An overview of integrated infrared (IR) array technology is presented. Although the array pixel formats are smaller, and the readout noise of IR arrays is larger, than the corresponding values achieved with optical charge-coupled-device silicon technology, substantial progress is being made in IR technology. Both existing IR arrays and those being developed are described. Examples of astronomical images are given which illustrate the potential of integrated IR arrays for scientific investigations

Goebel, J. H.

Mccreight, C. R.

English

NASA Technical Reports Server

I i 6 . >  NASA Technical Memorandum 88357 , L 
Integrated Infrared Array 
Technology 
J.H. Goebel and C.R. McCreight 
(NASA- IB-88357)  I N T E G R A T E D  IhEEieEED ALRRAY N87-1E475 
lECHNCLCGY ( # M A )  14 p CSCL 03A 
Unclas 
G3/89 43513 
October 1986 
I 
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National Aeronautics and 
Space Administration 
https://ntrs.nasa.gov/search.jsp?R=19870009042 2020-03-20T11:25:15+00:00Z
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NASA Technical Memorandum 88357 
. 
Integrated Infrared Array 
Technology 
J. H. Goebel, 
C. R. McCreight, Ames Research Center, Moffett Field, California 
October 1986 
National Aeronautics and 
Space Admin 1st ration 
Ames Research Center 
Moffett Field California 94035 
INTEGRATED INFRARED ARRAY TECHNOLOGY 
J. H. Coebel and C. R. McCreight 
NASA Ames Research Center 
Moffett Field, CA 94035 USA 
Abstract 
An overview of integrated infrared (IR) array technology is presented. 
Although the array pixel formats are smaller, and the readout noise of IR arrays 
is larger, than the corresponding values achieved with optical charge-coupled- 
device silicon technology, substantial progress is being made in IR 
technology. Both existing IR arrays and those being developed are described. 
Examples of astronomical images are given which illustrate the potential of 
integrated IR arrays for scientific investigations. 
Introduction 
Recent advances in microelectronics technology have made possible the 
development of multiple-element arrays of infrared (IR) detectors. Although 
these arrays are much smaller than the optical arrays which preceeded them (and 
which are the primary focus of this workshop), IR arrays have reached a 
significant level of sophistication, and have already produced some substantial 
scientific results. 
A brief outline of the terminology and technology of IR arrays will be given 
in this paper. It is assumed that the reader is familiar with optical 
charge-coupled-device (CCD) array concepts and technology, so emphasis will be 
placed on the unique aspects of IR-arrays. The material presented here is drawn 
primarily from the work of companies and investigators in the United States, 
although there are now important IR array evaluation and utilization projects 
under way in Europe and elsewhere in the world. 
Infrared Arrav Concepts and Terminology 
Infrared-detector arrays have evolved through a synthesis of discrete IR- 
detector technology and the CCD multiplexer schemes developed for optical 
arrays. Termed integrated arrays (since the detection and readout/multiplexing 
functions are performed in a integral package), these devices can in principle 
use a wide variety of intrinsic and extrinsic IR-sensitive detector materials 
(Fig. 1 ) .  The intrinsic materials such as InSb and HgCdTe are generally used in 
photovoltaic detectors; the extrinisic materials such as Si:Ca and Si:As are 
used in the form of photoconductors or photocapacitors. The crystal growth, 
material purity, and contacting techniques of discrete IR detectors have become 
quite advanced, and it is common for both discrete IR detectors and the detector 
elements of integrated arrays to perform in nearly ideal fashions.’ The 
limitations to sensitivity are commonly imposed by the readout, which in the I R  
has been accomplished with a number of approaches, including CCD, 
charge-injection device (CID), and switched field-effect-transistor (FET-) 
rnult iplexers. 
The early IR CCD arrays were produced in monolithic designs, with the 
detection and readout performed in a common (usually silicon) substrate. This 
0 
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Si Ge 
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GaSb InAs InSb 
m- Y 
II-PI HgCdTe 4 rn 
PbTe 
PbGeTe I PbSnTe 
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4 Se: X Cu Zn Be i’ 1 I 1 “PAd 
