I104 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 38. NO. 5. MAY 1991

256 x 256 Hybrid HgCdTe Infrared Focal Plane Arrays

Robert B. Bailey, Lester J. Kozlowski, Jenkon Chen, Duc Q. Bui, Kadri Vural, Dennis D. Edwall, R. Victor Gil, A. Ben Vanderwyck, Edward R. Gertner, and Michael B. Gubala

Abstract-Hybrid HgCdTe 256 X 256 focal plane arrays have been developed to meet the sensitivity, resolution, and field-of- view requirements of high-performance medium-wavelength infrared imaging systems. The detector arrays for these hy- brids are fabricated on substrates that reduce or eliminate the thermal expansion mismatch to the silicon readout circuit. The readouts are foundry-processed CMOS switched-FET circuits that have charge capacities greater than lo’ electrons and a single video output capable of 20-MHz data rates. The high quantum efficiency, tunable absorption wavelength, and broad operating temperature range of these large HgCdTe staring fo- cal plane arrays give them significant advantages over compet- ing Sensors. The mature PACE-1 technology, using sapphire detector substrates, has demonstrated 256 x 256 MWIR arrays with mean laboratory NETD’s of 9 mK for a 4.9-pm cutoff wavelength, 40-pm pixel size, and 80-K operating temperature. RMS detector response nonuniformities are less than 4%, and pixel yields are greater than 99%. The newly developed PACE-3 process uses silicon for the detector substrate to completely eliminate the thermal mismatch with the silicon readout cir- cuit. It has the potential for similar performance in even larger arrays sizes. A 640 x 480 hybrid array is under development.

I . INTRODUCTION YBRID photovoltaic HgCdTe 256 x 256 focal plane H arrays (FPA’s) have been fabricated and field tested

to demonstrate the advantages of large staring infrared de- tector arrays in high-performance thermal imaging sys- tems. HgCdTe is widely used for infrared detectors be- cause of its high quantum efficiency, tunable absorption wavelength, and wide operating temperature range [I], [2]. Two-dimensional arrays of infrared detectors oper- ated in a staring mode have sensitivity advantages over optically scanned linear arrays and can reduce system complexity, cost, and weight [2]. Very large HgCdTe staring FPA’s provide a combination of sensitivity, res- olution, and field of view that are unmatched by other infrared imaging sensor technologies.

The hybrid FPA architecuture [3] is shown in Fig. 1. The detector array and the readout circuit are fabricated on different substrates. The two substrates are mated through an array of indium columns that electrically con-

Manuscript received August 31. 1990; revised December 3. 1990. R. B. Bailey, L. J . Kozlowski, J . K. Chen, D. Q. Bui, K. Vural, D. D.

Edwall, R. V. Gil, A. B. Vanderwyck, and E. R. Gertner are with the Rockwell International Science Center, Thousand Oaks, CA 9 1360.

M. B. Gubala is with the Rockwell International Tactical Systems Di- vision, Duluth. GA 30136.

IEEE Log Number 9143229.

nect each detector to an input cell of the readout circuit. This architecture permits independent optimization of the materials parameters and device fabrication processes for the detectors and the signal-processing electronics. In- frared radiation is incident through the transparent detec- tor substrate, and the optical fill factor is nearly 100%. Fig. 2 shows a photograph of a packaged 256 X 256 hy- brid FPA. The pixel size is 40 pm X 40 pm. The 10.24 mm X 10.24 mm HgCdTe detector array is bonded to a slightly larger silicon readout circuit that intercon- nects to 16 of the 68 package pins.

The key to producing the very large hybrid HgCdTe FPA’s required for high-resolution thermal imagers is the use of substrates other than CdTe for fabrication of the HgCdTe detector array. Section I1 describes Rockwell’s Producible Alternative to CdTe for Epitaxy (PACE) tech- nology [4]-[6] that has produced 256 x 256 hybrid FPA’s using both sapphire and silicon for the detector substrates [7]-[9]. These are the largest HgCdTe FPA’s that have been reported. They have been optimized for either the 1- 2.5-pm short-wavelength infrared (SWIR) band or the 3- 5-pm medium-wavelength infrared (MWIR) band. Sec- tion I11 discusses the foundry-processed CMOS readout circuits used for high background MWIR imaging. Sec- tion IV presents performance and reliability data for 256 x 256 MWIR FPA’s.

