Abstract: A plasma processing apparatus comprises a plasma chamber having a gas inlet opening and a gas outlet opening. A first high-frequency energy source supplies accelerating energy to a holder that supports a semiconductor specimen within the chamber to produce a high-frequency accelerating electric field. Gas is introduced to the chamber through the inlet opening and accelerated by the electric field toward the specimen. An antenna structure is connected to a second high-frequency energy source which supplies exciting energy at a frequency in the range between 100 MHz and 1 GHz which is higher than the frequency of the accelerating energy. The antenna structure has radially outwardly extending, circumferentially equally spaced apart elements of length equal to the quarter wavelength of the exciting energy so that there is a phase difference of 180 degrees between adjacent ones of the antenna elements. The accelerated gas is uniformly excited and converted to high-density plasma.