Surface Preparation Techniques for the Atomic Layer Deposition of Hafnium Oxide

Abstract

Atomic layer deposition (ALD) has been successfully used over the years for the deposition of conformal dielectric layers with precise thickness down to the nanometer scale. Therefore, optimization of the growth behaviour of the dielectric is mandatory. Since ALD is a surface sensitive growth technique, determined by the amount of available reaction sites at the starting surface, the impact of various wet chemical and thermal Si treatments on the HfO² growth was evaluated. Thin SiO² starting layers, based on wet chemical processing, were prepared by using a diluted peroxide mixture and an ozone/DI-water treatment. The thermally grown oxides were gradually etched by slowly immersing the oxidized Si substrate into a diluted HF solution, creating a thickness range on a single wafer. Our results demonstrate that the HfO₂ deposition is more dependent on the thickness of the SiO₂ layer than on the chemistry used to grow the oxide layer. For all studied oxides, two regions can clearly be distinguished. First, a linear relationship between the oxide thickness and the amount of deposited HfO₂ is seen. Because chemical oxides tend to grow in islands, this trend can be explained by an increase in density of surface OH groups when the oxide grows thicker. When an ellipsometric thickness of ~0.8 nm is reached, saturation of the HfO₂ growth is obtained. We believe that, from this thickness on, the starting surface is completely covered with hydroxyl groups, leaving the HfO₂ growth only dependent on the ALD process itself. Since both the wet chemical as the thermal oxides are showing the same trend in HfO² deposition, it can be stated that surface preparations can be selected solely based on their ease of processing. However, the electrical results show that there may be a difference between the ozone based wet chemical oxides and the etched thermal oxides, since the latter seems to possess slightly more leakage current. The electrical results will be discussed in more detail during the presentation.