11. HgCdTe PHOTOVOLTAIC DETECTOR ARRAYS

CdTe and CdZnTe are the most widely used substrates for back-side-illuminated HgCdTe detector arrays [lo]. Their metallurgical compatibility permits the growth of very low defect density epitaxial layers of HgCdTe. How- ever, the available substrates are relatively small, expen- sive, and fragile. Also, their thermal expansion coeffi- cients [ l l ] are much larger than those of the silicon substrates used for the readout circuits of hybrid FPA’s. This thermal expansion mismatch causes a lateral dis- placement of the detector and readout Substrates when a hybrid FPA is cooled to its operating temperature, typi- cally 77 to 120 K. The relative displacement of the sub- strates is largest at the edges of the FPA and is propor- tional to the size of the array. The resulting stress on the indium columns connecting the two substrates causes them to fail if the array is too large. The largest hybrid FPA’s fabricated with CdTe or CdZnTe detector substrates have

0018-9383~91/0500-1104$01.00 0 1991 IEEE

BAILEY et al . : HYBRID HgCdTe INFRARED FOCAL PLANE ARRAYS I105

\ ..-. ..-..-- RAD I AT ION

SI SIGNAL PROCESSOR

HgCdTe EPITAXIAL LAYER

INDIUM BUMP

Fig. 1 . Hybrid focal plane array architecture.

IMPLANTATION I I I I L N +

ZnS

(3) PASSIVATION ?, ZnS OR Cdre .. .

RADIATION

N CONTACT HOLE

(4) (5) (6)

Fig. 3 . HgCdTe MWIR processing sequence.

Fig. 2 . Photograph of packaged 256 X 256 hybrid FPA.

128 x 128 pixel formats. To produce larger hybrid FPA’s and to lower their cost, the PACE-I process was devel- oped using rugged, inexpensive sapphire detector sub- strates whose thermal expansion mismatch to silicon is 40% less than that of CdTe.

The PACE-1 fabrication process [4], [5] is summarized in Fig. 3. A CdTe epitaxial layer is first grown by MOCVD on a 2-in-diameter sapphire substrate and then etched back to a thickness of approximately 3 pm. A p-type HgCdTe layer is then grown from the Te-rich melt using liquid-phase epitaxy. The composition x of the Hg,_,Cd,Te alloy is adjusted to tune the infrared absorp- tion cutoff wavelength to the desired value. A layer thick- ness between 8 and 12 pm is chosen to maximize the quantum eqciency in the wavelength band of interest. An

array of n-on-p junction diodes is formed by boron-ion implantation and thermal annealing. The planar junctions are passivated with a ZnS or CdTe film. Metal contacts and indium columns are then deposited and patterned. A matching array of indium columns is patterned on the readout circuit before it is hybridized to the detector ar- ray.

Infrared absorption by sapphire limits the use of PACE-1 FPA’s to wavelengths less than 5 . 5 pm. For the 8-12-pm, long-wavelength IR band, the PACE-2 process has been developed with detectors fabricated on GaAs substrates [12]-[14]. Both the CdTe buffer layer and the HgCdTe detector layer in this process are grown by MOCVD. High-performance 64 x 64 LWIR PACE-2 ar- rays have been fabricated. Because GaAs has a thermal expansion comparable to CdTe, these PACE-2 FPA’s will have the same size limitations as CdTe based hybrids un- less GaAs readout circuits are used

Several laboratories [ 141-[ 161 have begun fabricating HgCdTe detector arrays on silicon substrates with the same thermal expansion coefficient as the silicon readout circuit. Rockwell’s PACE-3 process [ 141 uses 3-in-di- ameter GaAs/Si substrates purchased from Kopin Corp. [17] as the starting material. The thin buffer layer of GaAs

I106 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 38. NO. 5 . MAY 1991

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Mean I 0.609 - stddev .0.0234

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0 0 0.5 1 1.5 0.5 0.55 0.6 0.65 0.7 2

Dark current (loe e-/-)

Fig. 6 . Dark-current histogram of MWIR PACE-I FPA. Fig. 4 . QE histogram of 256 X 256 MWIR PACE-I FPA.

Quantum Efficiency

Fig. 5 . QE histogram of 256 X 256 MWIR PACE-3 FPA. Fig. 7 . Dark current versus temperature for MWIR PACE-I FPA.

on these substrates overcomes the nucleation and anti- phase domain problems encountered when HgCdTe is grown directly on silicon. The PACE-3 CdTe and Hg- CdTe layers are grown by MOCVD.

