Simultaneous two color IR detector having common middle layer metallic contact

An array of dual-band HgCdTe radiation detectors (10) wherein individual detectors include a first layer (14) having a first type of electrical conductivity and a bandgap selected for absorbing radiation within a first spectral band. The radiation detectors also each include a second layer (16) overlying the first layer. The second layer has a second type of electrical conductivity that is opposite the first type of electrical conductivity. Each radiation detector further includes a third layer (18) overlying the second layer, the third layer having the first type of electrical conductivity and a bandgap selected for absorbing radiation within a second spectral band. The first and second spectral bands are selected from SWIR, MWIR, LWIR, and VLWIR. The first, second and third layers are contained within at least one mesa structure (10a, 10b) that supports on a top surface thereof a first electrical contact (24) to the first layer and a second electrical contact (28) to the third layer. The at least one mesa structure further supports on a sidewall region (10b') thereof an electrical contact (30) to the second layer. The sidewall electrical contact is coupled to an electrically conductive bus that is conductively coupled in common to mesa structure sidewall electrical contacts of the plurality of radiation detectors. As a result, each radiation detector site is simplified in construction, and may be reduced in area over a site wherein a separate (third) contact, such as an indium bump, is required to contact the second layer.

FIELD OF THE INVENTION 
This invention relates generally to solid state radiation detectors and, in 
particular, to radiation detectors that are sensitive to radiation within 
a plurality of spectral bands or "colors". 
BACKGROUND OF THE INVENTION 
A desirable type of photodetector is a two-color infrared radiation (IR) 
detector having simultaneous sensitivity in two spectral bands. The 
spectral bands may include short wavelength IR (SWIR), medium wavelength 
IR (MWIR), long wavelength IR (LWIR), and very long wavelength IR (VLWIR). 
An array of two-color IR detectors may be employed in a number of imaging 
applications wherein it is required to simultaneously detect radiation 
within two spectral bands from a scene within a field of view of the 
array. By example, the array may simultaneously detect LWIR and MWIR, or 
LWIR and SWIR. 
Referring to FIG. 1, commonly assigned U.S. Pat. No. 5,113,076, issued May 
12, 1992, entitled "Two Terminal Multiband Infrared Radiation Detector" to 
E. F. Schulte, discloses a radiation detector having two heterojunctions 
that function in a manner analogous to two back-to-back photodiodes. Each 
of the photodiodes is responsive to radiation within a different IR 
spectral band, such as LWIR and MWIR. Detection of a particular wavelength 
band is achieved by switching a bias supply. Disclosed configurations 
include an n-p-n configuration, a p-n-p configuration, and a p-n-p-n 
configuration. 
Reference is also made to commonly assigned U.S. Pat. No. 5,149,956, issued 
Sep. 22, 1992, entitled "Two-Color Radiation Detector Array and Methods of 
Fabricating Same", by P. R. Norton. This patent describes the formation of 
a substantially continuous common layer between semiconductor regions 
responsive to different wavelength bands (e.g., MWIR and LWIR). A contact 
28 is made to the common layer for coupling same to readout electronics. 
Reference can also be made to the n-p+-n dual-band detector described by J. 
M. Arias et al. in the Journal of Applied Physics, 70(8), 15 Oct. 1991, 
pgs. 4820-4822. This triple-layer n-p+-n structure assumes that MWIR 
absorption occurs in the bottom n-type layer, and LWIR absorption in the 
top n-type layer. 
In U.S. Pat. No. 4,847,489, Jul. 11, 1989, entitled "Light Sensitive 
Superlattice Detector Arrangement with Spectral Sensitivity" Dietrich 
discloses a detector arrangement comprising a plurality of photosensitive 
detector elements. Each of the detector elements has a multilayer 
structure of alternating positively and negatively doped photosensitive 
semiconductor material having a superlattice structure. A control voltage 
is said to control spectral light sensitivity, and an optical filter 
arrangement is provided for dividing the photodetectors into an upper and 
lower effective spectral range group. 
In U.S. Pat. No 4,753,684, Jun. 28, 1988, "Photovoltaic Heterojunction 
Structures" Ondris et al. describe a three layer, double heterojunction 
Group II-VI photovoltaic structure. 
In Japanese Patent No. 55-101832, Aug. 4, 1980, Makoto Itou discloses, in 
the Abstract, an infrared detector comprised of n-type HgCdTe having 
electrodes 2 and 3 arranged on opposite surfaces. A polarity of a bias 
voltage is switchably coupled to the electrodes 2 and 3. This device is 
said to enable rays of wide wavelength ranges to be detected by only one 
semiconductor detector. 
