The first step comprises of preparing Si or Ge wafer with desired purity level.
The processing characteristics and some material properties of silicon wafers depend on its orientation.The <111> planes have the highest density of atoms on the surface, so crystals grow most easily on these planes and oxidation occurs at a higher pace when compared to other crystal planes.Traditionally, bipolar devices are fabricated in <111> oriented crystals whereas <100> materials are preferred for MOS devices.
Electronic-grade silicon (EGS), a polycrystalline material of high purity, is the starting material for the preparation of single crystal silicon. EGS is made from metallurgical-grade silicon (MGS) which in turn is made from quartzite, which is a relatively pure form of sand.
The Czochralski(CZ) process, which accounts for 80% to 90% of worldwide silicon consumption, consists of dipping a small single-crystal seed into molten silicon and slowly withdrawing the seed while rotating it simultaneously.Impurities, both intentional and unintentional, are introduced into the silicon ingot. Intentional dopantsare mixed into the melt during crystal growth, while unintentional impurities originate from the crucible, ambient, etc.
The float-zone process has some advantages over the Czochralski process for the growth of certain types of silicon crystals.The molten silicon in the float-zone apparatus is not contained in a crucible, and is thus not subject to the oxygen contamination present in CZ-Sicrystals.The float-zone process is also necessary to obtain crystals with a high resistivity(>> 25 W-cm)
Silicon, albeit brittle, is a hard material. The most suitable material for shaping and cutting silicon is industrial-grade diamond. Conversion of silicon ingots into polished wafers requires several machining, chemical, and polishing operations.
The wafer as cut varies enough in thickness to warrant an additional lapping operation that is performed under pressure using a mixture of Al2O3and glycerine. Subsequent chemical etching removes any remaining damaged and contaminated regions.Polishing is the final step. Its purpose is to provide a smooth, specular surface on which device features can be photo engraved.