X-ray imaging has become a key enabling technology for a wide range of industrial, medical and scientific applications since it allows studying the inside of objects without the need to destroy or dismantle them. In this sense there is a growing research interest in literature to develop advanced X-ray systems capable of obtaining high quality images while reducing the total radiation dose._x000D_ Currently, X-ray imagers are dominated by hybrid systems, built from a pixel array of direct conversion X-ray detectors and its corresponding readout integrated circuit (ROIC). Despite their higher cost and limited area compared to classical indirect counterparts, the advantages of these systems are clear in terms of radiation dose reduction, signal integrity improvement and spatial resolution scaling. Concerning the readout method used by the ROICs, the most common design strategy is based on photon-counting, due to its advantages regarding circuit noise immunity and photon classification. However, these X-ray imaging systems tend to experience from information losses caused by charge-sharing and pile-up effects._x000D_ In this context, the goal of the presented thesis work is to propose specific analog and mixed circuit techniques for the full-custom CMOS design of low-power and compact pitch digital pixel sensors (DPS) for ROICs targeting hybrid and direct conversion X-ray imagers._x000D_ The proposed pixel architecture, based on the charge-integration readout method, avoids information losses experienced by photon-counting and contributes to X-ray image quality by a compact pixel area and low-power consumption to improve image resolution and reduce heating of X-ray detectors, respectively. In this sense, the proposed CMOS DPS circuits feature in-pixel A/D lossless charge conversion for extended dynamic range, individual gain tuning for pixel array FPN compensation, self-biasing capability and digital-only interface for inter-pixel crosstalk reduction, built-in test capability for costs reduction, selectable electron/hole collection to wide the applications range and in-pixel dark current cancellation. Furthermore, the proposed design techniques are oriented to the future development of truly 2D modular X-ray imager systems with large scale and seamless sensing areas._x000D_ All the above circuit design research has been materialized in several generations of DPS demonstrators, with pitch values ranging from 100μm down to 52μm, all of them integrated using standard 0.18μm 1P6M CMOS technology._x000D_ Extensive analysis of both electrical and X-ray measurements on the pixel circuit prototypes have been done to proof their validity. Experimental results align this work not only within but also beyond the state-of-the-art active pixels in terms of spatial resolution, power consumption, linearity, SNR and pixel flexibility. This last point makes the proposed pixel design techniques specially suitable for a wide range of X-ray image applications.
Low-Power and Compact CMOS Circuit Design of Digital Pixel Sensors for X-Ray Imagers
Figueras Bagué, R. (Author). 10 Apr 2015
Student thesis: Doctoral thesis
Student thesis: Doctoral thesis