Patent GB-1475431-A (GB)
Title: Exposure apparatus for printing
IPC: G03F7/20
Filed: 1975-01-10
Abstract: 1475431 Exposure apparatus for printing CANON KK 10 Jan 1975 [12 Jan 1974 29 March 1974] 1060/75 Heading G2A Exposure apparatus for printing an integrated circuit on a semi-conductor wafer 18 bearing a photo-resist layer 17 comprises a light source 11, a convergent optical system, e.g. a condenser lens 12, to collimate light from the source 11, a multiple flux generating assembly, e.g. a fly's eye lens assembly 13, to form a plurality of divergent component fluxes, a collimating lens system, e.g. a condenser lens 15, having its primary focal point lying in the plane F in which lie the focal points of the lens elements of the lens assembly 13 and a photomask 16 having an integrated circuit pattern formed in it and supported by a member 19, the mask being within 6 to 30 microns, for example, from the wafer 18 and substantially at the entrance pupil of the lens 15. The light source 11 is preferably a mercury vapour lamp and it is provided with a concave mirror 10, and an adjustable diaphragm 14 controls the maximum angle of incidence of the rays reaching the mask 16. The lens elements of the lens assembly 13 are sandwiched between spaced metal plates having openings slightly smaller in diameter than the lens elements, and they may be positive lens elements, Fig. 5 (not shown), in which case the focal plane F would be at the opposite side of the assembly 13. The multiple flex generating assembly 13 may be a ground or opal glass screen with the primary focal point of the lens 15 lying in the plane of the diffusing surface of the screen. In a second embodiment, Fig. 6 (not shown), the convergent optical system consists of a positive lens (23a) and a negative lens (23b). A standing wave compensating filter (24) is disposed between the negative lens and the multiple flux generating assembly (25) which may be a fly's eye lens assembly, an optical fibre plate, the fibre having a refractive index varying with the radius of the plate, a diffusing plate, or a direction selective diffusing element, and a diaphragm (26) is disposed adjacent, the assembly. Between the collimating lens system (27) and the photomask (29) is an illumination compensating filter (28). The standing wave compensating filter (24) is chosen from a plurality of such filters which can be selectively positioned in the optical path, and it acts to produce light of two or more different prominent wavelengths and intensities selected in accordance with the spectral characteristics of the light source and the spectral sensitivity of the photo-resist layer. The lights are incoherently superposed so that the peaks and valleys of the individual standing waves produced by the interaction of the light entering the photoresist layer and the light reflected from the surface of the wafer, which is usually of silicon or a metal such as aluminium, cancel each other out over at least a portion of the thickness of the photo-resist layer. A transparent layer is provided between the wafer 18 and the photo-resist layer so that the position of the photo-resist layer can be adjusted to its best position depending upon the interaction of the standing waves. The illumination compensating filter (28) comprises a plano-convex lens element (31) of a material having a larger absorbance for rays to which the photo-resist layer is exposed, the element (31) being in contact with a plano-concave lens element (32) of a material having a smaller absorbance, and the different axial thickness and radii of curvature of the elements (31) and (32) are chosen to suit, Fig. 14 (not shown). Alternatively, the filter (28) may be a transparent plate on which a metal such as aluminium or chromium is evaporated so as to impart to the plate a continuous or discontinuous distribution of density.