Patent GB-1532901-A (GB)
Title: Inspection of masks and wafers by image dissections
IPC: G03F1/84
Filed: 1976-05-05
Abstract: 1532901 Photo-electric detection of defective components WESTERN ELECTRIC CO Inc 5 May 1976 [5 May 1975] 18357/76 Heading G1A A member, such as a mask for micro-miniature semi-conductor component manufacture or the semi-conductor wafer itself, which comprises a  plurality of pattern areas e.g. 1 to 10, Fig. 1, which are nominally identical, is inspected for differences between corresponding elemental regions of said pattern areas indicative of defects in particular pattern areas, by an image dissection method which involves projecting images of said corresponding elemental regions, in vertical columns thereof, sideby-side on to an image storage medium, and reading out the storage medium by scanning in a direction perpendicular to the columns. Thus, in the arrangement of Fig. 1, for inspecting a mask 13, the latter is mounted on a continuously movable glass table so that light transmitted through transparent sections of the patterns on the mask can be imaged on medium 40, which may be a vidicon camera or a multi-element charge-coupled silicon detector array and which is moved in steps under the control of processor 20. This also controls flashing of lamp 16. At time t 1 , the table is positioned such that the first pattern 1 is flash illuminated. The projected image thereof is interrupted by plate 38, comprising 50 spaced slots, such that only the images of 50 columnar spaced apart regions of pattern 1 reach the medium 40 where they are stored (columns 1-1-n, 1-11-n to 1-491-n), Fig. 6. The table moves continuously to bring pattern 2 into the illumination zone at  time t 2 . Between flashes at t 1  and t 2 , the image stored medium is stepped sideways. At time t2 therefore, 50 spaced columnar image regions of pattern 2 (i.e. 2-1-n, 2-11-n to 2-491-n) are imaged adjacent the image columns for pattern 1. This sequence is repeated for all 10 patterns. The stored image when completed is then analyzed by scanning elemental areas in rows of 500, displaying 10 elemental areas i.e. 1-1-1 to 10-1-1, 1-11-1 to 10-11-1 &c., at a time. The amplitude for each elemental area is compared with those for adjacent areas or with the weighted average of all ten areas; if gradually increasing change from pattern to pattern is allowable, the threshold level is suitably set. After this first analysis, the cycle is repeated the table and medium having been returned to their original positions. This time however the flash illuminations are delayed with respect to the times t 1 -t 10  originally used sufficient to allow the table to have moved one element width sideways. Eight similar subsequent cycles are repeated so that an analysis of the whole of the patterns has been carried out. Wafer inspection is similar in method to the above but illumination is by reflection from the wafer surface, Fig. 3 (not shown). If spectral reflective qualities are to be utilized, the apparatus is modified, Fig. 8 (not shown), by the insertion of a member carrying three adjacent colour filters, and instead of slotted member 38, a member comprising an array of lenslets, the arrangement being such that elemental images are projected in groups of three, differently coloured, on to the image storage medium.