GB1328976A - Electronic scanning devices - Google Patents

Abstract

1328976 Optical positioning MARCONI CO Ltd 22 Dec 1970 [15 May 1970] 23620/70 Heading H4D [Also in Division G4] A pattern of square marks, Fig. 2, is applied to a specimen to assist in aligning the specimen in the beam of a scanning electron microscope. The output from the microscope is applied to a display device via an amplifier 1 Fig. 3, and lead 2. An oscillator 7 controls the scanning circuitry 4 of the microscope to give line frequency and field frequency outputs 4L, 4F which are fed to the deflection coils via leads 3X, 3Y. The circuitry 4 also supplies an output to a gate 8 which is gated on when the beam is scanning the specimen to allow through the output from amplifier 1 to a matched filter 9 which passes signals at the frequency corresponding to scanning of a line or column of the marks on the specimen. For aligning the specimen in the x direction, Fig. 2, switches 5, 6, Fig. 3 are first set to their a positions which causes the scanning raster to scan in lines L 1 , L 2 , L 3 parallel to the y-axis. The output of filter 9 is fed to a detector 10 which, when a signal of sufficient strength is passed by filter 9 gates on a gate 11 which passes the signal from a line counter 12 to a comparator 13 where it is compared with a predetermined number set in register 14, and an output is given on one of the lines 131, 132, 133, depending on whether the line being scanned is to the right of, the same as, or to the left of the predetermined line with which it is desired to align the first column of marks. The alignment of the specimen is then coarsely adjusted manually or automatically. Fine adjustment of the specimen is achieved by first switching switches 5, 6 to position b, the scanning raster then scanning in lines H 1 , H 2 , H 3 parallel to the x-axis. The output from filter 9 is fed to a phase comparator 15 where the phase of the signal due to the first line of specimen marks is compared with the phase of a signal from oscillator 7 via a variable delay 16. The output is passed via a low pass filter 17 and manual or automatic adjustment is carried out to finely align the specimen e.g. with scanning line L 3 running down the left-hand vertical edge of the first column of marks. Two circuits such 'as that shown in Fig. 3, one for aligning parallel to the x-axis and the other for aligning parallel to the y-axis may be used or one circuit with additional switches may be used for both x and y-axis alignment.