Mechanisms of cross-talk in large area a-Si:H continuous image sensors
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Mechanisms of cross-talk in large area a-Si:H continuous image sensors M. Mulatoa), J. P. Lu, S. E. Ready, K. Van Schuylenbergh, J. Ho, R. Lau, J. B. Boyce and R. A. Street Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA a) Present Address: Departamento de Física e Matemática, FFCLRP-USP, Ribeirão Preto, SP, 14040-901, Brazil
Abstract We report studies of the image-blur effects caused by lateral cross-talk between neighboring pixels of large-area amorphous silicon (a-Si:H) image sensors. The lateral conduction is attributed to three effects: conduction along the interface between the a-Si:H film and the underlying passivation; fielddependent electron injection at the edge of the sensor; and a field enhancement of the interface conduction due to the bias applied to the address lines. We show that the cross-talk can be controlled by choice of the operating conditions and optimization of the materials.
INTRODUCTION High sensitivity and resolution (pixel pitch smaller than 100 µm) large area twodimensional imagers, used for X-ray medical applications such as mammography and also CMOS sensors, can only be satisfactorily manufactured through the use of the high fill factor design. The increased performance is achieved by depositing a continuous a-Si:H layer over the whole surface area of the array [1,2,3]. An effective fill factor (defined as the ratio of the sensitive area to the total pixel area) of > 90 % can be achieved with a pixel size of 75µm [4]. A buried material with low dielectric constant insulates the sensors from the TFT addressing electronics. This structure brings the possibility of extra parasitic capacitance [5,6] in the addressing data lines and lateral conduction between pixels [1,6,7]. We have shown that the quality of the interface between the a-Si:H and the buried insulator layer is more relevant than the buried insulator material itself, and that cross-talk values below 1% can be achieved [6]. Optimization of the array is a complex materials problem involving the dielectric, the interface to the a-Si:H sensor and the array operation. This paper presents more detailed information about the origin of the lateral conduction and magnitude of the cross-talk, and in particular we identify a dynamical effect arising from the imager operation conditions associated with switching the gate-lines, which can be suppressed by suitable design of the readout. EXPERIMENTAL The structure of the high fill factor array is presented in detail elsewhere [1,5,6]. The aSi:H TFT and metal address lines are deposited and patterned first. The buried insulator material covers these structures and is patterned to create a contact between the TFT and the bottom electrode of the a-Si:H sensor. The arrays used in the present work have 1 µm-thick amorphous silicon oxynitride as the buried insulator material. A metal and the n+ a-Si:H layer are deposited to define the sensor contact, and then the intrinsic and p+ a-Si:H layers, as well as the top ITO metal contact are deposited as a continuous film over the
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