General information regarding IR-responsive materials may be found in an 
article entitled "HgCdTe and Related Alloys" D. Long and J. L. Schmit, 
Semiconductors and Semimetals, Vol. 5, IR Detectors, Academic Press 1970. 
An article entitled "Some Properties of Photovoltaic Cd.sub.x Hg.sub.1-x Te 
Detectors for Infrared Radiation", by J. M. Pawlikowski and P. Becla, 
Infrared Physics, Vol. 15 (1975) pp. 331-337 describes photovoltaic p-n 
junction detectors constructed of HgCdTe crystals and epitaxial films. The 
authors report that the position of a photosensitivity maximum is shifted 
within a spectral region of 1-9 microns by changing a molar fraction of 
cadmium. 
Also of interest are commonly assigned U.S. patent application Ser. No. 
08/014,939, filed Feb. 8, 1993, entitled "Method of Fabricating a Two 
Color Detector Using LPE Crystal Growth", by P. R. Norton (allowed); and 
commonly assigned U.S. patent application Ser. No. 08/045,741, filed Apr. 
8, 1993, entitled "Dual-Band Infrared Radiation Detector Optimized for 
Fabrication in Compositionally Graded HgCdTe", by K. Kosai and G. Chapman 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved two color IR 
detector of reduced complexity over conventional approaches. 
It is another object of this invention to provide a two color IR detector 
array having a common contact for each radiation detector or pixel of the 
array. The common contact takes the form of a bus that is threaded through 
the array for contacting a common layer within a mesa structure or 
structures of individual ones of the detectors. In this manner an 
associated readout device is enabled to contact a middle, common layer of 
each detector at a periphery of the array, as opposed to within each 
pixel. 
The foregoing objects of the invention are realized by an array comprised 
of a plurality of radiation detectors, and by a method for fabricating the 
array. In a presently preferred embodiment individual ones of the 
radiation detectors include a first layer comprised of Group II-VI 
semiconductor material. The first layer has a first type of electrical 
conductivity and a bandgap selected for absorbing radiation within a first 
spectral band. The radiation detectors also each include a second layer 
overlying the first layer. The second layer is comprised of Group II-VI 
semiconductor material and has a second type of electrical conductivity 
that is opposite the first type of electrical conductivity. Each radiation 
detector further includes a third layer overlying the second layer, the 
third layer also being comprised of Group II-VI semiconductor material. 
The third layer has the first type of electrical conductivity and a 
bandgap selected for absorbing radiation within a second spectral band. 
The first and second spectral bands are selected from the group consisting 
essentially of SWIR, MWIR, LWIR, and VLWIR. 
The first, second and third layers are contained within at least one mesa 
structure that supports on a top surface thereof a first electrical 
contact to the first layer and a second electrical contact to the third 
layer. The first electrical contact includes a first indium bump and the 
second electrical contact includes a second indium bump for coupling to an 
external readout integrated circuit. 
In accordance with this invention the at least one mesa structure further 
supports on a sidewall thereof an electrical contact to the second layer. 
The sidewall electrical contact is coupled to an electrically conductive 
bus that is conductively coupled to mesa structure sidewall electrical 
contacts of the plurality of radiation detectors. 
The array of radiation detectors further includes a substrate having a 
first surface underlying a surface of the first layer. The substrate is 
selected from a material that is substantially transparent to 
electromagnetic radiation within the first and the second spectral bands. 
The array may further include an antireflection coating that is disposed 
upon a second surface of the substrate. The second surface is a radiation 
admitting surface of the array that is opposite the first surface. The 
array may further include, for each radiation detector, at least one 
microlens disposed adjacent to the second, radiation admitting surface of 
the substrate. The microlens focusses incident radiation into at least one 
of the first layer and the third layer. 
A dielectric layer, preferably a layer of wide bandgap semiconductor 
material, is disposed over surfaces of the at least one mesa structure. In 
order to avoid diode gating and leakage effects, a layer of high 
resistivity dielectric material is preferably interposed between the 
passivation layer and the electrically conductive bus.

DETAILED DESCRIPTION OF THE INVENTION 
The disclosures of the above-referenced commonly assigned U.S. Pat. Nos. 
5,113,076 and 5,149,956 are incorporated by reference herein in their 
entireties. 
