The small helium cooled infrared telescope for Spacelab 2 is designed for sensitive mapping of extended, low-surface-brightness celestial sources as well as highly sensitive investigations of the shuttle contamination environment (FPA) for this mission is described as well as the design for a thermally isolated, self-heated J-FET transimpedance amplifier. This amplifier is Johnson noise limited for feedback resistances from less than 10 to the 8th power Omega to greater than 2 x 10 to the 10th power Omega at T = 4.2K. Work on the focal plane array is complete. Performance testing for qualification of the flight hardware is discussed, and results are presented. All infrared data channels are measured to be background limited by the expected level of zodiacal emission

Fazio, G. G.

Gautier, T. N.

Hoffmann, W. F.

Koch, D.

Low, F. J.

Poteet, W.

Rieke, G. H.

Traub, W. A.

Urban, E. W.

Young, E. T.

English

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I-/6602/6
(NASA-Td-8J45o)
	 IEST iiCaULTj
INFRARED '1t;LESCOPE F O CAL PLANE.
(Arizoud Uuiv., Tucsur,.)
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NA S A- 10'36
Test results of Spacelab 2 Infrazed Telescope focal plane
E. T. Young, G. H. Rieke, T. N. Gautier, W. F. Hoffmann, F. J. Low, W. Poteet
Steward Observatory, University of Arizona, Tucson, Ari::nna 85721
G. G. Fazio, D. Koch, W. A. Traub
Smithsonian Astrophysical Observatory, 69 Garden Street, Cambridge, Massachusetts 02138
E. W. Urban, L. Katz
Space Sciences Laboratory, Marshall Space Flight Center, Huntsville, Alabama 35812
Abstract
The small helium cooled infrared telescope for Spacelab 2 will permit sensitive mapping
of extended, low-surface-brightness celestial sources as well as highly sensitive investiga-
tions of the shuttle contamination environment. The developnent of the focal plane array
for this instrument is described. A number of the Dhotoconductive detectors used in the
focal plane were made at the University of Arizona, and we describe the fabrication proce-
dure and some test results. A design for a thermally isolated, self -heated J-FET transim-
pedance amplifier is presented. This amplifier is Johnson noise limited for feedback resis-
tances from less than 10 8 n to greater than 2 x 10 13 r at T - 4 . 2{. Work on the focal
plane array is now complete. Performance testing for qualification of the flight hardware
is discussed, and results of this work are presented. All infrared data channels are
measured to be background limited by the expected level of zodiacal emission.
I. Introduction
The small helium-cooled infrared telesco pe ( IRT) for Spacelab 2 is a collaborative effort
on the part of the University of Arizona W of A), t`ie lord thsonian Astrophvsical 0bservatory
(SAO), and the Marshall Space Flight Center (MSFC). This instrument will pe rmit very high
sensitivity observations of astronomical o::ijects et a variety of infrared wavelengths and
will make important engineerin g measurenents that have bearing on future s pace infrared
missions. The U of A is building the telescope, focal plane array, and focal plane elec-
tronics, MSFC is handling the cryogenic system, and SAO is responsible for overall project
management, the data and control electronics, and data reduction.
The experiment has three primary goals. First, an infrared sky survey to high sensitiv-
ity and modest angular resolution is desired. Presently, virtually nothing is known about
the diffuse, very-low surface brightness emission from the Galaxy and from extragalactic
sources. Additionally, present measurements of the zodiacal background, especially at the
longer wavelengths, are highly uncertain. Second, we plan to measure the contamination
environment of the space shuttle. Particulate and gaseous contamination (especially NO
and CO-)) from the shuttle may degrade future infrared observations, so we have designed the
IRT to be especially sensitive to these backgrounds. To meet the first two experimental
goals a highly sensitive, multi -band instrument is required, preferably backgrounc limited
