A signal processing technique for measurement of multimode waveguide signals: An application to monitoring of reaction bonding in silicon nitride

A system that uses ultrasonic techniques to monitor the reaction bonding of silicon nitride is described. Reaction bonding of silicon nitride takes place in a nitrogen atmosphere at temperatures up to 1400°C. As with many sensors used in hostile environments, it is difficult to design the ultrasonic sensor in a way that provides optimal clarity of the signal. The sensing system must be designed within the physical limitations on access to the furnace. Ultrasonic probes that accommodate limited access to the silicon nitride sample have been designed and ultrasonic signals acquired during processing, albeit with significant noise and complexity in the signal. Signal processing techniques are used which make it possible to measure changes in phase velocity and attenuation during reaction bonding. Because of variability in the measured velocity and attenuation, the method of signal processing presented is applicable to those cases where it is not possible to redesign the probe for optimal clarity of the ultrasonic signal. This technique demonstrates the potential to perform measurements using signals that would have been considered intractable in the past. Data obtained from ultrasonic monitoring are suitable for use as input to a manufacturing process control feedback loop.