As employed herein Short Wavelength Infrared (SWIR) radiation is considered 
to include a spectral region extending from approximately 1000 nanometers 
(nm) to approximately 3000 nm. Medium Wavelength Infrared (MWIR) radiation 
is considered to include a spectral region extending from approximately 
3000 nm to approximately 8000 nm. Long Wavelength Infrared (LWIR) 
radiation is considered to include a spectral region extending from 
approximately 7000 nm to approximately 14000 nm. Very Long Wavelength 
Infrared (VLWIR) radiation is considered to include a spectral region 
extending from approximately 12000 nm to approximately 30000 nm. Although 
the bands overlap to some extent, for the purposes disclosed herein the 
overlap is not considered to be significant. Also, as employed herein a 
semiconductor material is considered to exhibit significant responsivity 
to a given spectral band if the semiconductor material exhibits a maximum 
or substantially maximum photosensitivity to wavelengths within the given 
spectral band. 
Reference is first made to FIG. 2 for showing a cross-sectional view, not 
to scale, of an embodiment of a dual-band radiation detector 1. Detector 1 
includes a substrate 2 over which is grown a multilayered HgCdTe detector 
structure. The detector 1 may be grown by Liquid Phase Epitaxy (LPE), 
Molecular Beam Epitaxy (MBE), Vapor Phase Epitaxy (VPE), or by any process 
that is suitable for forming high quality layers or films of Hg.sub.1-x 
Cd.sub.x Te, where the value of x is selected to set the bandgap energy of 
the Hg.sub.1-x Cd.sub.x Te to provide a desired spectral response for a 
given layer or film. 
By example, suitable LPE growth techniques are described in the following 
two articles: T. Tung, M. H. Kalisher, A. P. Stevens and P. E. Herning, 
in: Materials for Infrared Detectors and Sources, Mater. Res. Soc. Symp. 
Proc., Vol. 90 (Mater. Res. Soc., Pittsburg, Pa., 1987), p. 321; and T. 
Tung, Infinite-Melt Vertical Liquid-Phase Epitaxy of HgCdTe from Hg 
Solution: Status and Prospects, J. Crystal Growth 86 (1988), pgs. 161-172. 
The multi-layered HgCdTe detector structure is comprised of a first layer 3 
which is an n-type MWIR-responsive radiation absorbing layer. Overlying 
the first layer 3 is a p-type second layer 4. Overlying the second layer 4 
is a third layer 5 which is a n-type LWIR-responsive radiation absorbing 
layer. Layers 3, 4 and 5 are differentiated into three mesa structures 6, 
7 and 8. 
Mesa structure 6 includes an indium bump 6' for connecting to a 
corresponding indium bump on a readout integrated circuit (not shown). 
Indium bump 6' is electrically connected to a metal lead 6" that makes an 
ohmic connection to the n-type MWIR-responsive layer 3. Mesa structure 7 
includes an indium bump 7' for connecting to a corresponding indium bump 
on the readout integrated circuit. Indium bump 7' is electrically 
connected to a metal lead 7" that makes an ohmic connection to the p-type 
middle layer 4. Mesa structure 8 includes an indium bump 8' for connecting 
to a corresponding indium bump on the readout integrated circuit. Indium 
bump 8' is electrically connected to the n-type LWIR layer 5. 
In operation, IR radiation is incident upon a bottom surface 2a of the 
substrate 2. Substrate 2 is thus comprised of a material, such as CdZnTe, 
that is substantially transparent to IR radiation having wavelengths of 
interest, or MWIR and LWIR in this case. An antireflection coating may be 
applied to the bottom surface of the substrate 2 to improve efficiency. 
Suitable reverse bias potentials are applied between the middle layer 
indium bump 7' and the MWIR layer indium bump 6' and the LWIR layer indium 
bump 8', enabling IR-induced photocurrents to be read out and subsequently 
processed by the readout integrated circuit (not shown). 
It can be appreciated that the presence of the mesa 7 and the indium bump 
7' increases both the complexity and the size requirement for the detector 
1. As such, when fabricating an array of detectors a minimum size of the 
individual detectors (pixels) cannot be reduced beyond what is required to 
accommodate the three mesas and the three indium bumps. 
This invention thus addresses and solves this problem, and enables a 
reduction in both the complexity and the minimum size of each detector or 
pixel element of an array of IR-responsive two color detectors. 
Reference is now made to FIGS. 3 and 4 for showing a first embodiment of a 
two color IR responsive radiation detector array in accordance with this 
invention. The detector array is comprised of a plurality of photodetector 
sites each comprised of a radiation detector or pixel 10 having, by 
example, a 50 micrometer pitch. As is evident from FIG. 3, the 
cross-sectional view of FIG. 4 is taken diagonally across a single one of 
the radiation detector pixels 10. 
