Wide-range humidity sensing and monitoring applications including instrumentation, agriculture, meteorology, biomedicine, and food processing have attracted long-standing interests, where recently substantial progress is made in both sensing-material science and microfabrication technologies to achieve portable, reliable and low-cost humidity sensing instruments. Due to their high sensitivity, enormous miniaturization potential, and well-developed high-volume microfabrication technologies, microelectromechanical systems (MEMS)-based piezoresistive cantilever devices covered by large-surface-area nanostructures of hygroscopic materials offer an ideal platform for highly sensitive humidity detection. Since resonant gravimetric sensing is the dominant humidity sensing technique in recent research works, in this paper, resonant actuation principles for microcantilevers (i.e. the dynamic operation mode) are addressed and compared with respect to the quality of the amplitude and phase signals, as required for on-line frequency tracking using a phase-locked loop circuit. Parasitic feedthrough effects are considered between the resonance-mode ( f ₀ ) excitation element and the piezoresistive detection circuit, which can lead to a reduction of stop-band attenuation, the generation of a parallel resonance in close vicinity of f ₀ , a hardly detectable 90° phase jump, and a long-term drift of resonance frequency and phase shift. Methods for eliminating these parasitic feedthrough effects have been considered, including de-embedding of the motional signal by later data processing and the integration of a reference cantilever or circuit. Then, different concepts of environmental sensing using microcantilevers are described, including detection of particulate matter and gas molecules/volatile organic compounds. Depending on the condition of the cantilever during sensing operation, two different modes have been used to sense the target analyte (i.e. static and dynamic modes). In a static operation mode, mass change of the cantilever, surface stress, or swelling of a layer on top related to the uptake and binding of particles or molecules on the cantilever are detectable via a deformation of the cantilever (i.e. by deflection or strain), which can be sensed by an integrated piezoresistive strain gauge. Quasistatic bending of the cantilever as well as frequency down-shift of an excited resonance mode are normally used for detection. Humidity adsorption/desorption characteristics of such piezoresistive microcantilevers can be modified using hygroscopic layered materials deposited on the cantilever, among which we address metal oxides, ceramics, organics, or organic/inorganic composites. These thin layers comprise preferentially concave or convex nanostructures (e.g. pores, particles, colloids, rods, or fins), which provide a sensing surface of large surface-to-volume ratio and thus a large number of binding sites for highly efficient adhesion of water molecules. Finally, fabrication processes of integrated piezoresistive microcantilever-based humidity sensors, including micromachining/MEMS technology, integration of nanostructures and their combination with deposited hydrophilic materials are described. Lastly, their humidity sensing performance is compared with competing state-of-the art and advanced MEMS devices, e.g. capacitive micromachined ultrasonic transducers, quartz crystal microbalance, thin-film bulk acoustic resonator, surface acoustic wave resonator and complementary metal oxide semiconductor-MEMS for gravimetric sensing with respect to, e.g. sensitivity, hysteresis, and response time.

The pursuit of sculpting materials at increasingly smaller and deeper scales remains a persistent subject in the field of micro- and nanofabrication. Anisotropic deep-reactive ion etching of silicon at cryogenic temperatures (cryo-DRIE) was investigated for fabricating arrays of vertically aligned Si nanowires (NWs) of a large range of dimensions from micrometers down to 30 nm in diameter, combined with commonly used wafer-scale lithography techniques based on optical, electron-beam, nanoimprint, and nanosphere/colloidal masking. Large selectivity of ∼100 to 120 and almost 700 was found with resists and chromium hard masks, respectively. This remarkable selectivity enables the successful transfer of patterned geometries while preserving spatial resolution to a significant extent. Depending on the requirements by applications, various shapes, profiles, and aspect ratios were achieved by varying process parameters synchronously or asynchronously. High aspect ratios of up to 100 comparable to the best result by metal-assisted wet-chemical etching and sub-μm trenches by DRIE were obtained with NW diameter of 200 nm, at an etch rate of ∼4 μm/min without being collapsed. At the same time, low surface roughness values were maintained on the NW top, sidewall, and bottom surface of ∼0.3, ∼13, and ∼2 nm, respectively, as well as high pattern fidelity and integrity, which were measured using angle-resolved Fourier microscopy, combined atomic force, and scanning electron microscopy on selected NWs. This work establishes the foundation in the controllable development of Si nanoarchitectures, especially at sub-100 nm structures, for energy-harvesting and storage, damage-free optoelectronics, quantum, photovoltaics, and biomedical devices.

In this study, we investigate the performance of two piezoresistive micro-electro-mechanical system (MEMS)-based silicon cantilever sensors for measuring target analytes (i.e., ultrafine particulate matters). We use two different types of cantilevers with geometric dimensions of 1000 × 170 × 19.5 µm³ and 300 × 100 × 4 µm³, which refer to the 1st and 2nd types of cantilevers, respectively. For the first case, the cantilever is configured to detect the fundamental in-plane bending mode and is actuated using a resistive heater. Similarly, the second type of cantilever sensor is actuated using a meandering resistive heater (bimorph) and is designed for out-of-plane operation. We have successfully employed these two cantilevers to measure and monitor the changes of mass concentration of carbon nanoparticles in air, provided by atomizing suspensions of these nanoparticles into a sealed chamber, ranging from 0 to several tens of µg/m³ and oversize distributions from ~10 nm to ~350 nm. Here, we deploy both types of cantilever sensors and operate them simultaneously with a standard laboratory system (Fast Mobility Particle Sizer, FMPS, TSI 3091) as a reference.

The asymmetric resonance response in electro-thermal piezoresistive cantilever resonators causes a need of an optimization treatment for taking parasitic actuation-sensing effects into account. An electronic reference circuit for signal subtraction, integrated with the cantilever resonator has the capability to reduce the effect of parasitic coupling. Measurement results demonstrated that a symmetric amplitude shape (Lorentzian) and an optimized phase characteristic (i.e., monotonically decreasing) were successfully extracted from an asymmetric resonance response. With the monotonic phase response, real-time frequency tracking can be easier to implement using a phase-locked loop (PLL) system. In this work, an electro-thermal piezoresistive cantilever resonator functionalized with self-assembled monolayers of chitosan-covered ZnO nanorod arrays as sensitive layers has been investigated under different relative humidity (rH) levels. Enhancement of resonance phase response has been demonstrated by implementing the reference signal subtraction. Subsequently, a lock-in amplifier integrated with PLL system (MFLI, Zurich Instruments, Zurich, Switzerland) was then employed for continuously tracking the resonant frequency. As a result, we find a good correlation of frequency shift (∆f₀) with change in rH monitored using a commercial reference sensor.