U.S. patent number RE36,706 [Application Number 08/601,411] was granted by the patent office on 2000-05-23 for microstructure design for high ir sensitivity.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to Barrett E. Cole.
United States Patent |
RE36,706 |
Cole |
May 23, 2000 |
Microstructure design for high IR sensitivity
Abstract
A microstructure design for high IR sensitivity having a two
level infrared bolometer microstructure, the lower level having a
reflective metal film surface such as Pt, Au, or Al to reflect IR
penetrating to that level, the upper level being separated from the
lower level by an air gap of about 1-2 microns which allows the
reflected IR to interfere with the incident IR and increase the
sensitivity to a higher level.
Inventors: |
Cole; Barrett E. (Bloomington,
MN) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
23108254 |
Appl.
No.: |
08/601,411 |
Filed: |
February 14, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
288701 |
Nov 7, 1988 |
05286976 |
Feb 15, 1994 |
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Current U.S.
Class: |
250/349;
250/338.1; 250/338.4; 250/342 |
Current CPC
Class: |
G01J
5/20 (20130101); H01L 27/16 (20130101) |
Current International
Class: |
G01J
5/20 (20060101); H01L 27/16 (20060101); G01J
005/20 (); G01J 005/22 (); G01J 005/02 () |
Field of
Search: |
;250/338.1,338.4,330,332,349,370.01,370.08 |
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 |
|
Other References
Kurt Peterson & Anne Shartel, Micromechanical Accelerometer
Integrated with MOS Detection Circuitry, IBM Research Facility,
1980, pp. 673-675. .
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. .
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, 72, and 74, 1990.
.
A. Tanaka, et al., Infrared Linear Image Sensor using a Poly-Si pn
Junction Diode Array, 33 Infrared Phys., 229-231, and 234-236,
1992. .
Thin-Film Resistance Bolometer IR Detectors, K.C. Liddiard,
Infrared Phys., vol. 24, No. 1, pp. 57-64, 1984. .
Thin-Film Resistance Bolometer IR Detectors II, K.C. Liddiard,
Infrared Phys., vol. 26, No. 1, pp. 43-49, 1986..
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Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Mackinnon; Ian D. Jensen; Roger
W.
Claims
The embodiments of the invention in which an exclusive property or
right is claimed are defined as follows:
1. A two-level microbridge infrared bolometer structure
comprising:
a bolometer structure on a semiconductor substrate, said structure
having a lower section on the surface of the substrate and a
microbridge upper detector plane structure spaced from and
immediately above the lower section;
an infrared-reflective thin film metal coating on the surface of
said lower section;
said upper microbridge detector plane structure comprising a planar
sandwich structure including a .Iadd.first .Iaddend.supporting
dielectric thin film layer, and a thin film temperature responsive
resistive element having first and second terminals;
downwardly extending dielectric leg portion means which are a
downwardly extending continuation of said .[.upper structure.].
.Iadd.first .Iaddend.dielectric .Iadd.layer .Iaddend.supporting
said upper microbridge detector plane structure above said lower
section so that a thermal isolation gap exists between said upper
.[.and.]. .Iadd.detector plane structure and said .Iaddend.lower
.[.sections.]. .Iadd.section.Iaddend.; and,
electrically conductive paths included in said downwardly extending
leg portion means connecting said first and second terminals to
said lower section.
2. The microbridge structure according to claim 1 wherein said
reflective thin film metal coating is selected from the group
consisting of Au, Pt, and Al.
3. The microbridge structure according to claim 1 wherein said
dielectric is of silicon nitride.
4. The microbridge structure according to claim 1 wherein said thin
film resistive element is selected from the group consisting of
vanadium oxide and titanium oxide.
5. The microbridge structure according to claim 1 wherein said thin
film resistive element is V.sub.2 O.sub.3.
6. The microbridge structure according to claim 1 wherein said gap
between said lower section and said upper detector .Iadd.plane
.Iaddend.structure is in the range of about 1-2 microns.
7. The microbridge structure according to claim 2 wherein the
coating is about 50 nm in thickness.
8. The microbridge structure according to claim 1 and further
comprising, in said planar sandwich structure, a second dielectric
thin film layer and a thin film absorber layer.
