Effect of temperature and gas velocity on growth per cycle during Al2O3 and ZnO atomic layer deposition at atmospheric pressure

The growth per cycle as a function of temperature during atomic layer deposition (ALD) of Al2O3 and ZnO at atmospheric pressure follows very closely the trend measured at typical (∼2 Torr) process pressure. However, the overall growth rate is found to be nearly 2 × larger at higher pressure and the magnitude of the growth increase can be adjusted by controlling the gas velocity near the growth surface. The growth increase at high pressure is approximately independent of process temperature at T < 200 °C for Al2O3 and ZnO, but the effect begins to become less pronounced at T > 150 °C, especially for Al2O3. The relatively high growth/cycle measured at 760 Torr and T < 150 °C suggests that excess physisorbed water remains on the alumina or zinc oxide surface after the water purge step. Increasing the gas velocity in the growth zone reduces the growth rate, consistent with more efficient removal of excess water. To better understand the observed trends, we present analytical expressions for the boundary layer thickness and species diffusivity and describe how these parameters are affected by reactor pressure and bulk gas velocity in the low temperature regime. To optimize ALD films and products, new ALD schemes at ambient pressure will need to understand the interaction between reactor pressure, gas velocity near the growth surface, fluid boundary layer thickness, and product desorption and diffusivity to achieve controlled growth.