Patent GB-1468892-A (GB)
Title: Method and apparatus for electron beam alignment with a  substrate
IPC: H01J37/305
Filed: 1974-06-03
Abstract: 1468892 Positioning WESTINGHOUSE ELECTRIC CORP 3 June 1974 [15 June 1973 1 Oct 1973] 24457/74 Heading G3N [Also in Division H1]  In the alignment of an electron beam with selected areas of a substrate, the beam is moved until it is located and orientated with at least one alignment portion of the beam in predetermined positional relation with a detector mark utilizing a Schottky barrier contact providing a control signal corresponding to the degree of overlap between the beam portion and the detector. Electron image projection device, Fig. 1. An n or p doped single crystal silicon, or metal, substrate 15 together with Schottky barrier detector marks 39, 40 is bombarded by a patterned electron beam generated by photocathode source 14, layers 30 of Ti O 2  overlaid with gold or palladium layer 28 defining the pattern. The beam is adjusted in orientation and position, preferably automatically, by means of X and Y deflection coils 25 1 , 25 2 , 26 1 , 26 2 , and focusing coils 24 1 -24 3  adjusting rotation and size, until alignment beam portions (43, 44) Fig. 3 (not shown) are either coincident with the detector marks 39, 40 when beam portions and the latter have the same shape, or the portions are aligned with edges of the marks. Alternatively the photocathode, or substrate may be moved by servo-mechanisms for alignment. The photocathode source is irradiated through transparent quartz layer 29, by light from mercury source 27. The marks may be circular, rectangular or triangular in shape. The substrate may support an electroresist, in the making of integrated circuits using successive exposures over the same area. Schottky barrier detector marks, Figs. 3-9 (not shown). Possible constructions comprise: (i) see (Fig. 3). A layer of Si O 2  covering the Si substrate except in the detection area, the whole being covered with conductive material { platinum silicide, chromium-gold alloy, chromium silicide, titanium silicide, or molybdenum-gold alloy } connected to a current indicator or control system. (ii) Sputtered on conductive material, having lesser thickness in the detection area. (Fig. 4) (iii) An epitaxially grown semi-conductor layer exposed for contact with the conductive material in the detection areas, otherwise separated therefrom by an oxide layer. The detection circuit may make contact with the epitaxial layer, by-passing the substrate, which may be sapphire. (Fig. 5). (iv) An epitaxial layer on the substrate overlain by conductive material, thinner in the detection area, (Fig. 6).  (v) An oxide layer may mask the conductive layer except in the detection area, and an area for ohmic contact to the conductive layer, (Fig. 7). In constructions (i) - (v), alignment is achieved when the current through the detector, (54 Fig. 3), is maximum, or when the beam portion is positioned corresponding to the mean point of a current plateau. For alignment of the beam portion with an edge of a detection area, the current attains the mean value of a rise to a current plateau at alignment. Detector marks in which alignment is indicated by minimum or null current may comprise: (v) A conductive layer, forming a Schottky barrier contact with the substrate, and removed in the detection area, (Fig. 8). (vi) An extra thickness of conductive material in the detection area, inhibiting electron induced conduction there. (Fig. 9). Automatic control of alignment, Fig. 10. The output of Schottky barrier detector 39 is modulated (means not shown) and processed in units 71, 73, 75, 77 to produce X and Y alignment error signals gated, when an alignment is started, to integrators 88, 89 providing signals modulating the quadrature output of oscillator 78 to provide signals to the X and Y controls for the beam. Beam angular orientation, and size, error signals are similarly derived from barrier detector 40 to control motor driven potentiometers which control the focusing coils for the beam. All error signals are fed to unit 115, and, when all are null, a reset signal R causes, via flip-flop 117, closing of gates 85, 105, the whole alignment taking place quickly enough to avoid smearing of any images on a photoresist.