Atmospheric Microplasma Sputter Deposition of Interconnects

We have preliminarily developed an apparatus that allows for the continuous, direct writing of interconnect-quality conductive lines. An atmospheric-pressure microplasma obviates the need for a vacuum while allowing for fine resolution imprints. We tested and characterized a novel focusing mechanism in which collisions with the working gas are harnessed to transfer electrostatic force to neutral sputtered atoms. This method compresses the deposit’s width in one dimension while expanding its length in the perpendicular dimension. We find that for an ideal set of parameters, the imprint is narrower than the sacrificial sputtering target (i.e., 9 μm wide imprint from a 50 µm diameter target). Other sets of parameters lead to other results, as computer simulation predicted, ranging from an unfocused spot 400 µm in diameter to a narrow line with 20:1 compression in the direction of focus, i.e., width, and 20:1 expansion in length as compared to the unfocused spot.

The microstructure of the deposit is of particular interest. As is typical of sputterers, the deposit could be smooth (55 nm roughness), and the resistivity can be as low as 1.1 µ-omega * m (with no annealing). However, the resistivity greatly depends on the microstructure, which in turn depends on the deposition conditions. It is well known that sputtering at high-pressure results in a grain structure, as the early deposits shadow parts of the bare substrate, keeping sputtered material from fully coating the substrate. Traditionally, vacuum sputtering prevents this problem by allowing the sputtered material to impact the substrate normal to the surface; however, we sputter at atmospheric pressure, and thus, the sputtered material is redirected by random collisions. In this case, we use a combination of directed gas flow and electrostatic forces to prevent this shadowing effect.