Initial Growth Mechanism and Microstructural Evolution of Sub-10 nm Hydrogenated Amorphous Silicon Films

Thin-film components employed in modern semiconductor fabrication are becoming increasingly ultrathin, often exhibiting porous, nonuniform structures with open surfaces that exhibit properties distinct from those of the bulk materials. Precise characterization of these ultrathin layers is essential to optimize device performance. However, conventional characterization techniques often lack the sensitivity necessary for the analysis of films thinner than 10 nm. Notably, thin films generally exhibit porous, open-surface structures during the initial growth stage and transition to denser, closed-surface morphologies as the thickness increases. However, direct measurement of this microstructural evolution in ultrathin films is challenging. To address these limitations, this study developed a high-sensitivity H exodiffusion setup to investigate the initial growth mechanism of hydrogenated amorphous silicon (a-Si:H) films. This method enabled accurate tracking of H desorption behavior in a-Si:H films with sub-10 nm film thicknesses. Distinct low- and high-temperature features were found to be associated with H desorption from interconnected and isolated voids, respectively. Hence, the microstructural evolution of sub-10 nm thin film was successfully characterized, showing a transition from the open surfaces found during the initial growth stage to closed surfaces and uniform growth. These results were validated by spectroscopic ellipsometry and Fourier-transform infrared spectroscopy. Unlike conventional methods that require specialized substrates and are hindered by limited sensitivity to ultrathin films, our approach enables characterization without the need for special sample preparation. The proposed method enables direct, substrate-independent characterization of ultrathin films, thereby elucidating thin-film growth and microstructural transitions relevant to nanoscale semiconductor applications.