What is a silicon wafer?

A wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serves as the substrate for microelectronic devices built in and upon the wafer. It undergoes many microfabrication processes, such as doping, ion implantation, etching, thin-film deposition of various materials, and photolithographic patterning. Finally, the individual microcircuits are separated by wafer dicing and packaged as an integrated circuit.

Formation

Wafers are formed of highly pure, nearly defect-free single crystalline material, with a purity of 99.9999999% (9N) or higher. One process for forming crystalline wafers is known as Czochralski growth invented by the Polish chemist Jan Czochralski. In this process, a cylindrical ingot of high purity monocrystalline semiconductor, such as silicon or germanium, called a boule, is formed by pulling a seed crystal from a melt. Donor impurity atoms, such as boron or phosphorus in the case of silicon, can be added to the molten intrinsic material in precise amounts in order to dope the crystal, thus changing it into extrinsic semiconductor of n-type or p-type. The boule is then sliced with a wafer saw (a type of wire saw) and polished to form wafers. The size of wafers for photovoltaics is 100–200 mm square and the thickness is 100–500 μm. Electronics use wafer sizes from 100–450 mm diameter. The largest wafers made have a diameter of 450 mm but are not yet in general use.

Cleaning, texturing and etching

Wafers are cleaned with weak acids to remove unwanted particles, or repair damage caused during the sawing process. There are several standard cleaning procedures to make sure the surface of a silicon wafer contains no contamination. One of the most effective methods is RCA clean. When used for solar cells, the wafers are textured to create a rough surface to increase their efficiency. The generated PSG (phosphosilicate glass) is removed from the edge of the wafer in the etching.