Craig O. Staller

CMOS active pixel image sensors for highly integrated imaging systems

A family of CMOS-based active pixel image sensors (APSs) that are inherently compatible with the integration of on-chip signal processing circuitry is reported. The image sensors were fabricated using commercially available 2-/spl mu/m CMOS processes and both p-well and n-well implementations were explored. The arrays feature random access, 5-V operation and transistor-transistor logic (TTL) compatible control signals. Methods of on-chip suppression of fixed pattern noise to less than 0.1% saturation are demonstrated. The baseline design achieved a pixel size of 40 /spl mu/m/spl times/40 /spl mu/m with 26% fill-factor. Array sizes of 28/spl times/28 elements and 128/spl times/128 elements have been fabricated and characterized. Typical output conversion gain is 3.7 /spl mu/V/e/sup -/ for the p-well devices and 6.5 /spl mu/V/e/sup -/ for the n-well devices. Input referred read noise of 28 e/sup -/ rms corresponding to a dynamic range of 76 dB was achieved. Characterization of various photogate pixel designs and a photodiode design is reported. Photoresponse variations for different pixel designs are discussed.

CMOS active pixel sensor star tracker with regional electronic shutter

A 64/spl times/64 element CMOS active pixel sensor (APS) for star tracker applications is reported. The chip features an innovative regional electronic shutter through the use of an individual pixel reset architecture. Using the regional electronic shutter, each star in the field of view can have its own integration period. This way, simultaneous capture of bright stars with dim stars is accommodated, enabling a large increase in tracker capability. The chip achieves 80 dB dynamic range, 50 e-rms read noise, low dark current, and excellent electronic shutter linearity.

128 x 128 CMOS photodiode-type active pixel sensor with on-chip timing, control, and signal chain electronics

A 128 X 128 element CMOS active pixel image sensor (APS) with on-chip timing, control, and signal chain electronics has been designed, fabricated and tested. The chip is implemented in 1.2 micrometers n-well process with a 19.2 micrometers pixel pitch. The sensor uses a photodiode-type CMOS APS pixel with in-pixel source follower, row selection and reset transistors. The sensor operates from a +5 V supply and requires only a clock signal to produce video output. The chip performs correlated double sampling (CDS) to suppress pixel fixed pattern noise, and double delta sampling (DDS) to suppress column fixed pattern noise. The on-chip control circuitry allows asynchronous control of an inter frame delay to adjust pixel integration. On-chip control is also provided to select the readout of any size window of interest.

Wide-dynamic-range APS star tracker

Design considerations and description of a CMOS Active Pixel image sensor (APS) star tracker are reported. APS technology has been thought of being most appropriate for guidance and navigation. However, making APS useful for future star tracker missions means a few challenges have to be overcome. A wider dynamic range is required, while fill factor ought to be high and simple geometry of the pixel active area is desired. These requirements are analyzed and tradeoff considerations are explained for a practical future celestial tracker. A 64 X 64 element CMOS APS array with individual pixel reset is described.

A radiation-hard, low-background multiplexer design for spacecraft imager applications

A possible multiplexer design for the focal plane for the Cassini Visible and Infrared Mapping Spectrometer (VIMS) is reviewed. The instruments requirements for the multiplexed array are summarized. The VIMS instrument has a modest radiation-hardness requirement due to the trajectory and planetary environments in which the instrument will be required to operate. The total ionizing dose hardness requirement is a few tens of kilorads. A thin-gate oxide of a few hundred angstroms thickness is to be used. Field hardness is to be achieved by guard bands or hardened dielectric isolation. The design is argued to meet the low-noise and radiation-hardness required for imaging at Saturn. The design is versatile enough to provide double-correlated and double-uncorrelated sampling, which is accomplished in the signal processing electronics outside the focal plane.

InGaAs detectors for miniature infrared instruments

In the past year, there has been substantial impetus for NASA to consider missions that are of relatively low cost as a trade off for a higher new mission launch rate. To maintain low mission cost, these missions will be of short duration and will use smaller launch vehicles (e.g., Pegasus). Consequently, very low volume, very low mass instrument (a.k.a. miniature instrument) payloads will be required. Furthermore, it is anticipated that the number of instruments flown on a particular mission will also be highly constrained; consequently increased instrument capability will also be desired. In the case of infrared instruments, focal planes typically require cooling to ensure high performance of the detectors, especially in the case of spectrometers where high D* is necessary. Since a major portion of an instruments mass and power budget is consumed by the focal plane cooler, detector technologies that require only modest or no cooling can contribute significantly to the realization of a miniature infrared instrument. InGaAs detectors feature high D*, low dark current, and response not only in the 1 - 3 micrometers SWIR regime, but also in the visible regime as well. The latter feature can extend the versatility of a given spectrometer by enabling greater spectral band response while maintaining focal plane simplicity. In this paper, we discuss the InGaAs detector technology and its potential application in miniature infrared instruments.

