High Fill Factor a-Si:H Sensor Arrays with Reduced Pixel Crosstalk
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High Fill Factor a-Si:H Sensor Arrays with Reduced Pixel Crosstalk Y. Vygranenko1, A. Sazonov2, D. Striakhilev2, J. H. Chang3, G. Heiler3, J. Lai3, T. Tredwell3, and A. Nathan4 1 Electronics Telecommunications and Computer Engineering, ISEL, Lisbon, 1950-062, Portugal 2 Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 3 Carestream Health, Rochester, NY, 14652-3487 4 London Centre for Nanotechnology, UCL, London, WC1H OAH, United Kingdom
ABSTRACT In this paper, we report on low noise, high fill factor amorphous silicon (a-Si:H) image sensor structures for indirect radiography. Two types of the sensor arrays comprising n-i-p photodiodes and m-i-s photosensors have been fabricated. The device prototypes contain 100 x 100 pixels, with a pixel pitch of 139 µm. The active-matrix addressing is provided by low offcurrent TFTs. The sensors are vertically integrated onto the TFT-backplane, by implementing a 3-µm-thick low-k interlayer dielectric. This dielectric layer serves to reduce the data line capacitance and to planarize underlying topography. The detector was designed for reduced dataline resistance and parasitic coupling. Details of the device design and fabrication, along with sensor performance characteristics, are presented and discussed. INTRODUCTION Active-matrix, flat-panel X-ray detectors based on amorphous silicon (a-Si:H) technology are widely used for medical imaging [1]. Here, an X-ray phosphor screen is coupled to an array of a-Si:H sensors and an a-Si:H thin-film transistor (TFT) backplane, which, in turn, are connected to external electronics. The design of the array needs to targets the most important performance features, such as the sensitivity, dynamic range and spatial resolution. In planar architecture imaging arrays, where the TFTs and photo-sensors are juxtaposed on the same plane, the fill factor is inherently low due to the limited area available for the sensor. Alternatively, in high fill-factor array designs, the sensor array is vertically integrated on top of the TFT backplane. In this paper, we report on active matrix X-ray detectors, with high fill-factor a-Si:H sensor arrays. The novelty of our array design is that the data lines are formed with the top metallization layer in order to reduce the data line resistance and parasitic coupling. Furthermore, we used a segmented sensor design to reduce pixel crosstalk for improved spatial resolution. Two types of device prototypes, one with n-i-p photodiodes and one with m-i-s photosensors, were designed, and characterized for their electrical and optical performance.
EXPERIMENT The detector prototype has 100 x 100 pixels, with a pixel pitch of 139 µm. The active-matrix addressing is provided by low off-current TFTs. Figure 1(a) shows a cross-section view of the pixel. Here, a photosensor (either n-i-p or m-i-s structure), with metal bottom electrode and ITO top electrode, is located on top of a TFT array and separated by an insulation-planarization layer. The gate line serves to bias the TFT
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