CRYSTAL PREPARATION. The substrate of bipolar and MOS integrated circuits is generally a single crystal of silicon that is lightly doped with either n- or p- typc impurities. The substrate serves both as the physical medium upon and within which the IC is built and as part of the electrical circuit itself. These cry stals arc sliced from large right-circular cylinders of crystalline silicon, which are carefully grown to lengths up to 2 m and which vary in diameter from I to several inchcs. The slices are typically 250µ to 400µ thick. From an electrical viewpoint much thinner slices would be acceptable; however, the thicker slices have been adopted because they are more practical to handle (less breakage) and are less likely to warp during processing. The size of the wafers has been increasing rapidly with time to allow for both large chips and a larger number of chips per wafer. As of 1989, many of the older processing lines were using 4 inch wafers, but the newer lines are typically using 5 and 6 inch wafers. The crystals are often cut so that the surface is oriented approximately in the <100> direction.
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