Extended contact area using undercut silicide extensions

1. A method of forming a contact silicide extension comprising:

forming an undercut region below a dielectric layer and above a source-drain region, the undercut region located directly below a bottom of a contact trench and extending below the dielectric layer to a gate spacer formed on a sidewall of a gate stack wherein the forming the undercut region below the dielectric layer and above the source-drain region comprises:

forming a liner layer on the gate spacer and the source-drain region;

forming an etch stop layer on the liner layer and the gate stack;

forming the dielectric layer on the etch stop layer;

forming the contact trench in the dielectric layer, the bottom of the contact trench exposing a portion of the etch stop layer;

removing the exposed portion of the etch stop layer; and

removing a portion of the liner layer, the portion of the liner layer extending from directly below the bottom of the contact trench and below the dielectric layer to the gate spacer; and

forming a contact silicide in the undercut region, the contact silicide in direct contact with the source-drain region.

2. The method of claim 1 , wherein the forming the undercut region below the dielectric layer and above the source-drain region comprises:

forming a liner layer on the gate spacer, and the source-drain region;

forming the dielectric layer on the liner layer and the gate stack;

forming the contact trench in the dielectric layer, the bottom of the contact trench exposing a portion of the liner layer; and

removing a portion of the liner layer, the portion of the liner layer extending from directly below the bottom of the contact trench and below the dielectric layer to the gate spacer.

3. The method of claim 2 , wherein the removing the portion of the liner layer comprises:

performing an isotropic wet etching process to remove the portion of the liner layer selective to the dielectric layer, the source-drain region, and the gate spacer.

4. The method of claim 1 , wherein the removing the exposed portion of the etch stop layer comprises:

performing an anisotropic etching process to remove the etch stop layer selective to the dielectric layer and the liner layer.

5. The method of claim 1 , wherein the removing the portion of the liner layer comprises:

performing an isotropic wet etching process to remove the portion of the liner layer selective to the dielectric layer, the etch stop layer, the source-drain region, and the gate spacer.

6. The method of claim 1 , wherein the forming the contact silicide on the source-drain region comprises:

depositing a metal layer in the undercut region; and

performing an annealing process to cause the metal layer to react with silicon in the source-drain region.

7. The method of claim 1 , further comprising:

forming an electrical contact in the contact trench and on the contact silicide.

8. A method comprising:

forming a gate stack on a substrate;

forming a gate spacer on a sidewall of the gate stack;

forming a source-drain region adjacent to the gate spacer;

forming a liner on the source-drain region, wherein the forming a liner on the source-drain region comprises:

depositing a liner layer comprising a metal nitride; and

depositing a etch stop layer on the liner layer, the etch stop layer comprising an insulator;

forming a dielectric layer over the liner;

forming a contact trench in the dielectric layer, a bottom of the contact trench exposing a portion of the liner;

removing a portion of the liner below the contact trench and below the dielectric layer to form an undercut region, the undercut region extending from below the bottom of the contact trench and below the dielectric layer to the gate spacer; and

forming a contact silicide in the undercut region, the contact silicide in direct contact with the source-drain region.

9. The method of claim 8 , wherein the forming a source-drain region adjacent to the gate spacer comprises:

growing a doped epitaxial semiconductor material on the substrate.

10. The method of claim 8 , wherein the forming a liner on the source-drain region comprises:

depositing a liner layer comprising a metal nitride.

11. The method of claim 8 , further comprising:

removing a portion of the etch stop layer directly below the bottom of the contact trench selective to the dielectric layer and the liner layer using an anisotropic etching process.

12. The method of claim 8 , further comprising:

forming the liner on the gate spacer and the gate stack.

13. The method of claim 8 , wherein the removing the portion of the liner below the contact trench and below the dielectric layer to form the undercut region comprises:

removing the portion of the liner selective to the dielectric layer, the source-drain region, and the gate spacer using an isotropic wet etching process.

14. The method of claim 8 , wherein the forming the contact silicide in the undercut region comprises:

depositing a metal layer in the undercut region; and

performing an annealing process to cause the metal layer to react with silicon in the source-drain region.

15. The method of claim 8 , further comprising:

forming an electrical contact in the contact trench and on the contact silicide.