Quasi atomic layer etching of SiO₂ using plasma fluorination for surface cleaning

In this work, quasi atomic layer etching process using surface fluorination was investigated for native oxide (SiO₂) cleaning with CHF₃, O₂, and Ar in inductively coupled plasmas. SiO₂ surface was fluorinated by fluorocarbon radicals generated from CHF₃ plasma in the first step. In the following step, the fluorinated SiO₂ was removed by O₂ or Ar plasma. O₂ plasma was used to minimize carbon contamination caused by fluorocarbon layer, and Ar plasma was used to prevent the risk of surface reoxidation by O₂ plasma. The effects of various process variables were investigated such as radio frequency source power, bias power, and processing time of each step. As the exposure time of fluorination step increased, the etch rate increased gradually and saturated. As the exposure time of O₂ plasma processing time increases, SiO₂ removal rate was also saturated and self-limited characteristic was confirmed. For surface cleaning, the authors achieved the atomic layer removal rates of 6.8 Å/cycle with the controlled small source power of 25 W and with the bias power of 5 W. When Ar plasma was applied instead of O₂ plasma for the SiO₂ removal, the similar self-limited etch rate was achieved and the etch rate was 4.0 Å/cycle. The self-limited atomic layer etching process makes discrete removal rate control possible.