U.S. patent application number 10/092887 was filed with the patent office on 2002-07-25 for amorphous silicon carbide thin film articles.
Invention is credited to Brandes, George R., Christos, Chris S., Xu, Xueping.
Application Number | 20020096684 10/092887 |
Document ID | / |
Family ID | 30003375 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020096684 |
Kind Code |
A1 |
Brandes, George R. ; et
al. |
July 25, 2002 |
Amorphous silicon carbide thin film articles
Abstract
Amorphous silicon carbide thin film structures, including:
protective coatings for windows in infrared process stream
monitoring systems and sensor domes, heated windows,
electromagnetic interference shielding members and integrated
micromachined sensors; high-temperature sensors and circuits; and
diffusion barrier layers in VLSI circuits. The amorphous silicon
carbide thin film structures are readily formed, e.g., by
sputtering at low temperatures.
Inventors: |
Brandes, George R.;
(Southbury, CT) ; Christos, Chris S.; (Brookfield,
CT) ; Xu, Xueping; (Stamford, CT) |
Correspondence
Address: |
ATMI, INC.
7 COMMERCE DRIVE
DANBURY
CT
06810
US
|
Family ID: |
30003375 |
Appl. No.: |
10/092887 |
Filed: |
March 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10092887 |
Mar 7, 2002 |
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09557165 |
Apr 25, 2000 |
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09557165 |
Apr 25, 2000 |
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09461693 |
Dec 14, 1999 |
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6268229 |
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09461693 |
Dec 14, 1999 |
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08575484 |
Dec 20, 1995 |
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6031250 |
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Current U.S.
Class: |
257/77 ;
257/E21.004; 257/E31.049 |
Current CPC
Class: |
H01J 1/3042 20130101;
H01J 2201/319 20130101; H01J 2201/30403 20130101; Y02E 10/548
20130101; H01L 28/20 20130101; H01L 31/03765 20130101; H01J 2329/00
20130101 |
Class at
Publication: |
257/77 |
International
Class: |
H01L 031/0312 |
Claims
What is claimed is:
1. An article comprising an amorphous silicon carbide thin film on
a structure selected from the group consisting of: substrates that
are transmissive to at least one of light and infrared radiation;
structures adversely affected by exposure to radio frequency and/or
microwave radiation thereon; shielding members for protection of
structures adversely affected by exposure to radio frequency and/or
microwave radiation thereon; structures susceptible to chemical
attack and/or thermal degradation in their end use environments;
and electronic circuitry structures susceptible to diffusional
release and/or diffusional receipt of atomic species in use
thereof.
17. An electromagnetic interference shielded assembly, comprising:
a structure disposed in protective relationship to a region to be
shielded; and an electrically conductive thin film of amorphous
silicon carbide on at least a portion of said structure.
18. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film exhibits sufficient conductivity to provide a
ground path for electromagnetic interference induced currents and
exhibits sufficient optical transparency to pass optical signals
through the window without substantial attenuation.
19. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film exhibits an electrical resistivity in the
range from about 10 m.OMEGA. cm to about 25 m.OMEGA. cm.
20. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film has been deposited on the structure by a
process selected from the group consisting of chemical vapor
deposition, plasma enhanced chemical vapor deposition, RF glow
discharge, RF sputtering, ion cluster beam deposition, ion beam
sputtering, sol gel coating, reactive sputtering, plasma spray,
reactant spraying, microwave discharge, and photo CVD.
21. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film comprises a sputtered thin film.
22. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film has a thickness in the range from about 0.025
micron to about 10 microns.
23. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film has a thickness in the range from about 0.05
micron to about 1.0 micron.
24. The electromagnetic interference shielded assembly of claim 17,
wherein the thin film is formed with a thickness in the range from
about 0.1 micron to about 0.5 micron.
25. The electromagnetic interference shielded assembly of claim 17,
further comprising a glue layer between the structure and the thin
film.
26. The electromagnetic interference shielded assembly of claim 25,
wherein the glue layer comprises a material selected from the group
consisting of Ti, Si, Cr, and Zr.
27. A sensor assembly, comprising: a sensor; and an amorphous
silicon carbide thin film on at least part of the sensor.
