Temperature measurement in rapid thermal processing using the acoustic temperature sensor

Acoustic techniques are used to monitor the temperature of silicon wafers in rapid thermal processing environments from room temperature to 1000°C with ±5°C accuracy. Acoustic transducers are mounted at the bases of the quartz pins that support the silicon wafer during processing. An electrical pulse applied across the transducer generates an extensional mode acoustic wave which is guided by the quartz pins. The extensional mode is converted into Lamb waves (a guided plate mode) in the silicon wafer which acts as a plate waveguide. The Lamb wave propagates across the length of the silicon wafer and is converted back into an extensional mode at the other pin. The extensional mode acoustic wave is detected and the total time of flight is obtained. The time of flight of the extensional mode in the quartz pin is measured using pulse echo techniques and is subtracted from the total time of flight. Because the velocity of Lamb waves in the silicon wafer is systematically affected by temperature, the measurement of the time of flight of the Lamb wave provides the accurate temperature of the silicon wafer. The current implementation provides a ±5°C accuracy at 20 Hz data rate. Further improvements in electronics and acoustics should enable ±1°C measurements. The acoustic temperature sensor (ATS) has several advantages over conventional temperature measurement techniques. Unlike pyrometric measurements, ATS measurements are independent of emissivity of the silicon wafer and will operate down to room temperature. ATS also does not have the contact and contamination problems associated with thermocouples