Fast, In Situ Gas Analysis during Atomic Layer Deposition through Optical Emission Spectroscopy and Non-Negative Matrix Factorization

In situ characterization of atomic layer deposition (ALD) processes allows to study reaction mechanisms and kinetics and enables industrial process control. It has been proposed that gas species in the reactor can be monitored in a nonintrusive way through capturing the optical emission from a cold cathode discharge. We critically evaluated this approach by characterizing the trimethylaluminum (TMA)-H₂O ALD chemistry. We found that a naive analysis method for tracking channel intensities corresponding to specific emission lines was not sufficient to identify reaction products. Therefore, we adopted non-negative matrix factorization (NMF) as a holistic way to analyze the obtained data sets. We were able to identify spectra and time signatures from precursor fragments and reaction products, confirming the established reaction mechanism between TMA and H₂O yielding CH₄. The NMF analysis also suggests that the discharge generates spurious H signals that can give rise to a wrong interpretation of the H emission lines. Finally, we present a critical assessment of continuous optical emission spectroscopy for ALD process characterization and NMF as a tool for unraveling and visualizing complex data sets.