Patent GB-1484788-A (GB)
Title: Catadioptric projection printer
IPC: G03F7/20
Filed: 1975-08-27
Abstract: 1484788 Projection printing of circuit patterns TROPEL Inc 27 Aug 1975 [2 Oct 1974] 35363/75 Heading G2A An arrangement for printing a circuitry pattern from a reticle on to a wafer includes means for initially adjusting the registry of wafer and reticle, and means for then projecting a beam of ultraviolet light through the reticle which is reflected by a polarizing beam-splitter to a focusing mirror and then back to the wafer. As shown, the system comprises a source 11 of ultraviolet light, a carriage 33 for alternately moving a microscope 32 or a mirror 14 into operative position, a polarizing beam-splitter prism 16 having a splitting plane 17, an adjustable optical wedge 21, a quarter-wave plate 23, a lens system 22 and a focusing mirror 20. The reticle 15 and the wafer 25 coated with a photo-resist material are disposed adjacent the sides of the prism, as shown. Before exposure, the wafer and reticle are accurately registered by a preliminary operation. A beam of visible light from a source 26 passes through a circular polarizer 27 and a quarter-wave plate 28, and is reflected downwards from a mirror 29, preferably positioned eccentrically to the main optical axis 19. The light is plane polarized by plate 23 to a proper orientation for passing straight through beamsplitter plane 17 on to wafer 25. From here it is reflected back through plane 17 up to mirror 20, and then down again. The upward and downward passes of light through plate 23 alters the polarization so that. this time plane 17 reflects about half the incident light on to the reticle 15. The wafer is thus now imaged on the reticle and is inspected through the microscope 32. Either the reticle or the wafer is adjusted to achieve the desired registration. After registration has been effected, light from source 26 is cut off, or the mirror 29 is moved out of the light path. The wedge 21, which can be moved transversely to axis 19 to various positions to adjust the length of the optical path between plane 17 and mirror 20 to accommodate differences between visible and ultraviolet light, or the different wavelengths to which the photoresist material on wafer 25 is sensitive, is suitably adjusted, and carriage 32 is moved so that the microscope is replaced by mirror 14. Shutter 12 is opened for a predetermined period to allow a beam of ultraviolet or near ultraviolet light from source 11 to fall on mirror 14 and be reflected therefrom. This passes through the reticle to fall on the splitter plane 17 of the prism 16. The prism preferably has a non-square configuration, so that the light is incident thereon at more than 45 degrees. Approximately half the light is unused and passes out along axis 18. The remainder is polarized and reflected upwards to mirror 20, the plate 23 altering the direction of polarization by 90 degrees. From mirror 20 the light is reflected back, passing straight through plane 17, to fall on wafer 25 to effect an exposure. The lens 22 is a multi-element system designed to correct for chromatic and spherical aberration, astigmatism, and field curvature. The use of a projection printing method avoids the possibility of damage to the reticle if it were placed in contact with the wafer.