Mechanism of pyridine-catalyzed SiO2 atomic layer deposition studied by Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) investigations were performed to study the mechanism of catalytic SiO2 atomic layer deposition (ALD) using pyridine as the catalyst. Pyridine adsorption on hydroxylated SiO2 surface was examined by monitoring both the changes to the O-H stretching vibrations and the appearance of pyridine molecular vibrations. The strong hydrogen bonding of pyridine to the isolated hydroxyl groups with a desorption energy of 9 +/- 2 kcal/mol is believed to make oxygen a stronger nucleophile for nucleophillic attack on the SiCl4 reactant. The SiCl4 reaction with the hydroxylated SiO2 surface was then investigated by monitoring the disappearance of O-H stretching vibrations and appearance of Si-Cl stretching vibrations. These FTIR results revealed that the SiCl4 reaction completely removed the isolated hydroxyl species and left a small fraction of hydrogen-bonded hydroxyl species. The H2O reaction was studied by observing the disappearance of the Si-Cl stretching vibration and the appearance of the O-H stretching vibration. To understand the role of pyridine during the H2O reaction, the Si-Cl stretching vibration of the SiClx surface species was monitored during pyridine adsorption. The weak interaction between pyridine and the SiClx surface species suggested that the pyridine catalyzes the H2O reaction by hydrogen bonding to the incoming H2O reactant. The FTIR spectra also revealed that a pyridinium salt was left behind on the SiO2 surface at lower temperatures after both the SiCl4 and H2O reactions. The pyridinium salt can desorb from the SiO2 surface and no pyridinium salt was observed for surface temperatures > 340 K after either the SiCl4 or H2O reactions.