by the zodiacal emission. Finally, we will study the thermal performance of the telescope
to better understand the properties of large, superfluid helium systems in the space
environment.
Briefly the IRT consists lsee Figure 1) of a cryostat (with sunshade) which contains a
cooled 15 cm Herschelian telescope, a heliur transfer assembly, a 250 1 helium storage
dewar, and the associated electronics. The cryostat scans in a 90 0 arc along an axis
parallel to the long axis of the shuttle orbiter. Thus, overlapping strips of the sky are
mapped as the orbiter circles the B rth. More detailed discussions of the telescope and
mission are given by Gautier et al . ^ and Koch et a1. 2 while the cryogenic system has been
described by Urban et al.
The present paper will describe the development of the ten-detector focal plane array
(FPA) for this mission. A number of the infrared detectors were fabricated at the Univer-
sity of Arizona, and we will describe this work in the next section. The focal plane 	 :"0
electronics are of unusual design and are considered in section IV. The final section 	 ^j
presents some of the results of the testing program. 	 ;	
^O
ro
II. Infrared Detectors	 ^' J
For the low background conditions to be encountered in Spacelab, the most sensitive c5-	 ^` ^ ^^
^,^	 ^ ^Q
280-02
jN •. i t	 -	 `	 r	 `
^. t	 - 43	
- 
r	
^	
^ \	 rw i t ^{	
.w
Figure 1. Scale model of Spacelab IRT showing cryostat,
dewar, and mount. Actual experiment height is 340 cm.
infrared detectors are impurity doped silicon and germanium photoconductors. In support of
the Spacelab IRT and the Infrared Astronomy Satellite (I%%S) projects, we have investigated
the performance of a large number of detectors under low backgzound conditions. Addition-
ally, we have fabricated boti silicon and germanium detectors successfully.
Photoconductors exhibit the highest quantum efficiency near the energy cutoff of the
particular impurity level that is being excited. Thus, a multiband focal plane will likely
use a number of detector materials. The Spacelab IRT will incorporate three detector types
as indicated in Table 1. The gallium-doped silicon CA :Ga) detectors were made by the
Santa Barbara Research Center (SBRC). The antimonv-doped silicon detectors were made at
Table 1. Spacelab Detectors
Detector
Band A(ym) Type Source Thickness (mm)
S 2-3 Si:Ga SBRC 0.5
A 4.5-9.5 Si:Ga SBRC 0.5
8 6-7 Si:Ga SBRC 0.5
C 9-16 Si:Ga SBRC 0.5
D1,D2,D3	 1e-30	 Si:Sb	 UA/RIC	 1.0	 ORIGINAL PAGE IS
E1,E2,E3	 70-120	 Go:Ga	 LBL	 1.0	 OF POOR QUALITY
SBRC - Santa Barbara Research Center
UA/RIC - University of Arizona, material from Rockwell
International Corp.
LBL - Lawrence Berkeley Laboratory, made by Dr. E. Haller
280-02
TRANSPARENT CONTACT
DETECTOR CRYSTAL
SAPPHIRE SUBSTRATE
	 Figure 2. Detail of infrared detector.
TUNGSTEN MOUNT
ELECTRICAL FEEDS
the U of A from dies prepared by Dr. E. Haller of Lawrence Berkeley Laboratory. The prep
-aration technique is similar to that for Ge:Sb described in {7rtson et al.4
Detector construction
Figure 2 shows the details of a Spacelab detector. The single crystal photoconductor is
mounted on a sapphire substrate to provide electrical isolation fron the tungsten mount.
Tungsten is used throughout the Spacelab focal plane to provide a degree of shielding from
cosmic radiation. The quality of the contacts on the semiconductor material is critical to
the performance of the detector. Although this technology has been something of an art in
the past, consistently successful contacts are now possible with ion implantation. The ion
implanted front and back surfaces are then metalized with vacuum evaporated (or sputtered)
titanium-gold for connection to leads.
We have obtained several slices of antimony do ped silicon (Si:Sb) from the I-RAS project
for detector research. This material was manufactured by the Rockwell International Corpor-
ation. The slices were ion implanted at the Microelectronics Laborator of the University
of Arizc,.-.s with 100 KeV phosphorous ions at a concentration of 10 14 cm- . Subsequent steps
are:
