Rapid SiO₂ Atomic Layer Deposition Using Tris(<i>tert</i>-pentoxy)silanol

Abstract

Rapid SiO2 atomic layer deposition (ALD) can deposit very thick and conformal SiO2 films by silanol exposure to surfaces covered with Al catalysts. In this study, we have explored the growth of rapid SiO2 ALD films using liquid tris(tert-pentoxy)silanol (TPS). The SiO2 film thicknesses were determined using quartz crystal microbalance and X-ray reflectivity measurements. The SiO2 film thicknesses deposited during one silanol exposure were dependent on the substrate temperature, silanol pressure, and silanol exposure time. The dependence of the SiO2 growth on these parameters helped to establish the mechanism of rapid SiO2 ALD. For TPS exposures of 1 s, the SiO2 ALD growth rate was larger at lower temperatures and higher TPS pressures. SiO2 ALD thicknesses of 125−140 Å were observed at the highest TPS pressures of ∼1 Torr at lower temperatures of 150 and 175 °C. Rapid SiO2 ALD is believed to result from the growth of siloxane polymer chains at the Al-catalytic sites and the cross-linking of these polymer chains to form a dense SiO2 film. The results indicated that higher TPS fluxes increase the siloxane polymerization rates. Likewise, the lower temperatures reduce the cross-linking rates between the siloxane polymers that self-limit the SiO2 deposition. To explore the rate of cross-linking between the siloxane polymers, experiments were conducted where small TPS micropulses were employed with different delay times between the micropulses. The SiO2 ALD thicknesses decreased with increasing delay times. This behavior suggested that the longer delay times produced more cross-linking that self-limits the SiO2 deposition. Other experiments showed that higher temperatures produced faster nucleation of the rapid SiO2 ALD. The nucleation was nearly immediate at the higher temperatures and could be as long as 10 s at the lower temperatures. The growth kinetics of rapid SiO2 ALD can be understood in terms of the temperature dependence of nucleation and cross-linking and the pressure dependence of the siloxane polymerization rate. Rapid SiO2 ALD was also dependent on the presence of pyridine derivatives in the TPS. These Lewis base impurities catalyze both the initial nucleation and the cross-linking reaction during rapid SiO2 ALD.