The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) instrument uses four separate focal plane assemblies consisting of line array detectors that are multiplexed to a common J-FET preamp using a FET switch multiplexing (MUX) technique. A 32-element silicon line array covers the spectral range from 0.41 to 0.70 microns. Three additional 64-element indium antimonide (InSb) line arrays cover the spectral range from 0.68 to 2.45 microns. The spectral sampling interval per detector element is nominally 9.8 nm, giving a total of 224 spectral channels. All focal planes operate at liquid nitrogen temperature and are housed in separate dewars. Electrical performance characteristics include a read noise of less than 1000 e(-) in all channels, response and dark nonuniformity of 5 percent peak to peak, and quantum efficiency of greater than 60 percent

Bailey, Gary C.

English

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- - . I  
SLf-35- 
Visible and infrared linear detector arrays for the 
Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) 
Gary C. Bailey 
Jet Propulsion Laboratory, California Institute of Technology 
4800 Oak Grove Drive, Pasadena, California 91109 
ABSTRACT 
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) instrument uses four 
separate focal plane assemblies consisting of line array detectors that are multiplexed to 
a common J-FET preamp using a FET switch multiplexing (MUX) technique. A 32-element 
silicon line array covers the spectral range from 0.41 to 0.70 pm. Three additional 
64-element indium antimonide (InSb) line arrays cover the spectral range from 0.68 to 
2.45 pm. The spectral sampling interval per detector element is nominally 9 . 8  nm, 
giving a total of 224 spectral channels. All focal planes operate at liquid nitrogen 
temperature and are housed in separate dewars. Electrical performance characteristics 
include a read noise of < 1000 e- in all channels, response and dark nonuniformity of 5% 
peak to peak, and quantum efficiency of > 60%. 
1. INTRODUCTION 
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) instrument is the first 
flight instrument (flown in a U2 aircraft) to utilize FET switch multiplexing (MUX) 
techniques developed at JPL for readout of indium antimonide (InSb) detector material that 
covers the 0.7- to 5.2-pm spectral range. The technology described in the following 
sections has undergone significant developmental refinement to the point where high 
reliability and performance characteristics can be obtained at reasonable cost. 
2. DESIGN 
The AVIRIS focal plane design uses a new Reticon MUX that features lower video line 
capacitance along with an integral reset switch. These two improvements lower noise and 
simplify the hybrid design as described in previous papers.1#2#3 
the result of considerable prototype hybrid experience and reflects all the improvements 
found to be desirable from early work. 
The new MUX design is 
The integral reset switch and tighter design rules reduce video line capacitance, which 
gives a reduction in KTC noise. A dummy pixel has been added at the beginning and end of 
the MUX to minimize first and last pixel offsets induced from the geometry discontinuity 
at the start and finish of the structure. Other improvements include mirror image MUXs 
of 32, 64, 128, and 256 inputs, along with wider scribe clearance adjacent to the detector 
input pads to improve wire-bonding yield. 
The focal plane hybrid design consists of three subassemblies: 1) a detector line 
array with a fan-in interconnect to match the MUX input bonding pad pitch, 2) a MUX and 
thick-film circuit pattern, and 3) a J-FET follower preamp with thermal isolation and bias 
load resister. Each subassembly can therefore be tested prior to final assembly into the 
Kovar package. The result is a high-reliability hybrid that is 100% functional at initial 
test and requires no rework! This is a prime requirement for any high-reliability 
hybrid: It must operate properly at initial test. 
(silicon and InSb line arrays) are connected to the input bonding pads. The detectors are 