9. The microbridge structure according to claim .[.3.]. .Iadd.8
.Iaddend.wherein the first .Iadd.supporting .Iaddend.dielectric
.Iadd.thin film .Iaddend.layer .[.in.]. .Iadd.is .Iaddend.on the
order of 100 nm in thickness and the second dielectric .Iadd.thin
film .Iaddend.layer is on the order of 250 nm in thickness.
10. The microbridge structure according to claim 4 wherein the
.[.resistive element film.]. .Iadd.thin film resistive element
.Iaddend.is on the order of 50-75 nm in thickness.
11. The microbridge structure according to claim 8 wherein the
absorber layer is on the order of 30 nm in thickness.
12. A two-level microbridge infrared bolometer structure
comprising:
a bolometer microstructure on a semiconductor substrate, said
structure having a lower section on the surface of the substrate
and a microbridge upper detector plane structure spaced from and
immediately above the lower section;
an infrared reflective thin film metal coating on the surface of
said lower section, said metal being selected from the group
consisting of Au, Pt, and Al;
said upper microbridge detector plane structure comprising a planar
sandwich structure including a first bridging dielectric thin film
layer, a thin film temperature responsive resistive element
selected from the group consisting of vanadium oxide and titanium
oxide, said resistive element having first and second terminals, a
second dielectric thin film layer over said first dielectric layer
and resistive layer, and a thin film absorber layer;
downwardly extending dielectric leg portion means which are a
downwardly extending continuation of said .[.upper structure.].
.Iadd.first .Iaddend.dielectric .Iadd.layer .Iaddend.supporting
said upper microbridge detector plane structure above said lower
section so that an air gap on the order of 1-2 microns exists
between said upper .[.and.]. .Iadd.detector plane structure and
said .Iaddend.lower .[.sections.]. .Iadd.section.Iaddend.; and,
electrically conductive paths included in said downwardly extending
leg portion means connecting said first and second terminals to
said lower section. .Iadd.13. A two-level microbridge uncooled
infrared thermal detector means comprising:
a pixel on a semiconductor substrate, said pixel having a lower
section on the surface of said substrate and a microbridge upper
detector section spaced from and immediately above the lower
section;
said lower section including integrated circuit means and
infrared-reflective means coating said integrated circuit
means;
said microbridge upper detector section comprising a bridging
dielectric layer having mounted thereon temperature responsive
means having first and second terminals, said microbridge upper
detector section being supported above said lower section by
dielectric leg portions which are downward extending continuations
of the bridging dielectric layer to thereby support said upper
section and so that a thermal isolation gap is defined between said
upper and lower sections;
and said first and second terminals being continued down said leg
portions to said integrated circuit means; and
said two-level microbridge uncooled infrared thermal detector means
being further characterized by the size of said gap, i.e., the
distance between said upper and lower sections, being selected so
that infrared radiation which initially passes through said upper
section to said infrared-reflective means is then reflected toward
and is intensified at said upper section to optimize infrared
absorption over a preselected band
of infrared wavelengths. .Iaddend..Iadd.14. The thermal detector
means of claim 13 wherein said bridging dielectric layer comprises
a first dielectric layer beneath said temperature responsive means
and a second dielectric layer over said first dielectric layer and
said temperature responsive means. .Iaddend..Iadd.15. The thermal
detector means of claim 14 wherein said dielectric layers are of
silicon nitride. .Iaddend..Iadd.16. The thermal detector means
according to claim 15 wherein the first dielectric layer is on the
order of 100 nm in thickness and the second dielectric layer is on
the order of 250 nm in thickness. .Iaddend..Iadd.17. The two-level
microbridge uncooled infrared thermal detector means of claim 13
wherein:
said temperature responsive means is a thin film resistive
element;
said upper detector section includes absorber means covering said
resistive element; and
said gap is selected so that infrared radiation reflected from said
infrared-reflective means is intensified at said absorber means, to
thereby optimize the absorption of infrared radiation in said upper
detector section. .Iaddend..Iadd.18. The thermal detector means
according to claim 17 wherein the absorber means is a layer on the
order of 30 nm in thickness. .Iaddend..Iadd.19. The thermal
detector means of claim 13 wherein said infrared-reflective means
is a thin film metal coating.
.Iaddend..Iadd.20. The thermal detector means of claim 13 wherein
said infrared-reflective means is a thin film metal coating
selected from the group consisting of Au, Pt, and Al.
