US Patent 9754768 Plasma processing method and plasma processing apparatus

At a time point T0 when starting a process, a duty ratio of a high frequency power RF1 to which power modulation is performed is set to be an initial value (about 90%) which allows plasma to be ignited securely under any power modulating conditions. At the substantially same time of starting the process, the duty ratio of the high frequency power RF1 is gradually reduced from the initial value (about 90%) in a regular negative gradient or in a ramp waveform. At a time point t2 after a lapse of a preset time Td, the duty ratio has an originally set value Ds for an etching process. After the time point t2, the duty ratio is fixed or maintained at the set value Ds until the end (time point T4) of the process.