In situ examination of tin oxide atomic layer deposition using quartz crystal microbalance and Fourier transform infrared techniques

Abstract

The atomic layer deposition (ALD) of tin oxide thin films has been examined using in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) techniques. The ${\mathrm{SnO}}_x$ films were deposited using sequential exposures of ${\mathrm{SnCl}}_4$ and ${\mathrm{H}}_2{\mathrm{O}}_2$ at temperatures from 150 to 430 °C. The linear growth of the tin oxide ALD films was observed by both the mass gain during QCM measurements and the background infrared absorbance increase during FTIR investigations. The FTIR spectra revealed the loss and gain of the O–H stretching vibrations of the hydroxyl group for the ${\mathrm{SnCl}}_4$ and ${\mathrm{H}}_2{\mathrm{O}}_2$ exposures, respectively. The background infrared absorbance also oscillated after each ${\mathrm{SnCl}}_4$ and ${\mathrm{H}}_2{\mathrm{O}}_2$ exposure. The background absorbance increased after ${\mathrm{SnCl}}_4$ exposure and decreased after ${\mathrm{H}}_2{\mathrm{O}}_2$ exposure. QCM measurements were consistent with a tin oxide ALD growth rate of $\sim 60\mathrm{ng}{\mathrm{cm}}^{-2}$ per cycle. This mass change corresponds to a growth rate of $\sim 0.7\mathrm{\AA }∕\text{cycle}$ at 325 °C assuming a ${\mathrm{SnO}}_2$ density of $6.9\mathrm{g}{\mathrm{cm}}^{-3}$ . Additional ex situ surface analysis using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) revealed that the ${\mathrm{SnO}}_x$ films grown at 325 °C were defined by $x<2$ . Atomic force microscope (AFM) results also showed that the ${\mathrm{SnO}}_x\mathrm{ALD}$ films deposited on Si(100) wafers have a very rough surface. Understanding and controlling the growth of tin oxide ALD films should be useful to enhance the sensitivity of ${\mathrm{SnO}}_x$ gas sensors.