In situ metrology of direct-write laser ablation using optical emission spectroscopy

Direct-write laser ablation is an effective manufacturing method for etching complex microscale patterns, especially on hard ceramics such as sapphire that are difficult to machine using traditional mechanical or micromachining methods. However, the variability of the laser-matter interaction causes inconsistencies that prevent this process from moving beyond the research realm. This work presents the real-time monitoring of the ablation process in sapphire using optical emission spectroscopy to assess the key wavelengths that exhibit strong correlations to the fabricated features. In this process, a focused ultrafast laser is used to create microscale features and morphological changes in sapphire substrates, which are studied by a subsequent wet etching in a hydrogen fluoride solution. The etched sapphire samples are observed to have amorphous sapphire removed, resulting in microstructures with higher profile fidelity. Furthermore, principal component analysis of the measured spectral obtained during the etch process indicates that the emission from a few key wavelengths exhibits strong correlations to the etched sapphire patterns. This result indicates that the use of data-driven techniques to assess the spectral emissions of direct-write laser ablation can be a useful tool in developing in situ metrology methods for laser-matter interactions.