Metal gate
Encyclopedia
A metal gate, in the context of a lateral Metal-Oxide-Semiconductor MOS stack, is just that—the gate material is made from a metal.
For decades, the industry had moved away from metal as the gate material in the MOS stack due to fabrication complications. A material called polysilicon (silicon
, highly doped with donors or acceptors, polycrystalline
) was used instead because it can be deposited easily and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900-1000 °C), where metal was not. Polysilicon is also attractive for the easy manufacturing of self-aligned gate
s. The implantation of source and drain is carried out with the gate in place, leading to a channel perfectly aligned to the gate without more lithographic steps. Furthermore, metal has a tendency to disperse into silicon during these thermal annealing steps.
However, polysilicon doesn't offer the near-zero electrical resistance
of metals, and is therefore not ideal for charging and discharging the gate capacitance
of the transistor
.
From the 45 nm node onward, the metal gate technology returns, together with the use of high-k materials, pioneered by Intel announcements.
For decades, the industry had moved away from metal as the gate material in the MOS stack due to fabrication complications. A material called polysilicon (silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, highly doped with donors or acceptors, polycrystalline
Polycrystalline
Polycrystalline materials are solids that are composed of many crystallites of varying size and orientation. The variation in direction can be random or directed, possibly due to growth and processing conditions. Fiber texture is an example of the latter.Almost all common metals, and many ceramics...
) was used instead because it can be deposited easily and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900-1000 °C), where metal was not. Polysilicon is also attractive for the easy manufacturing of self-aligned gate
Self-aligned gate
A self-aligned gate, in electronics, is a transistor manufacturing feature whereby a highly doped gate electrode region of a MOSFET transistor is used as a mask for the doping of the source and drain around it...
s. The implantation of source and drain is carried out with the gate in place, leading to a channel perfectly aligned to the gate without more lithographic steps. Furthermore, metal has a tendency to disperse into silicon during these thermal annealing steps.
However, polysilicon doesn't offer the near-zero electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of metals, and is therefore not ideal for charging and discharging the gate capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
of the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
.
From the 45 nm node onward, the metal gate technology returns, together with the use of high-k materials, pioneered by Intel announcements.