28. The sensor assembly of claim 27, wherein the thin film has a
thickness in the range from about 0.025 micron to about 10
microns.
29. The sensor assembly of claim 27, wherein the thin film has a
thickness in the range from about 0.05 micron to about 1.0
micron.
30. The sensor assembly of claim 42, wherein the thin film has a
thickness in the range from about 0.1 micron to about 0.5
micron.
31. A sensor assembly, comprising: a sensor including sensing
element(s) formed of amorphous silicon carbide, whereby the sensor
assembly is operable at temperatures up to 1000.degree. C.
32. A high-temperature sensor assembly, comprising: a sensing
element formed of amorphous silicon carbide; and electrical
circuitry operatively coupled with the sensing element, said
electrical circuitry comprising amorphous silicon carbide doped
with at least one dopant selected from the group consisting of
n-type and p-type dopants, whereby the sensor assembly is operable
at temperatures up to 1000.degree. C.
33. A high-temperature pressure sensor, comprising: a substrate
including a reference cavity region; a first highly resistive
amorphous silicon carbide thin film deposited on the substrate, but
not the reference cavity region; a second highly resistive
amorphous silicon carbide thin film deposited over the first highly
resistive thin film, and additionally over the reference cavity
region, to form a sealed reference cavity; a low resistivity
amorphous silicon carbide thin film deposited over the second
highly resistive thin film, over the region of the sealed reference
cavity; and electrodes contacting the low resistivity amorphous
silicon carbide thin film, and operatively coupled to a
resistance-sensing electrical circuit, whereby changes in
resistivity of the low resistivity amorphous silicon carbide thin
film incident to changes in strain in the low resistivity amorphous
silicon carbide thin film are sensed by the resistance-sensing
circuit.
34. The high-temperature pressure sensor of claim 33, comprising
amorphous silicon carbide doped with a dopant species comprising a
material selected from the group consisting of hydrogen, halogen,
nitrogen, oxygen, sulfur, selenium, transition metals, boron,
aluminum, phosphorus, gallium, arsenic, lithium, beryllium, sodium
and magnesium.
35. A VLSI circuit assembly, comprising: a VLSI electronic circuit
including an active circuit structure and a metalization
interconnect layer; and a thin film of amorphous silicon carbide
between the active circuit structure and the metalization layer, as
a diffusion barrier against diffusion of atoms from the
metalization layer into the active circuit structure.
36. A method of forming an article as in claim 1, comprising
deposition of said amorphous silicon carbide thin film by a process
selected from the group consisting of chemical vapor deposition,
plasma enhanced chemical vapor deposition, RF glow discharge, RF
sputtering, ion cluster beam deposition, ion beam sputtering, sol
gel coating, reactive sputtering, plasma spray, reactant spraying,
microwave discharge, and photo CVD.
37. The method of claim 36, wherein said process comprises
sputtering.
38. The method of claim 36, wherein the amorphous silicon carbide
thin film forms a protective coating for the structure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of U.S. patent application Ser. No.
09/557,165 filed on Apr. 25, 2000, filed in the names George R.
Brandes, et al. for Amorphous silicon carbide thin film articles,
which is a continuation-in-part of U.S. patent application Ser. No.
09/461,693 filed Dec. 14, 1999 in the names of George R. Brandes,
et al. for "Integrated Circuit Devices and Methods Employing
Amorphous Silicon Carbide Resistor Materials," which is a
divisional application of U.S. patent application Ser. No.
08/575,484 filed Dec, 20, 1995 in the names of George R. Brandes,
et al. for "Integrated Circuit Devices and Methods Employing
Amorphous Silicon Carbide Resistor Materials," and issued on Feb.
29, 2000 as U.S. Pat. No. 6,031,250.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to amorphous silicon
carbide thin films and articles comprising amorphous silicon
carbide thin films, as well as to methods of making and using the
same.
[0004] 2. Description of the Art
[0005] Silicon carbide (SiC) is an extremely hard, mechanically
strong, and chemically inert ceramic material. Silicon carbide
exhibits good oxidation resistance and corrosion resistance, high
heat transfer coefficient compared to metals, low expansion
coefficient compared to metals, high resistance to thermal shock
and high strength at elevated temperatures.