The detector 10 is formed over a transparent substrate 12, e.g., a CdZnTe 
substrate. Over a surface of the substrate 12 is grown an n-type MWIR 
responsive radiation absorbing layer 14. Layer 14 has a thickness of 
approximately 11 micrometers and is doped n-type with, by example, indium 
at a concentration of approximately 3.times.10.sup.15 atoms/cm.sup.3. 
Overlying the first layer 14 is a p-type second layer 16. Layer 16 has a 
thickness of approximately 3.5 micrometers and is doped p-type with, by 
example, arsenic at a concentration greater than 10.sup.16 atoms/cm.sup.3. 
Overlying the second layer 16 is an n-type LWIR responsive radiation 
absorbing layer 18. Layer 18 has a thickness of approximately 8.5 
micrometers and is doped n-type with, by example, indium at a 
concentration of approximately 3.times.10.sup.15 atoms/cm.sup.3. 
It is pointed out the foregoing layer thicknesses, dopant types, and dopant 
concentrations are exemplary, and are not to be construed in a limiting 
sense upon the practice of the teaching of this invention. 
In the embodiment of FIGS. 3 and 4 the layers 14, 16 and 18 are 
differentiated with orthogonally disposed trenches to form a plurality of 
mesa structures, wherein a single photodetector is comprised of one (FIG. 
5) or two (FIGS. 3 and 4) mesa structures, such as the two mesa structures 
10a and 10b. The mesa structure 10a has a top surface and downwardly 
sloping sidewalls that terminate in the substrate 10 and within the MWIR 
layer 14. The mesa structure 10b also has a top surface and downwardly 
sloping sidewalls that terminate in the MWIR layer 14 and substrate 10. 
One sidewall of the mesa structure 10b is preferably formed to have a step 
10b' at the level of the p-type layer 16. The step 10b' facilitates access 
to the common p-layer 16, as will be described below. Overlying exposed 
surfaces of the mesa structures 10a and 10b is an electrically insulating 
dielectric layer, preferably a wide-bandgap passivation layer 20, such as 
a layer of CdTe. The passivation layer 20 beneficially reduces surface 
states and improves the signal-to-noise ratio of the detector 10. A 
suitable thickness for the passivation layer 20, when comprised of CdTe, 
is approximately 5000 .ANG.. 
A first contact electrode 22 is electrically coupled to the MWIR layer 14 
at contact region 22a and to a first bump 24. The bump 24 is preferably an 
indium bump, although other suitable metals or metal alloys can be 
employed. A second contact 26 is electrically coupled to the LWIR layer 18 
and to a second, preferably indium, bump 28. The indium bumps 24 and 28 
enable the array to be subsequently hybridized with an associated readout 
integrated circuit, and are cold welded to corresponding indium bumps on a 
surface of the readout integrated circuit. Techniques for hybridizing 
radiation detector arrays to readout integrated circuits are known in the 
art. 
In accordance with this invention, a third electrical contact 30 is 
electrically coupled to the p-type layer 16 upon the mesa sidewall step 
10b', and is conductively coupled to a common bus 32 that runs through the 
entire array of detectors 10. A significant advantage of the middle layer, 
sidewall contact 30 is that it requires significantly less space than 
would a separately provided mesa and indium bump (see, for example, the 
mesa 7 of FIG. 2). The bus 32 may be comprised of a layer of 
photolithographically defined and sputtered aluminum, and has the general 
form of a metallic grid that is threaded through the array of detectors 
10. The metallic grid or bus 32 is provided with an indium bump (not 
shown) at a peripheral region of the array for contacting the external 
readout integrated circuit (not shown). 
Exemplary dimensions for the detector 10 of FIGS. 3 and 4 are as follows. 
The top surface of mesa 10a is 12.times.16 micrometers and the base of the 
indium bump 24 is 10.times.8 micrometers. The MWIR contact metal 22 is 
8.times.16 micrometers and the contact portion 22a is 10.times.18 
micrometers. The top surface of mesa 10b is 16.times.16 micrometers and 
the base of the indium bump 28 is 12.times.12 micrometers. The LWIR 
contact metal 26 is 8.times.8 micrometers. An etched aperture made through 
the passivation layer 20 for the middle layer contact 30 is 3.times.25 
micrometers, and the width of the common bus 32 is 5 micrometers. 
