Christopher J. Wrigley

CMOS active pixel sensor specific performance effects on star tracker/imager position accuracy

This paper gives the status of theoretical and experimental efforts at JPL in the development of environmentally robust (Radiation Hard and radiation Tolerant), ultra-low power, high performance CMOS active pixel sensor (APS) imagers for start tracker/imager applications. The work explores the effect of imager performance on star position accuracy, specifically examining the performance of JPL designed APS imagers. Accuracy is estimated as a function of star magnitude for a nominal star tracker optical design. Using these APS sensors, which have wide dynamic range and no blooming, simultaneous imaging of widely differing star magnitudes during the same observation is possible. It is shown that prototype Rad Hard APS imagers meet many next generation, star tracker/imager mission performance requirements when operated at reduced temperatures. These imagers also provide excellent performance at cryogenic operating temperatures appropriate to some anticipate flight missions. APS imagers with their high level of integration, on-chip timing and control, ultra-low power, and environmental robustness are excellent candidates for NASAs earth observing, interplanetary and deep space exploration missions.

Multi-megarad (Si) radiation-tolerant integrated CMOS imager

The paper describes the design, operation, and performance of integrated CMOS imagers that withstand multi-megarad(Si) total dose of ionizing radiation. It reports test result from two imagers - one with on-chip integrated timing and control, and the other with a variety of pixel structures for parametrically investigating the effects of radiation ion imager performance. The CMOS Imager has been shown to response only to ionizing radiation, and is able to withstand high proton fluence. Minimal change in imager performance is observed after being subjected to a proton fluence of 1.2 X 1012 protons/cm2. The imager also exhibits minimal change in optical response after being dosed with 1.5 Megarad(Si). The radiation-induced dark current ins small and is well-behaved over the entire dose range. No change in operation bias is needed either for operating the imager at low-temperature or after irradiation. The parametric test chip indicates that the LOCOS region plays a significant role in determining the total-side-hardness of the pixel. Based on test results, most promising pixel structures for imaging under high radiation environments have been identified.

High dynamic range low-noise focal plane readout for VLWIR applications implemented with current mode background subtraction

Design and operation of a low noise CMOS focal pa;ne readout circuit with ultra-high charge handling capacity is presented. Designed for high-background, VLWIR detector readout, each readout unit cell use an accurate dynamic current memory for automatic subtraction of the dark pedestal in current domain enabling measurement of small signals 85 dB below the dark level. The redout circuit operates with low-power dissipation, high linearity, and is capable of handling pedestal currents up to 300 nA. Measurements indicate an effective charge handling capacity of over 5 X 109 charges/pixel with less than 105 electrons of input referred noise.

Hardening CMOS imagers: radhard-by-design or radhard-by-foundry

A comparative study between radhard-by-design and radhard-by-foundry approaches for radiation hardening of CMOS imagers is presented. Main mechanisms for performance degradation in CMOS imagers in a radiation environment are identified, and key differences between the radiation effects in CMOS imagers and that in digital logic circuits are explained. Design methodologies for implementation of CMOS imagers operating in a radiation environment are presented. By summarizing the performance results obtained from imagers implemented in both radhard-by-design and radhard-by-foundry approaches, the advantages and shortcomings of both approaches are identified. It is shown that neither approach presents an optimum solution. The paper concludes by discussing an alternate pathway to overcome these limitations and enable the next-generation high-performance radiation-hard CMOS imagers.

Modulation transfer function of active pixel focal plane arrays

Modulation Transfer Function (MTF) is an important figure of merit in focal plane array sensors, especially for accurate target positions such as star trackers. In-situ evaluation by MTF in different stages of imager system developments is necessary for an ideal design of different sensors and their signal processing. Understanding the tradeoff between different figures of merit will enable designers to achieve the most efficient design in specific missions. Advanced active pixel test sensors have been designed and fabricated where different pixel shapes were placed. Research on analyzing the MTF for the proper pixel shape is currently in progress for a centroidal configuration of a star. Explicit formulas for the modulation transfer function have been studied for the rectangular shaped pixel array. MTF will give us a more complete understanding of the tradeoffs opposed by the different pixel designs and by the signal processing conditions. In this paper, preliminary results of two different active pixel sensor (APS) focal plane arrays are presented in terms of crosstalk using a laser. MTF measurements of the APS arrays are achieved by applying only a single image. A rising or falling edge rather than the conventional bar target of slit scanning is needed to perform the measurement in each direction for the evaluation of the design efficiency.

