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Concept
Metal gate
Summary
A metal gate, in the context of a lateral metal–oxide–semiconductor (MOS) stack, is the gate electrode separated by an oxide from the transistor's channel – the gate material is made from a metal. In most MOS transistors since about the mid 1970s, the "M" for metal has been replaced by a non-metal gate material.
The first MOSFET (metal–oxide–semiconductor field-effect transistor) was made by Mohamed Atalla and Dawon Kahng at Bell Labs in 1959, and demonstrated in 1960. They used silicon as channel material and a non-self-aligned aluminum gate. Aluminum gate metal (typically deposited in an evaporation vacuum chamber onto the wafer surface) was common through the early 1970s.
By the late 1970s, the industry had moved away from aluminum as the gate material in the metal–oxide–semiconductor stack due to fabrication complications and performance issues. A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum.
Polysilicon can be deposited easily via chemical vapor deposition (CVD) and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900–1000 °C), where metal was not. Particularly, metal (most commonly aluminum - a Type III (P-type) dopant) has a tendency to disperse into (alloy with) silicon during these thermal annealing steps. In particular, when used on a silicon wafer with a < 1 1 1 > crystal orientation, excessive alloying of aluminum (from extended high temperature processing steps) with the underlying silicon can create a short circuit between the diffused FET source or drain areas under the aluminum and across the metallurgical junction into the underlying substrate - causing irreparable circuit failures. These shorts are created by pyramidal-shaped spikes of silicon-aluminum alloy - pointing vertically "down" into the silicon wafer. The practical high-temperature limit for annealing aluminum on silicon is on the order of 450 °C.
Official source
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