Contactless Measurement of Sheet Resistance Using Impulse Voltage

A novel method for the contactless sheet resistance measurement is presented. The sheet resistance of a silicon wafer has been conventionally measured using the four-point probe technique. However, the high mechanical pressure of the probe causes pitting and structural damage in semiconductor wafers. Therefore, to prevent such damage, a contactless measurement technique was devised that employs electromagnetic induction. The conventional contactless method is implemented using high frequency sinusoidal magnetic excitation with a Robinson marginal oscillator as an excitation current supply. However, the oscillator gain drifts due to an increase in temperature during continuous scanning of the wafer, which causes an error in the resistance measurement. The proposed method does not require an analogue electronic circuit; therefore, more precise measurement is expected. This novel method utilizes an impulse voltage to generate an eddy current in the sheet under test. However, no high voltage source is required, because the impulse voltage is generated by cutting off the DC current flowing through the winding of a ferrite core facing the sheet.