2938 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 5, OCTOBER 2009 Characterization and Testing of LAS: A Prototype ‘Large Area Sensor’ With Performance Characteristics Suitable for Medical Imaging Applications S. E. Bohndiek, A. Blue, J. Cabello, A. T. Clark, N. Guerrini, P. M. Evans, E. J. Harris, A. Konstantinidis, D. Maneuski, J. Osmond, V. O’Shea, R. D. Speller, R. Turchetta, Member, IEEE, K. Wells, H. Zin, and N. M. Allinson Abstract—The Large Area Sensor (LAS) is a 1350 1350 array of active pixels on a 40ìm pitch fabricated in a 0.35ìm CMOS process. Stitching technology is employed to achieve an area of 5.4 cm 5.4 cm. The sensor includes ’regions of reset’, whereby three different integration times can be set on the array to achieve a large imaging range for static scenes. Character- ization of the noise performance included temporal and fixed pattern sources. LAS was found to have a read noise of 62 , a full well capacity of 61 and a conversion gain of 5 per digital number (DN). The fixed pattern noise (FPN) was evaluated at half saturation; within a single stitched section of the array, column-to-column FPN was found to be 0.6%, while the pixel-to-pixel FPN was 3%. Both FPN sources were found to be gain related and could be corrected via flat fielding. Based on the results of characterization, LAS was coupled to a structured CsI:Tl scintillator and included in an X-ray diffraction system developed for the analysis of breast biopsy samples. Data acquired with plastic test objects agrees with that acquired by a previous prototype sensor. It is demonstrated that an imaging output range of 140 dB can be achieved using integration times of 0.1 ms to record the transmitted X-ray beam and 2.3 s to record the lower intensity scattered radiation. Index Terms—CMOS, image sensors, semiconductor devices. Manuscript received April 09, 2009; revised July 03, 2009. Current ver- sion published October 07, 2009. This work was supported by the RC-UK Basic Technology Multidimensional Integrated Intelligent Imaging (MI-3) programme (GR/S85733/01). S. E. Bohndiek was with the Department of Medical Physics and Bio- engineering, University College London, London WC1E 6BT, U.K. (e-mail: sbohndiek@googlemail.com). A. Blue, D. Maneuski, and V. O’Shea are with the Physics and Astronomy Department, University of Glasgow, Glasgow G12 8QQ, U.K. (e-mail: v.oshea@physics.gla.ac.uk). J. Cabello and K. Wells are with the Centre for Vision, Speech and Signal Processing, University of Surrey, Guildford GU2 7XH, U.K. (e-mail: K.Wells@eim.surrey.ac.uk). A. T. Clark, N. Guerrini, and R. Turchetta are with the CMOS Sensor De- sign Group, Rutherford Appleton Laboratory, Didcot OX11 0QX, U.K. (e-mail: r.turchetta@rl.ac.uk). P. M. Evans, E. J. Harris, J. Osmond, and H. Zin are with the Joint Physics Department, Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Surrey SM2 5NG, U.K. (e-mail: phil.evans@icr.ac.uk). A. Konstantinidis and R. D. Speller are with the Department of Medical Physics and Bioengineering, University College London, London WC1E 6BT, U.K. (e-mail: rspeller@medphys.ucl.ac.uk). N. M. Allinson is with the Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K. (e-mail: n.allinson@shef.ac.uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2009.2029575 I. INTRODUCTION A CTIVE Pixel Sensors (APSs) have emerged in the last two decades as a valuable alternative to the CCD in many scientific imaging applications. Fabricated in complementary metal oxide semiconductor (CMOS) technology, the APS can include complex circuits, and in turn functionality, at the pixel level. CMOS processes also confer other advantages, such as low power consumption, potential for low cost and fast-scaling technology [1]. Compared to the CCD, the APS provides high frame rates and the potential for true random access via column parallel readout. Recent developments in APS design and fabrication technology have helped to reduce read noise, improve quantum efficiency and achieve wafer size sensors via “stitching”. These developments, together with the ability to tailor image sensor functionality, have stimulated the uptake of the APS among the scientific community. Particular interest has arisen in “stitching” technology, which enables an APS design to be seamlessly scaled up to 12 cm 12 cm (standard wafer limits) without loss of performance [2]. This could be of great benefit in medical imaging, where large pixels, high noise, low frame rates and artefacts such as image lag limit conventional flat panel imagers. Improved tu- mour detection could be achieved with pixel sizes as small as 50 , as well through advanced digital mammography techniques such as dual energy contrast enhanced kinetics, or tomosyn- thesis, which require: fast image acquisition without image lag, full field coverage of the breast, good conversion gain and high detective quantum efficiency [3]. A number of commercial large area Active Pixel Sensors are available. For example, the Hamamatsu C9732 DK [4] measures 120 mm by 120 mm with 50 pixels and has 75 dB dynamic range despite 1250e- read noise. The RadIcon RadEye 100 [5] has dimensions of 98 mm by 49 mm with 96 pixels and has been designed to be ’3-sides’ buttable, so it can reach much greater areas when tiled. This design has a much lower read noise of 250e- but suffers poor conversion gain of 0.2 . Although these provide the large area required by medical im- agers, they suffer relatively high read noise, poor conversion gain and limited dynamic range. The development of the present APS has been primarily driven by the need for a sensor to perform X-ray diffraction studies of biological tissue, in particular for breast cancer 0018-9499/$26.00 © 2009 IEEE Authorized licensed use limited to: CAMBRIDGE UNIV. Downloaded on November 9, 2009 at 06:00 from IEEE Xplore. Restrictions apply.