Large area focal plane arrays of unprecedented performance were developed for use in Near Infrared Camera and Multi-Object Spectrometer (NICMOS), a proposed Hubble Space Telescope refurbishment instrument. These FPAs are 128x128-element, HgCdTe hybrid arrays with a cutoff wavelength of 2.5 microns. The multiplexer consists of a CMOS field effect transistor switch array with a typical mean readout noise of less than 30 electrons. The detectors typically have a mean dark current of less than 10 electrons/s at 77 K, with currents below 2 electrons measured at 60 K (both at 0.5 V reverse bias). The mean quantum efficiency is 40 to 60 percent at 77 K for 1.0 to 2.4 microns. Functional pixel yield is typically greater than 99 percent, and the power consumption is approximately 0.2 mW (during readout only)

Blessinger, Michael

Kleinhans, William

Rasche, Robert

Rieke, Marcia J.

Thompson, Rodger

Vural, Kadri

English

NASA Technical Reports Server

N90-21337
LOW NOISE llgCdTe 128 X 128 SWIR FPA
FOR IIUBBLE SPACE TELESCOPE
Michael Blessinger and Kadri Vural
Rockwell International Science Center
Thousand Oaks, CA 91360
William Kleinhans
Valley Oak Semiconductor
Westlake Village, CA 91362
Marcia Rieke, Rodger Thompson and Robert Rasche
University of Arizona
Tucson, AZ 85721
Abstract
Large area focal plane arrays of unprecedented performance have
been developed for use in NICMOS, a proposed Hubble Space
Telescope refurbishment instrument. These FPAs are
128x128-element, HgCdTe hybrid arrays with a cutoff wavelength of
2.5 _m. The multiplexer consists of a CMOS FET switch array with
a typical mean readout noise of less than 30 electrons. The
detectors typically have a mean dark current of less than I0
electrons/s at 77K, with currents below 2 electrons/s measured at
60K (both at 0.5 V reverse bias). The mean quantum efficiency is
40-60% at 77K for 1.0-2.4 _m. Functional pixel yield is
typically greater than 99%, and the power consumption is
approximately 0.2 mW (during readout only).
Focal plane array design
The NICMOS focal plane array (FPA) is a hybrid structure
consisting of a silicon multiplexer mated through indium columns
to a HgCdTe detector array. The detectors are an array of
photodiodes formed by ion implantation in a layer of HgCdTe grown
by liquid phase epitaxy on a substrate of sapphire. The bandgap
of the detector material is chosen such that it responds to
infrared radiation in the 0.8 to 2.5 _tm wavelength region. The
format of the array is 128 x 128 elements on 60 _m centers.
Each detector is connected to a source follower amplifier in the
input cell of the multiplexer through an indium column. This
amplifier can be connected to the multiplexer output amplifier
through a set of row and column select switches (Figure i). In
normal operation, the reset FETs, M2 and M4, are turned on to
reset the voltage at the cathode of the detector, and are then
turned off, allowing the cathode to float. Photo and dark
current is then integrated into that capacitance which is the sum
of the detector capacitance and the input capacitance of the
multiplexer. Since the detector substrate voltage is fixed, the
cathode voltage (and thus the voltage at the gate of the source
follower MI) changes, reflecting the integrated current. Since
the integration capacitance is small (approximately 0.i pf), a
small amount of integrated charge produces a significant voltage
311
https://ntrs.nasa.gov/search.jsp?R=19900012021 2020-03-19T23:17:03+00:00Z
change at the source follower gate. When M3 and M5 are turned
on, the voltage at the unit cell source follower is sampled at
the output amplifier, which is another source follower.
Control of the select and reset switches, M2 through M5, is
accomplished by the operation of two shift registers driven by
user supplied clocks. Additionally, the reset is gated by an
enable llne, REN, which allows one to optionally reset or not
reset a particular pixel while it is selected. This not only
allows monitoring of the reset level at the output, but also
allows the pixel to be sampled non-destructively. Consequently,
sophisticated sampling schemes can be used to improve the
signal-to-noise ratio.
Since the multiplexer is CMOS in design, and because the input
cell source folllower only draws power when it is accessed, the
design is inherently low power. Turning off the output amplifier
when readout is not occurring reduces power consumption to zero.
During readout, only the output amplifier and one input cell
amplifier are on at any time, so that the power consumed is a
minimal 0.2 mW when 5V power supplies are used.
Performance results
Completed focal plane arrays were tested extensively with
particular attention paid to their noise and dark current
characteristics.
Dark current. The dark current was characterized at a wide
variety of reverse biases at 77K. Figure 2 shows a dark current
histogram at 79K and 0.5 V reverse bias which has a mean value of
4.3 electrons/s. Figure 3 shows the mean array dark current
versus bias at 79K. Even at large biases of several volts,
currents less than I00 electrons/s were achieved. At 60K the
dark current is _xen lower as shown in Fig. 4 where it is 1.6
electrons/s (i0 -_= amps).
Noise. The multiplexer readout noise was characterized in a
variety of readout schemes. A simple model of the multiplexer
circuit based on I/f and thermal MOSFET noise indicates that the
noise level should be about 4 electrons distributed fairly
equally between the input cell amplifier and the output
amplifler. Since this represents a level of about 3 _V, in
practice other sources such as switching transients and crosstalk
will in fact dominate.
Three sampling schemes were examined. By sampling the signal,
without any multiple sampling{ one would expect the noise to be
dominated by the reset kTC nolse of the integration capacitance,
about 60 electrons, which is indeed the case. By sampling both
the signal level and the reset pedestal immediately following,
one would expect some suppression of low frequency noise, but no
removal of the reset noise since the samples are not truly
correlated. This results in a noise somewhat worse than the
preceding case due to the two samples.
312
A better sampling scheme is triple correlated sampling where the
pixel is sampled immediately before, during, and immediately
after reset. By properly correlating samples between frames, a
removal of reset kTC noise is accomplished. In this way noise
levels below 30 electrons are achieved (Figure 5). From this
data a total FPA noise based upon readout noise and dark current
can be projected (Figure 6).
Quantum efficiency. The mean FPA quantum efficiency was measured
at various wavelengths using bandpass filters, and the results
are shown in Figure 7.
Conclusions and future work
The FPAs developed for the NICMOS program have achieved an
unprecedented level of performance for a large format array.
Readout noises under 30 electrons and dark currents under i0
electrons/s at 77K (<2e-/s at 60K) were achieved using a design
which was simple, easy to operate, and easy to produce.
Furthermore, the design can be extrapolated to larger sizes
(256x256 for example) with low risk and similar performance.
Acknowledgements
The authors thank their colleagues at Rockwell for their work in
fabricating the FPAs, particularly Jenkon Chen, victor Johnson,
and Matthias Blume. This work was funded by NASA (NAS5-30008)
through the University of Arizona NICMOS project.
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Low noise HgCdTe 128 x 128 SWIR FPA for Hubble space telescope

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