[0006] Although silicon carbide has come into use as a wafer
material for semiconductor device manufacturing, it has not been
extensively utilized in a manner that reflects its commercial
potential and variant properties.
[0007] The present invention is directed to novel applications of
silicon carbide in the form of amorphous silicon carbide thin
films.
SUMMARY OF THE INVENTION
[0008] The present invention relates to amorphous silicon carbide
thin films in a variety of products and end use applications, in
which the physical, chemical, electrical and/or optical properties
of amorphous silicon carbide are utilized to advantage.
[0009] In one aspect, the present invention relates to an article
comprising an amorphous silicon carbide thin film on a structure
selected from the group consisting of:
[0010] substrates that are transmissive to at least one of light
and infrared radiation;
[0011] structures adversely affected by exposure to radio frequency
and/or microwave radiation thereon;
[0012] shielding members for protection of structures adversely
affected by exposure to radio frequency and/or microwave radiation
thereon;
[0013] structures susceptible to chemical attack and/or thermal
degradation in their end use environments; and
[0014] electronic circuitry structures susceptible to diffusional
release and/or diffusional receipt of atomic species in use
thereof.
[0015] Another aspect of the invention relates to a window
assembly, comprising:
[0016] a window; and
[0017] an optically transparent thin film of amorphous silicon
carbide on a surface of said window.
[0018] In a further aspect, the invention relates to a window
assembly, comprising:
[0019] a window;
[0020] an optically transparent and electrically conductive thin
film of amorphous silicon carbide deposited on a surface of said
window; and
[0021] a power supply operatively coupled to the thin film, and
selectively actuatable so that the power supply when actuated
causes an electrical current to flow through the thin film to
generate heat, whereby the window may be selectively defogged or
de-iced.
[0022] Another aspect of the invention relates to an
electromagnetic interference shielded assembly, comprising:
[0023] a structure disposed in protective relationship to a region
to be shielded; and
[0024] an electrically conductive thin film of amorphous silicon
carbide on at least a portion of said structure.
[0025] A further aspect of the invention relates to a sensor
assembly, comprising:
[0026] a sensor; and
[0027] an amorphous silicon carbide thin film on at least part of
the sensor.
[0028] Yet another aspect of the invention relates to a sensor
assembly, comprising:
[0029] a sensor including sensing element(s) formed of amorphous
silicon carbide, whereby the sensor assembly is operable at
temperatures up to 1000.degree. C.
[0030] In another aspect, the invention relates to a
high-temperature sensor assembly, comprising:
[0031] a sensing element formed of amorphous silicon carbide;
and
[0032] electrical circuitry operatively coupled with the sensing
element,
[0033] said electrical circuitry comprising amorphous silicon
carbide doped with at least one dopant selected from the group
consisting of n-type and p-type dopants, whereby the sensor
assembly is operable at temperatures up to 1000.degree. C.
[0034] A further aspect of the invention relates to a
high-temperature pressure sensor, comprising:
[0035] a substrate including a reference cavity region;
[0036] a first highly resistive amorphous silicon carbide thin film
deposited on the substrate, but not the reference cavity
region;
[0037] a second highly resistive amorphous silicon carbide thin
film deposited over the first highly resistive thin film, and
additionally over the reference cavity region, to form a sealed
reference cavity;
[0038] a low resistivity amorphous silicon carbide thin film
deposited over the second highly resistive thin film, over the
region of the sealed reference cavity; and
[0039] electrodes contacting the low resistivity amorphous silicon
carbide thin film, and operatively coupled to a resistance-sensing
electrical circuit,
[0040] whereby changes in resistivity of the low resistivity
amorphous silicon carbide thin film incident to changes in strain
in the low resistivity amorphous silicon carbide thin film are
sensed by the resistance-sensing circuit.
[0041] Another aspect of the invention relates to a VLSI circuit
assembly, comprising:
[0042] a VLSI electronic circuit including an active circuit
structure and a metalization interconnect layer; and
[0043] a thin film of amorphous silicon carbide between the active
circuit structure and the metalization layer, as a diffusion
barrier against diffusion of atoms from the metalization layer into
the active circuit structure.