Figs. 4 and 5 show histograms of the quantum efficien- cies (QE) measured on MWIR PACE-1 and PACE-3 256 x 256 hybrid FPA's operated at 80 K. Both arrays were fabricated with a 40-pm detector spacing and the detector process outlined in Fig. 3. The PACE-1 array had a cutoff wavelength of 4.9 pm and was illuminated through a 4.3- 4.8-pm bandpass cold filter. In this band, QE measure- ments were not affected by variations in the cutoff wave- length, which are typically less than 0.1 pm. The PACE-3 array had a cutoff wavelength of 4.6 pm and was mea- sured without an IR filter. The short wavelength cutoff of 0.8 pm for this measurement was determined by IR ab- sorption in the CdTe buffer layer of the detector substrate. The QE measurements in Figs. 4 and 5 show the relative maturities of the PACE-1 and PACE-3 processes. The mean PACE-1 QE of 60.9% and rms nonuniformity of 3.8% of the mean show that the HgCdTe thickness, junc- tion depth, surface recombination rate, and carrjer life- time are well controlled so that most photoelectrons are

The mean R& values measured on test diodes at 77 K were 3 x lo6 fl cm2 for PACE-1 and 2.4 X lo4 Q - cm2 for PACE-3. The PACE-3 detectors are surface limited. A dark current histogram of a PACE-1 256 X 256 FPA operated at 77 K with a reverse detector bias of approxi- mately 0.05 V is shown in Fig. 6. The mean dark current is 3.5 x lo5 e- /s (56 fA). The peak of the distribution is at 1.5 x 10' e - / s (24 fA). The temperature depen- dence of the dark current, shown in Fig. 7, suggests that the detectors are limited by generation-recombination cur- rents in the 77-125-K temperature range. Other mecha- nisms, such as tunneling, are dominant at lower temper- atures [ 181 and account for the broad tail of high dark- current pixels in Fig. 6.

PACE- 1 detector noise at 80 K for a 5-pm cutoff wave- length is typically lower than 0.3 fA/Hz'/* at 1 Hz under moderate bias (- 50 mV). Excess detector low-frequency noise is observed only under conditions of severe electri- cal stress (prolonged exposure to detector reverse bias of greater than 200 mV).

111. 256 x 256 CMOS READOUT CIRCUITS

collected by the detector junction. The PACE-3 QE of 24.6% is relatively low but is expected to improve rapidly as the PACE-3 process is optimized. The yield of pixels with QE greater than half the mean was 99.3% for the PACE-1 array and 98.4% for the PACE-3 array.

A schematic diagram of the 256 x 256 readout used in MWIR PACE-1 and PACE-3 hybrid FPA's is shown in Fig. 8. It consists of an aray of direct injection input cir- cuits coupled to a single output via a switched FET ar- chitecture. Each unit cell in the readout has an input FET,

BAILEY et al . : HYBRID HgCdTe INFRARED FOCAL PLANE ARRAYS

SYNC

Output ampliier

Mirror

Fig. 8 . 256 X 256 direct-injection readout circuit schematic

an integration capacitor, a reset FET, and a cell access FET. The signal voltage from each pixel is successively read through the cascaded source followers by clocking the appropriate switches. Dynamic CMOS shift registers generate the various clock signals so that only three ex- ternal clocks are required. The only critical adjustment needed to operate an FPA is the direct injection gate bias, labeled IG in Fig. 8. This sets the reverse bias of the de- tector diodes. The integration time of the array is adjust- able from 0.4 to 99% of the frame time.

Two versions [8] of the 256 X 256 direct injection readout design, each with a 40 pm X 40 pm cell size, have been fabricated at a commercial silicon foundary. The initial design used a 2-pm CMOS process to achieve a 1.0 x lo7 e- well capacity and 3.7-MHz data rate. An experimental lot of this readout design was processed with a thinner gate oxide to increase the well capacity to 2.3 X lo7 e-. A redesigned version, optimized for speed and testability, was fabricated with a 1.5-pm CMOS process. Its maximum data rate is greater than the 20-MHz limit of the test electronics.

IV. HYBRID FPA PERFORMANCE AND RELIABILITY Fig. 9 is a DSTAR histogram of an MWIR 256 x 256

FPA measured at a background flux of 1 x pho- tons/cm2 * s, an integration time of 4 ms, and an oper- ating temperature of 80 K. The mean value of 1.17 x 10l2 cm - Hz'12/W corresponds to about 90% of BLIP. Near- BLIP operation is typically achieved in MWIR imaging systems with HgCdTe FPA's operated at temperatures up

TEST NO. 241 -T

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Fig. 9 . DSTAR histogram for representative 128 X 128 quadrant of 256 x 256 FPA at 1 x I O i 4 photons/cm* . s and 4-ms integration time.

