In Subsurface Scanning Probe Microscopy (SSPM), Atomic Force Microscopy (AFM) is combined with ultrasound. The AFM cantilever is used as a receiver. At low frequencies (O(MHz)) the method can be used to measure the stiffness contrast in a sample and at high frequencies (O(GHz)) to measure scattering based contrast. Both variants use modulated excitation signals in combination with the nonlinear tip-sample interaction to downmix the sample top surface displacement close to the resonance frequency of the cantilever. This concept has three advantages: 1) the resonance of the cantilever is used to boost the sensitivity, 2) the downmixing allows the system to record signals in an extremely wide range of carrier frequencies (kHz - GHz), and 3) the cantilever tip-sample contact diameter is usually much smaller than 10 nm. The latter implies that spatial averaging effects are negligible up to 100 GHz. Said advantages mean that AFM cantilevers could be useful to characterize acoustic sources. Here, we investigate the suitability of AFM cantilevers as acoustic point receivers to characterize acoustic sources. Investigation with an AFM setup and two sources - one O(MHz) and one O(GHz) - show a good match between the measured response, KLM simulations and the transducers bandwidth, which are promising results for the use of a cantilever as a wideband receiver for high resolution acoustic (transducer) characterization.

The characterization of buried nanoscale structures nondestructively is an important challenge in a number of applications, such as defect detection and metrology in the semiconductor industry. A promising technique is Subsurface Scanning Probe Microscopy (SSPM), which combines ultrasound with Atomic Force Microscopy (AFM). Initially, SSPM was used to measure the viscoelastic contrast between a subsurface feature and its surrounding medium. However, by increasing the ultrasonic frequency to >1 GHz, it has been shown that SSPM can also measure acoustic impedance based contrasts. At these frequencies, it becomes difficult to reliably couple the sound into the sample such that the AFM is able to pick up the scattered sound field. The cause is the existence of strong acoustic resonances in the sample, the transducer, and the coupling layer-the liquid layer used to couple the sound energy from the transducer into the sample-in combination with the nonlinearity of the tip-sample interaction. Thus, it is essential to control and measure the thickness of the coupling layer with nanometer accuracy. Here, we present the design of a mechanical clamp to ensure a stable acoustic coupling. Moreover, an acoustic method is presented to measure the coupling layer thickness in real-time. Stable coupling layers with thicknesses of 700 ± 2 nm were achieved over periods of 2-4 h. Measurements of the downmixed AFM signals showed stable signal intensities for >1 h. The clamp and monitoring method introduced here makes scattering based SSPM practical, robust, and reliable and enables measurement periods of hours.