[0044] In another aspect, the invention relates to a method of
forming an article comprising an amorphous silicon carbide thin
film, including deposition of the amorphous silicon carbide thin
film by a process selected from the group consisting of chemical
vapor deposition, plasma enhanced chemical vapor deposition, RF
glow discharge, RF sputtering, ion cluster beam deposition, ion
beam sputtering, sol gel coating, reactive sputtering, plasma
spray, reactant spraying, microwave discharge, and photo CVD.
[0045] Other aspects, features and embodiments of the invention
will be more fully apparent from the ensuing disclosure and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a schematic representation of a process of coating
a window with a thin film of amorphous silicon carbide, according
to one embodiment of the present invention.
[0047] FIG. 2 is a schematic representation of a process of coating
a window with a thin film of amorphous silicon carbide,
additionally employing a glue layer, according to another
embodiment of the present invention.
[0048] FIG. 3 is a schematic diagram depicting a smoothing-out
effect of amorphous silicon carbide deposition, according to
another embodiment of the present invention.
[0049] FIG. 4 is a schematic representation of various coated
window shapes, according to additional embodiments of the present
invention.
[0050] FIG. 5 is a schematic representation of two types of
infrared process stream monitoring systems utilizing windows coated
with thin films of amorphous silicon carbide.
[0051] FIG. 6 is a schematic representation of a pressure sensor
assembly, according to one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS
THEREOF
[0052] As used herein, "thin film" refers to material layers having
a thickness less than about 1,000 microns. Amorphous silicon
carbide in the practice of the present invention refers to silicon
carbide whose x-ray diffraction scans do not show the discrete
sharp peaks of scattered radiation that are observed from
crystalline or semicrystalline solids of silicon carbide. Amorphous
silicon carbide is sometimes hereinafter identified as "a-SiC" for
ease of reference.
[0053] The disclosure of U.S. Pat. No. 6,031,250 issued Feb. 29,
2000 in the names of George R. Brandes, et al. for "Integrated
Circuit Devices and Methods Employing Amorphous Silicon Carbide
Resistor," is hereby incorporated herein by reference in its
entirety.
[0054] The features and advantages of the present invention are
illustrated with reference to various specific embodiments
hereinafter described. It will be recognized that the invention is
not thus limited, but extends in utility to other embodiments,
devices, assemblies and corresponding structures and methods,
wherein amorphous silicon carbide thin film materials and
structures are used to advantage.
[0055] The amorphous silicon carbide films of the present invention
may be formed in any suitable manner, e.g., by sputtering in a
sputter tool using either a beta silicon carbide target with argon
sputter gas or a silicon target with a mixture of methane and argon
sputter gas. Dopants, such as nitrogen, can be introduced in the
films by adding a controlled amount (partial pressure) of the
dopant gas into the sputter gas. Amorphous silicon carbide films
produced in the practice of the present invention may be annealed
up to a temperature of about 1100.degree. C. to produce films that
are optically transparent and chemically inert. The optically
transparent amorphous silicon carbide films can attenuate optical
or infrared radiation compared to the uncoated window, but the
transmitted signal will be detectable.
[0056] In one embodiment, the present invention contemplates the
deposition of amorphous silicon carbide coatings on conventional
infrared windows in infrared spectroscopy process stream monitoring
and analysis systems, to reduce chemical reaction at the window
surface and to fill window surface disparities. Amorphous silicon
carbide is chemically inert and transparent over wide bands in the
infrared region. The properties of a-SiC do not change over
temperatures typically encountered in industrial chemical process
streams or in subsequent cleaning stages of such processes.
[0057] Amorphous silicon carbide coatings for infrared windows in
accordance with the present invention may be formed by any suitable
deposition or film forming technique. Such techniques
illustratively include chemical vapor deposition (CVD), plasma
enhanced chemical vapor deposition (PECVD), RF glow discharge, RF
sputtering, ion cluster beam deposition, ion beam sputtering, sol
gel coating, reactive sputtering, plasma spray, reactant spraying,
microwave discharge, and photo CVD. Preferably, low temperature
deposition is employed to enable the selection of the window from a
wide variety of window types.
