Infrared radiation imaging array with compound sensors forming each pixel

An infrared imaging array of thermoelectric sensors has a plurality of electrically connected microbridge subsensors comprising each sensor of the array. Each subsensor consists of a short span microbridge lying across a relatively small pit. The use of many of such subsensors for each sensor rather than a single large area microbridge sensor for a single pixel allows each pixel to be made large enough to give good sensitivity in either vacuum or gas-filled designs, and at the same time avoid the reduced fabrication yield which results when sensors span large pits.

BACKGROUND OF THE INVENTION 
Quite sensitive and small infrared (IR) sensors which operate on the 
principles of a thermopile are now available. Such sensors provide a small 
electrical signal which varies with the relative strength of the IR 
radiation impinging on them, and can be used to measure the temperature or 
change in temperature of an object on which a sensor of this type is 
focussed. The most sensitive of these sensors can detect differences in 
temperature of a few thousandths of a degree Celsius in the object from 
which the IR radiation emanates. Using common photolithographic processes, 
such sensors can easily be fabricated in situ in a matrix or array, each 
sensor forming one of the pixels in the array. 
It is known that most objects emit amounts and frequencies of infrared 
radiation which differ based on the emissivity, angle of the surface to 
the viewer, and temperature of the body itself. The variations in this 
radiation when allowed to impinge on an array of IR sensors produces 
corresponding differences in the signals from the individual sensor 
elements in the array. The individual signals from the sensors in the 
array can therefore collectively encode an image of the field of view from 
which the IR radiation emanates. Typically, the individual output signals 
from the sensors are scanned in some sequential manner to form a composite 
signal encoding the image and changes in it in real time. In this way, 
such an array can form the IR radiation-sensitive, image-forming element 
of a camera which produces images based on the infrared radiation 
emanating from the field of view. The signal can be used to form a visible 
image in a display which accurately represents the spatial relationship of 
objects in the field. Such encoding of an image has been done for many 
decades in the imaging of visible light from a field of view as in 
television technology. 
A recently developed preferred design for IR sensors depending on a 
thermoelectric mechanism to provide the signal voltage output, has thin 
layers of conductive materials of various types and insulating material 
which are deposited in appropriate patterns on a silicon sensor substrate 
using well-known photolithographic techniques. Thermoelectric junctions 
are formed by overlapping conductors during the deposition. Such sensors 
will be called hereafter microbridge sensors. The junctions of these 
microbridge sensors are of two kinds, sensor junctions and reference 
junctions. The reference junctions are in close thermal contact with the 
substrate. Each sensor junction is within a small, discrete, area which 
overlays a pit or depression formed in the sensor substrate, and is of an 
area conforming to the footprint of the sensor junction. In cross section, 
these sensors look much like a bridge spanning a valley, hence the term 
"microbridge". The pits provide a measure of thermal isolation from the 
substrate for their associated sensor junctions. Thus, changing IR 
radiation impinging on both the sensor and reference junctions causes the 
temperature of the sensor junction to change more rapidly than does the 
reference junction, resulting in a temperature differential between the 
junctions which generates a signal. These known photolithographic 
techniques allow individual sensors to be easily fabricated in an array so 
as to allow imaging of the IR radiation in a field of view. Leads from the 
elements forming the junctions are led to electronic circuitry which may 
be formed in layers below the sensor substrate. This circuitry scans and 
amplifies the signals from the individual sensors to provide a signal 
which may be used to reproduce the field of view in a way analogous to 
that of television. 
For maximum sensitivity microbridge sensors may be maintained in a low 
pressure gas atmosphere or in a vacuum by virtue of reduced heat transfer 
between the sensing junction and the substrate, but this requires a 
hermetically sealed enclosure which adds cost and reduces reliability. It 
is also possible to use a less tightly sealed enclosure containing air or 
other gas at or near atmospheric pressure, at the cost of less 
sensitivity. One should realize that these microbridge sensors are 
designed to produce a usable signal with but a few hundredths or 
thousandths of a degree Celsius temperature differential between the 
sensing and reference junctions. 
