Patent GB-1196314-A (GB)
Title: Beam Intensity Detecting Device
IPC: H01J37/244
Filed: 1967-08-01
Abstract: 1,196,314. Electron beam focusing systems. INTERNATIONAL BUSINESS MACHINES CORP. 1 Aug., 1967 [29 Aug., 1966], No. 35207/67. Heading H1D. [Also in Division G1] A beam of radiation is precisely focused at a predetermined point by scanning the beam across a grid member, positioned at the point at which the beam is to be focused, and behind which is arranged a radiation detector, the rate of change of the electric current output of the detector being detected as an indication of the intensity and thus the size of the beam when incident on the grid. This measure of the size of the beam can then be used to adjust the focusing system in order to precisely focus the beam at the point required. The invention is described with reference to the focusing of an electron beam 13 which is to be used to record information on a thermoplastic or silver halide memory element 15. The recording apparatus comprises a chamber 11 containing a cathode 12, an anode 18 through which an electron beam 13 passes, focusing coils 19, 20 and apertures 21, 22. The final focus of the beam is controlled by coarse and fine coils 24, 25. In recording data on memory device 15, scanning of the beam repeatedly across the plane of the device is provided by coil 26 and gating of the beam to reach the device is by electrostatic plates 29. During recording, precise focusing of the beam on to the plane of the device is necessary to produce the smallest spot possible in order to increase the density of information recorded. However, beam current level changes &c. occur and precise focusing is lost. Therefore, in between changing of memory devices 15, a grid member 41, Fig. 5, is placed in the position previously occupied by a memory device. The positioning of member 41 is sensed to cause energization of coil 100, closing of switches 98, 99 and commencement of a refocusing cycle. The beam is scanned across the grid 41 by coil 26 in the same manner as with the memory device and the output of a PN junction detector 43 behind the grid 41 is used to obtain a measure of the size of the beam cross-section and cause refocusing of the beam accurately in the plane of the grid by control of current to the fine focus coil 25. The circuit used includes an oscillator 49 providing a square-wave signal which is supplied to the coil 25 through amplifier 30 together with a signal 32 which depends on how far the beam is out of focus. The square-wave modulation causes the beam to be brought to a focus alternately at two points equal distances either side of the plane of focus determined by coarse coil 24. The size of the beam crosssection incident on the grid when the beam is focused at one of these secondary focus points is different from its size when focused at the other secondary focus point except when these secondary focus points are at equal distances either side of the grid 41. In order to measure the spot size and thereby detect when the spot size for the secondary foci are equal, the detector output, after amplification at 50, is differentiated in circuit 51. The output of circuit 51 is the rate of change of detector current and is dependent on the intensity or size of the beam passing through grid 41. This is because a smaller spot will pass the edge of one of the slots in grid 41 more quickly than will a larger spot, Fig. 6 (not shown). The signal is rectified at 52 and applied to a peak sensing circuit 59 which also receives the signals from oscillator 49. The circuit includes transistors 69, 70 which are gated on alternately, so that capacitors 78, 77 are alternately charged up and store values representing the spot sizes for the two secondary foci. These peak voltages are fed continuously to a differential amplifier which transmits the voltage difference representing the amount by which the beam is out of focus to a holding circuit 66. This comprises a capacitor 79 which stores the control (difference) signal from the amplifier 65 and controls the supply of current from terminal 81 to the coil 25 (via amplifier 30) by means of a field effect transistor 80. Thus after removal of the grid 41 and positioning of a new device 15, the beam is held at the required focus by the supply of the correct amount of current I FC  to fine focus coil 25 this being decided by the charge on capacitor 79.