[0058] Sputtering is a preferred coating process, which enables low
substrate temperatures to be utilized and which produces a film
that is smooth and fills window surface disparities. The sputtered
coating layer of a-SiC may be formed with a thickness in the range
from about 0.025 microns to about 10 microns, preferably in the
range from about 0.05 microns to about 1.0 micron, and most
preferably in the range from about 0.1 microns to about 0.5
microns. Substrate temperatures of less than 500.degree. C. during
deposition of silicon carbide produce amorphous films. Deposition
at a substrate temperature of about 200.degree. C. yields an
advantageously large optical gap. Low temperature deposition
processes are preferred, with the most preferable substrate
temperature being in the vicinity of room temperature (30.degree.
C.) or below. At low substrate temperature, smoother films are
obtained at lower pressures (15 milliTorr). A silicon carbide
target is advantageously used for the sputtering formation of the
amorphous silicon carbide film.
[0059] Referring now to the drawings, FIG. 1 schematically
illustrates the steps used in fabricating a window coating,
according to one embodiment of the present invention. FIG. 1a shows
the IR window 11, or substrate, prior to deposition of the a-SiC
film, and FIG. 1b shows the IR window 11 following deposition of
the a-SiC film 12 thereon.
[0060] FIG. 2 schematically illustrates steps used in fabricating a
coating of amorphous silicon carbide, according to another
embodiment of the present invention. In FIG. 2a, the IR window 21
is shown following a cleaning step to prepare its surface, e.g.,
argon bombardment to remove oxides. FIG. 2b shows the substrate 21
following deposition of an intermnediate "glue" or adhesion layer
22. The glue layer 22 may illustratively comprise Ti, Si, Cr, or Zr
and serves to provide a surface for deposition of the amorphous
silicon carbide to which the amorphous silicon carbide is more
strongly binding. FIG. 2c shows the substrate following deposition
of the amorphous silicon carbide layer 23 onto the glue layer 22.
The glue layer 22 may be of any suitable thickness, and may be
formed by sputtering, CVD, or any other useful technique. In window
and other optical applications of the present invention, the
thickness of the glue layer must allow light passage through the
film, and thicknesses on the order of approximately 250 Angstroms
are usefully employed.
[0061] FIG. 3 schematically illustrates the smoothing-out of
microscopic disparities on the surface of an IR window 31 with a
sputtered deposited coating 32, whereby the root mean square
roughness characteristic of the IR window surface may be
substantially improved.
[0062] FIG. 4 illustrates various window shapes with an amorphous
silicon carbide coating layer applied. FIG. 4a shows a planar
window 41 with a protective amorphous silicon carbide coating 42
deposited on a single side thereof. FIG. 4b shows a cylindrically
shaped window 43 with an amorphous silicon carbide coating 45
applied. FIG. 4c shows a curved IR window 46 with the amorphous
silicon carbide coating 47 applied to both main surfaces of the
window.
[0063] FIG. 5 schematically illustrates representative systems
employing the coated window. The system of FIG. 5a shows infrared
windows 51 with amorphous silicon carbide coating 52 applied
thereon. The windows are mounted on and sealed to a tubular flow
passage housing containing process stream 55. Infrared photons 56
generated by light source 53 pass through the first IR window and
into the process stream 55. Light that passes through the process
stream is detected by detector 54, and analysis of the absorption
spectra of the detected light is used to monitor and/or control the
process stream 55, such as by means of a computer or
microprocessor-based control system that responsively modulates an
upstream process producing the process stream, to achieve a desired
character of the process stream, or that controls the downstream
use or disposition of the process stream.
[0064] FIG. 5b depicts a system in which an infrared transparent
tube 57 with amorphous silicon carbide coating 52 thereon extends
transversely through an elongate tubular flow passage housing
containing the process stream 55. Infrared photons 56, generated by
light source 53, enter the tube 57. Infrared photons 56 can
interact with the process stream 55. If the photon interactions
with the process stream 55 result in light absorption, the fraction
of light reaching infrared detector 54 is reduced. This
absorption-induced change in intensity is used to monitor and/or
control the process stream 55.
