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Metal or Polysilicon Used as the Gate?

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Willt

Willt

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Hi friends,

In the old days, metal was used as the gate of MOS.
But now, polysilicon is used.

Metal has a lower resistivity than polysilicon.
What is the reason for using polysilicon now?

I think of a few reason:
1. Polysilicon is easier to stick to silicon oxide.
2. Polysilicon can achieve self-alignment but metal cannot.
(Metal will melt when S/D dopant is being doped [HIGH TEMPERATURE!]
under self-alignment.)

Disadv. of using polysilicon as the gate:
1. Higher resistivity
2. Higher cap.

I heard that metal will be used as the gate again
in the near future under 45nm-process. Is this true?

Please feel free to discuss together.

Your friend
Will
 

Metal was used, but due to various advantages offered by polysilicon mainly..ease of fabrication and good performance makes polysilicon the ideal gate .

Also there were many problems associated with metal gate, like high leakage, electomigration, impurity, high electricfiedl. that discouraged the use of metal as gate.

Now as 45nm is also available, it still uses low res - poly as gate. But yeah, some activities are happening in this direction.
 

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