A high-performance NEMS force sensor

This thesis presents the development of a pioneering NEMS force sensor that uses suspended SiNW as ultrasensitive piezoresistive elements for multidirectional force detection. The innovative sensor architecture features a centrally positioned shuttle suspended by springs and integrated with eight SiNWs along <110> crystallographic directions on a (100) silicon wafer, enabling simultaneous measurement of perpendicular and shear force components with exceptional spatial resolution. Through comprehensive design optimization, the sensor achieves remarkable miniaturization with a 10 × 10 μm2 shuttle and SiNWs down to 250 nm. This ultra-miniaturized structure surpasses state-of-the-art microscale force sensors and suspended-beam sensors from a gauge dimension perspective. A CMOS-compatible, top-down microfabrication process was successfully developed on SOI wafers, incorporating deep silicon etching, precise SiNW thinning, boron ion implantation, and HF vapor release. The challenging encapsulation and protection of non-intended released areas using photoresist, through extensive optimization, led to the successful implementation of negative photoresist nLOF 2020 for this critical fabrication step, ensuring the long-term stability of released SiNWs in the ambient environment. The sensor's superior performance was verified through extensive experimental validation. LDV measurements identified a resonant frequency of 12.35 MHz, with corresponding quality factors reaching approximately 700, while quasistatic mechanical characterization yielded GFs up to 42 for released SiNWs, demonstrating the high piezoresistive effect. The demonstrated gauge factors (GF ≈ 42 for transverse and 22 for axial load) are comparable to most top-down fabricated SiNW sensors reported in the literature, validating the good piezoresistive response and strain localization achieved by the released SiNW design. Given the test setup's strain increment limitation of 0.0025 and a resistance change detection limit of 1 ppm, the sensor achieves a theoretical minimum force resolution of 0.72 nN with GF = 22, demonstrating capability for ultra-low force detection suitable for nanomechanical testing applications. Flow sensing experiments with nitrogen gas at various pressures confirmed highly linear responses, revealing force detection capabilities in the μN range. The research culminates with a novel cantilever-based sensor design that achieves tenfold amplification factor improvement through optimized SiNW placement along a novel stress amplification bridge, establishing SiNWs as transformative sensing elements for next-generation MEMS force sensors and opening new frontiers in high-sensitivity, miniaturized sensing technologies for advanced engineering applications. The key contributions of this work include the development of a compact multi-axis MEMS force sensor architecture with a novel encapsulation strategy to improve the long-term stability of released nanostructures, the establishment of a reproducible methodology for correlating displacement with applied force through combined experimental and finite element analysis, and the identification of multiplexed readout integration as a promising direction for future scalability. Moreover, this work bridges the gap between MEMS scalability and NEMS, providing a unified platform compatible with CMOS and wafer-level processes, and establishes a robust foundation for next-generation SiNW-based MEMS force sensors for applications in flow mapping, tactile robotics, and biomedical micro operations.