One desirable application for these sensors is in arrays for forming images 
of relatively low contrast scenes or fields of view, such as may arise 
indoors in occupied rooms. In such fields of view, the inanimate, non-heat 
producing objects are all very nearly at the same temperature. 
Distinguishing such objects by use of IR imaging requires very sensitive 
sensors. The types of microbridge IR sensors formed according to today's 
technology cannot provide the high quality signals, i.e. resolve contrasts 
in impinging radiation adequately so as to clearly distinguish the typical 
variations in IR radiation in low contrast fields of view unless the total 
area of each individual sensor pixel is larger than a certain minimum 
area. Typically, a 6 mil.times.6 mil (0.15 cm.times.0.15 cm) or equivalent 
area is required for vacuum-packaged sensors. Even larger areas are 
required for sensors operating in gas-filled packages. Sensors having such 
areas are too large to reliably fabricate using current processes. In 
essence, the span necessary for the bridge which supports the sensing 
junction is too great for reliable fabrication and adequate resistance to 
shock and vibration. 
BRIEF DESCRIPTION OF THE INVENTION 
We have found that the capabilities of large area microbridge sensors can 
be duplicated with satisfactory yields by instead using a group of 
adjacent small microbridge sensors whose aggregate area is similar to that 
of a single large sensor. Each such group of sensors forms a single pixel 
for imaging purposes. The sensitivity of the device is not reduced 
significantly by the use of a number of smaller sensors instead of a 
single large one of similar area. The small microbridge sensors in a 
single group are connected electrically in series or parallel to form an 
equivalent large sensor. These smaller sensors need span only a small pit 
which thermally isolates the sensing junction, and this can be done with 
much higher fabrication yields. Further, if an occasional smaller sensor 
is defective, others in a single group will still provide a signal which 
can if necessary be enhanced so as to provide at least some contribution 
to the image. This approach allows the individual pixel area to be made as 
large as is desired for adequate sensitivity and resolution. Each sensor 
may have as few as three or four subsensors or as many as thousands. 
The small microbridge sensors form in effect a sub-array of subsensors 
which are connected electrically to form a single large area sensor of the 
required sensitivity. Each of the large sensor elements so formed may 
itself be arranged in an array of these larger sensors to allow an image 
of impinging radiation to be formed. 
Such an infrared radiation sensing array comprises a substrate having on a 
first surface, a plurality of pixel areas arranged in an array. Within 
each pixel area, a plurality of infrared radiation voltaic microbridge 
subsensors are arranged in a sub-array to comprise a single sensor. Each 
subsensor has first and second output terminals across which appears a 
voltage responsive to changes in infrared radiation impinging on the 
subsensor. A plurality of conductors connect the subsensors of each sensor 
together in a predetermined manner so as to provide a composite signal to 
which each subsensor contributes. The composite signal from each sensor is 
provided on its own conductor to a signal processor. Each composite signal 
has the information content that would be provided by a single large 
sensor occupying essentially the same space in the array. 
The subsensors comprising a sensor may be connected in either series or 
parallel as desired so as to match the input impedance of the amplifier to 
which they are connected. Most conveniently, all of the subsensors forming 
a sensor may be arranged in an orthogonal array within the pixel area and 
connected in a series arrangement with the voltage output and internal 
impedance of each additive to that of all the others comprising the 
sensor. To achieve this, the subsensors in each row of the pixel area 
array are connected in series, and the rows of series-connected subsensors 
are then connected in series with each other as well to create a series 
connection of all of the subsensors wherein the signal voltages of the 
sensors are additive.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2, which are different views of the same array device 10, will 
be described together. In FIG. 1, there are shown two separate pixel areas 
located at the upper left hand and upper right hand corners respectively 
of a representative infrared imaging array. There may be as many as 
several hundred pixel areas in a single row of an array, and there may be 
several hundred rows as well in a high resolution array 10. Each pixel 
area contains a single composite sensor 11, 12, etc. formed by 
photolithographic processes. The entire array 10 is carried on a base 64, 
which typically will be formed of a silicon material and cut from a larger 
silicon wafer. The array 10 is formed in and on a thermally conductive 
heat sink layer 19. It is possible to form electrical components and 
connections beneath heat sink layer 19 within a group of contiguous 
component layers 57 deposited on base 64. The individual components within 
component layers 57 if present, as well as the connections between them, 
will typically also be formed by these well-known photolithographic 
processes. These components and their connections can form circuits for 
processing the signals provided by individual sensors 11, 12, etc. 
