10.1.4 Deposition

Applying films to the surface of the wafer is used for creating insulators, conductors, and n- and p-type semiconductors. Thus various materials must be deposited with film thicknesses ranging from a few nanometers for certain SiO$_2$ layers to a few micrometers. There are two kinds of deposition techniques: PVD (physical vapor deposition), or sputtering, and CVD (chemical vapor deposition).

Physical vapor deposition means that the host material is bombarded with high energy ions in order to displace molecules which then reattach on the wafer surface and on the surface of the sputtering reactor. To obtain layers with more complicated chemical properties two or more different host materials are often simultaneously bombarded at different rates, which is then called cosputtering.

CVD can be done in two different ways. The first possibility is the reaction of two gas species near the substrate, where the resulting molecules subsequently adhere to the wafer surface. The second possibility is called pyrolytic (i.e., caused by heating) decomposition of a single gas, which yields the desired gas species that attaches to the surface.

During deposition processes the wafer is usually heated to several hundred degrees Celsius depending on the chemistry used. In order to achieve a high temperature gradient in the reactor space above the wafer it is sometimes rotated at thousands of revolutions per minute. Because the desired reactions can only take place at certain temperatures, this ensures that reactions only occur very near to the wafer surface and not needlessly in large part of the reactor.