The semiconductor industry needs to fit ever more devices per unit area to improve their performance; hence a trend towards increasingly complex structures by varying material combinations and 3D geometries with increasing aspect ratios. The new materials used may be optically opaque, posing problems for traditional optical metrology methods. One solution is to use acoustical waves, which present the double advantage of not being hampered by optically opaque layers and allowing for penetration depths of 10's of μm at sub-μm wavelengths; which is considerably larger than most traditional optical methods (O(100 nm's - μm's)). Here, we present a novel acoustic metrology method using GHz ultrasound waves to measure deeply buried subsurface features (>5 μm). The concept consisted of a GHz acoustic transducer integrated above the tip of a custom designed probe, which is then scanned across a sample. The method uses non-damaging solid-solid contact without the need for liquid coupling layers - in contrast to conventional acoustical microscopy. This allows for the use of much higher acoustic frequencies, hence higher on-axis resolutions. The transducer is used in pulse-echo mode and a stage controller is used to move the probe for scanning. An experimental setup was built with a 4 GHz transducer and tested successfully on 1.5-2 μm size features buried below a 5 μm PMMA or 10 μm SiO₂layer, respectively. A good match was further obtained between the measurements and the model predictions. These results demonstrated the feasibility of the new method, opening new opportunities for metrology and inspection applications.

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.