Nucleation and growth of copper selective-area atomic layer deposition on palladium nanostructures

The nucleation and growth of copper atomic layer deposition (ALD) on palladium have been investigated for applications in nanoscale devices. Palladium nanostructures were fabricated by electron beam lithography and range in size from 250 nm to 5 μm, prepared on oxidized silicon wafers. Copper ALD using Cu(thd)₂(s) and H₂(g) as reactants was carried out to selectively deposit copper on palladium seeded regions to the exclusion of surrounding oxide surfaces. Nuclei sizes and densities have been quantified by scanning electron microscopy for different growth conditions. It is found that growth occurs via island growth at temperatures of 150-190 °C and alloy growth at temperatures above 210 °C. In the lower temperature window, nucleation density increases with decreasing temperature, reaching a maximum of 4.8 ± 0.2 × 10⁹/cm² at 150 °C, but growth is too slow for significant deposition at the lowest temperatures. At higher temperatures, individual nuclei cannot be quantified due to extensive mixing of copper and palladium layers. For the lower temperatures where nuclei can be quantified, rates of nucleation and growth are enhanced at high H₂ partial pressures. At the smallest length scales, conformality of the deposited over-layers is limited by a finite nuclei density and evolving grain structure that cause distortion of the original nanostructure shape during growth.