[0065] In another specific embodiment of the present invention,
amorphous silicon carbide thin film coatings are applied to the
protective domes covering infrared imaging and communications
systems on airborne platforms. Deposition techniques and
methodology are directly analogous to those described above with
respect to infrared windows for process stream monitoring systems.
Sputtering with a silicon carbide target is a preferred method of
a-SiC film formation. Deposition at or below room temperature is
preferred, to allow use of a wide variety of dome substrate
materials of construction, such as soda lime glass.
[0066] In another embodiment of the present invention, amorphous
silicon carbide thin films are applied to windows and powered to
provide window heaters for defogging, deicing, etc. Amorphous SiC
thin films are transparent to visible light; robust to changing
environmental conditions; can be applied with a low-cost,
repeatable deposition technique; and are tunable for resistivity
within an appropriate range to take advantage of existing power
sources.
[0067] Due to a wide band gap, SiC is optically transparent when
deposited in amorphous form, as band tailing or midgap defect
states are spread far enough apart that sufficient light passes
through the film.
[0068] The resistivity of the a-SiC thin film can be tuned by
introducing dopant (n- or p-type) electrical impurities, impurities
that reduce (or increase) the number of defect states, or
impurities that permit/promote the formation of a slightly
different compound. Such impurities may, for example, comprise one
or more selected from the group consisting of hydrogen, halogen,
nitrogen, oxygen, sulfur, selenium, transition metals, boron,
aluminum, phosphorus, gallium, arsenic, lithium, beryllium, sodium
and magnesium. The resistivity of the heater coating is preferably
in the range from about 10 m.OMEGA. cm to about 100 m.OMEGA. cm,
and most preferably in the range from about 25 m.OMEGA. cm to about
50 m.OMEGA. cm.
[0069] Amorphous silicon carbide films are not highly reactive and
are typically extremely hard (scratch-resistant). For example,
a-SiC does not etch, or etches very slowly, in acid. The amorphous
silicon carbide film can be deposited uniformly over large areas
and at relatively low cost, particularly if the film is very thin.
Varying the thickness and resistivity permits the use of typical
voltage sources. DC to DC converters may be necessary in some
applications to obtain a sufficiently high voltage for proper
operation.
[0070] When large areas (i.e., the entire window) are coated, the
resulting heater is less susceptible to scratch damage than current
resistive wire window heaters. With large area coatings, the entire
window is heated uniformly. Additionally, the a-SiC heater coating
permits a 2-stage heating process comprising an initial high
current with high generated temperatures to clear or de-ice a
fogged or iced window, followed by a lower "maintenance current" to
maintain the window in an optically clear condition. The power
supply operatively coupled with the a-SiC film may therefore be
constructed for corresponding "two-step" power operation.
[0071] In a further embodiment of the present invention, amorphous
silicon carbide thin film coatings are applied to shield electronic
components from electromagnetic interference, while providing a
transparent path in the wavelengths of interest for optical
signals. To be effective for EMI shielding, the coating must have a
high enough resistivity to push the plasma resonance at which free
carrier absorption becomes significant to wavelengths beyond the
3-5 micron operational window. At the same time, however, the film
must remain conductive enough to enable adequate shielding. The
plasma resonance .lambda.p where free carrier absorption becomes
significant can be calculated using equations based on Drude
theory:
.lambda..sub.P.sup.2=4.pi..sup.2c.sup.2.epsilon..sub.1.epsilon..sub.0m*/Ne-
.sup.2
[0072] where N is the free carrier density, .epsilon..sub.1 is the
high-frequency dielectric constant, and m* is the effective carrier
mass. Lowering the free carrier density and hence the conductivity
of the a-SiC, then, will serve to shift the plasma resonance to
wavelengths longer than IR. This shift in plasma resonance
wavelength, however, has an impact on RF shielding effectiveness.