TEST NO. 241. 200Ol I I

M€AM = 9.05 STWEV = 0.942 S T W E V I U W = 0.104 163190LITOF 16384199.6%1

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Fig. 10. NETD histogram for representative 128 x 128 quadrant of 256 X 256 FPA at mean integrated signal level of 10 000 000 electrons.

to 120 K. In this temperature range, detector dark-current shot noise and 1 /f noise are less than the photon noise from the 300-K background signal. Fig. 10 shows a mea- surement of the noise equivalent temperature difference (NETD) of a 256 X 256 FPA. This measurement was done with an integrated signal level of lo7 e- using the low-speed thin-oxide readout circuit described in Section 111. The mean NETD is 9 mK. The background-limited NETD decreases as the square root of the integrated sig- nal level, so an NETD of 7.7 mK is predicted for the new high-speed readout that has a 37% greater well capacity. In both the DSTAR and NETD measurements the non- uniformity in sensitivity was limited by the measurement since only 32 frames were used to quantify pixel noise. Other measurements using smaller randomly selected samples had a nonuniformity less than' 5 % of the mean.

The sensitivity of a staring HgCdTe FPA will be lim- ited by the temporal noise shown in Fig. 10 only if a cal- ibration procedure is used to remove the spatial noise caused by pixel-to-pixel gain and offset variations. Gain and offset corrections can be done in real time by image processing circuitry based on calibraton measurements done with the FPA uniformly illuminated by blackbody

I 108 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 38. NO. 5 . MAY 1991

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5000

4000

3000

2000

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Source temp (K)

Fig. 12. Histogram of source temperatures measured by 256 x 256 PACE-I FPA calibrated at 300 and 310 K .

Fig. 13. Image from 256 X 256 PACE-I MWIR FPA taken with 4.3-4.8-pm 1R filter and f / 2 optics.

sources at two temperatures. The effectiveness of this two- point calibration procedure depends on the response lin- earity of the FPA, shown in Fig. 11. The high detector impedance and nearly ideal input MOSFET subthreshold

slope of the readout circuit combine to produce a response linearity greater than 99.5%. Fig. 12 shows a histogram of the source temperatures measured by a 256 x 256 MWIR FPA using gain and offset coefficients calculated

BAILEY el al . : HYBRID HgCdTe INFRARED FOCAL PLANE ARRAYS

from eight-frame averages of the signals measured at source temperatures of 300 and 310 K. The blackbody source temperature was set near the middle of the calibra- tion range for this measurement. The 8.3-mK rms devia- tion from the mean source temperature is 0.17% of the difference between the calibration and measurement tem- peratures. Most of this nonuniformity is caused by the 9-mK temporal NETD of the detectors. The calibration procedure has reduced the fixed pattern spatial nonuni- formity of this array from 4 % , measured without correc- tion in Fig. 4, to less than 0.17%.

High-resolution imaging video tapes have been re- corded in field tests of both PACE-1 and PACE-3 256 X 256 detector arrays. Fig. 13 is a photograph of a TV im- age from a PACE-1 array operating in the 4.3-4.8-pm wavelength band. Light areas correspond to objects emit- ting or reflecting large infrared signals. The minimum re- solvable temperature measured with this imaging system was 0.025 K for a test structure with a spatial frequency of 22 temperature cycles per millirad.

There is no evidence that PACE-1 256 X 256 hybrid FPA’s have reliability problems caused by stress on the indium columns when the devices are cooled to low tem- peratures. Several dozen 256 X 256 MWIR and SWIR PACE-1 arrays have been delivered to customers since 1988 for use in thermal imaging systems, missile seeker demonstrations, and infrared astronomical telescopes. Many of these are used routinely and have undergone more than 50 thermal cycles. There are no reports of sig- nificant degradation in detector performance or failure of the indium columns due to thermal cycling. Characteri- zation of laboratory test structures has shown no signifi- cant failure of indium columns after 100 thermal cycles. The maximum number of thermal cycles a PACE-1 FPA has undergone in the laboratory before delivery to a cus- tomer is 17. Quantum efficiency measurements, like the ones shown in Figs. 4 and 5 , showed during the first cool- down of this device that 65 282 of the 65 536 pixels (99.61 %) had a measured QE greater than half the mean. During the seventeenth cool-down the number of pixels with at least half the mean QE increased by 11 to 65 293. This small change is insignificant and comparable to the measurement error. It indicates that stress caused by the sapphire-to-silicon thermal expansion mismatch does not cause reliability problems for 256 x 256 arrays with 40-pm pixels.

