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

This paper describes a new type of compact megasonic transducer drive which can control both the frequency and power at a frequency of 1 MHz 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 full bridge inverter having a maximum output power of 60 W so that RF output power is set at the specified values. The output voltage of the full bridge inverter is fed to the megasonic transducer through both the output transformer and the low pass filter to eliminate harmonics from the RF voltage since the low pass 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 SW to 60 W when the duty cycle of the drive pulse width was changed from 20% to 49%. The frequency of operation was satisfactorily locked at 4/spl times/1.04 MHz when the frequency of free-run oscillation in the VCO was up to 4/spl times/1.15 MHz. The megasonic transducer was satisfactorily operated at an RF power of up to 60 W when put in water.