a radio-frequency (RF) power source unit configured to generate three RF powers having different frequencies;

a process chamber to which a process gas for generating plasma is supplied and to which the RF powers are applied to generate a plasma in the process chamber; and

an impedance matcher located between the RF power source unit and the process chamber, the impedance matcher being configured to adjust an impedance,

wherein:

the RF power source unit includes a first RF power source connected to a first electrode located in a lower portion of the process chamber and configured to apply a first RF power having a first frequency, a second RF power source connected to the first electrode and configured to apply a second RF power having a second frequency and to set the second RF power to a first on power level and a second on power level different from the first on power level, and a third RF power source connected to a second electrode located in an upper portion of the process chamber and configured to apply a third RF power having a third frequency and to set the third RF power to a third on power level and a fourth on power level different from the third on power level,

the first frequency is higher than the second and third frequencies, the second frequency is lower than the first and third frequencies, and the third frequency is between the first and second frequencies, and

the first frequency ranges from higher than 12.88 MHz to about 14.25 MHz.