Thin filmsThere are a number of different techniques that facilitate the deposition or formation of very thin films (of the order of micrometers, or less) of different materials on a silicon wafer (or other suitable substrate). These films can then be patterned using photolithographic techniques and suitable etching techniques. Common materials include silicon dioxide (oxide), silicon nitride (nitride), polycrystalline silicon (polysilicon or poly), and aluminium.A number of other materials can be deposited as thin films, including noble metals such as gold. Noble metals will contaminate microelectronic circuitry causing it to fail, so any silicon wafers with noble metals on them have to be processed using equipment specially set aside for the purpose. Noble metal films are often patterned by a method known as "lift off", rather than wet or dry etching. Often, photoresist is not tough enough to withstand the etching required. In such cases a thin film of a tougher material (eg, oxide or nitride) is deposited and patterned using photolithography. The oxide / nitride then acts as an etch mask during the etching of the underlying material. When the underlying material has been fully etched the masking layer is stripped away. |