Patent GB-1306115-A (GB)
IPC: H03K3/023
Filed: 1970-04-08
Abstract: 1306115 Photo-electric focusing conture MULLARD Ltd 8 April 1970 16672/70 Heading G1A [Also in Divisions G3 and H3] A beam of radiation is focused on a target by providing the latter with at least one mark having a different response to the radiation than has the surrounding portion of the target repetitively deflecting the beam on and off the mark(s) at a constant speed to obtain a signal dependent on the radiation variation between the target and mark(s), measuring the time taken for the instantaneous value of the signal to change between a first and a second value, which time is indicative of the dimension of the focused beam spot as it crosses the edge of the mark(s) and automatically varying the focus of the beam so that the time becomes a minimum which corresponds to the optimum focus condition, the D.C. level of the signal being set at substantially the same value prior to individual deflections on and off the mark(s), to make the time independent of the time spent on or off the mark(s) prior to each individual deflection. In the embodiment of Fig. 5, the electron gun 2 of electron beam machining apparatus produces a beam for focusing on a target 3 provided with at least one mark having an electrical resistance between its free surface and the target bulk which is different from that between the target free surface and its bulk. The beam is moved at a constant speed over the target by applying X and Y deflection signals to deflection coils 4, 5, and is focused by sequential adjustment of a main focusing coil 6 and astigmatism correction coils 7, 8, the coils 4--8 being fed with signals from a programmer 9 (which may be an on-line computer) through D/A converters 10--14 and amplifiers 15--19. Passage of the beam across the mark produces a flat-topped pulse signal, Fig. 1 (not shown), which is passed through an amplifier 21 whose output is A. C. coupled to the inputs of Schmitt trigger circuits 22, 23 via a capacitor 24, these inputs receiving an A.C. signal which is approximately triangular in shape, Fig. 2 (not shown). The A.C. coupling (differentiating) circuit has a time constant long relative to the rise and fall periods of the pulse signal and short relative to the period of its flat top. Each portion of the triangular signal has an initial steep side corresponding to a rise of fall period of the pulse, and the trigger values of each Schmitt trigger circuit correspond to a lower value of the steep side of one portion and an upper value of the steep side of the next portion, Fig. 2 (not shown). Thus the Schmitt trigger circuits produce pulse outputs having time periods respectively corresponding to the period between the lower value of the (one) first portion and the upper value of the next portion, Fig.3a (not shown), and to the period between the upper value of the (one) first portion and the lower value of the next portion, Fig.3b (not shown). Application of the outputs of circuits 22, 23 to an exclusive OR circuit 25 produces pulse outputs from the latter corresponding to the rise and fall periods of the pulse signal, Fig. 4 (not shown). To obtain the above mentioned time constant relationship without adversely altering the beam deflecting speed and/or the size of the mark, diodes 30, 31 are connected as shown and conduct only when the voltage across them becomes substantial, e.g. half the peak voltage from amplifier 21. Thus the input to the circuits 22, 23 is clamped to the voltages of the diodes 30, 31. The circuit shown in Fig. 6 enables the diodes 30, 31 to be omitted and comprises amplifiers 32, 33 which are connected as Schmitt triggers and have their outputs limited by diode chains 45, 47. During the operation of the amplifiers 32, 33, diode chains 51, 52 and diodes 49, 50 are forward or reverse biased to maintain the potential at terminal 48 or connect it to earth, a typical cycle of operation being explained to shown how the circuit produces accurate signals irrespective of whether the above mentioned time constant relationship holds or not. Fig. 8 is an alternative to Fig. 6 and employs two comparators 61, 62, a symmetrical limiter 60, a logic circuit 65 and a circuit 68 comprising a flip-flop followed by a limiting amplifier. A typical cycle of operation of this embodiment is also discussed. The output from OR circuit 25 is fed to a timer 26 whose output to programmer 9 takes the form of a measure of the slope of the rise and fall periods of the pulse signal. Programmer 9 then varies one of the coils 6, 7, 8 and the process of slope measurement and coil variation continues (the main focus and astigmatism controls being varied in turn) until optimum focus is obtained. The embodiment of Fig. 5 may be combined with the beam positioning apparatus described in Specification No. 1211529, Fig.7 (not shown). The radiation change between the mark and target may be detected by an electrode or a scintillator and photo-multiplier arrangement responsive to secondary emission from the mark and target. Several marks are provided with edges lying in various directions to enable focusing to be achieved in these directions.