V. CONCLUSIONS Reliable, high-performance 256 x 256 hybird HgCdTe

FPA’s have been produced using PACE detector technol- ogy. They offer a combination of sensitivity, resolution,

I IO9

and field-of-view unmatched by competing sensor tech- nologies. The key to producing these large hybrid arrays is the use of detector substrate materials, such as sapphire and silicon, that reduce or eliminate the thermal expan- sion mismatch to the silicon readout circuit. Successful imaging demonstrated with a thermally matched PACE-3 256 X 256 hybrid shows that high-sensitivity IR imaging with TV-quality resolution will be possible in the near future. A 640 x 480 hybrid FPA is under development.

REFERENCES

P. W. Kruse, “The emergence of HgCdTe as a modern infrared sen- sitive material,” in Semiconducrors and Semimetals, vol. 18, R. K . Willardson and A. C. Beer. Eds. New York: Academic. 1981. p. I . R. Balcerak, J. F. Gibson, W. A. Gutierrez, and J . H. Pollard, “Ev- olution of a new semiconductor product: HgCdTe focal plane ar- rays,” Opt. Eng.. vol. 26, p. 191, 1987. J . P. Rode, “HgCdTe hybrid focal plane,” Infrared Phys.. vol. 25. p. 443. 1984. E. R. Gertner, W. E. Tennant, J. D. Blackwell, and J. P. Rode, “HgCdTe on sapphire-A new approach to infrared detector arrays,“ J . Cryst. Growth, vol. 72, p. 462. 1985. L. 0. Bubulac, “The role of epitaxy and substrate on junction for- mation in ion-implanted HgCdTe.” J . Crysral Grovtith. vol. 72. p. 478, 1985. D. D. Edwall, J. Bajaj, and E. R . Gertner, “Material characteristics of metalorganic chemical vapor deposition HgCdTe/GaAs/Si.” J . Vac. Sci. Techno/.. vol. A8. p. 1045, 1990. K. Vural, “Mercury cadmium telluride short- and medium-wave- length infrared staring focal plane arrays,” Opr. Eng . , vol. 26, p. 201, 1987. L. J. Kozlowski er a l . , “256 x 256 PACE- I PV HgCdTe focal plane arrays for medium and short wavelength infrared applications.” Proc. SPIE, vol. 1308, 1990. K. Vural, M. Blessinger, and J . Chen. “256 X 256 short wavelength HgCdTe focal plane arrays,” presented at Conf. Lasers and Electro- Optics, Anaheim, CA 1990. S. Sen et al . , “Crystal growth of large-area single-crystal CdTe and CdZnTe by the computer-controlled vertical modified-Bridgman pro- cess,” J . Crysral Growth, vol. 86, p. 1 I I , 1988. Y. S. Touloukian, Ed., Thermophysical Properties of Matter, vol. 13. New York: Plenum, 1977. D. D. Edwall, E. R. Gertner, and L. 0. Bubulac, “Material char- acteristics of HgCdTe grown by organometallic vapor phase epi- taxy,” J . Crysrul Growth, vol. 86, p. 240, 1988. L. 0. Bubulac et a l . , “P-on-n arsenic-activated junctions in MOCVD LWIR HgCdTe/GaAs,” Semicond. Sei. Technol.. vol. 5 , p. S45, 1990. D. D. Edwall, J . S . Chen. J . Bajaj, and E. R. Gertner, “MOCVD HgCdTe/GaAs for 1R detectors,” Semicond. Sci. Technol.. vol. 5 , p. 221, 1990. S. M. Johnson, M. H. Kalisher, W. L. Ahlgren, J . B. James. and C. A. Cockrun, “HgCdTe 128 X 128 infrared FPAs on alternative sub- strates of CdZnTe/GaAs/Si.” Appl. Phys. Lett . . vol. 56. p. 946, 1990. K . Zanio, et a l . , “HgCdTe on GaAs/Si for mid-wavelength infrared focal plane arrays,” Appl. Phys. Let t . , vol. 56. p. 1207. 1990. J. W. Lee, J. P. Salerno, R. P. Gale, and J. C . C. Fan, “Epitaxy of GaAs on Si: MBE and MOCVD,” in Materials Res. Soc. Sytnp. Proc. , vol. 91, 1987, p. 33. R . E. DeWames, G . M. Williams, J. G . Pasko. and A. H. B. Van- derwyck, “Current generation mechanisms in small band gap Hg- CdTe p-n junctions fabricated by ion implantation,” J . Crystal Growrh. vol. 86. p. 849, 1988.