Angular dependences of SiO2 etch rates at different bias voltages in CF4, C2F6, and C4F8 plasmas

The angular dependences of SiO2 etch rates at different bias voltages for in CF4, C2F6, and C4F8 plasmas were investigated using a Faraday cage system. When the bias voltage was -400 V, the normalized etch yields (NEYs) reached a maximum at 70 degrees in CF4 and C2F6 plasmas, while they decreased monotonically with ion-incident angle in a C4F8 plasma. This was because the thickness of the steady-state fluorocarbon film formed on the SiO2 surface was minimized at an ion incident angle of 70 degrees in CF4 and C2F6 plasmas, while much thicker fluorocarbon films were deposited in a C4F8 plasma. When the bias voltage was as high as -1200 V, the thicknesses of the steady-state fluorocarbon films were very thin (less than 2 A) and nearly unchanged at all ion-incident angles for CF4 and C2F6 plasmas, resulting in nearly the same shape of the NEY curves. In a C4F8 plasma, the NEY showed a maximum at an ion-incident angle of 50 degrees because the thickness of the steady-state fluorocarbon film was minimized at this angle. (C) 2017 Elsevier B.V. All rights reserved.