Re: Basic MOS question
hi,
The primary criterion for the gate material is that it is a good conductor. Highly-doped polycrystalline silicon is an acceptable, but certainly not ideal conductor, and it also suffers from some more technical deficiencies in its role as the standard gate material. There are a few reasons why polysilicon is preferable to a metal gate:
The threshold voltage (and consequently the drain to source on-current) is determined by the work function difference between the gate material and channel material. When metal was used as gate material, gate voltages were large (in the order of 3V to 5V), the threshold voltage (resulting from the work function difference between a metal gate and silicon channel) could still be overcome by the applied gate voltage (i.e. |Vg - Vt| > 0). As transistor sizes were scaled down, the applied signal voltages were also brought down (to avoid gate oxide breakdown, hot-electron reduction, power consumption reduction, etc). A transistor with a high threshold voltage would become non-operational under these new conditions. Thus, poly-crystalline silicon (polysilicon) became the modern gate material because it is the same chemical composition as the silicon channel beneath the gate oxide. In inversion, the work-function difference is close to zero, making the threshold voltage lower and ensuring the transistor can be turned on.
In the MOSFET IC fabrication process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better performing transistors. Unfortunately these high temperatures would melt metal gates, thus a high melting point material such as poly-crystalline silicon is preferable to metal as gate material. However, polysilicon is highly resistive (approximately 1000 times more resistive than metals) which reduces the signal propagation speed through the material. To lower the resistivity, dopants are added to the polysilicon. Sometimes additionally, high temperature metal such as tungsten, titanium, cobalt, and more recently nickel, is layered onto the top of the polysilicon (as a side effect of layering metal on the source and drain contacts) and alloyed with the polysilicon to decreases the resistivity. Such a blended material is called silicide. The silicide-polysilicon combination has better electrical properties than polysilicon alone and still does not melt in subsequent processing. Also the threshold voltage is not significantly higher than polysilicon alone, because the silicide material is not near the channel. The process in which silicide is formed on both the gate electrond and the source and drain regions is sometimes called salicide, self-aligned silicide.
When the transistors are extremely scaled down, it is neccessary to make the gate dielectric layer very thin, around 1 nm in state-of-the-art technologies. A phenomenon observed here is the so-called poly depletion, where a depletion layer is formed in the gate polysilicon layer next to the gate dielectric when the transistor is in the inversion. To avoid this problem a metal gate is desired. A variety of metal gates such as tantalum, tungsten, tantalum nitride, and titanium nitride, usually in conjunction with high-k dielectrics. An alternative is to use fully-silicided polysilicon gates, and the prosess is refered to as FUSI.
In found this information in:
https://en.wikipedia.org/wiki/MOSFET
Regards.