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Thermal wavefront imaging using GHz ultrasonics

In this paper an ultrasonic reflectometry approach to measure the thermal conductivity and temperature of a material by contact is presented. This device can be utilized for monitoring material properties during manufacturing, and robotic handling of objects. The sensor comprises an AlN piezoelectric ultrasonic transducer operating at GHz frequencies, situated on one side of a silicon chip. The transducer transmits a N-period ultrasonic pulse that propagates through the silicon bulk, reflects from the opposing side, and is received by a secondary receiving transducer. The time of flight (TOF) of the ultrasonic pulse and its return amplitude are measured. As the temperature distribution in the silicon bulk changes, by contacting the sensed object, the TOF changes due to the thermal coefficient of expansion and speed of sound. This paper demonstrates that the TOF temperature dependence can be used to determine the thermal properties of the contacted object. Analysis show that the sensor can detect different materials like silicon and glass. Experiments show that, by measuring the thermal time constant, the sensor can differentiate between a bare finger and a finger with rubber gloves. The temperature coefficient of the TOF in the device is 6 ps/°C.