A family of CMOS-based active pixel image sensors (APS's) that are inherently compatible with the integration of onchip signal processing circuitry is reported. The image sensors were fabricated using commercially available 2-m CMOS processes and both p-well and n-well implementations were explored. The arrays feature random access, 5-V operation and transistortransistor 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 m2 40 m with 26% fill-factor. Array sizes of 28 2 28 elements and 128 2 128 elements have been fabricated and characterized. Typical output conversion gain is 3.7 V/e 0 for the p-well devices and 6.5 V/e 0 for the n-well devices. Input referred read noise of 28 e 0 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. Index Terms-Active pixel sensor, cameras, CMOS image sensor. I. INTRODUCTION I N many imaging systems, integration of the image sensor with circuitry for both driving the image sensor and performing on-chip signal processing is becoming increasingly important. A high degree of electronics integration on the focal-plane can enable miniaturization of instrument systems and simplify system interfaces. In addition to good imaging performance with low noise, no lag, no smear, and good blooming control, it is desirable to have random access, simple clocks, and fast readout rates. The development of a CMOScompatible image sensor technology is an important step for highly integrated imaging systems since CMOS is well suited for implementing on-chip signal processing circuits. CMOS is also a widely accessible and well-understood technology.
The fluorescence spectrum of chromium-doped strontium titanate was measured as a function of temperature. The low-energy vibronic sideband of the R lines at low temperatures was analyzed to determine the onephonon and multiphonon contributions to the observed spectral profile. The one-phonon spectrum appears to be predominantly forced electric dipole in nature. Vibronic selection rules appropriate for such transitions were determined, and by comparison with neutron scattering and infrared absorption data more than 30 peaks in the vibronic spectrum can be tentatively identified with transitions involving specific phonon modes.Numerous low-frequency peaks are observed in the high-energy vibronic sideband, many of which cannot be associated with known vibrational modes. Using a simple long-wavelength phonon approximation, a phonon density of states is obtained and found to compare quite well to that determined from analyzing neutron scattering data. The temperature dependences of the widths of the zero-phonon lines and local mode were investigated using several phonon distributions including the effective density of phonon states obtained from the vibronic sideband. Low-frequency modes appear to make the dominant contribution to the broadening of the zero-phonon lines, whereas both lowand high-frequency phonons are active in broadening the local mode.
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 tradeoffbetween 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 anay. 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.
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