materials suitable as thermoresistive material in Miro-Electro-Mechanical-Systems (MEMS) devices for infrared light emission and heating. The infrared emitting light sources can emit light in the range of 1-20 micrometers and can pulse up to 100 Hz and are suitable as light sources in IR gas sensors and infrared signalling devices. A suitable thickness of the nanocomposite thermoresistive film as an infrared light source is in the range 0.05-1.0 microns. The nanocomposite materials include a mixture of finely dispersed phases of materials formed of the chemical elements Mo, Si, C, O and Me, where Me is one of the metallic elements Ti, Zr, Al, B, or their combinations. Films of the nanocomposite material are produced using vacuum magnetron sputtering in an atmosphere of pure Ar from a target including MoSi.sub.2+(10-85) mole % SiC+(0-30)% Y, where Y is selected from the metal-oxides SiO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3. Films can also be made by magnetron sputtering from a target including MoSi.sub.2+(10-85 mole %) SiC in an atmosphere of a gas mixture consisting of Ar+O.sub.2. The second variant of inventive films is produced by magnetron sputtering of target MoSi.sub.2+(1-100) mole % SiC+(0-60) mole % Z where Z gets out of a number borides: TiB.sub.2, ZrB.sub.2, HfB.sub.2, CrB.sub.2, SiB.sub.4, SiB.sub.6, CB.sub.2, or their combinations. The third variant of inventive films is produced by magnetron sputtering of target MoSi.sub.2+(1-100) mole % SiC+(0-60) mole % W. Films have a total emissivity equal to 0.8 in the infrared spectrum at temperatures of 700 C. 800 C. The range of compositions as atomic percentage of the elements which include the nanocomposite films, as well as the range of weight percentage ratios of the components constituting the sputtering target are defined. Also defined is the range of pressure of oxygen at which the films have optimum value of sheet resistance in a range 10150 Ohm/.

(10) The novel thin-film infrared emitting materials form the active, emitting element in infrared light sources. The inventive infrared light sources have a multitude of applications, including, but not limited to nondispersive infrared sensor (NDIR) gas sensors.

(11) Embodiments of the invention leverage known properties including:

(12) The atomic percentage ratio of the elements which are a part thin nanocomposite films produced by sputtering in a vacuum of a bulk material in the form of a mixture of MoSi.sub.2 and SiC have integral emissivity of approximately 0.8 in the infrared spectrum at a temperature of 700-800 C., and a sheet resistance R.sub.s=10150 Ohm/ (Ohm/square) with an accuracy of not less than 5%.

(13) The introduction in a structure of an initial mixture of metal oxides from the following: SiO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3 to the films improves the film's ability to stably operate as an IR emitter with a frequency range of up to 100 Hz.

(14) The introduction in a structure of an initial mix of borides, refractory and other metalsTiB.sub.2, ZrB.sub.2, HfB.sub.2, CrB.sub.2, SiB.sub.4, SiB.sub.6, CB.sub.2, where the introduction of these additives increases the lifetime performance of radiating and heating devices.

(15) The introduction in a structure of an initial mix of carbides and refractory metals TiC, ZrC, HfC, where these additives increases lifetime performance of radiating and heating devices. These properties are achieved with vacuum magnetron sputtering of source materials in inert gas of Ar, or a gas mixture of Ar and O.sub.2.

(16) The inventive thin-films have a high total emissivity of typically 0.8 in the infra-red region of the spectrum, as measured with the thin-films deposited on a ceramic substrate. The same ceramic substrate covered with soot was used as a comparison, where the emissivity was considered equal to 1. The long term high thermal stability of the thin-films has been experimentally confirmed up to at least 900 C.

