Tower Develops "Stitching" Technology
-- Semiconductor International, 3/1/1999
Tower Semiconductor Ltd. (Migdal Haemek, Israel) announced
the development of a process for fabricating CMOS image sensors physically
larger than a stepper's exposure field. It allows the physical merging of
multiple design structures onto a wafer during the lithography process. By
avoiding the size limitation imposed by the exposure field, a single image
sensor containing millions of pixels can be fabricated.
To achieve high resolution, image sensors integrate as many pixels as possible into a single chip. However, the more pixels required, the larger the die size. When using modern stepper equipment, which has a fixed, relatively small exposure field, the large die may easily exceed the limited exposure field.
Making pixels smaller to make more fit in a die is not practical, since the camera would require much higher quality lenses, leading to higher system cost. The company claims that making the die larger than the stepper exposure field is more cost effective.
A high-precision alignment method allows these structures to slightly
overlap, fusing them into a single integral chip. 
