Silicidization is the process of forming a surface layer of a refractory metal silicide on silicon. A metal, typically today titanium or cobalt, is deposited on the silicon (gate polysilicon and-or source drain regions), and a layer of silicide is formed when the two substances react at elevated temperatures. Why do it? To lower the resistance of the polysilicon interconnect and-or the source-drain contact and sheet resistances. In a “polycide” process only the polysilicon is silicided. In a “silicide” process (usually implemented as a self-aligned silicidization – from whence the term “salicide”) both gate polysilicon and source-drain regions are silicided. Some processes provide a silicide blocking mask so that resistors can be constructed from the higher-resistance non-silicided material.