Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Silicon Dioxide and Aluminum Oxide

Abstract

Atomic layer deposition (ALD) is a powerful deposition technique for the fabrication of highly conformal and precise thickness-controlled films. Recently, there has been a growing interest in low-temperature (<100˚C) ALD because of the emerging applications made of organic and polymeric materials, which are known to be thermally sensitive materials. Due to the inherent disadvantages of low-temperature thermal ALD, such as slow reaction rate, long cycle time, poor film density, high impurity level and poor encapsulation properties of ultra-thin films (<5nm), the plasma-enhanced atomic layer deposition (PEALD) becomes an attractive alternative. However, there is only limited data available to show the effect of plasma parameters on film properties at low process temperatures and thus its full potential remains largely unexplored. This thesis presents the process development for low-temperature PEALD SiO2 and Al2O3 aiming to have a deeper understanding of plasma parameters vs. film properties. The work started with a study on PEALD SiO2 thin film growth, where oxygen and carbon dioxide were used as oxidizing agents. The results showed that process temperature, plasma power and exposure time used in the deposition had a strong impact on the film growth rate as well as the film properties. For example, the growth-per-cycle (GPC) increased from 0.78 to 1.15 Å/cycle with a decrease of process temperature from 200 to 90 ˚C, plasma power from 300 to 50 W or plasma exposure time from 6 to 3 s, respectively. Although the high growth rate of ALD film is highly desired in the industry, the film quality should not be neglected. As shown in this work, there is a trade-off between the film properties and film growth rate during SiO2 thin film deposition. In order to balance the throughput and quality of PEALD SiO2 thin film, the plasma power of 180 W and plasma exposure of 3 s was suggested. The studied low-temperature ALD Al2O3 processes included either O2-based PEALD or a process that combines H2O-based thermal ALD with in-situ N2 plasma treatment. For O2-based PEALD, the moisture barrier properties of Al2O3 films were found to be improved with increasing plasma power when the plasma exposure time was relatively short (1 and 3 s). The best water vapor transmission rate of 4-nm thick Al2O3 film was 5×10-3 gm−2day−1. However, a degradation in the moisture barrier properties was observed if the plasma exposure time reached 6 s. This degradation did not occur in the combined process of H2O-based thermal ALD with in-situ N2 plasma treatment. In addition to the moisture barrier properties of Al2O3 thin films, the growth, refractive index, density, roughness and elemental composition were also investigated. Overall, the results demonstrate that with proper PEALD process optimization, it is possible to fabricate high-quality SiO2 and Al2O3 films for thermally sensitive materials/applications.