operated in the charge storage mode, in which each detector is supplied with a small 
reverse bias (-0.25 V), which charges the junction capacitance. After the signal 
integration interval ( 8 6  psec in the case for AVIRIS). each pixel is sequentially reset 
by the shift register, and the resulting Voltage excursion is applied to the gate of the 
J-FET preamp for impedance conversion. Signal amplification is accomplished off the focal 
plane, In Figure 2, a completed focal plane is shown with its blocking filter and 
field-of-view mask removed for clarity. The filter cuts off all input flux beyond 2.5 
pm and is cooled along with the mask to the focal plane temperature. 
Figure 1 shows a block diagram of the Reticon MUX, in which the detector materials 
3. FABRICATION 
Cincinnati Electronics manufactured the mesa InSb photodiodes, while United Detector 
Technology provided the silicon photodiodes with its standard DP-125 process. The detector 
line arrays are probed for proper I-V characteristics at the wafer level and bonded to the 
31 
https://ntrs.nasa.gov/search.jsp?R=19880004947 2020-03-20T09:00:17+00:00Z
SUBSTRATE 
YlOEO R E F E R E N C E  
' J l O E O R E S E T  
l lUMMI  VlDEO 
Figure 1. Simplified block 
diagram of the Figure 2. 64-element InSb AVIRIS 
Reticon multiplexer. focal plane. 
fan-in thin-film ceramic carrier. The subassembly is then retested at liquid nitrogen 
(LN2) temperature for proper detector leakage performance and set aside for final 
assembly into the focal plane. Spectral response measurements are performed on a sample 
of 5 to 10 random elements from the wafer to qualify that parameter. This test is not 
performed on the actual detectors that are bonded to the fan-in because of the high 
uniformity of this material (22.5%); thus the risk of ESD damage to the detectors is 
minimized by one less handling step. 
visually inspected and directly mounted to the thick-film ceramic circuit carrier for 
test. Since each input node has a 10 p x 10 p photodiode built in, a complete 
functional test can be carried out at room temperature before final assembly into the 
hybrid. Again, the handling and testing are held to a minimum, thus improving the 
reliability of the final hybrid assembly. 
Reticon supplies the MUX die 100% probed f o r  functionality. This allows the die to be 
Intersil supplies the 2N6483 J-FET 100% tested for D.C. parameters. A.C. parameters 
(En and In noise) cannot be accurately measured in die form; however, experience has 
shown that uniform D.C. parameters lead to good A.C. performance, and no J-FET has ever 
been removed from a hybrid because of poor noise or D.C. characteristics. The J-FET die 
is mounted on a hollow fiberglass post to provide thermal isolation, which causes the 
J-FET to operate at approximately 30K above the focal plane temperature due to the -15 
mW dissipation in the 2N6483. A ceramic cover is placed over the J-FET so the InSb 
detectors cannot "see" this heat source in low-background applications. This subassembly 
is set aside f o r  final hybrid fabrication after a D.C. probe test. 
All subassemblies are now bonded into the Kovar package and wire-bonded together along 
with the connection to the package pin-outs. The completed assembly is  mounted in a test 
dewar and cooled to LN2 temperature for a functional test. If the focal plane passes 
this test (all six units did the first time), it is removed from the test fixture and 
installed in the flight dewar. The blocking filter and mask are installed, and final 
acceptance tests are performed. 
4 .  PERFORMANCE 
The AVIRIS flight focal plane assemblies exceeded all performance specifications. The 
read noise specification of 1,200 e- was the main beneficiary of the improved MUX 
design, in which reduced capacitance resulted in lower noise. The flight units had a 
noise variation from 890 e- to 9 8 0  e-. Response nonuniformity and dark current 
nonuniformity were within specification at 5% peak to peak. Typical spectral response 
curves for InSb and the silicon detectors are shown in Figures 3 and 4, respectively. As 
