Non-contact interconnect for focal plane arrays

A focal plane array uses non-contact electrical interconnects instead of indium bumps. The interconnect bump comprises one or more vacuum microelectronics devices. The non-contact interconnect provides no themally conductive path between the detector and readout. For thermal detectors, the detector is not thermally connected to the readout and thus undergoes larger temperature changes in response to infrared radiation. For cryogenically cooled detectors, the detector and readout each have separate heat sinks with separate temperature controls. The readout may thus be operated at a higher temperature than the detector. The non-contact interconnect eliminates heat leakage from the readout to the detector enabling a thermal gradient to be maintained simultaneously with a net savings in refrigerator power. The non-contact interconnect also allows for differences in thermal expansion between the detector and the readout and thus increases the reliability of the focal plane array.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to focal plane array (FPA) technology. In 
particular, the present invention has utility for any spectral band FPA 
that benefits from the physical separation of the detector array from the 
readout circuit, e.g., pyroelectric and cryogenically cooled infrared 
FPAs. 
2. Background Art 
A hybrid FPA is comprised of an array of detectors and a readout circuit 
for sensing the photon or thermally generated charge on the detectors. 
Conventional FPAs use physically contacting interconnects, such as indium 
bumps, to connect the detector elements to the readout circuit. There are 
several undesirable consequences of this physical contact. 
The indium bumps create a high thermal conductance between the detector and 
readout substrates which force them to operate at near the same 
temperature. The substrates must be aligned and bonded (hybridized) in a 
difficult and low yield process. The indium bump bonds also suffer poor 
reliability particularly for materials of different thermal expansion 
coefficients. 
Thus, there is a great need for an interconnect for focal plane arrays 
which does not create a high thermal conductance between the detector and 
readout substrates. An interconnect for focal plane arrays is also needed 
which substantially eliminates poor reliability caused by structural 
failures. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a focal plane array is disclosed 
having a non-contact interconnection means for interconnecting the 
detector and readout which has no thermally conductive path between the 
detector and readout. For thermal detectors, the detector is essentially 
free standing being suspended from its edges. For cooled detectors, 
instead of using the readout as a heat sink for the detector, the detector 
and readout each has its own heat sink with its own temperature control. 
The readout may thus be operated at a higher temperature than the 
detector. The heat sink for the detector is made in such a way as to not 
obscure the active area of the detector, that is, it surrounds the active 
area of the detector. The non-contact interconnect means is employed to 
eliminate heat leakage from the readout to the detector thereby enabling a 
thermal gradient to be maintained simultaneously with a net savings in 
refrigerator power. 
The non-contact interconnect means comprises, in one embodiment, one or 
more vacuum microelectronics devices (VMDs) which produce an electron beam 
current when biased so that electrons tunnel from the VMD cathode. The 
basic VMD structure consists of a sharply pointed cathode tip in the 
vicinity of a surrounding gate electrode. When the VMD cathodes are 
fabricated on the detector array, the electron beam current is modulated 
by the infrared ("IR") generated electrons from each detector. When the 
VMD cathodes are fabricated on the readout, the electron beams serve to 
reset each detector of the array producing a displacement current 
proportional to the IR generated charge. 
By eliminating the physical contact between the detector and readout, an 
FPA is provided wherein the detector and readout operate at different 
temperatures, only gross alignment between the substrates is required, and 
the poor yield and reliability problems, due to failure of the indium bump 
bonds, are eliminated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, there is shown an enlarged side view of a slice 
through a portion of a conventional focal plane array 10. FIG. 1 shows a 
portion of one detector 11 from the array 10 that comprises a plurality of 
such detectors 11. The detector 11 may be a blocked impurity band (BIB), 
photovoltaic, photoconductive, Schottky barrier, pyroelectric, or 
bolometric detector material. The detector 11 normally detects infrared 
radiation from above as seen in FIG. 1. An electrical charge developed on 
the detector 11 is collected by a readout 12. In the conventional focal 
plane array 10, the charge is coupled from the detector 11 to the readout 
12 by an indium bump interconnect 13. 
