Patent GB-1274937-A (GB)
Title: Methods of and apparatus for visually comparing objects
Inventor: MONTONE LIBER JOSEPH
IPC: G03F1/84
Filed: 1969-07-16
Abstract: 1,274,937. Object comparison. WESTERN ELECTRIC CO. Inc. 16 July, 1969 [16 July, 1968], No. 35822/69. Heading H4F. Two objects e.g. masks used in the production of integrated circuits, are visually compared by generating an image of the first object on a display device e.g. on a television monitor, generating a composite image of the superimposed first and second objects on the display device with the portion of the composite image relating to the first object superimposed on the image of the first object, and displaying at least the composite image intermittently so that any difference between the objects is visually apparent as a flicker effect on the display device. In Fig. 1 a standard mask 10, comprising a transparent glass substrate 13 with opaque areas 14 on one surface, is aligned with a test mask 16, a layer of light selective material 15 having a large upper surface reflectivity coefficient being disposed between the two masks. In this embodiment mask 16 is symmetrical in that the pattern of opaque areas 19 is the same when viewed from either side. Since the patterns of opaque areas on the two masks are ideally identical, for every spot (opaque area) upon the surface of mask 10 there should be a spot on mask 16 in exact alignment. To detect misalignment of any two spots e.g. 14A and 19A, an image of the superimposed spots is formed intermittently (rotating light shutter 34) by light source 23, and an image of mask 16 is formed by light source 25, the light being reflected by layer 15. These images are detected by television camera 28 and displayed on a monitor 29 which may be covered with a transparent plastic grid pattern to facilitate measurement of any misalignment. Light source 25 may also be chopped in synchronism but out of phase with the chopping of light source 23. If misalignment is present a visible change (flicker effect) is visible on the monitor as it alternately displays the combined image 14A and 19A, and the single image 19A (or without shutter 35, the single image 19A is displayed continuously whilst the combined image 14A and 19A is displayed intermittently). When the first two opaque areas have been inspected for alignment the stage 12, in which the masks are held, is systematically stepped to inspect the remaining opaque areas of the two masks. In Fig. 2 the masks to be inspected are non- symmetrical i.e. they may not be inverted to be face to face, and to compensate for the thickness of test mask 43 and yet avoid changing the focal length of camera 28 in order that it alternately focusses upon the opaque areas 44A and 42A, a disc 46, having an open aperture 47 and an aperture 48 filled with a compensating material 50 having the same characteristics as test mask 43, is rotated in the light path in synchronism with the interruption of illumination from sources 25 and 23. Instead of using rotating disc 46 a layer of optical compensating material may be disposed between the lower mask and the layer of light selective material, Fig. 3 (not shown). The glass mask substrate, the layer of light selective material, or the interposed compensating layer may have grid lines formed on one surface. To compare treated areas 55, Fig. 4, on a semi-conductor wafer (germanium or silicon) with a, desired pattern 14 on a mask, infra-red radiation from source 52 is directed up through the wafer and mask combination. This infra-red radiation is alternated with visible radiation from source 25 (this visible light cannot penetrate the semi-conductor wafer and is scattered thereby). A layer of light selective material may be placed between the mask and wafer. Although Fig. 4 shows a symmetrical mask, non- symmetrical patterns may be compared using the techniques of Figs. 2 and 3. A multicolour image display to aid in detecting mask misalignment may be produced by using a CRT having two superimposed layers of phosphor which produce red and green displays when selected potentials are applied to the tube control grid in synchronism with illumination by either the upper or lower light source 25 or 23. The superimposed portions of the image appear yellowish and the non-superimposed portions green and red.