a point of interest, the 2.5-pm blocking filters were found to have small area (on the 
order of a few pixel dimensions) transmission variations as large as the detector response 
variation. 
Table 1 gives a summary of the flight focal plane performance variation with regard to 
important electrical parameters. 
Table 2 gives the important electrical and mechanical characteristics of the AVIRIS 
focal plane assemblies. 
32 
W A V E L E N G T H . p  rn 
Figure 3. Typical InSb spectral response. 
SPECTRAL RESPONSE 
0.6 
0 5  
0.4 ' 0.3 
0.2 
0.1 
I 1 1 I I 1 I I 1 I 
0 300 400 500 600 700 800 900 1000 1100 
WAVELENGTH, nrn 
Figure 4. Typical silicon spectral 
response. 
Table 1. Flight Focal-Plane Performance 
Response 
Nonuniformity output 
Flight Follower Capactive Read (Standard Transfer De t ec to r 
SIN Gain Division Noise Deviation) Function Materia 1 
1 0.915 0.572 918 e- 0.7% 8.73 x 104 e-/mV I nSb 
2 0.900 0.526 948 e- 0.7% 9.48 x lo4 e-/mV InSb 
3 0.909 0.587 887 e- 0.56% 8.52 x lo4 e-/mV InSb 
4 0.920 0.467 722 e- ~ 0 . 7 %  4.89 x 104 e-/mV Si 
5 0.910 0.468 750 e- ~ 0 . 7 %  4.84 x lo4 e-/mV Si 
6 0.878 0.579 972 e- 0.85% 8.64 x lo4 e-/mV InSb 
Table 2. AVIRIS Focal-Plane Electrical and Mechanical Characteristics 
Package Size 
Materia 1 
Electrical Connections 
Pixel Size 
Pixel Pitch 
Clocks 
Detector Material 
Response Nonuniformity 
Dark Current Nonuniformity 
Quantum Efficiency 
Operating Temperature 
Power Dissipation 
1.48 in. x 0.66 in. x 0.225 in. 
Kovar 
13 
200 x 200 pm 
230 pm 
3 (12-V swing) 
32-Element Silicon 
64-Element InSb 
5% Peak to Peak 
5% Peak to Peak 
>60% (see curve) 
80K 
15 mW 
33 
ORIGINAL PAGE IS 
OE POOR QUALITY 
Figure 5 shows the typical high image quality obtained from the AVIRIS system. 
These frames are from the second engineering flight and are representative of both the 
silicon and InSb signal channel performance. 
BAND 30 (0.686 pm) 
SAN FRANC I SCO 
BAY AREA 
BAND 68 (1.017 pm) 
BAND 125 (1.516 pm) 
BAND 188 (2.097 pm) 
Figure 5. Typical AVIRIS flight imagery, Rogers Dry Lake. 
3 4- 
5. CONCLUSIONS 
The AVIRIS instrument is the first operational system to provide a large, 614-pixel 
spatial cross-track format combined with 224 10-nm-wide spectral channels over the 0.4- to 
2.45-pm region. A typical flight path will extend to several thousand pixels. The 
sheer quantity of data created can tax the largest image processing facilities available. 
Without multiplexed line array focal planes, an instrument such as AVIRIS is not 
practical to construct. The high-quality image produced is testimony to how good the 
detector technology works. From a performance/cost point of view, this detector 
technology development can only be pronounced a success. 
6. ACKNOWLEDGMENTS 
The authors wish to express thanks to the many people who contributed to this 
successful technology development and for the continuing sponsor enthusiasm at NASA 
Headquarters. 
The research described in this paper was per<formed at the Jet Propulsion Laboratory, 
California Institute of Technology, and Cincinnati Electronics, under contract with the 
National Aeronautics and Space Administration. The funding for this research was provided 
by the Office of Space Science and Application. 
7. REFERENCES 
1. G.C. Bailey, "An Integrating 128 Element Linear Imager for the 1 to 5 pm Region," 
Proc. SPIE, Vol. 311, proceedings of the SPIE meeting held in San Diego, California, 
Aug. 27-28, 1981, pp. 32-37, 1982. 
2. G.C. Bailey, "An Integrating 128 Element InSb Array: Recent Results," Proc. SPIE, 
Vol. 345, proceedings of the SPIE meeting held in Arlington, Virginia, May 6-7, 1982, 
3. G.C. Bailey, K. Matthews, and C.A. Niblack, "Operating of Integrating Indium 
pp. 185-191, 1982. 
Antimonide Linear Arrays at 65 K and Below," Proc. SPIE, vol. 430, proceedings of the SPIE 
meeting held in San Diego, California, Aug. 23-25, 1983. 
35 


Visible and infrared linear detector arrays for the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)

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