Heat generated in the readout 12 is thermally coupled to the coldest stage 
of a refrigerator used to cool the focal plane array 10. FIG. 1 shows the 
readout 12 disposed on a readout heat sink 14 and the heat flow lines are 
indicated by arrows 15. The indium bump interconnect 13 that interconnects 
the detector 11 to the readout 12 has a high thermal conductance. 
Accordingly, the detector 11, the indium bump interconnect 13, and the 
readout 12 all operate at substantially the same temperature. More 
particularly, the readout 12 acts as the heat sink for the detector 11. 
For example, detector 11 made of a bolometric material produces an 
electrical signal proportional to its change in temperature resulting from 
IR exposure. When connected to the readout 12 by a high thermal 
conductance indium bump 13, the detector cannot undergo a large 
temperature change, thus its response is limited by the indium bump 
interconnect. 
Also, typically the desired operating temperature for a BIB detector 11 is 
approximately 10-20 degrees Kelvin. Unfortunately, this temperature is too 
cold for the readout 12 which typically provides optimum performance at a 
temperature above 40 degrees Kelvin. The noise generated by the readout 12 
typically increases monotonically as the temperature thereof is decreased 
for temperatures below about 50 degrees Kelvin. Also, the heat generated 
by the readout 12 places a significant load on the refrigerator. Thus, 
conventional interconnect arrangements as illustrated in FIG. 1 result in 
a focal plane array 10 that is limited by the noise and power dissipation 
of the readout 12. 
The indium bump 13 is also a potential point of failure in the electrical 
connection between the detector 11 and the readout 12. For example, in 
focal plane arrays where the detector 11 is made of mercury cadmium 
telluride and the readout 12 is made of silicon, the two materials 
contract to different sizes when cooled to an operating temperature which 
typically is 77 degrees Kelvin. This creates stress across the indium bump 
13 that can cause detector damage, stress related detector noise, and 
indium bump failure. 
Referring now to FIG. 2a and FIG. 2b, there is shown an improved focal 
plane array 20 constructed in accordance with the principles of the 
present invention. FIG. 2a is an enlarged side view of a slice through a 
portion of the focal plane array 20 showing a portion of a detector means, 
a portion of a readout means and a non-contact interconnect means for 
interconnecting the detector means and readout means. The detector means 
is any suitable detector for receiving radiation and generating a signal 
in response thereto. In the embodiment of FIG. 2a, the detector means is 
illustrated by detector 21 which is a thermally sensitive pyroelectric or 
bolometric material and has no detector heat sink. The readout means is 
any suitable means for receiving the signal generated by the detector, 
such as readout amplifier or transimpedience amplifier. In the embodiment 
of FIG. 2a, the readout means is illustrated by readout 22 and senses the 
photon or thermally generated charge on the detector means. 
The focal plane array of the present invention further comprises means for 
supporting the detector means and readout means in close proximity with 
each other and without physical contact. In the embodiment of FIG. 2a, the 
readout 22 is in intimate thermal contact with its heat sink 23 and 
detector 21 is suspended from its edges in any suitable manner. Arrows 26 
show the heat flow from the detector 21. Arrows 27 show the heat flow from 
the readout 22 to the readout heat sink 23. 
The focal plane array also comprises a non-contact interconnect means which 
is any suitable means for causing the signal generated by the detector 
means to be readout, without physical contact between the detector and the 
readout. In the embodiment of FIG. 2a, the non-contact interconnect means 
25 comprises one or more vacuum microelectronics devices ("VMD") which 
produce electron beams 24, when biased, which are proportional to the 
intensity of the impinging radiation on the detector 21. These will be 
described in more detail below. 
