Jan Bogaerts

CMOS image sensors with logarithmic response are attractive devices for applications where a high dynamic range is required. Their strong point is the high dynamic range. Their weak point is the sensitivity to pixel parameter variations introduced during fabrication. This gives rise to a considerable fixed pattern noise (FPN) that deteriorates the image quality unless pixel calibration is used. In the present work a technique to remove the FPN by employing on-chip calibration is introduced, where the effect of threshold voltage variations in pixels is cancelled. An image sensor based on an active pixel structure with five transistors has been designed, fabricated, and tested. The sensor consists of 525/spl times/525 pixels measuring 7.5 /spl mu/m/spl times/10 /spl mu/m, and is fabricated in a 0.5-/spl mu/m CMOS process. The measured dynamic range is 120 dB while the FPN is 2.5% of the output signal range.

A 512/spl times/512 CMOS active pixel sensor (APS) was designed and fabricated in a standard 0.5-/spl mu/m technology. The radiation tolerance of the sensor has been evaluated with Co-60 and proton irradiation with proton energies ranging from 11.7 to 59 MeV. The most pronounced radiation effect is the increase of the dark current. However, the total ionizing dose-induced dark current increase is orders of magnitude smaller than in standard devices. It behaves logarithmically with dose and anneals at room temperature. The dark current increase due to proton displacement damage is explained in terms of the nonionizing energy loss of the protons. The fixed pattern noise does not increase with total ionizing dose. Responsivity changes are observed after Co-60 and proton irradiation, but a definitive cause has not yet been established.

Difficulty and challenges of implementing time-delay-integration (TDI) functionality in a CMOS technology are studied: synchronization of the samples forming a TDI pixel, adder matrix outside the array, and addition noise. Existing and new TDI sensor architecture concepts with snapshot shutter, rolling shutter, or orthogonal readout are presented. An optimization method is then introduced to inject modulation transfer function and quantum efficiency specification in the architecture definition. Moderate spatial and temporal oversamplings are combined to achieve near charge-coupled device (CCD) class performances, resulting in an acceptable design complexity. Finally, CCD and CMOS dynamic range and signal-to-noise ratio are conceptually compared.

The dark current increase due to proton-induced displacement damage is studied in a standard and a radiation-hardened CMOS active pixel sensor (APS). Several devices have been irradiated with protons of different energies and fluences. The influence of the proton energy and fluence on the mean dark current increase and the dark current nonuniformity is investigated. Dark current density histograms are obtained by the theory based on collision kinematics. They are determined by the number of elastic and inelastic collisions and the damage these interactions create in the pixel sensitive volume. It is shown that field enhanced emission has to be taken into account to predict accurately the distribution of the dark current density increase. We also compare the results with data found in literature for charge coupled devices (CCD) and charge injection devices (CID).

The random telegraph signal (RTS) behavior of the dark current has been studied in a radiation-hardened CMOS active pixel sensor (APS). Several devices have been irradiated with protons of different energies and up to different fluences. The influence of the proton energy, fluence, and operating temperature on the amplitude, time constants, and occurrence of the RTS is investigated. Mechanisms for this behavior are discussed and several suggestions are made for possible defect types.