Frequency-selective smoothing of amorphous silicon via femtosecond laser polishing

Amorphous silicon (a-Si) thin films are fundamental to thin-film electronics, photovoltaics, and optoelectronics, where surface roughness critically governs device efficiency and reliability. In this work, we introduce a non-ablative femtosecond laser polishing (Fs-LP) technique for surface smoothing. By systematically optimizing pulse fluence, scanning speed, and number of passes, we identify a narrow processing window that reduces the roughness from RMS = 1.84nm to 0.18nm (via white light interferometer) and Ra = 0.824nm to 0.128nm (via atomic force microscope). root mean square density (RMSD) analysis reveals that this roughness suppression results from a spectral weight shift from low- and mid-frequency components to higher frequencies. Our study not only elucidates the mechanism underlying femtosecond laser polishing of a-Si, but also establishes RMSD analysis as a powerful framework for assessing surface finishing methodologies. The proposed approach can be extended to other thin-film systems whose performance is critically influenced by surface morphology.