In the embodiment of FIG. 2b, a detector array 30 has a separate detector 
heat sink 29 and a separate readout heat sink 23. In this embodiment of 
the invention, the detector 21 is an IR photon sensitive BIB, 
photovoltaic, or photoconductive detector, which requires cooling. The 
readout 22 is in intimate thermal contact with its heat sink 23, while the 
detector 21 is in intimate thermal contact with its heat sink 29. The 
detector heat sink 29 is made in such a way as to not obscure the active 
area of the detector 21, that is, the heat sink 29 is made annular in 
shape so as to surround the active area of the detector 21. For example, 
in an embodiment shown in FIG. 2c, an annular shaped heat sink 29 
surrounds the detector element 21. Infrared radiation illuminates the 
detector 21 from the top as seen in FIG. 2b. 
Instead of using the readout 22 as the heat sink for the detector 21 as in 
FIG. 1, the embodiment of FIG. 2b illustrates that both the detector 21 
and readout 22 have their own heat sinks with their own temperature 
controls. The readout 22 may thus be operated at a higher temperature than 
the detector 21. Typically, for BIB detectors, the readout 22 is 
maintained at a temperature above 40 degrees Kelvin, while the detector 21 
is maintained at about 10 degrees Kelvin. 
Arrows 28 in FIG. 2b show the heat flow from the detector 21 to the 
detector heat sink 29. Arrows 27 show the heat flow from the readout 22 to 
the readout heat sink 23. 
In the embodiment of the invention illustrated in FIGS. 2a and 2b, the VMD 
electron beam current is modulated by the IR generated signal from the 
detector which is sensed by the readout 22. In this embodiment, the 
readout circuit may be one of many circuits commonly used to sense 
photogenerated current, such as switched FET. In another embodiment, an 
amplification device, such as a microchannel plate, can be used which 
excites phosphors on a display device. 
In accordance with the principles of the present invention, the non-contact 
interconnect 25 between the detector 21 and the readout 22 provides no 
thermally conductive path from the readout 22 to the detector 21 . High 
thermal resistance to heat flow 26 through the detector 21 in FIG. 2a 
allows thermal detectors, such as pyroelectric and bolometric detectors, 
to experience larger changes in temperature as a result of exposure to 
infrared radiation. This feature produces increased thermal detector 
response. The non-contact interconnect 25 also enables a desired thermal 
gradient to be maintained between the detector 21 and the readout 22 in 
cryogenically cooled systems. This feature reduces noise in the readout 22 
and permits a net savings in refrigerator power. 
In addition, the present invention eliminates the process of bonding the 
detector 21 to the readout 22, such as by the indium bump shown in FIG. 1. 
This increases yield and reduces the cost of focal plane arrays. Also, 
because there is no physical contact between the readout and detector, the 
problem of thermal expansion mismatch is eliminated and, thus, reliability 
is increased. 
Referring now to FIG. 3, there is shown another embodiment of a focal plane 
array 40. In this embodiment, a cathode of a non-contact interconnect is 
constructed on the readout 22, instead of the detector 21. FIG. 3 is an 
enlarged side view of a slice through a portion of the focal plane array 
40 showing a portion of a detector 21, a portion of a readout 22 and a 
non-contact interconnect means 41. Electron beams 42 emitted from the 
cathode are illustrated therebetween. In this embodiment, the benefits of 
thermal isolation provided by the non-contact interconnect are the same as 
those described above. In addition, a means of non-destructively testing 
conventional detector arrays, i.e. those with indium bumps as shown in 
FIG. 1, is provided. 
Referring now to FIG. 4, an enlarged side view of the non-contact 
interconnect 41, illustrated in FIG. 3, is shown. The non-contact 
interconnect means 41 comprises one or more vacuum microelectronics 
devices (VMDs) 41a. The VMD comprises a cathode 44, a gate 43, and an 
interlayer dielectric 45. In the illustrated embodiment, the cathode is 
fabricated having a very sharply pointed cathode tip 47. When the gate 43 
is biased to a voltage sufficiently high to cause electron tunneling from 
the cathode tip 47, an electron beam 42 is produced which traverses the 
space between the readout 22 and the detector 21 and impinges on the anode 
46. The voltage on the gate 43 must be positive relative to the cathode 
tip 47 and produce an electric field sufficient to cause tunneling from 
the cathode tip 47. In this embodiment, the electric field sufficient to 
cause electron tunneling is approximately 5.times.10.sup.7 V/cm. The 
current of the electron beam 42 from the VMD 41a is related to the 
gate-to-electrode voltage by the Fowler-Nordheim equation: 
EQU I.sub.c =aVgc.sup.2 e.sup.(b/Vgc) 
where I.sub.c is the cathode electron beam current 42, V.sub.gc is the 
gate-to-electrode voltage, and a and b are factors related to the cathode 
tip 47 geometry and materials. 
