Thin layer acoustic image interpretation and metrology for microelectronics using a broadband model

A broadband model is proposed to describe the nature of ultrasonic pulses in multilayered systems with a sub-wavelength thickness layer. Experimental results are presented to illustrate how delaminations and cracks with foreign material or moisture ingress can appear to be well-bonded and why acoustic images of interfaces with thin layers can sometimes give erroneous indications of the bond state. Applications of this model for delamination analysis of a geometrically complex package are demonstrated. The model can not only predict ultrasonic pulses in the time and frequency domain accurately (the forward problem), but can also provide a theoretical framework for solving the inverse problem, namely, measurement of the thickness and material properties of sub-wavelength thick coatings and layers.