For exemplary purposes, FIGS. 1 and 2 show individual sensors 11 and 12, 
with sensor 11 shown as comprising four essentially identical individual 
subsensors 14-17. In point of fact sensor 11 may comprise as few as three 
or four subsensors 14-17 or as many as thousands of subsensors. The number 
of subsensors 14-17 depends on the sensitivity and resolution required and 
whether the array 10 is of the vacuum or gas-filled type. Each subsensor 
14-17 comprises a complete microbridge infrared sensor with a sensing 
junction 47 and a reference junction 50 as is shown for subsensor 14. With 
present technology, each sensor 11, 12, etc. may occupy a pixel area which 
is on the order of 6 mils (0.15 mm) square. Within each pixel area, the 
individual subsensors 14-17 may be arranged in orthogonal rows and columns 
as shown, although other spatial arrangements are possible as well. The 
subsensors 14-17 are formed and supported on a thin layer 42 comprising an 
electrically insulating material such as photoresist having relatively 
good thermal conductivity. 
The reference numbers identifying each of the various elements which 
comprises each of the subsensors 14-17 have been applied to subsensor 14. 
Explanation of the subsensors forming the device will be with reference to 
subsensor 14, and unless otherwise stated comments with respect to it are 
true also for each of the other subsensors 15-17 comprising sensor 11 and 
the subsensors comprising sensor 12, etc. 
Subsensor 14 is shown in FIG. 1 as having a cold, or reference junction 50 
and a hot or sensing junction 47. Reference junction 50 is formed by a 
first thermocouple element 34 which slightly overlaps a second 
thermocouple element 33 as shown in FIG. 2, creating intimate electrical 
contact between them. Each of elements 33 and 34 comprise a different 
conductive material. Sensing junction 47 is formed by a third thermocouple 
element 29 which slightly overlaps a fourth thermocouple element 28 as 
shown in FIG. 2, and also creates intimate electrical contact between 
them. Thermocouple elements 28 and 34 both comprise a part of layer 26 and 
are formed of the same thermocouple material. Thermocouple elements 28 and 
34 also serve as first and second output terminals for the signal from 
each subsensor. Elements 29 and 33 both comprise a part of layer 32 and 
are formed of the same thermocouple material, a material which is 
different from that forming layer 26. The material forming layer 32 may be 
nickel-iron for example, and that forming layer 26 may be chromium. Other 
pairs of metals or semiconductors are also known to be suitable for use as 
thermocouple elements. However, it is important that they be suitable for 
use in photolithographic or other types of deposition processes which may 
be used in forming these arrays. 
Thermocouple layers 26 and 32 and the elements comprising junctions 47 and 
50 are formed on the underlying support layer 42 as a part of the 
photolithographic manufacturing process. Layer 42 lies on the thicker heat 
sink layer 19. Junction 47 and the portion of support layer 42 underlying 
junction 47 extend over a small cavity or pit 24 in substrate layer 55 
which provides a measure of thermal insulation between junction 47 and 
layer 55. Layer 42 adds strength to the portion of subsensor 14 overlying 
cavity 24. The surface of subsensor 14 facing away from cavity or pit 24 
forms a radiation-responsive surface of subsensor 14. 
