Dielectric separator layer

1. A method comprising:

providing an inlaid interconnect in an interlevel dielectric layer, said inlaid interconnect comprising a via and an overlying trench;

forming a lower layer over said inlaid interconnect and said interlevel dielectric, the lower layer comprising SiCH or SiCNH;

densifying said lower layer to convert said lower layer to a modified lower layer with a k value of between 3.4 and 6.1;

forming an upper layer over said modified lower layer, the upper layer comprising SiCNH or SiNH and being at least five times as thick as the lower layer and having a k value between 4.3 and 7.0;

forming a second interlevel dielectric layer on the upper layer, the second interlevel dielectric layer having a thickness at least twenty times as thick as the upper layer and having a k value less than 2.75;

etching the second interlevel dielectric layer to form a via opening, the upper layer acting as an etch stop layer for the etch; and

filling the via opening with a barrier layer and conductor layer, the barrier layer extending from at least the top of the second interlevel dielectric layer to at least the lower layer.

2. The method of claim 1 further comprising coating and baking a spin-in dielectric to form said interlevel dielectric layer.

3. The method of claim 1 wherein said interlevel dielectric layer is formed with plasma enhanced chemical vapor deposition (PECVD) comprising:

a substrate temperature of about 250-400 degrees Centigrade; and

a reactor pressure of about 20-250 Torr.

4. The method of claim 1 wherein said densifying of said lower layer comprises a plasma pretreatment.

5. The method of claim 4 wherein said plasma pretreatment removes Carbon from a top surface of said lower layer.