Method of detecting and monitoring stresses in a semiconductor wafer

A method of detecting and monitoring elastic strains in a semiconductor wafer (12) comprising the steps of coupling the wafer (12) to a transducer (10) having a periphery (11). This is followed by operating the transducer (10) to produce ultrasonic vibrations at a predetermined wavelength λ and propagating a standing wave through the wafer (12) in response to the ultrasonic vibrations. The method is characterized by extending the wafer (12) in a cantilevered section L from the periphery (11) of the transducer (10) to a distal end (13), and measuring the amplitude of the standing wave λ in the cantilevered section L. For maximum efficiency, the cantilevered section L is substantially one quarter of the predetermined wavelength (λ/4)