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Chapter 5:
Wafer Steppers and Scanners
Prior to the advent of wafer steppers, circuit patterns were transferred from masks to wafers by contact or proximity printing, or by using full-wafer scanners. In contact printing, a mask that had the pattern for all chips on the wafer was brought into contact with a resist-coated wafer. The mask was illuminated, thereby exposing the resist under the clear features on the mask. This method of exposure was used in the earliest days of the semiconductor electronics industry, but the mechanical contact caused defects on both the mask and wafer, reducing productivity. Proximity printing, where masks and wafers were brought close to each other, but not into contact, was one approach to reducing the problem of defects that arises with contact printing. Unfortunately, resolution was poor when the gap between the mask and the wafer was a practical size, as a consequence of diffraction. The first workable solution to this problem was the full-wafer scanner, which also used a mask that contained the patterns of all chips that were transferred 1:1 to the wafer.
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