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.

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.

The number of chemical processes transferred from a batch-wise approach to continuous flow is increasing, due to several advantages of continuous over batch: processes can be operated at more extreme conditions, resulting in higher speed and efficiency. Thus it is critical to evaluate key performance indicators real-time and in-line. For fluid handling processes like mixing and filling, the viscosity of the process fluid is a critical parameter. Also, for non-Newtonian fluids the viscosity varies with the shear rate. Hence the measured rheology is affected by intrusive sensor designs. Moreover, in view of fouling prevention and safety, the pipe wall should not be punctured. We propose a new concept to measure the viscosity as a function of shear rate by measuring the liquid velocity profile in and the pressure drop over the sensor. The concept is in-line, real-time, does not puncture the pipe wall and is non-intrusive. Here, we report on the development and performance of the tomographic ultrasonic velocity meter, which is part of said concept. The device consisted of 9 transducers distributed along the outer surface of a pipe. Tomographic time delay inversion was used to extract the liquid velocity profiles. The performance of the entire measurement chain was predicted using simulations. The transfer functions and acoustic wave fields were measured using a hydrophone setup. The device was tested with water and a high viscosity Newtonian liquid. The sensor successfully measured liquid velocity profiles.