FIG. 5 illustrates in cross-section a second embodiment of a detector 10' 
wherein a single mesa 11 is employed to support the indium bumps 24 and 
28. In this case the mesa 11 has a first sidewall step 11a for providing 
the contact 22a to the MWIR layer 14 and a second sidewall step 11b for 
providing the common contact 30 to the p-type layer 16. 
FIG. 5 also illustrates the use of a microlens 34 for focussing or 
concentrating incident IR radiation into at least the LWIR-absorbing layer 
18. Preferably, the microlens 34 also provides some focussing of the MWIR 
radiation into the layer 14. The microlens 34 may be a binary diffractive 
type and can be fabricated within or upon the radiation receiving surface 
12a of the substrate 12, or may be fabricated within or upon a separate 
transparent substrate that is subsequently disposed over the radiation 
receiving surface 12a. The microlens 34 may also be used with the detector 
10 of FIGS. 3 and 4. Techniques for specifying and fabricating such 
microlenses are known in the art. By example, reference may be had to an 
article entitled "Binary Optics" by W. B. Veldkamp et al., Scientific 
American, 5/92, pgs. 92-97; and to Report No SD-TR-87-54 (AD-A188 705), 
"Optical Technique for Increasing Fill Factor of Mosaic Arrays", W. A. 
Garber et al., prepared for the Space Division, Air Force Systems Command, 
and dated 10 Nov. 1987. The use of the microlens 34, in combination with 
an antireflection coating 12b, enables a significant portion of the 
incident MWIR and LWIR radiation to be converted to detectable signal 
charge. 
In operation, the LWIR photodetector comprised of a first p-n photodiode 
(layers 16 and 18) and the MWIR photodetector comprised of a second p-n 
photodiode (layers 16 and 14) are preferably operated simultaneously to 
read out the LWIR and MWIR photoinduced charge carriers. This is 
accomplished by selectively applying reverse bias potentials across the 
p-n junctions with the associated readout integrated circuit, and then 
reading out the charge signal. It is noted that the charge signals can be 
readout sequentially, as opposed to simultaneously, if this mode of 
operation is desirable. 
As is seen in FIG. 6, the array of detectors 10 can be viewed electrically 
as an array of back-to-back photodiodes having a common anode (the p-type 
common layer), wherein all of the anodes are electrically bussed together 
in common by the common grid metalization or bus 32, and wherein the 
cathodes are all separately contacted through the indium bumps 24 and 28 
from the readout integrated circuit. 
One area of possible concern in operating the detectors of FIGS. 3-5 is 
that the common middle layer grid metalization or bus 32 could degrade the 
MWIR junction by a diode gating effect as it runs over the edge of diode 
mesa. This effect would be more pronounced with a dielectric material such 
as SiO.sub.2. As such, the use of a wide bandgap semiconductor passivation 
material (e.g., CdTe) is preferred for the passivation layer 20. To even 
further inhibit any gating effect and/or electrical leakage through the 
passivation layer 20, it is preferred to apply an electrically insulating 
layer 36 of dielectric material, such as SiO.sub.2, between the surface of 
the passivation layer 20 and the metal grid 32. 
Although described in the context of a MWIR-LWIR radiation responsive 
device, it should be realized that the detector 10 can be constructed to 
be responsive to other combinations of wavelength bands, such as 
SWIR-MWIR, SWIR-LWIR, or MWIR-VLWIR. The arrangement of the radiation 
absorbing layers in these alternate embodiments is such that the incident 
radiation first encounters the wider bandgap semiconductor material. 
Furthermore, in these alternate embodiments the material of the substrate 
12 is selected so as to be substantially transparent to the wavelength 
bands of interest. Also, the substrate 12 can be comprised of a material 
other than a Group II-VI material (CdZnTe). For example, the substrate 12 
can be comprised of a Group IV material, such as Si, or a Group III-V 
material, such as GaAs. It also within the scope of this invention to 
provide a p-n-p layer arrangement within the at least one mesa structure, 
as opposed to the n-p-n layer arrangement that is shown in FIGS. 4 and 5. 
For this case the schematic diagram of FIG. 6 would be redrawn accordingly 
to show the back-to-back photodetectors as having their cathodes connected 
in common to the grid metalization or bus 32, and their anodes separately 
contacted through the indium bumps 24 and 28. 
Furthermore, it should be realized that the various material types, 
dimensions, and thicknesses are exemplary, and should not be read in a 
limiting sense upon the practice of the teaching of this invention. 
Thus, while the invention has been particularly shown and described with 
respect to preferred embodiments thereof, it will be understood by those 
skilled in the art that changes in form and details may be made therein 
without departing from the scope and spirit of the invention.