We present our experimental results on ultrasound transducers based on Photonic Integrated Circuits. We have fabricated and tested devices based on Mach Zehnder Interferometers and Ring Resonators, in the thick-Silicon on Insulator platform (VTT, Finland) and Si3N4 platform (Ligentec, Switzerland). We have obtained a Noise Equivalent Pressure which is two orders of magnitude lower than conventional State-Of-The-Art transducers, clearly demonstrating the huge potential of this concept.

Echography is an important medical diagnostic technique. Historically, the key improvement driver was the hypothesis that higher image quality leads to better diagnoses and increased patient health. Here, a major parameter is the signal to noise ratio (SNR). Diffraction and attenuation reduce pressure levels during propagation. Thus, an SNR increase yields detection at larger depths benefitting traditionally difficult to image patients (eg large/obese patients). Peak pressures are limited by safety standards (mechanical/thermal index). Thus, to increase SNR more sensitive transducers are required. The state of the art Noise Equivalent Pressure (NEP) for piezotransducers / cMUTs / pMUTs is ~0.5 Pa at 1 MHz [1],[2],[3]. A recent innovation is the Integrated Photonic Ultrasound Transducer (IPUT), which combines a membrane and a photonic waveguide to measure ultrasound waves. Literature [5] reported such a device producing a 0.38 Pa NEP at 0.47 MHz and 21% -6 dB bandwidth with a 169x smaller spatial footprint compared to 0.5 x 0.5 wavelength2. Here, we cascade IPUTs into array elements and transform IPUT sensitivity per area into high absolute sensitivity. Several transducer elements were created by cascading up to 16 IPUTs. After designing these devices, their performance was predicted, and subsequently they were fabricated via VTT’s 3 µm thick silicon-on-insulator (SOI) waveguide platform. IPUT performance was measured in a water tank using a custom calibrated source transducer. The transfer functions and noise of each signal chain component was measured and analyzed. The results showed for a 5 cascaded IPUT element a measured NEP of 4 mPa at 0.54 MHz with a 13% -6 dB bandwidth. This improves on the state-of-the-art by a factor of 90-116x.

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.

Several methods are being researched to detect and characterize buried nanoscale structures in hard solid samples. The most common acoustic method is acoustic microscopy. An acoustic microscope is based on a single element transducer operating in pulse-echo mode. The acoustic waves are coupled into a sample using a liquid couplant (eg water) and the beam is focused using a geometric lens to obtain a good lateral resolution. Thus, the frequency is limited by the attenuation in the coupling layer (water 3.5\{dB}/{m} at 4 GHz) and the typically low transmission coefficients at the transducer-liquid couplant and liquid-sample interfaces. Here, we present a novel method for high frequency acoustic metrology of buried structures in solid samples. The concept consisted of a 4 GHz acoustic transducer integrated above the tip of a custom designed probe. It operated in pulse-echo mode, and used solid-solid contact with the sample without the need for liquid coupling layers. A prototype was built and successfully tested experimentally on samples consisting of silicon with 1D and 2D arrays ofmu\{m} sized features buried below 5-10{m} of PMMA or SiO2 top layers. Moreover, a good match was obtained between model predictions and measurements of the pulse-echo performance of the novel GHz acoustic metrology method. The technique features a penetration depth of O(10s ofmu\{m}), is nondamaging and is not hampered by optically opaque layers.

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.