Probing the Chemistry of Alumina Atomic Layer Deposition Using Operando Surface-Enhanced Raman Spectroscopy

This work demonstrates for the first time the capability of measuring surface vibrational spectra for adsorbates during atomic layer deposition (ALD) reactions using operando surface-enhanced Raman spectroscopy (SERS). We use SERS to study alumina ALD growth at 55 degrees C on bare silver film-over nanosphere (AgFON) substrates as well as AgFONs functionalized with thiol self-assembled monolayers (SAMs). On bare AgFONs, we observe the growth of Al-C stretches, symmetric C-H and asymmetric C-H stretches during the trimethylaluminum (TMA) dose half-cycle, and their subsequent decay after dosing with H2O. Al-C and C-H vibrational modes decay in intensity with time even without H2O exposure providing evidence that residual H2O in the ALD chamber reacts with -CH3 groups on AgFONs. The observed Al-C stretches are attributed to TMA dimeric species on the AgFON surface in agreement with density functional theory (DFT) studies. We observe Al-C stretches and no thiol vibrational frequency shifts after dosing TMA on AgFONs functionalized with toluenethiol and benzenethiol SAMs. Conversely, we observe thiol vibrational frequency shifts and no Al-C stretches for AgFONs functionalized with 4-mercaptobenzoic acid and 4-mercaptophenol SAMs. Lack of observed Al-C stretches for COOH- and OH-terminated SAMs is explained by the spacing of Al-(CH3)(x) groups from the SERS substrate. TMA penetrates through SAMs and reacts directly with Ag for benzenethiol and toluenethiol SAMs and selectively reacts with the -COOH and -OH groups for 4-mercaptobenzoic acid and 4-mercaptophenol SAMs, respectively. The high sensitivity and chemical specificity of SERS provides valuable information about the location of ALD deposits with respect to the enhancing substrate. This information can be used to evaluate the efficacy of SAMs in blocking or allowing ALD deposition on metal surfaces. The ability to probe ALD reactions using SERS under realistic reaction conditions will lead to a better understanding of the mechanisms of ALD reactions.