1. Thermal annealing of implantation layer
2. Cleaning with H2 SO4 + H2 02 solution
3. HF etch
4. Titanium + gold metalization
S. Dicing into detectors
6. Polish etch of detector sides
7. Mounting on substrate:;
Testing of these detectors has shown them to be very high quantum efficiency devices,
and four of them are used in the Spacelab TPA.
III.• Detector testing
Test conditions
The detector test unit is designed to accurately simulate conditions expected in the
Spacelab mission, in particular the low background conditions. The a pparatus consists of a
side-looxing Infrared Laboratories dewar and an externally chopped blackbody. We have
employed a combination of neutral density attenuators and ver7 restricted fields of view to
limit background irradiation to zodiacal levels. For example, the Si:Sb detectors are
tested at backgrounds of ti 1 x 10 8 photons s'	 The test unit has been described in greater
detail by Young and Low s with the only enhancement being the replacement of the MOSFET
electronics with a J-FET equivalent. The use of J-FET's in photoconductor circuits is
discussed in Section V.
We have measured responsivity and noise as functions of background power, bias voltage,
and frequency. Additional characteristics that have been investigated include the tempera-
ture dependence of the detectivity, the spontaneous spiking behavior, and anomalous time
280-02
I
Figure 3. Completed focal plane
array attached to fiberglass
mount.
Table 2. Detector Test Results
Detector Type Bias(V) Background (W) Responsivity (A/W)	 yEP (W Hz
- ^) n	 (t)
SBRC 726 Si:Ga 12 3.3 x 10-14 2.0 9.3 x 10-17 25
SBRC 727 Si:Ga 12 3.3 x 10-14 2.1 8.2 x 10-17 32
UA 113 Si:Sb 4 6.4 x 10-12 0.98 5.1 x 20-16 83
UA 117 Si:Sb 4 6.4 x 10-12 0.80 5.0 x 10-16 85
LBL 2 Ge:Ga 0.15 3.2 x 10-13 3.8 4.2 x 10-16 3
LBL 4 Ge:Ga 0.15 3.2 x 10-13 3.8 3.3 x 10-16 5
constants in the detectors. In general, we have found that problems with spontaneous spik-
ing and anomalous time constants are minimized at reduced bias.
Results
:able 2 presents some representative zes ,ilts of the detector testing at the U of A. The
backgrounds are typical for Spacelab cond:ti.ons and the bias voltages are chosen as a best
compromise between responsivity and anomalous behavior.
In all cases the detector active area is 2.5 = x 2.5 mm. For the Ge:Ga testing, the
detectors were placed in an integrating cavity with a 2 mm entrance aperture. The detec-
tive quantum efficiency n is defined by:
r = REP BLIP2
REP
IV. Spacelab focal plane optics
The Spacelab IR:' focal plane consists of ten channels. These ten channels are distri-
buted among four independent modules. The modular a pproach was chosen to allow the great-
est flexibility during constr6,c,.jn and testing. Each module, wit:i two or three detectors,
has its own electronics module and interface connector so that the units can be qualified
for the mission individually. Figure 3 is a photograph of the completed FPA mounted on
the fiberglass support fixture.
r
.SS BOX
'ET
AL FEED
SS TUBE
RESISTOR
BLOCK
NS
IG BAR
280-02
Figure 4. Cross sectional view of channel S-A focal plane module.
Figure 4 is a cross-sectional view of a representative module, the S-A segment. sand-
passes are determined with multilayer interference filters, and additional blocking is
accor.,plished with crystaline elements. The Fabry optics are germanium meniscus lenses
except for the D-channels which are silicon lenses. Each detector has a nominal field of
view of 10 x 0.60 on the sky, however, a 12 arc minute wide strip is blocked by an occult-
ing bar. This bar, which is perpendicular to the scan direction, modulates a point source
into two pulses during a scan. This technique allows point sources to be located to with-
in several arc minutes in the scan direction.
The Ge:Ga detectors used in the E channels have fairly low absorption coefficients, and
hence show improved responsivities when placed in integrating cavities. A Spacelab far
infrared channel uses a hemispherical cavity around the detector that is fed by a horn of
