Real-time monitoring of thin film thickness and surface roughness using a single mode optical fiber

Abstract

This research introduces an innovative method for real-time monitoring thin film growth and surface roughness using a single mode optical fiber without any additional treatment. The cleaved end of the fiber was installed within the deposition chamber, allowing the thin film to be deposited directly onto the fiber tip. During the deposition process, a Fabry-Pérot interferometer was formed with its cavity length equal to the film thickness. As the thin film grows, it alters the cavity length, resulting in a measurable interferogram. In cases where the film surface is not optically smooth, surface roughness becomes a function of deposition time. By leveraging this phenomenon, this research demonstrates a method for calculating film thickness and surface roughness using Hilbert Transform along with an iterative method. It was found that the measured film thickness fluctuates around the ground truth given by a simulation, with an error on the nanometer scale (≤4 nm) and the reflectivity error is less than 0.004 (maximum error percentage of 5.86 %). Compared to traditional quartz crystal microbalance counterpart, the proposed method directly measures the film thickness rather than mass of the thin film. Furthermore, the compact probe design allows it to be placed closer to the substrate, enhancing monitoring precision. This method offers a simple, quick, and affordable approach to monitor film thickness and surface roughness, effectively addressing the current challenges in the field.