1 I I 1 I I I l l 1  I I I I I 1  I l l  1 
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Fig. 1 Long-wavelength cutoff of various intrinsic and extrinsic IR detector 
materials. 
2 
monolithic approach proved to be only partially successful, since the processing 
steps required by the multiplexer circuit produced low responsivity in the 
adjacent detector region of the substrate. Monolithic IR array technology has 
been superseded by hybrid techniques, wherein two separate substrates are used 
(Fig. 2). By designing and processing the detector and multiplexer substrates 
separately and "bump-bonding'' them together, optimum performance can be obtained 
from each. The indium bump technology is now well established, and interconnect 
yields of 100% for 32-x-32-element hybrid arrays are not uncommon. 
The size of integrated IR arrays is determined by the maximum wafer dimen- 
sion of uniformly doped, perfectly crystalline semiconductor material, and the 
design rules (i.e., minimum trace dimensions) for the transistors on the 
companion multiplexer chip. The latter constraint generally limits array pixel 
sizes to 250 urn. Infrared arrays with 64-x-64-element format are now comm0n,~1~ 
and 128-x-128-element devices are in the active laboratory-development stage. 
As just mentioned, the first multiplexed readouts for IR arrays were CCDs. 
Since many IR detectors must be cooled .to low (<20 K) temperatures, the 
surface-channel CCD structure must be used to avoid carrier freeze-out effects 
in silicon. Although CCD arrays offer large well capacities and good frequency 
response, they suffer from relatively high read-noise levels (-1000 e-). 
INFRARED 4 ILLUMINATION 
\ MULTIPLEXED .^ .-. .- 
Fig. 2 Hybrid IR detector array. 
3 
Accumulation-mode C I D  readouts were developed around a simpler (detector- 
substrate/oxide-layerhetal-gate) structure, and read noises in the 100-200 e- 
range have been achieved. 
elements in a row are connected in parallel, and charge-trapping in C I D s  has 
been found to limit their sensitivity under low-flux conditions. 
They suffer from higher capacitance, since all 
4 
At present, switched-FET multiplexers are used for  best sensitivity. In 
this device, each unit cell of the array includes an individual charge- 
integration and reset circuit, which typically involves 3-5 metal-oxide 
semiconductor field effect transistors (MOSFETs) behind each detector element. 
Figure 3 illustrates a common switched-FET [or direct-readout ( D R O ) ]  circuit. 
As with the other two types of multiplexers, a correlated-double-sampling 
technique is used to read out the array. Photocurrent and leakage accumulate on 
the gate of the sense MOSFET, and the correlated-double-sampled signal is 
proportional to the difference of the voltage samples taken just before and just 
after the reset pulse.5 The DRO arrays have achieved readout noises in the 
-100-e- range, and provide the ability to randomly access and nondestructively 
read the elements in the array. This technology is now the baseline for the 
VRESET "RESET VDD 
/ 
/ 
/ 
MOSFET 
1-1 ![' F4;;2b-l CONVERTER 
PROPORTIONAL 
TO AV 
- -  / VDET 
1 2 3 4 5 6  I AT TIME 5 
ON 
%SET n FF 
TIME - 
Fig. 3 hitched-FET multiplexer and correlated-double-sampling electronics. 
The "delta-reset" sampling scheme is shown, with the signal proportional to the 
difference between voltage samples 1 and 5. 
. 
b 
4 
. 
4 
6 
Integrated IR arrays typically have properties which make them useful 
astronomically. The uniformity of responsivity is commonly 2-51, and crosstalk 
is generally <5%. The linearity of IR arrays has been found to be very good; 
for example, departures of less than 1% from linear have been reported for a 
HgCdTe array. 
integrated-array focal planes in the Space Infrared Telescope Facility (SIRTF). 
The use of integrated IR arrays in ground-based observing, particularly near 
10 urn, requires highly sophisticated, computer-based data-acquisition and 
control systems. The large IR photon arrival rates require that the arrays be 
read out at many hundreds of frames per second, which places challenging 
requirements on the drive-and-read electronics. Also, one must develop effi- 
cient means to correct (e.g., flat-field), organize, and archive data from a 
large number of frames, and to produce "quick-look" images or spectra to guide 
the course of the observation. 8 
