See Wafer (cooking) for the original meaning of the word.

In microelectronics, a wafer is a thin slice of semiconducting material, such as a silicon crystal, upon which microcircuits are constructed by diffusion (or other doping techniques, such as ion implantation) and deposition of various materials. Wafers are thus of key importance in the fabrication of semiconductor devices.

They are made in various sizes ranging from 1 inch (25.4 mm) to 300 mm (11.8 inches), and thicknesses of the order of 0.5 mm. Generally, they are cut from a boule of semiconductor using a diamond saw , then polished on one or both faces.

Wafers under 200mm generally have flats indicating crystallographic planes of high symmetry (usually the {110} face) and, in old-fashined wafers (those below about 100mm diameter), the wafer's orientation and doping type (see illustration for conventions). Modern wafers use a notch to convey this information, in order to waste less material [1].

Orientation is important since many of a single crystal's structural and electronic properties are highly anisotropic. For instance, wafer cleavage typically occurs only in a few well-defined directions. Scoring the wafer along cleavage planes allows it to be easily diced into individual chips ("dies") so that the billions of individual circuit elements on an average wafer can be separated into many individual circuits.

Sources

1. A website with semiconductor lore.
2. Federal Standard 1037C