Efficient and Stable Acoustical Coupling for GHz Subsurface Probe Microscopy

Many techniques are under development for the detection of subsurface defects or cracks. One such method is GHz subsurface probe microscopy (GHz SSPM). Here, modulated GHz elastic waves are transmitted through the bottom of a sample. The waves are scattered by buried features and detected using the probe of an atomic force microscope (AFM), which scans the top of the sample. To ensure that sufficient acoustical energy reaches the sample surface, an efficient and stable acoustical and mechanical coupling between the transducer and the sample is essential. As the GHz excitation is continuous, the attenuation and the standing wave patterns lead to a requirement on the coupling layer thickness and its stability in the micrometer and nanometer order, respectively. This study describes the design and modelling of a clamp and the associated acoustic measurement method to monitor and control the coupling thickness with nanometer accuracy. A 1 GHz transducer transmitted modulated elastic waves through the bottom of a sample. The out-of-plane sample surface displacement was read-out with an AFM. Samples typically measured 10×10x1 mm
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, The coupling layer was water. Evaporation of the coupling layer was counteracted using a basin and the capillary action. The combined acoustic behavior of the piezo, delay line, coupling layer and sample was modeled using the KLM model. The time dependent behavior of the coupling layer liquid was modeled analytically. The coupling layer thickness was controlled using the custom designed clamp and monitored in real-time using pulse-echo measurements and custom signal processing based on detecting the acoustic resonances of the transducer-coupling layer-sample stack. The acoustic measurements showed that the clamp allowed for stable coupling layers (<;1μm in thickness with 3a variations <; 8 nm) over a span of hours, allowing for successful BA GHz SSPM measurements.