Detection Limits of a nip a-Si:H Linear Array Position Sensitive Detector
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Detection Limits of a nip a-Si:H Linear Array Position Sensitive Detector R. Martins1, D. Costa1, H. Águas1, F. Soares1, A. Marques2, I. Ferreira1, P. Borges2, E. Fortunato1, (1) Materials Science Department of, Faculty of Sciences and Technology of New University of Lisbon and CEMOP/UNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal; (2) Tekelec, Chão de Murches 2750 Cascais, Portugal ABSTRACT This paper presents results of the spatial and frequency detection limits of an integrated array of 32 one-dimensional amorphous silicon thin film position sensitive detectors with a nip structure, under continuous and pulsed laser operation conditions. The data obtained show that 0.45×0.06 cm arrays, occupying a total active area of about 1 cm2 have a spatial resolution better than 10 µm (modulation transfer function of about 0.2), with a cut-off frequency of about 6.8 KHz. Besides that, under pulsed laser conditions the device non-linearity has its minimum (about 1.6%), for a frequency of about 200Hz. Up to the limits of the cut-off frequency, the device nonlinearity increases to values above 4%. INTRODUCTION The present demand of unmanned systems automation and image processing in real time makes the analogue optical sensors based on laser triangulation principles one of the main candidates able to satisfy these requirements. These sensors to be integrated in an inspection camera should be miniaturized and guaranteed their reliable. Similarly to what happens with Charge Coupled Device Detectors (CCD) [1] the data recorded are computer processed, but now with a major advantage: the detection can be performed continuously and not limited to the 50 or 100 frames per second. Besides that, if an array is used for 3D profile measurements, the number of arrays will supply the lateral bits resolution. For a 32 linear array position sensitive detector (LAPSD) although the lateral resolution is only of 5 bits, the acquisition of a surface/contour is larger than 5×103 measurements per second, since we have to take into account the analogue longitudinal detection. Nevertheless, the lateral bit resolution should be improved by having arrays with more lines (say, 256, for 8 bits resolution). Besides that, the device static (spatial frequency) and dynamic limit detections should be well-known in order to determine the device performances. This is the objective of the present paper concerning LAPSD of 32 elements based on nip a-Si:H structures. 2. OPERATION PRINCIPLES The operation principle of a detector array is that an image line projected in the array induces photocurrents (∆I ) or lateral photovoltages (∆V) in the illuminated elements [2]. All elements are then scanned to determine the position of the image line. In this case the angle of incidence that the laser line makes with the surface to be inspected should be such that allows its detection along the length of the detector. That is, the maximum length dd of each element of the array has to be such that dd(max)≥d0(max.)/cos(φ), where d0 is the distance between the
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