128x128 InGaAs detector array for 1.0-1.7 μm

A two-dimensional 128 x 128 detector array for the 1.0 - 1.7 micron spectral region has been demonstrated with indium gallium arsenide. The 30 micron square pixels had 60 micron spacing in both directions and were designed to be compatible with a 2D Reticon multiplexer. Dark currents below 100 pA, capacitance near 0.1 pF, and quantum efficiencies above 80 percent were measured. Probe maps of dark current and quantum efficiency are presented along with pixel dropout data and wafer yield which was as high as 99.89 percent (7 dropouts) in an area of 6528 pixels and 99.37 percent (103 dropouts) over an entire 128 x 128 pixel region.

Implementation of a noise reduction circuit for spaceflight IR spectrometers

The paper discusses the implementation and analysis of a correlated triple sampling circuit using analog subtractor/integrators. The software and test setup for noise measurements are also described. The correlation circuitry is part of the signal chain for a 256-element InSb line array used in the Visible and Infrared Mapping Spectrometer. Using a focal-plane array (FPA) simulator, system noise measurements of 0.7 DN are obtained. A test setup for FPA/SPE (signal processing electronics) characterization along with noise measurements is demonstrated.

LWIR, photovoltaic, Hg1-xCdxTe, and FPA performance for remote sensing applications

Focal plane arrays (FPAs), used for remote sensing applications, are required to operate at high temperatures and are subject to high terrestrial background fluxes. Typical remote sensing applications like cloud/weather imagery, sea- surface temperature measurements, ocean color characterization, and land-surface vegetation indices also require FPAs that operate from the visible through the LWIR portion of the spectrum. This combination of harsh requirements have driven the design of a unique LWIR FPA, that operates at 80 K under 300 K background conditions, with an operating spectral range from 11.5 micrometers to 12.5 micrometers , and a spectral cutoff of 13.5 micrometers . The FPA consists of 2 side by side arrays of 1 X 60 HgCdTe, (grown by molecular beam epitaxy) photovoltaic, detector arrays bump bonded to a custom CMOS Si readout. The 2 arrays are completely independent, and can be operated as such. The readout unit cell uses two, current-mode, analog building blocks; a Current Conveyor (CC1) and a dynamic current mirror. The CC1 has input impedance below 300 Ohms and an injection efficiency that is independent of the detector characteristics. This combination extracts high performance and excellent sensitivity from detectors whose average RoA values are approximately 1.7 Ohm-cm2 at T equals 80 K. The dynamic current mirror is used to subtract high background photocurrent while preserving excellent dynamic range. In addition to the performance enhancing readout, the detectors are manufactured with integral microlenses and operated in reverse bias to take advantage of their increased dynamic impedance. The dark currents associated with reverse bias operation are subtracted along with the background photocurrents by the dynamic current mirror. The expected and measured LWIR FPA performance will be presented. Measurements were performed on an LWIR FPA. Expected and measured FPA results are shown in the table below. The expected data are calculated from FPA models and compared to the measured values.

VLWIR HgCdTe photovoltaic detectors performance

Very Long Wavelength InfraRed (VLWIR; (lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detector operating in the 78 K range are needed for remote sensing applications. This temperature range permits the use of passive radiators in spacecraft to cool the detectors. VLWIR ((lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detectors in a range of sizes (8 micrometer diameter to 1000 micrometer diameter) have been fabricated and their performance measured as a function of temperature. Molecular Beam Epitaxy (MBE) was used to grow n-type VLWIR Hg1-xCdxTe on lattice matched CdZnTe. Arsenic was implanted and the wafer was annealed to provide the p-type regions. All the material was grown with wider bandgap cap layers and consequently the detector architecture is the Double Layer Planar Heterostructure (DLPH) architecture. Id - Vd versus temperature curves for 8 and 1000 micrometer diameter, (lambda) c equals 17 micrometer at 78 K detectors indicate that the 8 micrometer diameter detector is diffusion limited for temperatures greater than 63 K even at a -200 mV bias. There is no appreciable tunneling at T equals 50 K and at -200 mV applied bias. At T equals 40 K tunneling commences at a bias approximately equals -80 mV. Below T equals 30 K, the diode is tunneling limited. The 1000 micrometer diameter detector is diffusion limited at bias values less than -50 mV at 78 K. At zero bias, the detector impedance is comparable to the series/contact resistance. Interfacing with the low (comparable to the contact and series resistance) junction impedance detector is not feasible. Therefore a custom pre- amplifier was designed to interface with the large VLWIR detectors in reverse bias. The detector is dominated by tunneling currents at temperatures less than 78 K. The 1000 micrometer diameter, (lambda) c approximately equals 17 micrometer at 78 K detectors have dark currents approximately equals 160 (mu) A at a -100 mV bias and at 78 K. Detector non-AR coated quantum efficiency > 60% was measured at -100 mV bias in these large detectors and the response was constant across the (lambda) equals 7 micrometer to 15 micrometer spectral band. With AR- coating the quantum efficiency will be > 70%. Response was measured and non-linearity < 0.15% was calculated for the 1000 micrometer detectors. The flux values were in the 1017 ph/cm2/sec range and were changed by varying the blackbody temperature. In addition, a linear response was measured while varying the spot size incident on the 1000 micrometer detectors. This excellent response uniformity measured as a function of spot size implies that, low frequency spatial response variations are absent, for the 1000 micrometer detectors.