In-situ ultrasonic thermometry of semiconductor wafers

We report a temperature measurement technique based on the temperature dependence of acoustic wave velocity in silicon wafers. The zeroth order antisymmetric Lamb wave is excited in the wafer using the quartz pins which support the wafer during processing. Extensional waves are generated in the quartz pin by a PZT-SH transducer and the acoustic energy is coupled to the Lamb wave at the contact between the sharpened end of the pin and the wafer surface. The detection is done by a similarly modified quartz pin. By measuring the time of flight of the Lamb wave in the wafer between the two pins, we have monitored the wafer temperature in-situ in the 20-1000°C range with ±1 accuracy. With 8 spring loaded pins placed on a circular array around the wafer, 28 different measurements are made giving information about different regions. Using tomographic reconstruction techniques, the spatial temperature distribution with a 2 cm pixel size is then calculated. Excellent agreement is observed between the readings of thermocouples attached to the wafer, and calculated values for ultrasonic tomography