Metal gate structure and method of manufacturing same

1. A method of manufacturing a metal gate structure, the method comprising:

providing a substrate having formed thereon a gate dielectric, a work function metal adjacent to the gate dielectric, and an aluminum gate electrode adjacent to the work function metal;

selectively forming a sacrificial tungsten capping layer centered over the aluminum gate electrode;

forming a low-k dielectric material over the sacrificial tungsten capping layer such that the low-k dielectric material at least partially surrounds the sacrificial tungsten capping layer;

selectively removing the sacrificial tungsten capping layer in order to form a trench aligned to the aluminum gate electrode in the low-k dielectric material;

filling the trench with an electrically insulating material in order to form an electrically insulating cap centered on the aluminum gate electrode; and

depositing an electrically insulating layer over the low-k dielectric material.

2. The method of claim wherein:

selectively forming the sacrificial tungsten capping layer is performed at a temperature between approximately 200 degrees Celsius and approximately 275 degrees Celsius.

3. The method of claim 1 further comprising:

exposing the gate metal to a buffered hydrofluoric acid solution prior to selectively forming the sacrificial tungsten capping layer.

4. The method of claim 3 wherein:

the buffered hydrofluoric acid solution comprises a buffering agent; and

the buffering agent comprises ammonium fluoride.

5. The method of claim 4 wherein:

the aluminum gate electrode is exposed to the buffered hydrofluoric acid solution for between approximately ten and approximately sixty seconds.

6. The method of claim 1 further comprising:

exposing the aluminum gate electrode to a dilute hydrochloric acid solution prior to selectively forming the sacrificial tungsten capping layer.

7. The method of claim 6 wherein:

the dilute hydrochloric acid solution comprises one part per volume hydrochloric acid and 10 parts per volume de-ionized water.

8. The method of claim 1 wherein:

selectively removing the sacrificial tungsten capping layer comprises etching away the sacrificial tungsten capping layer using an etchant comprising a base and an oxidizer.

9. The method of claim 8 wherein:

the base comprises ammonium hydroxide and

the oxidizer comprises one of hydrogen peroxide and ozone.

10. The method of claim 9 wherein:

the etchant has a pH between 4 and 10.

11. A method of manufacturing a metal gate structure, the method comprising:

providing a substrate having formed thereon a high-k gate dielectric, a work function metal adjacent to the high-k gate dielectric, an aluminum gate electrode adjacent to the work function metal, spacers adjacent to the high-k gate dielectric, and an inter-layer dielectric adjacent to the spacers;

selectively forming a sacrificial tungsten capping layer centered over the aluminum gate electrode;

forming a silicon oxide film over the sacrificial tungsten capping layer such that the silicon oxide film at least partially surrounds the sacrificial tungsten capping layer;

selectively removing the sacrificial tungsten capping layer in order to form a trench aligned to the aluminum gate electrode in the silicon oxide film;

filling the trench with a silicon nitride cap centered on the aluminum gate electrode; and

forming an electrically insulating layer over the silicon oxide film.

12. The method of claim 11 wherein:

selectively forming the sacrificial tungsten capping layer is accomplished using a chemical vapor deposition process.

13. The method of claim 12 wherein:

the chemical vapor deposition process uses a molecular hydrogen precursor and a tungsten hexafluoride precursor.

14. The method of claim 13 wherein:

the molecular hydrogen precursor is introduced at a first flow rate into a chemical vapor deposition chamber;

the tungsten hexafluoride precursor is introduced at a second flow rate into the chemical vapor deposition chamber; and

the first flow rate is higher than the second flow rate.

15. The method of claim 14 further comprising:

exposing the aluminum gate electrode to a buffered hydrofluoric acid solution for approximately ten seconds prior to selectively forming the sacrificial tungsten capping layer.

16. The method of claim 15 wherein:

selectively removing the sacrificial tungsten capping layer comprises etching away the sacrificial tungsten capping layer using an etchant comprising a base and an oxidizer;

the base comprises ammonium hydroxide;

the oxidizer comprises one of hydrogen peroxide and dissolved ozone; and

the etchant has a pH between 6 and 8.