Examples of IR Arrays 
1. 32-X-32 InSb CCD 
W. Forrest and J. Pipher (University of Rochester) have used a 32-X- 
32-element IR array from Santa Barbara Research Center (SBRC) for laboratory and 
observational testing. The device, with 90-um pixels and a four-phase CCD 
multiplexer, has a quantum efficiency of 60-651, a well capacity of lo7 e-, and 
a read noise of -1400 e-. Its surface-channel CCD is operable down to -45 K, at 
which point the dark current is about 0.1 fA (1000 e-/~).~ This device has been 
used in a camera instrument at a variety of telescopes with great 
success.lO*ll An example of the imagery obtained is illustrated in Figs. 4 and 
5. This array has been proven to have a lower noise equivalent power (NEP) than 
the best discrete-detector InSb instrument presently in use. A limiting 
sensitivity of +20.9 magnitudelpixel (lo) has been achieved in a 1-hr 
integration at 2.2 um, with a bandpass of 0.23 A 58-x-62-element DRO InSb 
array is currently under development at SBRC for the next phase of this project. 
2. 16-X-16 Si:Bi CID 
We have found the Si:Bi accumulation-mode CID array technology from Aerojet 
ElectroSystems Company to have a quantum efficiency of -20% and read noise in 
the 100-300-e- range.4 Charge-trapping limits the applicability of these 
devices under low-background conditions, but a 16-x-16-element CID array (150-pm 
5 
Fig. 4 A 2.23-um image of the Sharpless 140 complex taken with the University 
of Rochester/SBRC 32-x-32-element InSb array (July 1985). 
pixels) has been successfully incorporated into a circular-variable-filter 
instrument, and a variety of objects have been imaged in both the (5- and 8-12- 
um On the 3-m telescope at Lick Observatory, the array system has 
achieved a sensitivity of +6 magnitudes/pixel in 30 min of integration at 10 um 
(0.2-urn bandpass). Figure 6 illustrates an image of NGC 7027 taken with this 
14 instrument at 11.50 ym, and Figure 7 shows an image of BD+30 3639 at 11.29 um. 
3. 58-x-62 Si:Sb DRO Array 
Si :Sb  DRO array technology has been developed by SBRC for potential appli- 
cation in SIRTF.3 Preliminary data from the supplier on a 58-x-62-elernent array 
with 75-urn pixels indicate that only 6 of the 3596 pixels were inactive on the 
6 
Fig. 5 Composite of images of the galactic center taken with 32-x-32-element 
InSb CCD array. The estimated position of the compact radio source SgrA* is 
indicated by the crosshairs. 
7 
? 
Fig. 6 NCC 7027 at 11.50 urn taken with Aerojet Si:Bi 16-x-16-element AMCID 
array. 
. 
. 
F i g .  7 An 11.29-um image of BD+30 3639 obtained w i t h  16-x-16-element Si:Bi 
AMCID. 
f i r s t  u n i t  tested.  Uncertainty i n  the  spectral  response of the Si:Sb detectors 
created uncertainty i n  the calculated responsivity, bu t  values f e l l  i n  the range 
1-6 A/W. e-/s) 
were established in limited tes t s  a t  device temperature o f  7K and a bias of 1.5 
V .  Figure 8 shows the NEP histogram for t h i s  array,  w i t h  an average of -5 Y 
4 Upper bounds on read noise (<600 e') and dark current ( < 3  x 10 
10-18 wt$z.  
9 
ORlGlMAh P2.EE IS 
OF POOR QUALITY 
Fig. 8 Noise-equivalent power obtained with 58-x-62-element Si:Sb DRO array. 
Future Directions 
The past few years have seen a rapid expansion in the number of groups which 
have acquired and used integrated I R  arrays. The rate of progress, both 
technically and scientifically, is expected to increase rapidly as the tech- 
nology continues to proliferate. However, the sophistication of the devices and 
the complexity of the electronics and computer systems required to operate them 
have created a lag time (of about 2-3 yr) between the time an array is acquired 
and the time astronomical results are obtained. We expect this delay to 
continue to decrease as a larger user community is developed. 
The sensitivity of many devices, particularly in observations of faint 
objects at moderate- to high-resolution, is presently limited by the multiplexed 
readout. The continued optimization of switched-FET multiplexers is expected, 
as is the continued development of advanced detector and readout concepts, such 
as the impurity band conduction arrays (10 50 format, with 125-urn pixels and 
10 
c 
I 
(NOT TO SCALE) 
INCIDENT 
RADIATION TRANSPARENT 
SUBSTRATE 
BURIED 
ELECTRICAL 
CONTACT - IR ACTIVE LAYER 
I ,BLOCKING LAYER 
METAL 
BUMP BONDS 
MULTIPLEXER 