(17) The inventive thin-films possess such high values of emissivity only for certain compositions of the initial sputtering target. If the composition of the initial sputtering target materials is represented by the formula (MoSi.sub.2+X.SiC+Y), then X=mole percentage SiC, Y=mole percentage of metal-oxides from the group Si, Ti, Zr, Al, B or their mixture. The composition will be the following: 85>X (mole %)>10, 30>Y (mole %)>0. If X>85 mole %, the films have sheet resistance R.sub.s>150 Ohm/. The R.sub.s of the films increases with increased quantity of SiC. Application of such high-resistivity films in thin-film IRE is undesireable since the films will require a high voltage to operate. A similar effect of an undesirable increase in R.sub.s is observed with an increase in the components Y to more than 30 mole %. For X<10 mole %, a decrease in the hardness of the films at high-temperature is observed. This effect is connected with oxidation of the MoSi.sub.2 phase with a low content of the phase SiC, whose presence slows down this negative process.

(18) The present invention has utility as a thin-film resistor in thin membrane infra-red emitters (IRE). Such devices are made in the form of thin-film structures. The typical thin-film structure is shown in FIGS. 1A and 1B as used from example as an infrared emitter (IRE).

(19) The basic element of the IRE structure 10 is a thin thermoresisitve film 14 of typical thickness of 0.05-0.4 m (micro-meters (microns)) that is deposited on top of a thin low stress (typically 50-150 MPa) insulating (dielectric) membrane 24 with a typical thickness of 0.5-1.5 microns. The thin film structure of the infrared emitter 10 is characterized by the following combination of properties: a thin (typically 1.5-2.5 m) free-hanging multilayer membrane supported by a silicon frame 18 (typically 0.4-0.5 mm thickness); and a thin low-stress (typically 50-150 MPa) dielectric membrane that is used as a substrate 24 for the thermoresistive film 14. The low thermal mass of the membrane coupled with high emissivity, allows the membrane to cycle in temperature with a frequency of 10-50 Hz, and up to 100 Hz.

(20) The membrane or substrate 24 is formed with low-stress, non-stoichiometric silicon nitride (SiN), and includes in some inventive embodiments multilayers or alternating films comprising SiO2, SiN and nanoamorphous carbon that have a typical thickness of individual layers of 0.05-1.0 microns, for a typical combined thickness of 0.5-1.5 microns, if such multilayers are present. This multilayer membrane typically includes a protective topcoat 12 of a dielectric material, typically low-stress SiN, and includes in some inventive embodiments multilayers or alternating films comprising SiN, SiO2 and nanoamorphous carbon, with a typical total topcoat thickness of 0.1-1.0 microns as protection of the thermoresistive film 14 against oxidation. The multilayer membrane is suspended by a silicon frame 18 and produced with microsystems or MEMS (Micro-Electro-Mechanical-Systems) technology by etching the Si from the back of a Si wafer. The silicon wafer is coated on the backside by a film of SiN 22. Reactive Ion Etching (RIE) process was used to open windows in the backside SiN for etching the Si wafer from the backside in order to produce the freehanging multilayer membrane. The thin film thermoresistor 14 may be patterned in such a manner as to have a gap 15 between the thermoresistor and the silicon frame 18 along the edges in order to minimize thermal loss by conduction to the silicon frame 18 as shown in FIG. 1b. The gap 15, which consists of the dielectric support membrane 24 and the topcoat 12 may be 0-400 microns, and is typically 50-300 microns. The thin film thermoresistor 14 in some embodiments has a sheet resistance in the range of R.sub.s=10-150 Ohm/. The thin film thermoresistor 14 is positioned between metal contact pads 16. The method of fabrication of such thin membrane infra-red emitters (IRE) structures is compatible with standard microelectronic methods of fabrication of thin film devices on a substrate with PVD and CVD methods of film deposition. Magnetron sputtering provides for the formation of thin-film structures as shown in FIGS. 1A and 1B without membrane destruction.

