Ultrasonic Shear Wave Spectroscopy for Real-Time Monitoring of Static and Dynamic Liquid-Solid Interfaces

Wettability and droplet evaporation are widespread phenomena vital in numerous natural processes and industrial applications. Understanding the interaction and wetting dynamics of droplets with various surfaces, as well as the complex mechanisms of the drying process, is essential. Moreover, the formation of drying residues, such as watermarks, has been identified as a serious problem. Quantifying these phenomena and correlating them to the surface properties will provide in-depth information on the quality of the materials and cleaning liquids used by the semiconductor and other industries. Studying the phenomena at the interface of droplets and solid surfaces is challenging due to their confined dimensions ranging from a few nanometres to micrometres. It requires novel, highly precise sensing methods for non-invasive dynamic characterisation of the boundary between liquid and solid.
This thesis focuses on the application of ultrasonic spectroscopy utilising the megahertz frequency shear waves for probing the properties of thin (200-60 nm thickness range) layers of liquid at the droplet bottom for static and dynamic quantification of droplet formation and drying phenomena with high precision. The developed methodology for the ultrasonic shear wave analysis and the prototype of the ultrasonic shear wave spectrometer are based on admittance measurements of resonances in piezoelectric sensors emitting ultrasonic shear waves through the solid-droplet interface. The methodology involves the determination of the response constants, and , for sensors of different electrode configurations, applied for the ultrasonic characterisation of droplet bottom surface area for surfaces of varying wettability, such as TiO2, Au, PTFE, PDMS, Si wafers and SiO2 surfaces.
The developed method discerns the various drying phases across all examined surfaces. It accurately identifies extreme and mixed modes of evaporation and the critical evaporating times; the durations between the transitions from one phase to another are pinpointed with high precision. The dynamic contact angles have been investigated, too. By translating the ultrasonically determined surface area to the dynamic contact angle, we can study the advancing and receding angles during droplet formation and evaporation. Using the developed shear wave spectrometer, we correlated the drying phases to the properties of the surface studied. The complex mechanics of drying and interfacial flows, such as Marangoni flow, are strongly connected to the drying residues left after evaporation, named watermarks, which can ultrasonically be detected and analysed. Moreover, energy-related phenomena are studied, confirming that the applied amplitudes of the shear wave do not affect our results, and the same is true for the roughness of the surfaces utilised. Measurements of different characteristics of resonances in piezoelectric sensors, such as frequency and bandwidth, allow distinguishing contaminants within the liquid/solid layers. Finally, a multiscale computational algorithm is built to analyse multilayer systems, including piezo and non-piezoelectric layers. It provides a tool for predicting the behaviour of materials tested with ultrasonic shear wave spectroscopy involving thin films, dust particles, and cleaning liquids used by the semiconductor industry. The results open the possibility for in-line ultrasonic shear wave monitoring of Si wafer surface quality at different cleaning and conditioning stages.