2.5 mm diameter entrance aperture and 1.57 mm diameter exit aperture. We find typically
a factor of 2 or 3 improvement in responsivity over a bare Ge:Ga detector.
V. Focal plane electronics
The transimpedance amplifier (TIA) (see Ref. 6, 7, and 8) used in the Spacelab IRT is
shown schematically in Figure S. This'circuit configuration was chosen primarily because
of the extent-d high frequency response. The effective input capacitance of the input
stage (the usuh! frequency limitation in a source follower) is reduced roughly by a factor
of the open loop gain of the operational amplifier. Additionally, the bias voltage on the
detector remains constant under varying signals, and the output is directly proportional to
the detector photocurrent.
Early versions of the TIA used MOSFET's operating at liquid helium temperatures. Because
of high-MOSFET voltage noise, however, (,. 300 nV Hz-^ 0 5 Hz in the best G116 devices) the
systems were usually limited by amplifier noise rather than the Johnson noise of the feed-
back resistor. A second problem associated with operating MOSFET's below 15 R is a lack of
D. C. stability in the source voltage.
The present Spacelab FPA uses Siliconix J230 J-FtTs as a first stage to the TIA. These
devices combine low noise (5 nV Hz- E 10 Hz) with low input capacitance (2 pF). The pri-
mary complication stems from the dual requirements of close proximity to the detectors but
operation at temperatures above 50 K. Figure 4 shows the thermal isolation procedure used.
ORIGINAL GAGE IS
OF POOR QUALITY
CRMSTAT	 ;	 [roam ELIM 0uICf
art
280-02
Figure 5. Schematic of transimpedance amplifier circuit used in IRT.
A substrata carrying three matched pairs of T_ZTs is supported on thinwall (2.2 mm diameter,
0.15 mm wall thickness) fiberglass tubes. Electrical connections are made with 3-mil con-
stantan wire. The internal power dissipation of 1 m'g for each J-FET is sufficient to heat
them to % 93 K. To prevent photons from the !seated JFETs from interfering with the detec-
tors, the electronics are surrounded by a copper-clad fiberglass enclosure.
We have operated these JFET amplifiers with feedback resistances ranging from ID S R to
2 x 1013 2 and find the noise to be Johnson limited by the feedback resistor. The noise
spectra are flat from low frequencies to a roll off at f :^ 1/2TrRf Cf where Rf is the feed-
back resistance and Cf the shunt capacitance of the feedback resistor. Cf is typically
0.35 pF.
A much more complete discussion of JFET use with infrared detectors is giver by Low.B
yI. System performances
Test conditions
Low background tests of the focal plane modules were performed in a large dewar that
contained a low temperature blackbody source with shutter and a variable background source.
Special precautions were taken to eliminate any proton leaks in the test cham^Der so that
QZILCH conditions (< 10 6 photons s' 1 ) were realized even in the far infrared bands with
the variable background source, in-band irradiances of greater than 10 9 photons s-1 were
achieved.
A typical test sequence consisted of measurements of signal and noise for the channels
as functions of background and detector bias voltage. *igure 6 is a representative plot of
noise voltage as a function of background for channel E1. Note that at very low background
levels the amplifier is Johnson noise limited by the feedback resistor. Second, the back-
ground noise increases as the square root of the background power, as expected for photon
statistics. Finally, the channel will be zodiacal background limited if the current models
for this background are correct. Here we use the revised IRAS models
2.5 x 10-6s 1
 (230)	 (w cm'2sr'lvm'1 )F 1 ^T 
Table 3 summarises the low background performance of the ipacelab 2 IRT. It should be
emphasized that the sensitivities quoted are measured for the actual flight hardware. All
channels except S, the aspect sensor, meet the sensitivity goal of being limited by the
280-02
10
e
s
C
1.0
113
_X-
x
.r'
_e-
Figure 6. Plot of noise
vs. background for E1
channel. Arrow indicate
predicted zodiacal back-
ground level for this
band.
	