.Iaddend..Iadd.21. The thermal detector means according to claim 20
wherein the coating about 50 nm in thickness. .Iaddend..Iadd.22.
The thermal detector means of claim 13 wherein said temperature
responsive means is a thin film resistive element.
.Iaddend..Iadd.23. The thermal detector means of claim 22 wherein
said thin film resistive element is selected from the group
consisting of vanadium oxide and titanium oxide. .Iaddend..Iadd.24.
The thermal detector means according to claim 23 wherein said thin
film resistive element is on the order of 50-75 nm in thickness.
.Iaddend..Iadd.25. The thermal detector means of claim 22 wherein
said thin film resistive element is V.sub.2 O.sub.3.
.Iaddend..Iadd.26. The thermal detector means of claim 22 wherein
said thin film resistive element is V.sub.2 O.sub.3 operated in its
semiconductor phase. .Iaddend..Iadd.27. The thermal detector means
according to claim 13 wherein said gap between said upper and lower
sections is in the range of about 1-2 microns. .Iaddend..Iadd.28. A
two-level microbridge infrared bolometer structure comprising:
a bolometer structure on a semiconductor substrate, said structure
having a lower section on the surface of the substrate and a
microbridge upper detector plane structure spaced from and
immediately above the lower section by a thermal isolation gap of
between about 1-2 microns, the upper microbridge upper detector
plane structure including a thin film resistive element having
first and second terminals;
an infrared-reflective thin film on the surface of said lower
section; and
electrically conductive paths connecting said first and second
terminals to said lower section. .Iaddend.
Description
FIELD OF THE INVENTION
The field of the invention is in a high sensitivity two-level
microstructure infrared bolometer array which can produce
absorptance levels of greater than 80% and also achieve high IR
sensitivity over a wavelength range from 8-14 microns.
BACKGROUND AND SUMMARY OF THE INVENTION
This invention is an improvement on co-pending application Ser. No.
887,495, filed Jul. 16, 1986, entitled "Thermal Sensor", and
assigned to the same assignee as the present invention. The
teachings of the co-pending application are incorporated herein by
reference. In the co-pending application the invention is directed
to a pixel size sensor of an array of sensors, for an infrared
microbridge construction of high fill factor, made possible by
placing the detector microbridge on a second plane above the
silicon surface carrying the integrated components and bus lines.
The improvement in the present invention is directed to a structure
which increases the sensitivity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are front and top views of a microstructure design
according to the invention.
FIG. 3 is a graphical plot of overall absorptance vs. wavelength of
the device over a selected wavelength including 8-14 microns.
FIG. 4 shows graphically the transmittance, absorption and
reflectance of the resistive layer.
FIG. 5 shows graphically absorption vs. air gap thickness.
FIG. 6 shows absorption of entire structure vs. metal absorber
thickness.
FIG. 7 shows measured optical properties of Si.sub.3 N.sub.4.
DESCRIPTION
A cross section view of the two-level microbridge bolometer pixel
10 is shown in FIG. 1. The device 10 has two levels, an elevated
microbridge detector level 11 and a lower level 12. The lower level
has a flat surfaced semiconductor substrate 13, such as a single
crystal silicon substrate. The surface 14 of the substrate has
fabricated thereon conventional components of an integrated circuit
15 such as diodes, bus lines, connections and contact pads (not
specifically shown), the fabrication following conventional silicon
IC technology. The IC is coated with a protective layer of silicon
nitride 16.
The elevated detector level 11 includes a silicon nitride layer 20,
a thin film resistive layer 21, preferably a vanadium or titanium
oxide (such as V.sub.2 O.sub.3, TiO.sub.x, VO.sub.x), .[.i.e..].
.Iadd.sometimes hereinafter referred to as .Iaddend.AB.sub.x a
silicon nitride layer 22 over the layers 20 and 21 and an IR
absorber coating 23 over the silicon nitride layer 22. The thin
absorber coating (approximately 20A thick) may be of a nickel iron
alloy, often called permalloy. Downwardly extending silicon nitride
layers 20' and 22' deposited at the same time as layers 20 and 22
during the fabrication make up the sloping supports 30 .Iadd.and
30' .Iaddend.for the elevated detector level. The cavity or gap 26
(approximately 1-2 microns high) between the two levels is ambient
atmosphere. During the fabrication process, however, the cavity 26
was originally filled with a previously deposited layer of easily
dissolvable glass or other dissolvable material until the layers
20, 20', 22 and 22' were deposited. Some other easily dissolvable
materials are quartz, polyimide and resist. Subsequently in the
process the glass was dissolved out to provide the thermal
isolation cavity or air gap (i.e., the air gap actually may be in
operation, a vacuum gap). In FIG. 1 the horizontal dimension, as
shown, is greatly foreshortened. That is, the height of FIG. 1 is
exaggerated in the drawing compared to the length in order to show
the details of the invention.
