Piezoelectric Suspended Microchannel Resonators for Calorimetry

The objective of this PhD thesis is to develop a microfluidic chip platform for real-time, multidimensional characterization of biological samples and, eventually, calorimetric measurements at the single-entity level. The developed devices are piezoelectrically transduced single- and double-clamped beams with embedded microchannels, commonly known as Suspended Microchannel Resonators (SMRs). Their operating principle relies on the flow of samples through the microchannels and on the real-time monitoring of the resonance frequency, which directly depends on the systemâ s mechanical properties and is therefore highly sensitive to analyte-induced variations. The microchannels are fabricated from low-stress silicon nitride (ls-SiNx), chosen for its excellent mechanical properties, temperature and chemical resistance, biocompatibility, and transparency. Silicon oxide is incorporated to improve the fabrication robustness without compromising the aforementioned nitride properties. The channel cross-sections are designed to maximize device responsivity, ensure fabrication feasibility, and accommodate a broad range of biological analytes. Device lengths range from 50 to 500 ÎŒm, covering several orders of magnitude in resonance frequency. Each device integrates 2â 4 resonator beams, enabling simultaneous measurement of a single analyte using multiple resonators. Fabrication relies on an 8-mask process flow organized into 4 main stages: fabrication of the embedded channels, fabrication of the piezoelectric (PZE) stack, definition of microfluidic access and chips, and release of the beams. The embedded microchannel fabrication results in a flat wafer surface, facilitating subsequent integration of the PZE stack. Microfluidic access and chip definition are achieved with a single backside lithography and etching step, and the beams are subsequently released through an isotropic plasma etching. The fabrication of embedded microchannels is identified as the most critical process step. A simplified approach based on a single Deep Ultraviolet Lithography (DUVL) and two depositions was developed, providing full dimensional control. As this step governs the overall process yield, multiple design and process optimizations are implemented, resulting in a robust and fully repeatable process achieving a 100% yield. The use of Al0.6Sc0.4N thin films for piezoelectric transduction is also investigated. Devices incorporating sub-200 nm AlScN films are fabricated, and the influence of deposition and process parameters on film quality is studied using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), extraction of the d31 piezoelectric coefficient, dielectric permittivity, and breakdown field measurements. A fourfold increase in d31 compared to AlN is demonstrated, confirming the suitability of Al0.6Sc0.4N for high-performance SMR transduction. Finally, experimental characterization demonstrates that the fabricated SMRs operate as multidimensional sensors capab