Patent GB-1211227-A (GB)
Title: Improvements in or relating to apparatus for positioning the area of incidence of a beam of radiation on a target
Inventor: UNDERHILL MICHAEL JAMES
IPC: H01J37/304
Filed: 1967-05-17
Abstract: 1,211,227. Template followers; programme control. MULLARD Ltd. Jan.26, 1968 [May 17, 1967], No.22960/67. Heading G3N. An apparatus for positioning the area of incidence of a beam of radiation on a target provided with a reference mark or marks having a different response to the radiation than the surrounding portion of the target and being at an angle to the beam axis and each mark having a first and second boundary edge portions at an angle to each other, has means scanning the beam over the edge portions and means for deriving an output signal in response to radiation incidence on the said target which is fed to first and second synchronous gates both coupled with the scanning means and means for feeding the outputs of the synchronous gates to deflection means to deflect the scanning beam in directions parallel to the edge portions so as to make the intersection between the beam and the edge portions coincide with a pair of unique points. The beam from an electron gun 2 is required to be located upon a point on a target 3 an area adjacent to the point having a different resistivity to the remainder of the target. The beam is positioned by a programme arrangement approxiamately on the point which is defined by the intersection of two lines so that when the beam is scanned preferably in a circular orbit by means of an oscillator 16 connected to X and Y deflection coils 4, 5 the beam will cut both lines. The electrons are collected on a lead 8 passed through an amplifier 9 and a reference level device 21 to synchronous gates 10, 11 which also have an input from the oscillator 16. If the centre of the beam scan lies other than in the position shown synchronous gates will have outputs other than zero which through deflection means 12, 13 move the beam into its correct position the gate outputs being stored at 14, 15 and added to that of the programme arrangement 17 so that the trace of the beam derived therefrom will start from its correct position relative to the remainder of the target 3. The synchronous gates may comprise phase detectors and if D.C. coupling is present between the lead 8 and the inputs of the gates, a component may be arranged to supply a D.C. bias so that peak level in its output are symmetrical about zero. In a preferred arrangement the coupling is A.C. The store outputs may apply a D.C. shift to the beam position via the deflection means 12, 13 or it may be arranged to alter the gain of the deflection means or it may be used as a control voltage for a correction term generator coupled with a deflection means. The beam may be used to cut out selected portions of the target or the target may be covered with photo-resist and the beam used to expose selected portions thereof, or the beam may be caused to deposit the chemical on the target surface by decomposing a chemical compound present in envolope 1 in the form of a vapour or gas. If the reference mark has different secondary emission characteristics to that of the surrounding target surface, a collector anode 21 may be provided inside the envelope to collect secondary electrons emitted from the target. The inputs to coils 4,and 5 may be alternatively applied to common coils 6 and 7.  More than one mark may be employed on any one target the scanning being switched at appropriate points in the programme by the arrangement 17. The beam need not scan in a circular arc of closed loop form and the mark 18 may be replaced by a pair of marks divided by a blank strip along a diagonal, in which case the gates must be muted when the scan passes over the strip. The scan should be switched off, when not required, by the arrangement 17.