FIG. 2 is a top plan view of the elevated detector level 11. This
drawing is made as though the overlying absorber coating 23 and the
upper silicon nitride layer 22 are transparent so that the
resistive thin film layer 21 can be shown. In one preferred
embodiment the material for the resistive layer 21 is a vanadium
oxide, preferably V.sub.2 O.sub.3. Vanadium oxides have very strong
changes in resistance with temperature allowing high sensitivity
microbolometer operation. It also has a low reflectance to IR in
the 8-14 micron range. In the preferred embodiment at this
time.Iadd., .Iaddend.the V.sub.2 O.sub.3 is operated in its
semiconductor phase. Its deposition is preferably by the process of
ion beam sputter which permits the deposition of very thin layers
such as 50-75 .[.mm.]. .Iadd.nm.Iaddend.. This material was thus
selected for its low IR reflectance together with a relatively high
temperature coefficient of resistance (TCR). The ends of the
resistive paths 21a and 21b are continued down the slope area 30
embedded in 20' and 22' to make electrical contact with contact
pads 31 and 32 on the lower level.
FIG. 2 also shows nitride window cuts 35, 36 and 37 which are
opened through the silicon nitride layers 20 and 22 to provide
access to the phos-glass beneath for dissolving it from beneath the
detector plane. The sloping supports may be of the necessary length
is provide adequate support and thermal isolation for the upper
level 11.
Although the description is basically in terms of individual
detector pixels, the invention is directed for use to an x,y array
assembly of adjoining pixels forming an imaging or mosaic detector
array. Each pixel assembly may cover an area about 50 microns on a
side, as an example.
Referring again to FIG. 1 a sequence of fabrication steps for the
upper level is described. Following the deposition of the silicon
nitride layer 16 in fabricating the lower level 12, a thin film
layer 18 of reflective material, such as a metal film like Pt or
Au, is deposited. The construction of the upper level can then
commence. The detectors presently being described are intended for
use in the 8-14 micron IR wavelength. The reflective layer 18 is on
the lower plane 12. The vertical distance between reflective layer
18 and upper level 11 is chosen so the reflected IR from layer 18
returned upwardly has interference properties such that significant
absorption is achieved for a wide range of wavelengths (8-14
microns) .[.and air gap spacing between the reflector and the
detector structure.]..
A layer of phos-glass or other easily soluble material in the range
of about 1-2 microns thick is deposited and the slopes 30 and 30'
are thoroughly rounded to eliminate slope coverage problems. The
upper level silicon nitride base layer 20 is then deposited, the
resistive film 21 is deposited, connections down the slope to lower
plane contact pads are made, and a silicon nitride passivation
layer 22 covers the layers 21 and 20. A thin metal absorber coating
23 (about 15-40A) is deposited on top of the upper level. The slots
35, 36 and 37, earlier mentioned are made and the phos-glass is
dissolved from beneath the detector plane. As earlier described, by
depositing Pt, Au or other reflecting thin film 18 on the substrate
before the stack is formed, it is possible to reflect transmitted
radiation reaching the reflecting film back to the absorber
coating.
The optical properties of the total structure are achieved by
careful selection of optical materials with the proper optical and
electrical properties. The top film must reflect little radiation
and generally transmit a significant percentage of the non-absorbed
radiation through to the reflected light at a nodal position in the
film determined by the air gap distance. .Iadd.Stated otherwise,
the air gap distance is determined so that the reflected light is
at a nodal position in the film. .Iaddend.An additional constraint
on the absorbing film is that to be compatible with the total
structure, the absorbing material must be very thin (and hence have
a low mass).
To optimize the absorption in the structure, the thickness of all
the absorbing layers and the air gap distance must be controlled.
The absorbing films in the present device consist of .[.ABx.].
