Piezoresistive microcantilevers for humidity sensing

Abstract

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 welldeveloped 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

A Multiparameter Gas-Monitoring System Combining Functionalized and Non-Functionalized Microcantilevers

“…In this study, we focus on the feasibility of humidity measurement at ppm level on synthetic welding gas mixtures with functionalized micro-cantilevers in an industrial environment, which means that gas composition, temperature and pressure may vary and, therefore, the sensor must be able to work under non-ideal conditions. This is in contrast to most previous studies on this topic, where the behaviour of the sensors was investigated under very stable and controlled conditions [3][4][5][6].…”

“…Palomino from Instituto de Tecnología Química (UPV-CSIC) Valencia -Spain for offering access to their instruments for water isotherm determination. [ * ] concentration of free sorption sites per m 3 alternative equilibrium adsorption constant (1/Pa) ∆G sorption adsorption free energy (J/mol) q isoteric isosteric isosteric heat of adsorption (J/mol)…”

In-Plane and Out-of-Plane MEMS Piezoresistive Cantilever Sensors for Nanoparticle Mass Detection

“…On the other hand, particle counting based on light scattering method leads to a lower limit of detectable particle size (~0.3 µm) [12]. Another approach is based on gravimetric measurements, in which resonant mass sensors are normally employed as the key monitoring elements that primarily rely on the principle of resonance-frequency shift, e.g., quartz crystal microbalance (QCM) [13,14] and cantilevers [15][16][17]. Gravimetric-based cantilever sensors have high potential and capability to detect small amounts of mass and are widely used in gas sensing [18], fluid density [19], and fluid viscosity measurements [20], etc.…”

Visible Light-Driven p-Type Semiconductor Gas Sensors Based on CaFe2O4 Nanoparticles

“…During the last decades, although other types of sensor systems were introduced to the environmental community like micro-/nanoelectromechanical gravimetric gas and nanoparticle sensors [33][34][35][36][37][38][39][40][41][42], conductometric gas sensors were still in high demand and widely investigated due to several advantages, e.g., low cost and flexibility in production, the large number of detectable gases/possible application fields, simplicity in measurement setup, and the ease of miniaturization for portable instruments [43]. However, most of the available conductometric sensing devices are based on n-type materials (e.g., ZnO and SnO 2 ) with wide band gap which needs UV light to activate the absorption and desorption processes occurring in the surface of the sensing material [44][45][46][47][48].…”