Real-time reconfigurable foveal target acquisition and tracking system

This paper presents a target acquisition and tracking system based on the biomimetic concept of foveal vision. The system electronically reconfigures the resolution, sizes, shape, and focal plane position of visual acuity to meet time- varying operational requirements while maximizing the relevance of acquired video. A reconfigurable multiresolution active pixel CMOS imaging array is integrated in a closed-loop fashion with video processing and configuration control. Imager and algorithm configuration is updated frame-by-frame and reactively to target and scene conditions. By dynamically tailoring the visual acuity of the senor itself, the relevance and acquired visual information is maximized and a fast update rate is achieved with reduced communications bandwidth and processing requirements throughout the entire system. The system also features small size and less power consumption, and does not require a pointing mechanism. The distinguishing features of reconfigurable foveal machine vision are presented, and the hardware and software architecture of the target acquisition and tracking system is discussed. Real-time experimental result for automated target search, detection, interrogation, and tracking are then presented.

Comparing the low-temperature performance of megapixel NIR InGaAs and HgCdTe imager arrays

We compare a more complete characterization of the low temperature performance of a nominal 1.7um cut-off wavelength 1kx1k InGaAs (lattice-matched to an InP substrate) photodiode array against similar, 2kx2k HgCdTe imagers to assess the suitability of InGaAs FPA technology for scientific imaging applications. The data we present indicate that the low temperature performance of existing InGaAs detector technology is well behaved and comparable to those obtained for state-of-the-art HgCdTe imagers for many space astronomical applications. We also discuss key differences observed between imagers in the two material systems.

Two-dimensional active pixel InGaAs focal plane arrays

Switching and amplifying characteristics of a newly developed 2D InGaAs Active Pixel Imager Array are presented. The sensor array is fabricated from InGaAs material epitaxially deposited on an InP substrate. It consists of an InGaAs photodiode connected to InP depletion-mode junction field effect transistors for low leakage, low power and fast control of circuit signal amplifying, buffering, selection and reset. This monolithically integrated active pixel sensor configuration eliminates the need for hybridization with a silicon multiplexer, and in addition, allows the sensor to be front illuminated, making it sensitive to visible as well as near IR signal radiation. Adapting the existing 1.55 micrometers fiber optical communication technology, this integration will be an ideal system of optoelectronic integration for dual band applications near room temperature, for use in atmospheric gas sensing in space and target identification on earth. In this paper, 4 by 4 test arrays will be described. The effectiveness of switching and amplifying circuits will be discussed in terms of circuit in preparation for 2D InGaAs active pixel sensor arrays for applications in multifunctional, transportable shipboard surveillance, night vision and emission spectroscopy.

Characterization of NIR InGaAs imager arrays for the JDEM SNAP mission concept

We present the results of a study of the performance of InGaAs detectors conducted for the SuperNova Acceleration Probe (SNAP) dark energy mission concept. Low temperature data from a nominal 1.7um cut-off wavelength 1kx1k InGaAs photodiode array, hybridized to a Rockwell H1RG multiplexer suggest that InGaAs detector performance is comparable to those of existing 1.7um cut-off HgCdTe arrays. Advances in 1.7um HgCdTe dark current and noise initiated by the SNAP detector research and development program makes it the baseline detector technology for SNAP. However, the results presented herein suggest that existing InGaAs technology is a suitable alternative for other future astronomy applications.

Target acquisition and tracking system based on a real-time reconfigurable multiwindow CMOS image sensor

This paper presents a prototype Dynamically Reconfigurable Vision (DRV) system. DRV is based on the intelligent, dynamic allocation of spatial and temporal resources in order to maximize system performance. Minimization of irrelevant data in the video processing chain reduces on- board processing requirements, power consumption, and payload, and increases the amount of relevant information that can be communicated over bandwidth-limited channels. Our DRV system employs a real-time reconfigurable CMOS image sensor which supports multiple variance-resolution, independently-configurable windows per exposure, operates in a snapshot capture mode to minimize smear, and outputs data through multiple video ports to minimize readout time. Multiple, time-correlated windows enable the vision system to better support multiple targeting functions concurrently, and achieve a maximum level of situational awareness. This imager is capable of reconfiguring itself in microseconds upon demand; frame-by-frame configurable parameters include frame rate, integration time, and parameters defining the position, shape, size, and resolution of each window. The system additionally features a small footprint and very low power, resulting from a CMOS implementation and on-chip signal processing using passive circuitry. The advantages of DRV over conventional imaging techniques are discussed, and the overall design and performance of our DRV sensor and camera are presented.