Thin, continuous, and conformal copper films by reduction of atomic layer deposited copper nitride

Thin and continuous copper films serve as seed layers for electrodeposition of interconnects in microelectronic devices. Gaps in the continuity of these Cu films must be avoided, because they can generate voids that later lead to failure of the devices. It is difficult to sputter completely continuous copper-seed layers into the increasingly narrow trenches and holes in modern interconnects. Here we report a method for producing thin, completely continuous, and highly conductive copper films conformally inside very narrow holes with aspect ratios of over 40:1. The first step in our process is atomic layer deposition (ALD) of copper(l) nitride, Cu3N, by the alternating reactions of copper(l) N,N'-di-sec-butylacetamidinate vapor and ammonia on surfaces heated to approximately 160 degrees C. At this temperature, Cu3N is thermally stable, but is readily reduced to copper metal by exposure to molecular hydrogen gas (H-2). Copper layers as thin as 0.8 nm (about 3 monolayers) are electrically continuous and show the electrical resistivity predicted by a grain-boundary-scattering model for continuous films of that thickness. 3 nm thick copper films on 2 nm of ruthenium have a sheet resistance of less than 50 Omega/square, a value low enough to serve as seed layers for advanced electroplating techniques.