Peter Rieve

Sensitivity of CMOS based imagers and scaling perspectives

CMOS based imagers are beginning to compete against CCDs in many areas of the consumer market because of their system-on-a-chip capability. Sensitivity, however, is a main weakness of CMOS imagers and enhancements and deviations from standard CMOS processes are necessary to keep up sensitivity with downscaled process generations. In the introductory section several definitions for the sensitivity of image sensors are reviewed with regard to their potential to allow meaningful comparison of different detector structures. In the main section, the standard CMOS sensor architecture is compared to detector structures designed to improve the sensitivity, namely the photogate (PG), the pinned photodiode (PPD) and the thin film on ASIC (TFA) approach. The latter uses a vertical integration of the photodiode on top of the pixel transistors. A careful analysis of the relevant electrical, optical and technological parameters and many previously published experimental data for different imagers reveals that only the PPD and the TFA enhancements provide satisfactory sensitivity and withstand scaling down to 0.18 /spl mu/ processes. Due to the higher fill factor and the higher quantum efficiency TFA provides significantly better values than PPD. The radiometric sensitivity of a 5 /spl mu/m/spl times/5 /spl mu/m TFA pixel is found to amount to 11.9 V/(/spl mu//cm/sup 2/) for a 0.25 /spl mu/m process and 27.5 V/(/spl mu/J/cm/sup 2/) for a 0.18 /spl mu/m process.

New type of thin film color image sensor

A new thin film color sensor array has been developed. In this device a single pixel consists of a combination of an amorphous silicon nipin detector and a crystalline operational amplifier. The carrier transport mechanisms of the diode under dark conditions as well as steady state opto electronic behavior of the nipin structure have been studied in theory and experiment in order to optimize the design of the image sensor. As a result, nipin structures with excellent dynamic range and linearity have been fabricated. Our study has also demonstrated that a three color detector can be obtained either by optimization of the design of the detector or by appropriate signal processing. The limitations arising from the design rules of the crystalline electronics for a single channel MOSFET process and their impact on readout performance and signal distortion are discussed. A novel two stage operational amplifier with optimized design and layout has been fabricated. Because of the superior performance of the amplifier and the diode this sensor is especially suitable for high sensitive color image processing applications.