Method for forming a via contacting several levels of semiconductor layers

1. A method for forming a via connecting a first upper level layer to a second lower level layer, both layers being surrounded with an insulating material, the method comprising the steps of:

a) forming an opening to reach an edge of the first layer, the opening laterally continuing beyond said edge;

b) forming a layer of a protection material on said edge only;

c) deepening said opening by selectively etching the insulating material to reach the second lower level layer; and

d) filling the opening with at least one conductive contact material.

2. The method of claim 1 , wherein the protection material is a conductive material.

3. The method of claim 2 , wherein, at step b), the layer of the conductive protection material is formed by electroless deposition.

4. The method of claim 1 , wherein the first and second layers are silicon layers covered with a metal silicide.

5. The method of claim 4 , wherein the metal silicide is based on a metal selected from the group comprising platinum and nickel.

6. The method of claim 1 , wherein:

the thickness of the insulating material separating the first layer from the gates of transistors formed on the second layer ranges between 10 and 500 nm;

the thickness of the first layer ranges between 5 and 150 nm; and

at step b), a layer of the protection material having a thickness ranging between 10 and 50 nm is formed.

7. The method of claim 2 , wherein the conductive protection material is selected from the group comprising platinum and nickel.

8. The method of claim 1 , wherein the conductive contact material is tungsten.