IN-SITU ACOUSTIC TEMPERATURE TOMOGRAPHY OF SEMICONDUCTOR WAFERS

Spatial temperature distribution in semiconductor wafers during rapid thermal processing is obtained by means of acoustic tomography. Ultras onic Lamb waves are excited in the wafer by acoustic transducers bonde d to spring-loaded quartz support pins located at the wafer periphery. The Lamb wave time of flight in the wafer is used to measure the aver age wafer temperature with +/- 0.5-degrees-C accuracy for a S/N ratio exceeding 55 dB. Spatial temperature information is gathered by electr onic switching of the transmitter and receiver transducers. Tomographi c reconstruction techniques are then used to calculate the temperature distribution from the measurements with different pixel maps. Using e ight transducers, the thermal image of a 10 cm, (100) silicon wafer is obtained with 2x2 cm2 pixel resolution. Thermal image rates of 2 imag es/s are achieved by the system enabling real-time control of wafer te mperature distribution during rapid thermal processing.