The equation describing shielding effectiveness of a thin (much
thinner than skin depth) conductive layer against wavelengths
longer than the plasma wavelength is:
SE=20 log[1+188.5.sigma.t]
[0073] where SE is shielding effectiveness in dB, .sigma. is
conductivity and t is layer thickness. For relatively thin films of
.about.5 .mu.m and assuming a mobility of 10 cm.sup.2/Vs, a film
with a resistivity of 10 m.OMEGA. cm has a plasma resonance above 5
.mu.m and a shielding effectiveness greater than 20 dB. If the
mobility of the a-SiC can be further increased, the SE
effectiveness increases substantially while the plasma wavelength
remains unchanged. The window coating requires a material that is
transparent to light in the relative wavelength range, resistant to
scratching, and readily deposited over large areas. A film
resistivity of approximately 10-25 m.OMEGA. cm is required, with
relatively high mobility.
[0074] The amorphous silicon carbide films of the present invention
provide the necessary optical and electrical properties and
otherwise are robust and easy to deposit. Low resistivity of the
amorphous silicon carbide film is readily achieved by nitrogen
doping and annealing in argon.
[0075] In another embodiment of the present invention, amorphous
silicon carbide is used as a coating on micromachined silicon
sensors, protecting the active sensor elements from exposure to
harsh environments. Amorphous SiC in such applications is highly
advantageous, as a consequence of its chemical inertness and
mechanical strength, and the fact that a-SiC can be sputtered
deposited, facilitating large area deposition and low cost. By
adjusting the deposition conditions, low stress films of a-SiC can
be produced on a variety of substrates and surfaces. Amorphous SiC
in accordance with the present invention allows sensors to be used
in harsh environments.
[0076] Amorphous silicon carbide is usefully employed in accordance
with the present invention as a replacement material for silicon in
micromachined sensors. Amorphous SiC has a larger band gap than
does silicon, which reduces the thermally induced carrier
generation rate by orders of magnitude. Hence, a-SiC does not
suffer from the introduction of defects at temperatures up to
1000.degree. C. Additionally, a-SiC has a relatively high Knoop
hardness and a much larger bond strength than silicon, resulting in
fewer thermally induced defects. Amorphous silicon carbide can be
doped with n- and p-type dopants to provide desired electrical
properties, it possesses a native oxide, and devices can be
fabricated using conventional microelectronic techniques. The
present invention enables the use of a-SiC in applications where
general electrical control components and sensors need to function
under widely varying environmental conditions.
[0077] The use of a-SiC permits on-chip integration of sensors with
associated electronics. Temperature compensation, calibration,
signal conditioning and interface functions, for example, can be
co-located at the sensor. Integration of the sensor and electronics
greatly improves sensor accuracy and reliability, and facilitates
sensor incorporation into electronics systems. When the a-SiC
sensor is formed on crystalline SiC, high performance control
electronics can readily be fabricated.
[0078] Amorphous silicon carbide based high-temperature, harsh
environment micromachined sensor devices in accordance with the
present invention find use in the automotive, aircraft, defense,
aerospace, oil and mining, and manufacturing industries. In
aircraft and turbine powered generators, pressure sensors
fabricated of amorphous silicon carbide are capable of withstanding
the harsh environment of a gas turbine engine, to provide valuable
information about the operating conditions of the engine during
use. The amorphous silicon carbide pressure sensor devices of the
invention may also be employed in the automotive industry, to
increase fuel economy and safety and to reduce vehicle exhaust
emissions.
[0079] FIG. 6 is a schematic representation of a silicon carbide
pressure sensor device according to one embodiment of the present
invention. On substrate 110 is deposited a first highly resistive
amorphous silicon carbide thin film 111 over the substrate top
surface, with the exception of a reference cavity region. A second
highly resistive amorphous silicon carbide thin film 113 is
deposited over the first thin film 111, and also over the reference
cavity region, forming a sealed reference cavity 112. A low
resistivity amorphous silicon carbide thin film 114 is deposited
over the second highly resistive amorphous silicon carbide thin
film 113, over the region of sealed reference cavity 112. Nickel
electrodes 115 are formed at the ends of low resistivity amorphous
silicon carbide thin film 114. The device of FIG. 6 operates as a
pressure sensor; the resistivity of a-SiC thin film 114 changes
with the film strain.