In this embodiment of the invention, anode 46 also serves as an electrode 
of the detector 21. A charge builds up on the anode 46 which is 
proportional to the intensity of the impinging radiation on detector 21. 
The interconnect means is activated and the electron beam 42 acts to reset 
the detector 21 producing a displacement current proportional to the IR 
generated charge which can be sensed by a suitable amplifier circuit 49 
connected to the detector bias electrode 48. An IR image can be 
constructed, for example, by matrix switching an array of non-contact 
interconnects, sensing the current on the detector bias electrode with a 
suitable amplifier, and using the signals to modulate the intensity of a 
display device such as a cathode ray tube. 
In an alternate embodiment of the present invention, the non-contact 
interconnect means utilizes an ion beam or a photon beam, i.e. a laser 
beam, to cause the charge generated on the detector means to be measured. 
In still another embodiment, the non-contact interconnect means 
interconnects the signal generated by the detector means to the readout 
means through an electric field, such as by capacitance coupling. Also, 
the non-contact interconnect means comprises magnetic coupling. 
Referring now to FIG. 5, a focal plane array 60 is illustrated in 
accordance with another embodiment of the present invention. Focal plane 
array 60 comprises a VMD 61 constructed so that a gate electrode 43 is 
connected to a detector electrode 62. In the embodiment shown in FIG. 5, 
the VMD structure 61 is fabricated by depositing an additional dielectric 
layer 63 on the detector electrode 62 and etching a via 64 through 
dielectric layers 45 and 63 so that contact is made between the gate 
electrode 43 and the detector electrode 62. In this embodiment, the 
non-contact interconnect means is connected so that the IR generated 
signal modulates the voltage on the gate of the VMD. Because the electron 
beam current 42 is related to the voltage on the gate 43 as described 
above, the non-contact interconnect means can be used to amplify the 
photocurrent before it is sensed by the readout circuit. This 
amplification results in higher sensitivity. 
Thus, there has been described a new and improved interconnect for an 
infrared focal plane array. The lack of physical contact between the 
detector and readout of the present invention enables the two devices to 
operate at different temperatures. This increases the responsivity of 
thermal detectors by allowing larger temperature changes as a result of 
exposure to IR. For cryogenically cooled detectors, the readout of a focal 
plane array is enabled to operate at a higher temperature which is more 
desirable, while allowing the detector to operate at its optimum 
temperature. This results in less readout noise. The readout is maintained 
at a higher temperature than in a conventional focal plane array. This 
reduces the required refrigerator capacity, thus effecting power and 
weight savings. Since readout noise typically increases monotonically as 
the temperature is decreased, for temperatures below about 50 degrees 
Kelvin, by operating the readout at a higher temperature than the 10-20 
degrees Kelvin typical of arsenic doped silicon (Is:As) BIB or gallium 
doped silicon (Is:Ga) BIB detectors, the present invention provides for 
significant noise reduction. 
The lack of physical contact allows the detector and readout to expand to 
different sizes without stress or failure of the electrical connection 
provided by the non-contact interconnects. Thus, the reliability of the 
focal plane is increased. And, because the hybridization process is 
eliminated, the focal plane array production cost is reduced and the yield 
is increased. 
It is to be understood that the above-described embodiment is merely 
illustrative of some of the many specific embodiments which represent 
applications of the principles of the present invention. Clearly, numerous 
and other arrangements can be readily devised by those skilled in the art 
without departing from the scope of the invention.