Surface roughness and power spectral density study of SHI irradiated ultra-thin gold films

Quantitative roughness and microstructural analysis of as-deposited and swift heavy ion (SHI) (107 MeV Ag and 58 MeV Ni) irradiated 10 and 20 nm thick Au films were performed by atomic force microscopy (AFM). Power spectral density (PSD) analysis was done from the AFM images. The energies chosen for the two different ions eliminated the velocity effect of SHI in materials modification. The rms roughness estimated from the AFM data did not show either monotonic increase or decrease with ion fluences. Instead, it increased at low fluences and decreased at high fluences for 20 nm thick film. In 10 nm film, the roughness first increased with ion fluence, then decreased and again increased at higher fluences. Though the 10 and 20 nm films exhibited very different patterns of rms roughness variation with ion fluence, the pattern of variation in both cases was identical for Ni and Ag beams. The PSD analysis for both 10 and 20 nm films (pristine and irradiated) showed similar variation of low frequency roughness with ion fluence as that of the rms roughness. In the high frequency regime, PSD analysis suggests that surface morphology of the irradiated samples is governed by the combined effect of evaporation-recondensation and diffusion dominated processes.