[0080] Thus, changes in ambient pressure exerted on the film 114
are manifested as altered film strain characteristics that change
the resistivity of the film and by coupling the FIG. 6 device to
suitable circuitry to permit sensing of the resistivity change, a
corresponding output can be effected for pressure monitoring
purposes, e.g., to alter a process system in response to the
pressure change.
[0081] In another embodiment of the present invention, amorphous
silicon carbide thin films are used as diffusion barrier layers in
VLSI electronic circuits. This aspect of the invention takes
advantage of the stability and chemical inertness of amorphous
silicon carbide and the fact that many elements conventionally used
in integrated circuitry have a low diffusion rate in silicon
carbide. The amorphous nature of the a-SiC barrier will further
reduce the diffusion of metals beyond the inherent low diffusional
rate, since a-SiC films have no grain boundaries, and diffusion
along grain boundaries is generally faster than diffusion in
bulk.
[0082] The amorphous silicon carbide diffusion barrier layer can be
deposited by sputtering a silicon carbide target, or by reactively
sputtering a silicon target in the presence of a hydrocarbon, or it
can be deposited by chemical vapor deposition or in any other
suitable manner.
[0083] The amorphous silicon carbide material used in various
embodiments of the invention may be in the form of amorphous
silicon carbide per se, or various alloy forms of amorphous silicon
carbide may be employed, such as for example amorphous silicon
carbide:nitride and amorphous silicon carbide:hydride alloys. In
window and protective coating applications, corresponding amorphous
carbide and amorphous carbide alloys can be employed, as well as
amorphous gallium nitride and amorphous aluminum nitride.
[0084] The amorphous silicon carbide materials of the invention may
be formed with desired resistivity characteristics by doping with
suitable dopant species, e.g., nitrogen, boron, phosphorus,
aluminum or arsenic, to produce an amorphous silicon carbide
material with suitable electrical properties for a given end use
application.
[0085] The features and advantages of the invention are more fully
shown with respect to the following illustrative examples.
[0086] EXAMPLE 1
[0087] An infrared analyzer of the type depicted in FIG. 5b is
arranged to continuously monitor organic and some inorganic
components in a liquid process stream. Infrared light is generated
(1-20 .mu.m), passed through a cubic zirconium cylinder (internally
reflected along the length), and focused onto detectors. The
process stream solution on the cylinder surface absorbs at specific
wavelengths and a reduction in signal intensity is detected and
compared to the signal intensity for a reference solution or to the
signal intensity at a wavelength where no absorption takes place.
The cubic zirconium rod requires a protective coating that is
robust, does not readily delaminate, and is chemically inert under
the following conditions:
[0088] 1-2% citric or phosphoric at or below room temperature
[0089] 100 ppm sodium hypochlorite at or below room temperature
[0090] 1-2% caustic soda and/or detergent at room temperature to
85.degree. C.
[0091] 1-2% phosphoric acid and/or detergent at room temperature to
85.degree. C.
[0092] The cubic zirconium rod was coated with amorphous silicon
carbide. The surface of the rod was smoothed considerably. SEM
images at 10K magnification prior to the a-SiC deposition showed a
rough surface morphology. After deposition of a-SiC film, a scan at
20K magnification appeared flat.
EXAMPLE 2
[0093] Amorphous silicon carbide was deposited by sputtering a
.beta.-SiC target. The sputtering process is scalable up to large
dimension substrates. Additionally, a low resistivity a-SiC film
was produced by nitrogen doping and activation at higher
temperatures. A low resistivity of 50 m.OMEGA. cm was obtained for
nitrogen-doped amorphous silicon carbide films. The films were
deposited in a Perkin-Elmer 4450 production sputter system using a
.beta.-SiC target under a condition of 1000 W magnetron power, 480
sccm argon flow, 16 mTorr argon pressure, and 6.times.10.sup.6 Torr
nitrogen partial pressure. The low-resistivity films were activated
by annealing in argon at 1100.degree. C. for several minutes.
[0094] The present invention extends to and encompasses other
features, modifications, and alternative embodiments, as will
readily suggest themselves to those of ordinary skill in the art
based on the disclosure and illustrative teachings herein. The
claims that follow are therefore to be construed and interpreted as
including all such features, modifications and alternative
embodiments, within their spirit and scope.
* * * * *