The reference junction 50 of each subsensor 14-17 is carried on an area of 
layer 42 which lies directly on and is in close thermal connection with 
support layer 42 and heat sink layer 19. There is good thermal 
conductivity between junction 50 and substrate layer 55, so that when 
radiation impinges on a subsensor, there will be a temperature 
differential between the sensing and reference junctions. 
To further increase the temperature differential between the sensing 
junction 47 on the one hand, and the reference junction 50 and the heat 
sink layer 19, during fabrication slots 35 and 36 are formed in layers 26 
and 42, and slots 37 and 38 are formed in layers 32 and 42 adjacent 
sensing junction 47. For maximum thermal isolation, It is important that 
slots 35 and 38 each straddle an edge of pit 24 as shown. This arrangement 
creates bridges 44 and 45 which suspend sensing junction 47 above pit 24 
so as to provide relatively long heat conduction paths of relatively small 
cross sectional area between layer 19 and sensing junction 47. There are 
other equally suitable configurations for providing thermal isolation of 
sensing junction 47 from heat sink layer 19. It should be noted that this 
thermal isolation feature is not the main feature of this invention. 
The four subsensors 14-17 comprising sensor 14 are shown in FIGS. 1 and 2 
in series connection with layer 26 connecting reference junction 50 of 
subsensor 14 to sensing junction 47 of subsensor 15. A bridge 38 forming a 
part of layer 26 connects reference junction 50 of subsensor 15 to sensing 
junction 47 of subsensor 17. Connectors 52 and 53 form vias passing 
through layers 42 and 19 to respectively connect sensing junction 47 of 
subsensor 14 and the reference junction of subsensor 16 to signal 
processing circuitry within layers 57. Connectors 52 and 53 may 
alternatively be used for connection to external circuitry. 
In operation, infrared radiation shown symbolically in FIG. 2 as rays 45 
impinges on both the sensing and the reference junctions of each of the 
subsensors 14-21. Because of the differing thermal conductivities between 
the sensing junction 47 of a subsensor 14-17 and heat sink layer 19, and 
the associated reference junction 50 of the same subsensor 14-17 and heat 
sink layer 19, a change in the intensity of infrared radiation which falls 
on the radiation-responsive surface of subsensor 14 and its sensing 
junction 47 and reference junction 50 will alter the temperature 
differential between the two junctions. As noted above, this temperature 
differential is very slight but is sufficient to create a small signal 
voltage between element 28 of subsensor 14 and element 32 of subsensor 16, 
which signal varies as a function of the change in intensity of the 
infrared radiation impinging on a sensor 11, 12, etc. The different 
temperature differentials within a pixel area are averaged by the signal 
resulting from the series connection between the subsensors 14-17. Where 
the changes in intensity of impinging infrared radiation on different 
sensors is different, the resulting voltage signals will also differ 
allowing an image of the infrared radiation pattern to be electronically 
constructed. In order to be able to provide real time images of the 
infrared radiation pattern, it is important that the thermal mass of the 
material suspended above cavities 24 be very small, so that the 
temperature of junction 47 can change very quickly relative to that of 
junction 50 in response to changes in the level of impinging radiation. 
FIG. 3 shows a series-parallel connection of four subsensors in each sensor 
11, 12, etc. Signal conductor 71 is connected to the connector 38 shown in 
FIGS. 1 and 2, and elements 28 and 34 of respective subsensors 14 and 16 
are connected electrically by a conductor 70 which may be a part of layers 
26 and 32. Connectors 53 and 71 for each sensor 11, 12, etc. then provide 
to a signal processor 75 the signal representing the changes in intensity 
of infrared radiation falling on the sensors 11, 12, etc. An advantage of 
such an arrangement is that if one of the subsensors should open 
electrically or be defectively manufactured, an attenuated signal will 
still be provided, which may then be enhanced if necessary in order to 
provide a reasonably good image. In this way, occasional defects in the 
subsensors will not result in high scrap rates for the complete array. It 
is also possible to connect all of the subsensors in parallel to further 
reduce the effect of electrically open subsensors. This configuration will 
reduce the internal impedance of the individual sensors.