An RBS study of interdiffusion across a brominated Si(111)/Cu interface with and without a barrier layer

Abstract

Cu thin films have been deposited on Si(111) substrates with two different initial conditions. In one, the substrates had a top native oxide layer, and in the other the native oxide layer was etched in hydrofluoric acid and the etched surface was treated with a bromine-methanol solution prior to the surface being exposed to atmosphere. This Br-treatment is known to saturate surface dangling bonds on an Si(111) surface. Vacuum annealing of the Cu/Si samples, followed by Rutherford backscattering spectrometry (RBS) measurements showed the onset temperature of interdiffusion to be ~220°C for the Si(111) substrate treated with bromine-methanol solution whereas the onset temperature of interdiffusion was found to be between 600°C and 700°C for Si(111) surfaces with native oxide on top. This shows that the native oxide layer works like a diffusion barrier. The effect of an interposed TiN_x layer on the diffusion behaviour has also been investigated. A thin layer (< 100Å) of TiN_x has been interposed between the Cu film and the Si substrate in each of the two above mentioned cases. With the TiN_x layer the onset temperature of interdiffusion has been found to be around ~500°C for Cu/TiNx/Br-Si(111) case, indicating an increase in the diffusion barrier height compared to that for Cu/Br-Si(111). For the Cu/TiN_x/SiO₂/Si(111) case, that is, when native oxide is present on the Si(111) substrate, the onset temperature of diffusion is somewhat higher (" 600°C) compared to Cu/TiN_x/Br-Si(111). This has been attributed to the possible formation of TiO_x at the TiN_x/SiO₂ interface. TiO_x is presumably a better barrier than TiN _x. However, a higher diffusion onset temperature (> 600°C) for the Cu/SiO₂/Si(111) suggests that SiO₂ introduces the highest barrier for Cu-Si interdiffusion among all the barrier combinations studied here. At annealing temperatures above the onset temperature of interdiffusion copper silicides are formed. RBS simulations have been used to study the compositions of the reacted films.