Paper PS-ThP16
Study of Tungsten Oxidation in Low-Temperature Plasma Processing

Thursday, October 18, 2007, 5:30 pm, Room 4C

As the device feature size of integrated circuit continues to be scaled down, metal or polysilicon/metal stack has been used as gate electrode for transistor formation. Among different metals investigated, tungsten meets various requirements and is gradually adopted for 45nm and below nodes. However, tungsten can be oxidized easily when exposed to oxygen plasma during process, leading to degradation of device performance. One example is post-implantation photoresist stripping in oxygen plasma where tungsten oxidation results in gate profile distortion and critical dimension change. Although an effective approach to solve this problem is available by using an oxygen-free reducing gas, such chemistry usually gives a very low photoresist removal rate and poor process uniformity. The second example is selective polysilicon oxidation over tungsten after gate etching to anneal etching damage where metal oxidation is difficult to be prevented with the presence of oxygen. The current common approach is using water vapor and hydrogen mixture to do thermal oxidation, but the process requires very high temperature and tends to cause contamination issue. In this paper, a detailed and systematic work has been conducted to study the tungsten oxidation in oxygen and oxygen-containing gases in an inductively-coupled plasma reactor operating at low temperature. By using various surface analytical methodologies, the oxidation of tungsten surface has been characterized and the oxide thickness has been measured. The dependences of tungsten oxide growth or tungsten loss on various process conditions, including RF power, pressure, temperature and exposure time, and process chemistry have been investigated. The experimental results show that tungsten oxidation occurs very fast at the top surface, but the oxide growth is mostly controlled by a few process parameters. The extent of tungsten oxidation is also found to change significantly with plasma chemistry and can be varied through post-treatment. Based on this work, mechanism of tungsten oxidation in high-density plasma has been discussed. Optimized process regime and chemistry have been identified to greatly reduce or even suppress tungsten oxidation for different process applications.