A toolbox for easy entry low wavenumber in situ atomic layer deposition transmission FTIR spectroscopy studies

A detailed description of a flexible and portable atomic layer deposition (ALD) system is presented for conducting in situ Fourier transform infrared (FTIR) absorption spectroscopy studies during the evolution and growth of ALD films. The system is directly integrated with a commercial FTIR spectrometer (Bruker Vertex 80V) to avoid the necessity of an external optical path to the instrument, thereby mitigating complexity and optical losses. In this work, we use potassium bromide (KBr) with a 5 nm layer of sputtered Si as a substrate due to higher infrared transmittance when compared to a single-side polished Si wafer. The FTIR absorption study is conducted at normal incidence in transmission mode using a deuterated L-alanine doped triglycine sulfate (DTGS) detector owing to its potential applicability for reliable measurements at wavenumbers below ∼700 cm^-1. We demonstrate this by measuring ex situ the transverse optical phonon of bulk Al₂O₃ centered at 680 cm^-1. The integrity and functionality of the system to track the nucleation stage are validated by conducting in situ FTIR absorption measurements of Al₂O₃ using tri-methyl aluminum (TMA) and H₂O. The measured IR absorption spectra for the Al₂O₃ growth after each cycle of TMA and H₂O show the formation and removal of CH₃ (2800-3000 cm^-1) groups on the substrate surface and CH₄ (3016 and 1306 cm^-1) molecules in the reactor, thus confirming the successful tracking of ligand exchange. Thus, this instrument, together with the choice of KBr as substrate, can enable straightforward ALD nucleation studies using a DTGS detector having sufficient signal without additional optical setup and modifications to off-the-shelf FTIR systems that allow low wavenumber experiments.