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.