Process to fabricate a metal high-K transistor having first and second silicon sidewalls for reduced parasitic capacitance

1. A method to form a metal high dielectric constant (MHK) transistor, comprising:

providing a MHK stack disposed on a substrate, the MHK stack comprising a layer of high dielectric constant material and an overlying layer comprised of a metal, the MHK stack having an overlying layer comprised of silicon;

selectively removing only the overlying layer comprised of silicon and the overlying layer comprised of metal, without removing the layer of high dielectric constant material, to form an upstanding portion of a MHK gate structure comprised of an underlying portion of the layer comprised of metal, and an overlying portion of the layer comprised of silicon;

forming a first sidewall layer comprised of silicon on sidewalls of the upstanding portion of the MHK gate structure;

forming a second sidewall layer on sidewalls of the first sidewall layer;

removing a portion of the layer of high dielectric constant material to form exposed surfaces of the layer of high dielectric constant material;

forming an offset spacer layer over the second sidewall layer and over the exposed surfaces of the layer of high dielectric constant material; and

forming in the substrate extensions that underlie the first and second sidewall layers and that extend under a portion but not all of the upstanding portion of the MHK gate structure, wherein each substrate extension overlaps the upstanding portion of the MHK gate structure by an overlap distance.

2. The method of claim 1 , wherein the overlap distance is about 5 to 10 nanometers.

3. The method of claim 1 , wherein forming a first sidewall layer further comprises:

forming the first sidewall layer on surfaces of the MHK stack and of the high dielectric constant material; and

selectively removing the first sidewall layer from surfaces of the MHK stack and of the high dielectric constant material without removing the layer of high dielectric constant material, and without removing all of the first sidewall layer from sidewalls of the MHK stack.

4. The method of claim 1 , wherein forming a second sidewall layer further comprises:

forming the second sidewall layer on surfaces of the first sidewall layer, of the MHK stack, and of the high dielectric constant material; and

selectively removing the second sidewall layer from surfaces of the first sidewall layer, of the MHK stack and of the high dielectric constant material without removing the layer of high dielectric constant material, and without removing all of the second sidewall layer from sidewalls of the first sidewall layer.

5. The method of claim 1 , where the layer of high dielectric constant material is comprised of at least one of HfO 2 , Ta 2 O 5 , TiO 2 , Al 2 O 3 , Y 2 O 3 and La 2 O 5 .

6. The method of claim 1 , where the layer comprised of metal is comprised of at least one of TiN, Ta, TaN, TaCN, TaSiN, TaSi, AIN, W and Mo.

7. The method of claim 1 , where the first sidewall layer is comprised of at least one of amorphous silicon or polycrystalline silicon.

8. The method of claim 1 , where the metal high dielectric constant (MHK) transistor is a P-type field effect transistor.

9. The method of claim 1 , where the metal high dielectric constant (MHK) transistor is an N-type field effect transistor.

10. The method of claim 1 , further comprising forming a final spacer layer over the offset spacer layer.

11. The method of claim 10 , further comprising, after forming the final spacer layer, forming source and drain regions in the substrate.

12. The method of claim 1 , wherein the second sidewall layer is an insulator.

13. The method of claim 1 , wherein the second sidewall layer is conductive.