(21) In specific embodiments, the sputtering of the films is carried out in an argon atmosphere. Thus, the vacuum is reduced to a pressure 10.sup.3 Pa. The sputtering process is conducted with Ar gas introduced into the vacuum chamber to a pressure P=8.Math.10.sup.22.Math.10.sup.1 Pa. The sheet resistance of films R.sub.s does not depend strongly on the pressure. For a film thickness of 0.5 microns, the films have values in the range of R.sub.s20-40 Ohm/ all values P in this range.

(22) It has surprisingly been discovered that if the sputtering is done in a mixture of argon and oxygen, the sheet resistance of the applied thin-film increases, and the thin-film resistance value depends on the partial pressure of oxygen. In FIG. 2, the dependence of sheet resistance of films produced from a target of MoSi.sub.2+35 mole % SiC in a partial pressure of oxygen (P.sub.oxygen) is shown. These tests are carried out for films of an identical thickness of 0.3 microns. For all experiments, the pressure of the gas mixture (Ar+O.sub.2), is equal to 1.Math.10.sup.1 Pa. As shown in FIG. 2, with a change in the partial pressure of oxygen in the range P.sub.oxygen=5.Math.10.sup.3-1.Math.10.sup.2 Pa, the sheet resistance varies in the range R.sub.s=30-100 Ohm/. Furthermore, for the films deposited in a partial oxygen atmosphere, the emissivity and thermal stability are measured, and it is shown that these parameters do not deteriorate.

(23) In inventive embodiments, the structure and phase composition of films are defined by the method of film deposition. At a thickness of 0.1-1 micron, the thin-films can be considered as quasi-two-dimensional objects and their properties are in many respects defined by properties of a surface of a substrate and the conditions of film growth. With ion-plasma methods of films deposition, and in particular magnetron sputtering, the target material is sputtered in a vacuum under the influence of high-energy ions of the inert gas (Ar) and deposited on a substrate in the form of a stream of neutral molecules, atoms, and ions. Condensation of this stream on a substrate at room temperature leads to formation of highly dispersed films with a complex multiphase composition and a quasi-amorphous structure. Furthermore, with the size of crystal grains in the nanometer range, the films are effectively a nanocomposite.

(24) An analysis of the atomic-molecular structure of the thin-films described in the present invention was carried out. Electron transmission diffraction patterns of the films made immediately after deposition on a substrate exhibited the presence of two wide rings, characteristic of quasi-amorphous highly dispersed structures with crystal size of 2-5 nanometers. The type of rings and their characteristics demonstrated the presence of a considerable fraction of amorphous phases. Exact electron-diffraction analysis of phases with a size of crystal areas in the range 2-5 nanometers against an amorphous phase is difficult. The stream of a sputtered material contains ions and atoms and neutral molecules. Their condensation on a substrate can lead to formation of new phases.

(25) Consider an example of a target having the following components: MoSi.sub.2, SiC and SiO.sub.2. If in a sputtered material the only phases present are -MoSi.sub.2 (tetragonal), -SiC (cubic) and -SiO.sub.2(trigonal), the formation of other double and threefold phases, in particular, -MoSi.sub.2 (hexagonal), -SiC (hexagonal), and Mo.sub.5Si.sub.3 (hexagonal), Mo.sub.2C (hexagonal), Mo.sub.24Sil.sub.5C.sub.3, and also complex phases containing oxygen, for example, MoOC, CSiO.sub.2, etc., is possible. Some of these phases can form solid solutions of a nonstoichiometric composition.

(26) By means of X-ray element analysis EDAX (Genesis 2000 system), the elemental composition and the atomic percentage ratios of elements of the films have been measured. FIG. 3 shows a typical X-ray spectrum measured for a film produced by sputtering of an initial material with composition (MoSi.sub.2+35 mole % SiC) that is carried out in a mix of gases, such as Ar and O.sub.2.

(27) A determination of the dependence of emissivity and thermal stability of films on elemental composition is shown in Table 1 for the ranges of atomic percentages of the elements in the films studied, produced by target sputtering in pure Ar and in a mix of Ar and O.sub.2. The films with such a ratio of elements in their composition have emissivity in the IR-region of the spectrum equal to approximately 0.8 and have a high thermal stability.

