Barrier layer stack to prevent Ti diffusion

1. A method for forming an improved barrier layer stack for preserving an aluminum alloy interconnect resistivity comprising the steps of:

providing a semiconductor process wafer comprising an exposed conductive region;

forming a first barrier layer comprising a barrier layer stack over the exposed conductive region comprising a titanium nitride (TiN) layer in contact with the conductive region;

forming at least one additional barrier layer comprising the barrier layer stack to form an alternating sequence of TIN and Ti layers;

forming an uppermost barrier layer of TiN comprising the barrier layer stack;

forming an overlying aluminum alloy region in contact with the uppermost barrier layer, wherein the exposed conductive region and the overlying aluminum alloy layer comprises an AlCu alloy; and,

subjecting the semiconductor process wafer to at least one process comprising a temperature of greater than about 300° C.

2. The method of claim 1 , wherein the at least one subsequent process comprises an AlCu deposition process, an AlCu reflow process, an AlCu annealing process, and a metal nitride deposition process.

3. The method of claim 2 , wherein the at least one subsequent process is maintained at a temperature of less than about 450° C.

4. The method of claim 1 , wherein the barrier layer stack sequence comprises at least one TiN/Ti sequence to stop with an uppermost layer of TiN.

5. The method of claim 1 , wherein the barrier layer stack sequence comprises at least one Ti/TiN sequence to stop with an uppermost layer of TiN.

6. The method of claim 1 , wherein the barrier layer stack is formed by at least one of a CVD and PVD process.

7. The method of claim 1 , wherein the barrier layer stack is formed by a reactive sputtering process comprising:

a) flowing N 2 and Ar to form a gas mixture to contact the process wafer for a period of time prior to initiating a glow discharge in a sputter deposition process wherein the gas mixture comprises a ratio of nitrogen to argon of about 1.3 to 1 to about 2.5 to 1;

b) initiating a glow discharge to sputter Ti;

c) forming a layer of TiN;

d) turning off the N 2 flow to form a sputter atmosphere consisting essentially of argon;

e) depositing a layer of Ti;

wherein at least steps a through c are repeated at least once with step c comprising the final step.

8. The method of claim 1 , wherein the barrier layer stack is formed by a reactive sputtering process comprising:

a) forming a sputter atmosphere consisting essentially of argon;

b) initiating a glow discharge to sputter Ti;

c) depositing a layer of Ti;

d) stopping the glow discharge;

e) flowing N 2 and Ar to form a gas mixture to contact the process wafer for a period of time prior to initiating a glow discharge in a sputter deposition process wherein the gas mixture comprises a ratio of nitrogen to argon of about 0.1 to 10;

f) initiating a glow discharge to sputter Ti;

g) forming a layer of TiN;

wherein steps a through g are optionally repeated at least once.

9. A method for forming a metallic inter-diffusion resistant barrier layer stack for preserving an AlCu interconnect resistivity comprising the steps of:

providing a semiconductor process wafer comprising a metal interconnect opening formed to extend through a thickness of at least one dielectric insulating layer to form closed communication with an underlying conductive region, said underlying conductive region comprises an AlCu alloy;

forming a titanium nitride (TiN) barrier layer over the conductive region to contact with the conductive region and line the metal interconnect opening;

forming at least one sequence of a titanium (Ti) barrier layer followed by a TiN barrier layer to end with an uppermost TIN barrier layer to form a barrier layer stack;

forming an AlCu layer over the barrier layer stack to fill the metal interconnect opening;

annealing the AlCu layer at a temperature of about 300° C. to about 450° C.

10. The method of claim 9 wherein the AlCu layer comprises a copper concentration of about 0.5 weight percent to about 4.0 weight percent.

11. The method of claim 9 , wherein the barrier layer stack is formed by at least one of a CVD and PVD process.

12. The method of claim 9 , wherein the barrier layer stack is formed by a reactive sputtering process comprising:

a) flowing N 2 and Ar to form a gas mixture to contact the process wafer for a period of time prior to initiating a glow discharge in a sputter deposition process wherein the gas mixture comprises a ratio of nitrogen to argon of about 0.1 to 10;

b) initiating a glow discharge to sputter Ti;

c) forming a layer of TiN;

d) turning off the N 2 flow to form a sputter atmosphere consisting essentially of argon;

e) depositing a layer of Ti;

wherein at least steps a through c are repeated at least once with step c comprising the final step.

13. The method of claim 12 , wherein the reactive sputtering process comprises one of a magnetron sputtering process and an ion metal plasma (IMP) process.

14. The method of claim 9 , wherein the TiN layer is formed having a ratio of Ti to N of about 0.1 to 10.

15. The method of claim 9 , further comprising the steps of:

planarizing the AlCu layer; and,

forming a second barrier layer stack over the AlCu layer comprising at least one stack sequence of TiN/Ti/TiN.

16. The method of claim 9 , wherein the TiN barrier layers and the Ti barrier layers are formed at a thickness between about 50 Angstroms and about 200 Angstroms.

17. The method of claim 9 , wherein the barrier layer stack thickness is between about 10 Angstroms to about 600 Angstroms.