Air-spacer MOS transistor

1. A method for manufacturing a MOS transistor comprising:

forming a gate stack having a height, a length, and a width and comprising a lower portion, a middle portion and an upper portion;

forming around the gate stack at least one first spacer surrounded with another material;

removing the first spacer to form a cavity on each side of the gate stack;

decreasing the length of the middle portion of the gate stack in said cavity such that the length of the middle portion is less than the lengths of the lower and upper portions;

closing an upper aperture of the cavity by a non-conformal method while depositing a dielectric layer on the walls of the gate stack with the decreased length middle portion so as to provide an air spacer;

removing the lower, decreased length middle and upper portions of the gate stack to provide a gate stack opening; and

filling the gate stack opening with a conductive metal to form a metal gate having a lower conductive portion, a middle conductive portion and an upper conductive portion, wherein the length of the middle conductive portion is less than the lengths of the lower and upper conductive portions.

2. The method of claim 1 , wherein decreasing the length of the middle portion produces a length equal to from 0.3 to 0.9 the length of the lower and upper portions of the gate stack.

3. The method of claim 1 , wherein the air spacer extends along said length of the gate stack and is provided on opposite sides of the gate stack.

4. The method of claim 3 , wherein the dielectric layer on the walls of the gate stack delimits said air spacer.

5. The method of claim 1 , wherein the lower portion of the gate stack comprises a first material and the middle portion of the gate stack comprises a second material.

6. The method of claim 5 , wherein the lower portion comprises a portion of a metal layer.

7. The method of claim 5 , wherein the middle portion comprises a portion of a doped semiconductor layer.

8. The method of claim 7 , wherein the doped semiconductor layer is one of a polycrystalline silicon layer or a silicon-germanium layer.

9. The method of claim 1 , wherein forming the gate stack comprises:

forming the lower portion of the gate stack with a layer of a metal or a metal alloy and a doped polysilicon layer; and

forming the middle portion of the gate stack with a doped polycrystalline silicon-germanium layer.

10. The method of claim 1 , wherein forming the gate stack comprises:

forming the lower portion of the gate stack with a layer of a first metal or metal alloy; and

forming the middle portion of the gate stack with a layer of a second metal or metal alloy.

11. A method for manufacturing a MOS transistor comprising:

forming a gate stack having a height, a length, and a width and comprising a lower portion, a middle portion and an upper portion;

forming around the gate stack a first spacer surrounded with another material;

removing the first spacer to form a cavity in said another material;

reducing a length of the middle portion of the gate stack in said cavity relative to the lengths of the lower and upper portions;

lining side walls of the cavity and the gate stack with an insulating material;

closing an upper aperture of the cavity so as to provide an air spacer defined by the lined side walls;

removing the gate stack to provide a gate stack opening; and

filling the gate stack opening with a conductive metal to form a metal gate having a lower conductive portion, a middle conductive portion and an upper conductive portion, wherein the length of the middle conductive portion is less than the lengths of the lower and upper conductive portions.

12. The method of claim 11 , wherein forming the gate stack comprises:

forming the lower portion of the gate stack with at least one layer of a metal material; and

forming the middle portion of the gate stack with a portion of a doped semiconductor layer, said doped semiconductor layer comprising one of a polycrystalline silicon layer or a silicon-germanium layer.

13. The method of claim 11 , wherein forming the gate stack comprises:

forming the lower portion of the gate stack with a layer of a metal or a metal alloy and a doped polysilicon layer; and

forming the middle portion with of the gate stack with a doped polycrystalline silicon-germanium layer.

14. The method of claim 11 , wherein forming the gate stack comprises:

forming the lower portion of the gate stack with a layer of a first metal or metal alloy; and

forming the middle portion of the gate stack with a layer of a second metal or metal alloy.

15. The method of claim 11 , wherein reducing the length of the middle portion of the gate stack in said cavity comprises etching material of the middle portion at a rate faster than the materials of the lower and upper portions.