.Iadd.AB.sub.x .Iaddend., .[.SIN.]. .Iadd.SiN.Iaddend., and the
thin absorbing metal described above. In practice, the .[.ABx.].
.Iadd.AB.sub.x .Iaddend.and .[.SIN nitride.]. .Iadd.SiN
.Iaddend.thicknesses are chosen by electrical and physical
requirements. Both have absorption levels ranging from 10-20% in
the spectral region of interest (FIGS. 4 and 7). A combination of
these materials produces an absorption of no more than about 30% in
the 8-14 micron region. This absorption level is very close to
ideal, however, for use with a Pt reflective layer and an air gap
which intensifies the field in the absorbing film, it is possible
to achieve absorptances in excess of 80% (FIG. 5) in this
configuration. The use of a thin absorbing metal which in the
standard design provides 50% absorption, here is used to fine tune
the absorption for maximum effect. FIG. 6 shows the small
absorption improvements that can be achieved by using this metal
film.
In this two-level structure, the low thermal mass structure 11 is
separated from the .[.Pt/substrate.]. .Iadd.Pt substrate
.Iaddend.layer by an air gap. The interference properties of this
reflected radiation are such that significant absorption is
achieved .[.by.]. .Iadd.for .Iaddend.a wide range of wavelengths
.[.and air gap spacing between the Pt reflector and the detector
structure.]..
For this optical interference to occur in the detector, it is
necessary to avoid other films in the detector structure which
reflect IR. The use of AB.sub.x which has both a high TCR and a low
IR reflectance (FIG. 4) ideally meets these requirements. Thus the
merging of this absorption phenomenon into a detector structure
which has a detector material processing both a high TCR and low
reflectance permits this interference effect to occur.
There is a substantial degree of variability of detector
absorptance with air gap in the structure. Referring to the table
below which shows wavelength in nanometers in the left column vs.
air gap in microns across the top it can be seen that with an air
gap of only 0.5 micron the detector absorptance varies widely with
wavelength and it is not very high. With air gaps of 1-2 microns
and especially at 1.5 microns the absorptance is relatively high
across the desired wavelength spread.
TABLE 1 ______________________________________ DETECTOR ABSORPTANCE
Air Gap (microns) Wavelength (NM) .5 .75 1.0 1.5 2.0
______________________________________ 8000 .89 .91 .9 .84 .76 9000
.84 .88 .86 .81 10000 .76 .82 .84 .84 .82 11000 .69 .77 .8 .82 .82
12000 .66 .74 .79 .83 .84 13000 .64 .78 .85 .93 .94 14000 .56 .72
.83 .95 .98 15000 .47 .64 .77 .92 .99
______________________________________
The effect of gap thickness on the absorptance vs. wavelength in
the regions of interest are further displayed graphically in FIG.
5. It can be seen in the curve of 1.5 microns gap thickness that at
8 microns the absorptance of the structure is climbing rapidly
towards 90% and more, and that it remains relatively high out to
about 14 microns. The curve for a gap of 2 microns shows that at IR
wavelengths of 14 microns the absorptance is better and well above
90%. In measuring the data for FIG. 5 the absorber film 23 was not
included in the stack structure.
Referring now to FIG. 6 there is shown graphically how the overall
absorption of the film structure varies across the IR wavelength of
8-14 microns as the thickness of the metal absorber film is
increased to 3 .[.mm.]. .Iadd.nm .Iaddend.and to 5 .[.mm.].
.Iadd.nm.Iaddend.. In this film stack design the Si.sub.3 N.sub.4
layer 22 is 250 .[.mm.]. .Iadd.nm.Iaddend., the resistive film 21
is 75 nm and the Si.sub.3 N.sub.4 film 20 is 100 nm with an air gap
of 1.5 microns and a reflective Pt layer
18 of 50 nm. This curve for 3 nm shows absorptance>90% between 8
and 14 microns.
The measured optical properties of reflectance R, transmissivity T,
and absorptance A of the silicon nitride layers 20 and 22 (800A
thick) are shown in FIG. 7 with percent of signal shown on the
ordinate axis and IR wavelength along the abscissa. It can be seen
that the transmissivity at 8 microns (about 90) and at 14 microns
(about 80) is quite high and that the reflectance R at both 8 and
14 microns is well under ten.
* * * * *