Summary: Driven by the growing demand for flexible electronics, this thesis investigates the integration of MEMS resonators onto flexible substrates to enhance device functionality, interaction, and adaptability in various, including dynamic and constrained, environments.
III-Nitrides, particularly Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN), offer a unique platform due to their efficient transduction properties, mechanical robustness, thermal stability, and chemical resistance, making them highly competitive compared to silicon- or polymer-based materials.
The main objective of this work was the design, fabrication, and transfer of III-N MEMS resonators onto different flexible platforms. To achieve this, an innovative epitaxial process named Selective Area quasi-Van der Waals Epitaxy (SAqVWE) was developed.
This approach combines quasi-Van der Waals epitaxy (qVWE) on a two-dimensional hexagonal boron nitride (2D h-BN) release layer with Selective Area Growth (SAG) defined by dielectric masks (SiN or SiO₂). The method enables precise control of resonator geometry and dimensions prior to growth, without relying on post-growth etching steps.
This combination of selective epitaxy with a 2D release layer provides a pathway for etch- free, scalable, and clean dry integration of III-N resonators onto a wide range of substrates, including rigid (silicon), flexible (polymers or metals) platforms.
Throughout this thesis, several transfer methodologies were explored, including wet, dry, and thermo-mechanical Self Lift-Off and Transfer (SLOT)-based processes. The acquisition and optimization of a high-precision pick-and-place system (Fineplacer Lambda 2) enabled controlled exfoliation and accurate positioning, placement, and assembly of microdevices onto pre-patterned host substrates. Functional devices were successfully demonstrated using the 2D Layer Assisted Transfer (2DLAT) approach, including PN junctions, heterojunctions, and UV/blue micro-LEDs, highlighting the feasibility of this method for flexible MEMS and optoelectronic integration.
Passive III-N membrane resonators were fabricated, transferred from growth substrates to silicon-etched substrates, and mechanically characterized, with resonance frequencies and mode shapes validated against FEM simulations. This approach was also used to determine the mechanical properties of the III-N layers. Additionally, active doped and undoped AlGaN/GaN and GaN MEMS resonators with integrated transduction mechanisms were realized on flexible metallic substrates (Cu and Ni), demonstrating both electrothermal and piezoelectric transduction schemes. A temperature sensor on a flexible metallic substrate was also demonstrated as a proof of concept. Furthermore, SAG-grown resonators integrating transduction electrodes for Lorentz-force actuation and induced-current detection were designed, fabricated, and successfully exfoliated from sapphire, representing initial steps toward application-driven demonstrators.
In conclusion, this thesis establishes a comprehensive technological platform for the realization of III-N MEMS resonators on flexible substrates, encompassing selective growth, quasi-Van der Waals epitaxy, transfer, device microfabrication, and performance characterization. The results provide a significant step toward the development of flexible GaN-based sensors and resonators for next-generation electronics, with potential applications in wireless communications, chemical and biological detection, and robust embedded systems.