-15
	 -14	
-13
	 -12
	
-11
log BACKGROUND POWER (W)
Table 3. Characteristics of Spacelab 1R Telescope Detector Channels
Band	 a	
V 
	 S	 n	 E	 R 
	 Q
	
NEPQ	 NEPD	 NEFD
(y)	 (volts) (A/W) (X) (Y)	 (tl)	 (W)	 (W/Hzi)	 (W/Hz^) (W CM
-2 
lim- 1Hz-k)
S	 2.0-3.0	 22	 0.03	 % 6	 70 1.7x1010	1.2x10 13 8x10
-16 3x10 15 2.42x10 17
A	 4.5-9.5
	
14	 0.45	 40	 50 3009	2.4x10-12 800-16 5x10 16 1.8x10-18
B	 6.1-7.1	 14	 0.45	 40	 70 1.7x1010	4.4x10 13 3.6x10
-16 2x10
-16 2.9x10 18
C	 8.5-14	 19	 0.85	 45	 70 9x106	4.7x10 12 9x10 16 500
-16 1.3x10 18
D	 18-29.5	 4	 1.4	 60	 40 3x109	1.1x10 12 3.4x10 16 2x10-16 4.0x10 19
E	 70-120	 0.15	 3	 3	 50 3x109	3.7x10 14 1.5x10-16 7x10-17 3.4x10-20
Definitions:
Da:	 wavelength range between half power points of effective filter transmission curve,
including spectral characteristics of detectors, blockers, and optics
VS *	 detector bias voltage
S	 detector responsivity in the spectral band
n	 detector quantum efficiency in the spectral band
c	 optical efficiency
Rf :	 feed back resistor
Q :	 background flux (zodiacal) incident on the detector
NEPQ :	 background limited NEP
NEP D:
	
zero-background NEP
NEFD:	 noise equivalent spectral flux density. background limited
280-02
zoc..acal background. The S channel will be able to detect a star of K-magnitude 3 at a
si gnal to noise ratio of 20 in 16 ms, meeting the design requirer, .ents. As an example of
what these sensitivity levels translate to during the mission, consider the detectability
Of H2O contamination f om une shuttle. Using the calculated H2O emission spectrum of
Simpson and Witteborn, the Sp-ceJab IRT B channel will be able to sense a column density
of less than 2 x 10 9 molecules/cm . This level is much lower than predicted for the actual
shuttle environment.
We have performed additional testing of other system characteristics. In particular the
frequency response of the detector and electronics is important in discriminating between
cosmic ray hits and genuine infrared sources. All the detector channels have useful res-
ponse beyond 1 KHz, so the spikes (which last < 1 ms) will be easily distinguishable from
the true sources (which last > 16 ms).
VII. Current status and continuing work
-work on the focal plane is now complete, and the unit has been qualified for the Space-
lab 2 mission. Integration with the other components in the system is in progress leading
to scheduled launch in November 1963.
We are extending our photoconductor work to significantly lower backgrounds. Initially,
Si:Sb are being investigated under these conditions. For these exverinents the feedback
resistor is increased 20-fold to 2 x 1011 R. The J-FET transimpedance amplifier functions
satisfactorily at these impedan es albeit with a !high frequency rolloff at ti 30 Hz. A
limiting NEP of 5 x 10- 18 W Hz' has been achieved in the ' 18-30 um band with 8V bias. The
main difficulty with operation in this regime is the strong tendency of Si : Sb detectors to
spike at !higher biases.
Acknowledgements
We wish to thank Dr. J. Fordemwalt of the U of A Microelectronics Laboratory and
especially Dr. E. Haller of Lawrence Berkeley Laboratory for invaluable assistance in
detector fabricj+tion. This work is supported by the National Aeronautics and Space
Adainistration.
References
1. Gautier, T. N., III, Rieke, G. H., Low, F. J., and Hoffmann, W. F. 1979, Proc. SPIE,
172, 264.
2. Koch, D., 7azio, G. G., Traub, W. A., Rieke, G. H., Gautier, T. N., HLifrlann, W. F.,
Low, ?. J., Poteet, W., Young, E. T., Urban, E. W. and Katz, L. 1981, Proc. SPIE, 265, 257.
3. Urban, E. W., Katz, L., Hendricks, J. B., and -Carr, G. R. 1973, roc. f E,—IE3, 40.
4. Watson, D. M., Storey, J. W. V., Townes, C. H., and Haller, E. E. 1980, Astrop^ys.
J. (Letters), 241, L43.S.
Young, E.T. and Low, F. J. 1979, Proc. SPIE, 172, 184.
6. Wyatt, C. L., Baker, D. J., and Fro sham,D. G. T974, I. R. Ph s., 14, 165.
7. Luinge, W., Wildenan, K. J., and van Duinen, R. J. 198 , 	 A. Phys., 20, 39.
8, Low, F, J. 1981, Proc. SPIE, 280 (this volume).
9. Simpson, J. P. and Witteborn, . C. 1977, Apol. Opt., 16, 2051.


Test results of Spacelab 2 infrared telescope focal plane

Test results of Spacelab 2 infrared telescope focal plane

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