A New Linear Array Thin Film Position Sensitive Detector (LTFPSD) for 3D Measurements
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The developed 3D profile
laer
inspection system based in arrays of 1D TFPSD can be used directly to inspect 3D production pieces or finished articles, in a non contact
manner, where the main limitation
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is the specifications of the mechanical tools and not the ones related to the detector (such as conveyors where quality control of each product or piece can be performed item by item, in real Figure 1 - Illustration of the 3D measurement performed by the time). LTFPSD. 797 Mat. Res. Soc. Symp. Proc. Vol. 377 @1995 Materials Research Society
OPERATION PRINCIPLE Each elemental 1D-TFPSD of the array has the end-terminals connected to an analogue shift register (SR-MX4 circuit) to process the information recorded in both end-contacts as shown in figure 2. Per stripe, we get the information concerning the "point" of the line projected on the array as: Pn(Yn) = 2(0, iiAn L(I0,n-+II,n) (1) 'nyj=2(Io,n + Ii,n) where L is the length of each stripe, the subscripts 0 and 1 refer to the position of each endcontact, n is an integer that varies from I to 128 and the reference is considered to be the centre of the gravity of each elemental ID TFPSD. The recorded values are processed in sequential series ([I0,1 ...10,n] and [II,1...II,n]) by the SR whose output correspond to a voltage signal that is fed to the SENSIT 2 card. From that card, a sequential voltage information is obtained concerning Pn as given by equation (1). After that, the information is supplied to an A/D card to digitise the analogue signal (Pn) prior to be received by the computer. The SR's and the A/D converters (12 bits) are commanded by a controller box that determines the required time delays and the number of scans allowed per second, between information pockets, using the same clock command at frequencies up to 1 MHz. The position of an image line projected in the plane z-y is determined by the Yn data obtained by the 128 stripes and related to the currents detected by both SR, as above explained. The third variable (x) is the angle between the light source (laser) and/or the detector (TFPSD array) with the object plane to be inspected and by so, the determination of the depth profile related to the image received by the TFPSD array. :;:i•:V
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Figure 2 - Schematic diagram of the sensor electronics. EXPERIMENTAL DETAILS Device fabrication steps The structure of the LTFPSD is presented in figure 3, which illustrates the plane (top) and cross-sectional views. The fabrication process in indicated in Table I, and has the following steps: I- deposition of an Indium Tin Oxide (ITO) onto a glass substrate (front equipotential) by reactive thermal evaporation and patterned using standard photolithography processes; 2- deposition of the p.i.n a-Si:H structure by PECVD techniques, using the procedure described elsewhere 3 . To prevent lateral leakage currents, 4 the p.i.n structure was also patterned; 3- finally an Alum
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