Temperature Measurement in Rapid Thermal Processing Using the Acoustic Temperature Sensor
Acoustic techniques are used to monitorthe temperature of silicon wafers in rapid thermal processing environments fromroom temperature to 1000" C with accuracy. Acoustic transducers are mounted atthe bases of the quartz pins that support the silicon wafer during processing.An electrical pulse applied across the transducer generates an extensional modeacoustic wave which is guided by the quartz pins. The extensional mode isconverted into Lamb waves (a guided plate mode) in the silicon wafer which actsas a plate waveguide. The Lamb wave propagates across the length of the siliconwafer and is converted back into an extensional mode at the other pin, Theextensional mode acoustic wave is detected and the total time of flight isobtained. The time of flight of the extensional mode in the quartz pin ismeasured using pulse echo techniques and is subtracted from the total time offlight. Because the velocity of Lamb waves in the silicon wafer issystematically affected by temperature, the measurement of the time of flightof the Lamb wave provides the accurate temperature of the silicon wafer. Thecurrent implementation provides a accuracy at 20 Hz data rate.Further improvements in electronics and acoustics should enable measurements. The acoustic temperature sensor (ATS)has several advantages over conventional temperature measurement techniques.Unlike pyrometric measurements, ATS measurements are independent of emissivityof the silicon wafer and will operate down to room temperature. ATS also doesnot have the contact and contamination problems associated with thermocouples.