(28) TABLE-US-00001 TABLE 1 Gas filled into the At. % ratio of elements in film chamber Mo Si C O Ar 5-25 10-35 30-85 0.5-1* Ar + O.sub.2 10-20 15-30 15-60 10-20 *attached foreign material

(29) Embodiments of the inventive thin film in the form of a nanocomposite radiative film may be fabricated on a substrate while the substrate has an electrical bias applied, where the bias may be in the range of 0-400V. In addition the bias may be applied at a radio frequency (RF), where the RF ranges between 100 KHz-13.6 MHz.

(30) Embodiments of the inventive nanocomposite radiative film may be thermal annealed at a temperature of between 800-950 C. in an atmosphere of air, argon, or a vacuum for a period of time of between 1-10 hours.

EXAMPLES

(31) The following examples are intended to further illustrate the aspects of the invention.

Example 1

(32) The target (a disk of diameter of 50 mm and thickness of 6 mm) was prepared by hot pressing and subsequent high-temperature annealing in a vacuum. The composition of the target (composition 1) is in mole %: MoSi.sub.270; SiC30. The substrate, a polished ceramic plate was placed in the vacuum chamber at a distance of 50 mm from the magnetron target. The vacuum chamber was pumped down to a pressure of 1.Math.10.sup.3 Pa. Ar gas was filled in the chamber to a pressure of 1.Math.10.sup.1 Pa. Sputtering was carried out with the following magnetron parameters: current0.3 A; voltage480V. The thickness of the film was 0.5 micron and the sheet resistance was 30 Ohm/. The emissivity in the range 2-20 microns was 0.8.

Example 2

(33) A target with the following composition in mole %: MoSi.sub.225; SiC75. The chamber is filled with Ar gas to a pressure of 1.Math.10.sup.1 Pa. Sputtering is carried out with the following magnetron parameters: current0.3 A; voltage500V. The thickness of the film is 0.5 micron and the sheet resistance is 80 Ohm/. The emissivity in the wavelength of 2-20 microns is 0.8.

Example 3

(34) A target with the following composition in mole %: MoSi.sub.234; SiC46; SiO.sub.220. The chamber is filled with Ar gas to a pressure of 1.Math.10.sup.1 Pa. Sputtering is carried out with the following magnetron parameters: current0.3 A; voltage490V. The thickness of the film is 0.5 micron and sheet resistance 50 Ohm/. The emissivity in the wavelength range of 2-20 micron is 0.79.

Example 4

(35) A target with the following composition in mole %: MoSi.sub.246; SiC54. Oxygen is filled in the chamber to a pressure of 7.Math.10.sup.3 Pa, and then Ar to a pressure of 1.Math.10.sup.1 Pa. Sputtering is carried out with the following magnetron parameters: current0.3 A; voltage500V. The thickness of the film is 0.5 microns and sheet resistance 100 Ohm/. The emissivity in the wavelength range 2-20 microns is 0.8.

Example 5

(36) A target with the following composition in mole %: MoSi.sub.233, SiC53, HfB.sub.214. The chamber is filled with Ar gas to a pressure of 1.Math.10.sup.1 Pa. Sputtering is carried out with the following magnetron parameters: current0.3 A, pressure440. The thickness of the film is 0.5 micron and the sheet resistance is 60 Ohm/. The emissivity in the wavelength of 2-20 microns is 0.8.

Example 6

(37) A target with the following composition in mole %: MoSi.sub.229, SiC60, TiC11. The chamber is filled with Ar gas to a pressure of 1.10.sup.1 Pa. Sputtering is carried out with the following magnetron parameters: current0.3 A, pressure440. The thickness of the film is 0.4 micron and the sheet resistance is 70 Ohm/. The emissivity in the wavelength of 2-20 microns is 0.8.

(38) Any patents or publications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof.