U.S. patent number RE36,615 [Application Number 08/920,367] was granted by the patent office on 2000-03-14 for use of vanadium oxide in microbolometer sensors.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to R. Andrew Wood.
United States Patent |
RE36,615 |
Wood |
March 14, 2000 |
Use of vanadium oxide in microbolometer sensors
Abstract
In a microbolometer infrared radiation sensor, a detector
material (VO.sub.2) having a high thermal coefficient of resistance
to increase the sensitivity of the apparatus.
Inventors: |
Wood; R. Andrew (Bloomington,
MN) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
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Family
ID: |
26711767 |
Appl.
No.: |
08/920,367 |
Filed: |
August 29, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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035118 |
Mar 11, 1987 |
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781557 |
Sep 30, 1985 |
4654622 |
|
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Reissue of: |
085243 |
Jun 29, 1993 |
05450053 |
Sep 12, 1995 |
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Current U.S.
Class: |
338/18; 250/250;
250/334; 250/370.14; 338/14; 338/22R; 343/700MS |
Current CPC
Class: |
G01J
5/20 (20130101); H01L 27/14603 (20130101); H01Q
1/38 (20130101); H01Q 9/065 (20130101); H01Q
9/16 (20130101); H01Q 15/02 (20130101); H01Q
19/062 (20130101); H01Q 21/062 (20130101); H01Q
5/42 (20150115) |
Current International
Class: |
G01J
5/20 (20060101); H01L 27/146 (20060101); H01Q
5/00 (20060101); H01Q 19/06 (20060101); H01Q
1/38 (20060101); H01Q 9/06 (20060101); H01Q
9/04 (20060101); H01Q 19/00 (20060101); H01Q
9/16 (20060101); H01Q 21/06 (20060101); H01Q
15/00 (20060101); H01Q 15/02 (20060101); H01L
031/08 (); H01C 007/00 (); G02B 026/10 () |
Field of
Search: |
;338/18,14,22R
;250/370.14,334,250 ;343/7MS |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2253214 |
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May 1974 |
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DE |
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58-131525 |
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Aug 1983 |
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JP |
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60-119426 |
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Jun 1985 |
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JP |
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61-170626 |
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Aug 1986 |
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JP |
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61-195318 |
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Aug 1986 |
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JP |
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1-136035 |
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May 1989 |
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JP |
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3-41305 |
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Feb 1991 |
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JP |
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91/16607 |
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Aug 1986 |
|
WO |
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Other References
E Bassous, Fabrication of Novel Three Dimensional Microstructures
by the Anisotropic Etching of(100) and (110) Silicon, 10 IEEE
Transactions on Electron Devices, 1178-1185, 1978 (FF). .
Kurt E. Peterson, Dynamic Micromechanics on Silicon: Techniques and
Devices, 10, IEEE Transactions on Electron Devices, 1241-1250,
1978. .
Kurt Peterson & Anne Shartel, Micromechanical Accelerometer
Integrated with MOS Detection Circuitry, IBM Research Facility,
1980. .
H. Elabd & W.F. Kosonocky, Theory and Measurements of
Photoresponse for Thin Film Pd.sub.2 Si and PtSi Infrared
Schottky-Barrier Detectors with Optical Cavity, 43 RCA Review,
569-588, 1982. .
K.C. Liddiard, Thin Film Resistance Bolometer IR Detectors,
Infrared Phys., vol. 24, No. 1, 57-64, 1984. .
M. Okuyama, et al., Si-Monolithic Integrated Pyroelectric Infrared
Sensor Using PbTiO.sub.3 Thin Film, 6. International Journal of
Infrared and Millimeter Waves, 71-78, 1985. .
W.F. Kosonosky, et al., 160 .times.244 Element PtSi
Schottky-Barrier IR-CCD Image Sensor, vol. Ed-32, No. 8, IEEE
Transactions on Electron Devices, 1564-1573, Aug., 1985. .
K.C. Liddiard, Thin-Film Resistance Bolometer IR Detectors--II,
Infrared Phys., vol. 26, No. 1, 43-49, 1986. .
Suzuki, et al, An Infrared Detector Using Poly-Silicon p-n Junction
Diode, Tech Digest of 9th Sensor Symposium, 71-74, 1990. .
A. Tanaka, et al., Infrared Linear Image Sensor using a Poly-Si pn
Junction Diode Array, 33 Infrared Phys., 229-236, 1992..
|
Primary Examiner: Buczinski; Stephen C.
Attorney, Agent or Firm: MacKinnon; Ian D. Shudy, Jr.; John
G. Jensen; Roger W.
Government Interests
The U.S. Government has certain rights in this invention pursuant
to the terms of a contract DAAL01-85-C-0153.
Parent Case Text
This application is a continuation, of application Ser. No.
07/035,118, filed Mar. 11, 1987, now abandoned, which is a
continuation in part of Ser. No. 781,557 filed 30 Sep. 1985, now
U.S. Pat. No. 4,654,622.
Claims
The embodiments of the invention in which an exclusive property or
right is claimed are defined as follows:
1. An infrared radiation detector comprising in combination:
a semiconductor body having a depression formed in a first surface
of the body;
a thin film dielectric member attached to the first surface at
least at one location and positioned to .[.suspend the dielectric
member.]. .Iadd.be suspended .Iaddend.as a thermally isolated
structure over said depression; and.[.,.].
a thin film layer of vanadium oxide .Iadd.operated in its
semiconductor phase and .Iaddend.embedded in said dielectric member
over said depression, said thin film layer having a high
temperature coefficient of resistance; and, contacts to said thin
film layer of vanadium oxide adapted to be connected to a measuring
circuit.
2. The detector according to claim 1 in which the thin film layer
of vanadium oxide is of a thickness <1000 angstroms.
3. The detector according to claim 1 in which the thin film layer
of vanadium oxide has a resistivity on the order of 1000 ohms per
square.
4. The detector according to claim 1 in which the thin film
dielectric is of silicon nitride.
5. The detector according to claim 1 in which the semiconductor
body is of single crystalline silicon.
6. The infrared detector according to claim 1 further comprising:
an antenna member coupled to the thin film layer of vanadium
oxide.
7. The detector according to claim 6 wherein the antenna member
comprises a metallic conductor shaped to optimize electromagnetic
radiation of a predetermined wavelength.
8. The detector according to claim 7 wherein the antenna member
comprises a dipole antenna disposed upon the thin film dielectric
member.
9. The detector according to claim 8 further comprising an
electrically conducting ground plane disposed on a second surface
of the semiconductor body opposite the first surface of the
semiconductor body.
10. The detector according to claim 9, wherein the ground plane is
composed of copper.
11. The detector according to claim 10, wherein the ground plane is
a thickness effective to reflect radiation incident on the first
surface of the semiconductor body back through the semiconductor
body and toward the antenna member.
12. The detector according to claim 11, wherein the copper ground
plane is approximately 2000 Angstroms thick.
13. The detector of claim 8 wherein the antenna member comprises
generally a bow-tie shape and wherein the bow-tie shaped antenna is
electrically coupled to the thin film layer of vanadium oxide at
the center, or "knot," of the bow-tie shaped antenna.
14. An infrared radiation detector comprising in combination:
a thin film resistor of vanadium oxide .Iadd.operated in its
semiconductor phase and .Iaddend.encapsulated in .Iadd.a
.Iaddend.thin film dielectric;
a semiconductor body having a depression therein;
the encapsulated thin film resistor of vanadium oxide and thin film
dielectric forming a thin film member bridged across the depression
so that at least a major portion of the thin film resistor is out
of contact with the semiconductor body; and,
contacts to said thin film resistor adapted to be connected to a
measuring circuit.
15. The detector according to claim 14 in which the thin film layer
of vanadium oxide is of a thickness <1000 angstroms.
16. The detector according to claim 14 in which the thin film layer
of vanadium oxide has a resistivity on the order of 1000 ohms per
square.
17. The detector according to claim 14 in which the thin film
dielectric is of silicon nitride.
18. The detector according to claim 14 in which the semiconductor
body is of single crystalline silicon.
19. An infrared radiation detector comprising in combination:
a single crystalline silicon substrate having a depression formed
in a first surface of the substrate;
a thin film silicon nitride member attached to the first surface at
least at one location and positioned to be suspended over said
depression as a thermally isolated structure;
a thin film layer of vanadium oxide .Iadd.operated in its
semiconductor phase and .Iaddend.embedded in said silicon nitride
member, said thin film layer having a high temperature coefficient
of resistance; and.[.,.].
contacts to said thin film layer of vanadium oxide adapted to be
connected to a measuring circuit.
20. An infrared radiation detector comprising in combination:
a thin film resistor of vanadium oxide .Iadd.operated in its
semiconductor phase and .Iaddend.embedded in a thin film silicon
nitride member;
a silicon substrate having a depression in the major surface
thereof;
the thin film resistor and the thin film silicon nitride member
forming a thin film member fastened to the surface and bridged
across the depression so that at least a major portion of the thin
film resistor is out of contact with the substrate; and,
contacts on said thin film resistor adapted to be connected to a
measuring circuit.
21. An infrared detector element comprising:
a cavity in a semiconductor structure across one surface of which
is suspended a resistor of vanadium oxide .Iadd.operated in its
semiconductor phase and .Iaddend.being suspended and supported by a
thin film of dielectric material,
said suspension being disposed such that at least a major portion
of said vanadium oxide resistor is out of substantial thermal
contact with said semiconductor structure.
22. An infrared detector as set forth in claim 21 wherein said
vanadium oxide resistor is a thin film resistor.
23. The infrared detector according to claim 21, further comprising
an antenna member coupled to the thin film layer of vanadium
oxide.
24. An infrared radiation detector element comprising a
semiconductor structure having a cavity across which is suspended a
highly sensitive detector material which has a thermal coefficient
of resistance greater than metal, a film thickness of less than
1000 .ANG., and having film impedance in the range of 100 to
100,000 ohms, said detector material .Iadd.being a metal oxide
semiconductor operated in its semiconductor phase and
.Iaddend.disposed upon a layer of a dielectric material which
supports said highly sensitive detector material so that a major
portion of said highly sensitive detector material is out of
contact with said semiconductor structure. .Iadd.25. A
high-sensitivity infrared radiation detector comprising in
combination:
a semiconductor body;
a thin film dielectric member attached to said body at least at one
location and positioned to be suspended as a thermally isolated
structure over said body;
a thin film layer of an oxide of vanadium operated in its
semiconductor phase and embedded in said dielectric member, said
thin film layer having a high temperature coefficient of
resistance; and
contacts to said thin film layer being adapted to be connected to a
measuring circuit. .Iaddend..Iadd.26. Apparatus of claim 25 wherein
said thin film layer of an oxide of vanadium is of a thickness
<1000 Angstroms.
.Iaddend..Iadd.27. Apparatus of claim 25 wherein said thin film
dielectric member is of silicon nitride. .Iaddend..Iadd.28.
Apparatus of claim 25 wherein said semiconductor body is of single
crystalline silicon. .Iaddend..Iadd.29. A high sensitivity
electromagnetic energy detector comprising in combination:
a supporting body;
a dielectric member attached to said body at least at one location
and positioned to be suspended as a thermally isolated structure
over said body;
a metal oxide semiconductor material supported by said dielectric
member, said semiconductor material being operated in its
semiconductor phase and having a high temperature coefficient of
resistance; and
contacts to said semiconductor material adapted to be connected to
a measuring circuit. .Iaddend..Iadd.30. Apparatus of claim 29
further characterized by said supporting body being a
semiconductor. .Iaddend..Iadd.31. Apparatus of claim 29 further
characterized by said dielectric member being a thin film.
.Iaddend..Iadd.32. Apparatus of claim 29 further characterized by
said metal oxide semiconductor material comprising oxides of
vanadium. .Iaddend..Iadd.33. Apparatus of claim 32 further
characterized by said metal oxide semiconductor material being a
thin film. .Iaddend..Iadd.34. Apparatus of claim 30 further
characterized by said dielectric member and said metal oxide
semiconductor material being thin films. .Iaddend..Iadd.35.
Apparatus of claim 34 further characterized by said metal oxide
semiconductor material comprising oxides of vanadium. .Iaddend.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
This invention is directed to the field of microbolometer infrared
radiation sensors. Particularly described herein is the use of a
new detector material (AB.sub.x) in the microbolometer sensor.
A monolithic integrated focal plane sensitive to both mm-waves
(typically 94 GHz) and (typically 3-5 and 8-12 micron) IR radiation
is constructed on a silicon wafer by selective anisotropic etching
to fabricate microbolometer radiation sensors in a linear or
two-dimensional array. Sensors intended for IR detection are coated
with an IR absorbing material. Those intended for mm-wave sensing
are connected to metal film antennas deposited on the surface of
the silicon wafer. In this structure there is combined known
silicon IC processing techniques with a rugged high-g-load-tolerant
structure that permits the thermal conduction losses to approach
the radiative losses of the element. Of particular importance is
the combining and interspersing of millimeter wave sensors with
high performance infrared sensors and electronics on the same
silicon chip, and fabricating in the same processing steps.
The fabrication of novel three-dimensional microelectronic devices
in a semiconductor crystal, typically silicon has been accomplished
by fabricating the device through many techniques including
isotropic and anisotropic etching. These techniques utilize the
cystalline structure of a single crystal semiconductor. An example
is the Johnson et al patent 4,472,239, "Method of Making
Semiconductor Device", assigned to the same assignee as the present
invention. The referenced patent shows that the technique is known
to manufacture micromechanical devices by etching into single
crystal silicon. The citation of this patent is provided merely as
background and is not deemed as prior art to the specific invention
claimed in this application.
In the prior art, such as patent 3,801,949, there has been taught
an infrared sensitive solid-state imaging device which is small in
size and which has a two-dimensional array of IR detector elements
in an integrated microcircuit. The detector array is fabricated on
a single crystal silicon substrate coated with a thin layer of
electrical insulating material, such as silicon dioxide or silicon
nitride. Etched openings are made in the silicon beneath the
insulating layer wherever a sensing element is desired for the
purpose of thermally isolating the sensing elements from their
surroundings. In the present invention an integrated dual-mode
IR/millimeter-wave sensor array is taught. The section of the
magnetic spectrum including millimeter waves and 3-5 or 8-12 micron
infrared radiation is shown in FIG. 1. The mm-waves of about 94 GHz
and the 3-12 micron IR are several orders of magnitude apart in
frequency and devices for sensing or detecting these two categories
differ substantially. It is desired to fabricate a monolithic
integrated two-dimensional focal plane array which has array
elements sensitive to 3-5 and/or 8-12 micron IR and elements
sensitive to mm-waves. The elements incorporate VO.sub.2. The
individual integrated sensors are about 0.1 mm in size and do not
effectively couple the energy from the mm-waves which are of a
greater wavelength. It has been discovered that when the integrated
sensor elements intended for mm-wave detection are provided with
antennas (such as full wave dipoles or bow-tie type) a successful
mm-wave energy coupling apparatus is achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a pertinent section of the electromagnetic
spectrum;
FIG. 2a is one embodiment of a microsensor linear array;
FIG. 2b discloses in two-dimensional geometry one embodiment of an
integrated dual-mode IR sensors and mm-wave sensors;
FIG. 3 shows a cross section of a microsensor structure;
FIG. 4 shows the front surface detail of full-wave dipole antenna
integrated IR/mm-wave array;
FIGS. 5a and 5b show detail of high thermal isolation
microsensor;
FIG. 6 shows the overall scanned array functional diagram;
FIGS. 7 and 8 show the dual mode sensor design using bow-tie
microantenna design.
FIG. 9 shows one embodiment of a dual-mode system illustrated
schematically in FIG. 6.
DESCRIPTION
Referring now to FIG. 2a there is shown a linear geometry version
of a monolithic integrated dual mode IR/mm-wave microsensor linear
array. Two-dimensional arrays mayb be obtained by constructing
several adjacent linear arrays. A focal plane sensitive to both IR
radiation (3-5 and/or 8-12 micron) and mm-waves is constructed on a
semiconductor substrate wafer 10, such as monocrystalline silicon.
The microsensors 12 intended for IR sensing are coated with an IR
absorbing material such as a thin metal film. The microsensors 13
intended for mm-wave sensing are connected to metal film antennas
14 deposited on the surface of the silicon wafer. A two-dimensional
geometry version is shown in FIG. 2b in which IR sensitive
microsensor arrays 15 are formed on one surface of the silicon
wafer 10 and antennas 16 are formed on the other surface of the
wafer. This embodiment will be described later.
In FIG. 3 there is shown a cross section of a microsensor structure
showing the thermal isolation configuration as taught in patent
4,472,239, above referenced. The microsensor imaging array is on a
silicon chip 19, based upon anisotropic silicon etching in which a
small mass, thin film radiation detector 20 is fabricated into a
thermally isolated dielectric cantilever structure 21 on the
surface of the silicon chip. The small mass and thermal isolation
provide arrays with excellent detector sensitivity and response
time. The millimeter-wave array uses planar dipole or bow-tie type
antennas to couple the mm-wave radiation to the thermally
integrating microsensors.
In FIG. 4 there is shown a detail of the front surface of a
full-wave dipole antenna type integrated infrared/millimeter wave
sensor electronically scanned linear array. Interspersed with the
multielement (ex.=300) IR detector elements 12' are a plurality
(ex.=10) of antenna coupled mm-wave elements 13'. Also shown in
block form is a bipolar pre-amp array 30 and an FET multiplexer 31.
An IR/mm-wave output signal is detected at 32. A partial cross
section of FIG. 4 cut through the detector array is shown in FIG.
5a. The silicon wafer 10 also includes a dielectric layer 33 and a
copper ground plane 34. A detail of one of the high thermal
isolation microsensors 35 is shown in the balloon of FIG. 5b in
which a resistor sensor 20 carrying dielectric cantilever 21 is
thermally isolated by the etch cavity in the silicon.
A structure which is required to couple efficiently to a mm-wave
radiation field must necessarily have dimensions of the order of
about the wavelength, e.g. 3 millimeters at 94 GHz). In the case of
an uncooled sensor, a sensitive area this large would lead to
degraded responsivity or response speed due to the increased
thermal mass of the sensor. We therefore require dimension of a few
mils, and must therefore couple the sensor to the radiation field
via an antenna structure with dimensions of the order of the
wavelength. Since microsensor arrays can be conveniently fabricated
on silicon substrates by photolithographic processes, we desire the
antenna and any coupling waveguides to be planar in design.
The mm-wave array portion, such as shown in FIGS. 2a and 4, is
further shown in FIG. 6 and consists of a silicon substrate 10',
upon which we use photolithography to fabricate an array of planar
microantennas 40, coupling waveguides 41 and microsensors 13, with
electrical leadouts to an electronic readout circuit as shown in
FIG. 6. MM-wave radiation is collected by the microantennas 40, and
coupled 41 to the dissipative load of the microsensors 13, whose
temperatures will rise causing the resistance to change. A
low-noise electronic circuit including a column address mux.42 and
a row address mux.43 monitors the resistances of the microsensor
elements and provides electrical signals 44 to output circuitry
dependent on the application such as target detection and
recognition.
The microsensor consists of a low-mass sensor element 20 which is
almost completely thermally isolated from its supporting structure
as shown earlier in FIG. 3. A resistance element is fabricated on
the sensor using a material whose resistance changes with
temperature. Any electrical power dissipated in this sensor
resistance (e.g. by direct infrared radiation on the sensor or by
mm-wave radiation coupled in from an antenna) heats the sensor
element 20 by an amount inversely proportional to the sensor
thermal mass and thermal conductance to the supporting structure.
The sensitivity of the microsensor requires a low thermal mass
sensor and good thermal isolation. The dissipated heat will flow to
the supporting structure with a time constant given by the sensor
thermal capacity times the thermal resistance to the surroundings.
This response time can be arranged to be milliseconds without
sacrificing sensitivity; faster response times can be achieved by
trading off sensitivity. The thin film resistance element has
contacts adapted to be connected to an output circuit. The
electrical output signals are obtained by the use of a readout
circuit which is sensitive to resistance changes in the microsensor
resistance.
The ultimate signal to noise ratio of such a microsensor is
achieved by the use of a very small sensor thermal mass, and very
high thermal isolation from the supporting structure. The minimum
noise level possible is due to Johnson noise in the sensor load
resistance, preamplifier noise and to fluctuations in the radiative
and conductive power interchanged between the sensor and its
surroundings. In the case of mm-wave radiation coupled electrically
into a microsensor from a microantenna, the sensor may be coated
with a highly reflective material so that radiation interchange
noise can be reduced to a low level. In this case the noise limits
would be due to a) Johnson noise, b) amplifier noise and c) thermal
conduction noise.
Of particular importance is the very low conduction noise which is
achieved by the excellent thermal isolation and low mass of the
proposed structure. Using typical parameter values demonstrated by
the prototype devices, we calculate that noise equivalent power
levels of 6.times.10.sup.-12 watts/.sqroot.Hz are expected,
assuming 75% coupling efficiency to the radiative mm-wave field.
This calculated figure is in close agreement with experimental data
obtained on prototype devices.
Experimental Results
Prototype devices have been connected to an electronic readout
circuit designed to display small resistance changes on an
oscilloscope. The sensors were installed in a metal chamber that
could be evacuated to vary the sensor thermal leak. Windows of ZnS
and glass were available to admit IR and mm-wave radiation into the
sensor chamber. A 10 Hz chopper was mounted in front of the sensor
window. A 1000.degree. K. black body IR source was used to
calibrate the sensor with an IR intensity of 7.times.10.sup.-4
W/cm.sup.2. A sensor response of about 100 mV was observed with the
sensor at atmospheric pressure, and about 400 mV with the sensor
cell evacuated. A 3.2 mm (94 GHz) CW oscillator source was used to
illuminate the sensor with a mm-wave intensity of about
2.times.10.sup.-3 w/cm.sup.2 at the sensor. The observed signal
amplitude from the sensor was measured at 280 mV. The mm-wave
signals increased in amplitude by about a factor of four as the
cell pressure was reduced from 760 to 0.5 torr, indicating that the
signal was due to the normal microsensor thermal response
mechanism.
Microantenna Considerations
The properties of planar antennas lying on dielectric (e.g. Si,
Si.sub.3 N.sub.4, SiO.sub.2) surfaces are quite different from
antennas in homogeneous media. The principal differences are 1) the
polar diagram is always heavily biased towards the dielectric, so
that efficient collection of radiation is biased towards radiation
incident from the dielectric side, and 2) additional peaks in the
polar diagram may occur: some peaks are found along the substrate
surface plane, indicating coupling to substrate surface waves which
will lead to cross-talk between adjacent antennas on that surface.
Although the polar diagram of a planar antenna on a dielectric
substrate is heavily biased towards the dielectric, this bias can
be reversed by depositing a metallic ground plane (e.g.
2000.degree. A. copper) on the back surface of the silicon
substrate as shown in FIG. 5a, so that all radiation is reflected
towards the air side, and the antenna only "looks" towards the air.
This arrangement is very desirable, since IR sensors receive
radiation from the airside, and common reflective optics can then
be used for an array of mm-wave and IR-sensors fabricated on the
same silicon wafer.
An alternate modification alluded to earlier is the use of
"bow-tie" antenna designs where the incident radiation is through
the dielectric substrate. Our tests have shown that bow-tie
antennas can be used in linear arrays to efficiently collect
mm-wave radiation incident through the substrate. In this
configuration the IR radiation is absorbed in the front side
detector elements while the mm-wave radiation passes through the
silicon wafer and is collected by the backside bow-tie antennas
(FIG. 8). In this approach through-the-wafer interconnects from
antenna to sensor are preferably used. This alternate approach
offers good performance, with
Simple, planar geometry fabricated from metal films deposited on Si
wafer surfaces.
A polar diagram heavily biased (by a factor n.sup.3), where n is
the refractive index, into the dielectric, with beam width
tailorable by adjustment of the bow-tie angle as shown in FIG.
8.
A resistive characteristic impedance, tailorable by adjustment of
the bow-tie angle, constant over wide frequency range.
Coupling of Antenna to Microsensors
The simplest way of coupling a dipole antenna to a radiation sensor
is to fabricate the sensor between the arms of the dipole and
metallize the antenna to the sensor load. The antenna impedance can
be matched to sensor loads in the 100 ohm range.
A High Sensitivity Detector Material
Referring again to FIG. 3 which shows a cross section of a
microsensor structure there is shown a detector element 20. The
requirements of a high sensitivity material for microbolometer
sensor detectors are
a) high thermal coefficient of resistance (TCR)
b) low 1/f noise
c) deposited in a thin film (<1000 angstroms)
d) process compatible with microbolometer fabrication
technology
e) no anomalous thermal capacity
f) film impedance compatible with microbolometer read-out circuitry
(100 to 100,000 ohms).
Vanadium oxides (preferably VO.sub.2) satisfy all these
requirements. Vanadium oxides have very strong changes in
resistance with temperature, allowing high sensitivity
microbolometer operation. The TCR is typically -0.01 to -0.04 per
degree Centigrade, and much higher at the semiconductor-to-metal
transition. This typical range is much higher than the TCR for the
detector material permalloy (previously used) which is
approximately 0.0035 per degree Centigrade. The 1/f noise can be
kept low by the use of high conductivity VO.sub.2, that is, about
1000 ohms/square in 1000 angstrom films. The VO.sub.2 thin film
detector 20 can be deposited directly onto the dielectric layer 21
with a chosen film impedance. This deposition is preferably by the
process of ion beam sputter which permits the deposition of very
thin layers using growth conditions compatible with the silicon
microbridge technology. In the preferred embodiment at this time
the VO.sub.2 is operated in its semiconductor phase. An increased
thermal capacity occurs at the semiconductor-to-metal phase
transition, but is acceptably low.
* * * * *