Note: Descriptions are shown in the official language in which they were submitted.
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MICROSTRUCTURE DESIGN FOR HIGH IR SENSITIVITY (U)
FIELD OF THE INVENTION
(U) 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
(U) In a related application filed by the present
applicant, 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.
In summary this invention seeks to provide 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
supporting dielectric thin film layer, and a thin film
temperature responsive resistive element having first and
second terminals; downwardly extending dielectric leg portion
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means which are a downwardly extending continuation of said
upper structure dielectric supporting said upper microbridge
detector plane structure above said lower section so that a
thermal isolation gap exists between said upper and lower
section; and, electrically conductive paths included in said
downwardly extending leg portion means connecting said first
and second terminals to said lower section.
This invention also seeks to provide 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 dielectric
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 lower sections; and,
electrically conductive paths included in said downwardly
extending leg portion means connecting said first and second
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terminals to said lower section.
BRIEF DESCRIPTION OF THE DRAWINGS
(U) Figures 1 and 2 are front and top views of a
microstructure design according to the invention.
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Figure 3 is a graphical plot of overall absorptance
vs. wavelength of the device over a selected wavelength
including 8-14 microns.
Figure 4 shows graphically the transmittance,
absorption and reflectance of the resistive layer.
Figure 5 shows graphically absorption vs. air gap
thickness.
Figure 6 shows absorption of entire structure vs.
metal absorber thickness.
Figure 7 shows measured optical properties of Si3N4.
DESCRIPTION
A cross section view of the two-level microbridge
bolometer pixel 10 is shown in Figure 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 li includes a silicon
nitride layer 20, a thin film resistive layer 21, preferably a
vanadium or titanium oxide (such as VZO3, TiOX, VOX) , i.e. ABX 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
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same time as layers 20 and 22 during the fabrication make up
the sloping supports 30 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 Figure 1 the horizontal dimension, as shown,
is greatly foreshortened. That is, the height of Figure 1 is
exaggerated in the drawing compared to the length in order to
show the details of the invention.
Figure 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 Vz03.
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 the Vz03 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 50-75nm. 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.
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Figure 2 also shows nitride window cuts 35, 36 and 37
which are opened through the silicon nitride layers 20 and 2 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 to 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 Figure 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
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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
5 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. An additional constraint on this
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, SIN, and the thin absorbing metal described
above. In practice, the ABX and SIN nitride thicknesses are
chosen by electrical and physical requirements. Both have
absorption levels ranging from 10-20% in the spectral region of
interest (Figures 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% (Figure 5) in
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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. Figure 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 layer by an air
gap. The interference properties of this reflected radiation
are such that significant absorption is achieved for 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 ABX which has both a
high TCR and a low IR reflectance (Figure 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 .5 micron the detector
absorptance varies widely with wavelength and 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.
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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 .89 .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 Figure 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 Figure 5 the absorber film 23 was not included in the
stack structure.
Referring now to Figure 6 there is shown graphically
how the overall absorptance of the film structure varies across
the IR wavelength of 8-14 microns as the thickness of the metal
absorber film is increased to 3nm and to 5nm. In this film
stack design the Si3N4 layer 22 is 250nm, the resistive film 21
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is 75nm and the Si3N4film 20 is 100nm with an air gap of 1.5
microns and a reflective Pt layer 18 of 50nm. This curve for
3nm shows absorptance > 90% between 8 and 14 microns.
The measured optical properties of reflectance R,
transmissivity T, and absorptance of the silicon nitride layers
20 and 22 (800A thick) are shown in Figure 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.
