Patent GB-1432797-A (GB)
Title: Aligning a radiation beam of predetermined cross-section with a detector mark of like cross-section
IPC: H01J37/04
Filed: 1973-06-05
Abstract: 1432797 Automatic control of position WESTINGHOUSE ELECTRIC CORP 5 June 1973 [20 June 1972] 26663/73 Heading G3N  The automatic alignment of an electron beam is effected by moving the beam across a mark in a predetermined overlapping pattern and positioning the beam in response to a comparison of the amplitudes of two derived frequency signals for a preselected alignment relationship.  An electron image projection system comprises a hermetically sealed chamber 10, Fig. 1, having end-caps 11, 12 and a port 13 for establishing a partial vacuum. Within the chamber, mounted on the end-cap 11 by supports 17, 18 and an insulating spacer 19, are an electromask 14 and a semiconductor wafer 15. The wafer 15 is mounted in a holder 16 within permissible limits. The mask 14 is made the cathode and wafer 15 the anode between which a high potential is applied. The mask comprises a gold or palladium layer 28 on a quartz substrate 29 which is overlaid with titanium dioxide negative of the component pattern and is irradiated by a mercury vapour lamp 27. Part of the electron beam emitted by the mask 14, which is circular in cross-section, impinges on conductive rectangular masks (not shown) on the wafer and the signals set up in the masks comprise a fundamental and a fourth harmonic, the relative amplitudes of which are indicative of electron beam alignment and are used to adjust the energization of electromagnet assemblies 24, 25, 26 whereby the electron beam from the mask is brought into alignment with the wafer. The circular positional modulation of the electron beam during alignment, whereby the fundamental and harmonic signals are generated, is effected by quadrature modulation of the energizing current to electromagnets 25, 26. The automatic alignment system, Fig. 6, is initiated by a signal 132 to a flip-flop 123 which, through a gate 124 causes an oscillator 62 to provide quadrature signals to adders 77, 78, determining currents to the X, Y electromagnets 24, 25 respectively, and open gates 70, 73, 98 and 101. The fundamental and harmonic signals are fed through pre-amplifiers 51, 82, separated by tuned amplifiers 54, 55, 85, 86. The fundamental signal from amplifier 54 is fed through a phase adjuster 59 and divider 60 which passes the signal or reduces it in a ratio to the harmonic, to a dual phase detector 61 providing X and Y error signals which, through gates 70, 74 are integrated 67, 68 to provide alignment signals to the electromagnets. Similarly the fundamental signal from amplifier 85 is processed to provide from a dual-phase detector 92 a 0-error signal on a line 97, which through a gate 98 is integrated 95 to provide signals to the electromagnets for rotating the image on the wafer, and a M-error on a line 100 which, through a gate 101  is integrated 96 to provide signals to the electromagnets for adjusting the scale M of the image. Alignment is determined by comparators 105, 112 which through a null-detector 121 reset the flipflop 123 and thereby open gates 70, 73, 124, 98, 101 and so hold the alignment signals to the electromagnets.