Mechanism of Pyridine-Catalyzed SiO₂ Atomic Layer Deposition Studied by Fourier Transform Infrared Spectroscopy

Abstract

Fourier transform infrared (FTIR) investigations were performed to study the mechanism of catalytic SiO 2 atomic layer deposition (ALD) using pyridine as the catalyst. Pyridine adsorption on hydroxylated SiO 2 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 SiCl 4 reactant. The SiCl 4 reaction with the hydroxylated SiO 2 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 SiCl 4 reaction completely removed the isolated hydroxyl species and left a small fraction of hydrogen-bonded hydroxyl species. The H 2 O 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 H 2 O reaction, the Si−Cl stretching vibration of the SiCl x surface species was monitored during pyridine adsorption. The weak interaction between pyridine and the SiCl x surface species suggested that the pyridine catalyzes the H 2 O reaction by hydrogen bonding to the incoming H 2 O reactant. The FTIR spectra also revealed that a pyridinium salt was left behind on the SiO 2 surface at lower temperatures after both the SiCl 4 and H 2 O reactions. The pyridinium salt can desorb from the SiO 2 surface and no pyridinium salt was observed for surface temperatures >340 K after either the SiCl 4 or H 2 O reactions.