Fig. 9 Integrated array of backside-illuminated impurity band conduction 
detectors. 
switched-FET readout) currently being researched by Rockwell (Fig. 9). This 
latter device offers the potential of superior sensitivity, linearity, and 
radiation-hardness over conventional bulk-extrinsic photoconductors. Another 
advanced concept of particular interest for space applications is the recently 
developed Si:As solid-state photomultiplier, which has demonstrated the ability 
to count single-photon events at wavelengths out to about 28 um. 
Two-dimensional array versions of  the device are expected to be achieved in the 
future. 
Sumnary 
A number of hybrid integrated-IR arrays have been developed and used in 
astronomical applications. The arrays have proved to have attractive sensi- 
tivity and imaging characteristics and to be productive scientifically. A 
variety of extrinsic and intrinsic detector materials are being used with 
low-noise, switched-FET, multiplexing readouts. A number of advanced arrays 
(e.g., impurity-band-conduction Si:As arrays) are under active development. 
While the achievements accomplished to date on IR astronomical arrays 
represent an exciting beginning, much work remains to be done in the areas of 
characterization and optimal use; e.g., device dark current, read noise, and 
imaging quality, need to be much more carefully established. 
11 
I 
References 
1. N. W. Boggess et al., "Infrared Receivers for Low Background Astronomy," 
NASA Technical Memorandum 78598, 1979. 
2. C. Orias et al., "58 x 62 InSb Focal Plane for IR Astronomy," 
SPIE Paper 627-52, March 1986. 
3. J. H. Coebel et al., "Low Background Direct Readout Array Performance," 
SPIE Paper 627-53, March 1986. 
4. M. E. McKelvey et al., Appl. Opt. Vol 24, 1985, p. 2549. 
5. H. H. White et al., IEEE J. Solid-state Circuits, Vol. SC-9, 1974, p. 1. 
6. R .  Ramos et al., "Space Infrared Telescope Facility (SIRTF) Focal Plane," 
AIAA Paper 86-0373, Jan. 1986. 
7. M. J. Lebofsky et al., "Proceedings of The Second Infrared Detector 
Technology Workshop, It NASA Technical Memorandum 882 13, Ch. 17, 1986. 
8. P. S. Stafford and D. A. Jared, Proceedings for AIAA Computers in 
Aerospace Conference, Hartford, CT, Oct. 1983, pp. 229-234. 
9. CJ. J. Forrest and J. L. Pipher, "Proceedings of The Second Infrared 
Detector Technology Workshop," NASA Technical Memorandum 88213. Ch. 11, 1986. 
10. 61.  J. Forrest et al., Publ. Astron. SOC. Pacific, Vol. 92, 1985, p. 183. 
11.  W. J. Forrest et al., Ap. J. Letters, Vol. 301, 1986, p. L28. 
12. J. H. Coebel et al., "Photometric Imaging Near Infrared Spectrometer," 
SPIE Paper 570-24, Aug. 1985. 
13. D. Y. Cezari et al., Ap. J., Vol. 299, 1985, P. 1007. 
L 
0 
. 
14. J. H. Goebel et al., "Narrow-Band Imagery in the 8-14 Micron Spectral 
Region," SPIE Paper 686-17, Aug. 1986. 
1 2  
1. Report No. 
NASA TM-88357 
c 
1 
2. Government Accssion No. 3. Reaplent's Catalog No. 
I 
4. Title and Subtitle 
INTEGRATED INFRARED ARRAY TECHNOLOGY 
5. Report Date 
6. Performing Orgnization Code 
OctoSer 1986 
7. AuthodsJ 
J. H. Goebel and C. R. McCreight 
Ames Research Center 
Moffett Field, CA 94035 
. 
2. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, DC 20546 
8. Performing Organization Report No. 1 A-86401 
t 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
505-45-3 1 
10. Work Unit No. 
9. Performing Organization Name and Address 
9. Security Qauif. (of this report) 
Unclassified 
20. Security Clauif. (of this page) 21. No. of Pq)n 22. Rice' 
Unclassified 15 A02 
5. Supplementary Notes 
Point of Contact: Craig McCreight, Ames Research Center, M/S 244-10 
Moffett Field, CA 94035 (415)  694-6549 or FTS 464-6549 
6. Abstract 
An overview of integrated infrared (IR) array technology is presented. 
Although the array pixel formats are smaller, and the readout noise of 
IR arrays is larger, than the corresponding values achieved with optical 
charge-coupled-device silicon technology, substantial progress is being 
made in IR technology. 
are described. Examples of astronomical images are given which illustrate 
Both existing IR arrays and those being developed 
the potential of integrated IR arrays for scientific-investigations. 
7. Key Words (Suggested by Authorts)) 
Infrared detectors 
Array technology 
Astronomy 
18. Distribution. Statement 
Unlimited -Unclassified 
Subject Category - 89 


Integrated infrared array technology

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