having a value greater than 20 w cm 2 may be applied depending on requirement fig5 is a graph plotting time versus voltage of power from a power mixer when the modulation index u201c m u201d is 0 5 an amplitude of power from the bias rf power supply 170 is modulated by the power mixer 320 to be various depending on time the power from the power mixer 320 has three frequencies of u201c u03c9 c u201d u201c u03c9 c u03c9 m u201d and u201c u03c9 c u2212 u03c9 m u201d and has a maximum power at a range of the angular frequency u201c u03c9 c u201d of the bias rf power supply 170 a waveform of the power is various depending on a value of the modulation index u201c m u201d for example if the modulation index u201c m u201d has a value of 1 the minimum amplitude u201c b u201d of the envelope becomes zero such that the power is not transferred and if the modulation index u201c m u201d has a value of 0 5 the maximum amplitude u201c a u201d of the envelope is three times as much as the minimum amplitude u201c b u201d of the envelope a 3b fig6 a and 6b show waveforms when a modulation index has a value of 0 5 and 1 respectively when the modulation index u201c m u201d has a relative.