A Quantum-Limited Highly Linear Monolithic CMOS Detector for Computed Tomography

Computed tomography (CT) detector requires quantizing the wide dynamic range (WDR) photocurrent with high speed. Existing CMOS CT detectors are limited by their low linearity. They also need high-resolution external analog-to-digital converters (ADC) to quantize the WDR. In this work, a novel synchronous partial quantization technique is developed to enable monolithic CMOS CT pixel detector. Improved from previous current quantization designs, the new pixel detector quantizes the photocurrent in the charge domain with constant charge packet size. As the result, the pixel detector can quantize the WDR photocurrent with improved linearity. The two-step charge-domain ADC scheme also allows low resolution single-slope ADCs for residual voltage quantization, which enables in-pixel integration. A prototype pixel detector is fabricated in a 0.35 mu m CMOS process. Silicon measurements show the detector can quantize signal currents from 6 pA to 63.4 nA with relative gain deviation lower than 0.06% at 1.1 kHz frame rate, which does not generate detector-induced artifact after the image reconstruction. The detector noise is smaller than the X-ray Poisson noise in the whole signal range. The minimum detector noise is 0.8 pA(rms). Hence, the detector achieves 16.6 bits dynamic range. The geometrical detective efficiency (GDE) of the detector is 75.5% including the band-gap voltage reference and ADC. The digital pixel detector can be easily tiled into a 2-D CT detection panel.