Fabrication of the single crystal

A single crystal (monocrystal), as required in semiconductor manufacturing, is a regular arrangement of atoms. In addition, there is polycrystalline silicon (composed of many small single-crystal grains) and amorphous silicon (disordered structure). Depending on the orientation of the lattice in space, silicon wafers exhibit different surface structures, which influence various device parameters, such as charge carrier mobility.

Crystal orientations

Chip manufacturing uses almost exclusively (100)-oriented silicon. The reason lies at the interface with the gate oxide: unsaturated bonds remain there after oxidation, and their number is lowest for this orientation. The (111) orientation has the densest arrangement of bonds and therefore the most defects; however, it oxidizes the fastest.

Crystal orientation is also of particular importance in micromechanics. For instance, microchannels with vertical walls can be produced on (110) silicon, whereas (100) orientation results in sidewall angles of 54.74°.

The orientation of crystal planes and crystal directions in semiconductor technology is described using so-called Miller indices. Parentheses such as (100) denote a single crystal plane, while curly braces such as {111} denote a family of symmetrically equivalent planes. Crystal orientation is important because many material properties — especially the etching behavior of silicon — strongly depend on the crystal direction. During anisotropic etching, for example, {111} planes are etched much more slowly than other planes and therefore form the characteristic inclined sidewalls of many MEMS structures.