New type of compact control system for frequency and power in megasonic transducer drive at 1MHz

This paper describes a new type of compact megasonic transducer drive which can control both the frequency and power at a frequency of 1MHz when the megasonic transducer is used for cleaning the surfaces of silicon wafers or LCD faceplates. A pulse width control is used to control the widths of the drive pulses for the fullbridge inverter having a maximum output power of 60W so that RF output power is set at the specified values; The output voltage of the fullbridge inverter is fed to the megasonic transducer through both the output transformer and the lowpass filter to eliminate harmonics from the RF voltage since the lowpass filter is the key circuit to effectively feed RF power to the megasonic transducer. The megasonic transducer current and voltage are compared with each other by the PLL so that the frequency of the VCO output voltage is set at four times the resonant frequency of the megasonic transducer. The RF output power was satisfactorily controlled from 5W to 60W when the duty cycle of the drive pulse width was changed from 20 % to 49 %. The frequency of operation was satisfactorily locked at 4x1.04MHz when the frequency of free-run oscillation in the VCO was up to 4x1.15MHz. The megasonic transducer was satisfactorily operated at an RF power of up to 60W when put in water.