Advancements in Microcantilever Sensor Technology: Gas Sensing Application

Abstract

Micro and nanoelectromechanical systems (M&NEMS) have great potential in sensor technologies, which are used for detecting mass or gas, measuring force and acceleration, and in applications related to memory storage and logic devices. Additionally, M&NEMS has been pivotal in developing ultra-stable oscillators that are crucial for precise timing purposes. Cantilever-based M&NEMS have emerged as versatile sensors, utilizing mechanical, optical, electrostatic, and electromagnetic methods for detecting gases, chemicals, and biological materials. These devices have now achieved sensitivities good enough to detect single molecules and gas concentrations in the parts-per-billion range. In this thesis, we explore gas sensing using self-sensing microcantilever-based sensors. The work focuses on how gas interactions with resonator surfaces alter their dynamics. The research involves the fabrication of these microcantilevers using surface micromachining, incorporating electrostatic actuation and piezoresistive transduction, along with a homodyne measurement technique. Real-time tracking of resonance frequencies using PLL circuits and simultaneous tracking of multiple cantilever modes and arrays is explored, highlighting their compatibility with array configurations. Frequency stability measurements using Allan deviation define the sensors' minimum sensitivity. A substantial part of this study investigates the impact of ultra-high purity (UHP) gas pulses on bare MEMS resonators. It explores how these pulses affect the temporal response of resonators' frequency and how these are related to changes in surface concentrations, temperature, binding energies, and the chamber's initial and final pressures. A key observation is the identification of a critical transition temperature around 109 Kelvin under specific pressure conditions, marking a shift from transient to permanent frequency changes in the microcantilever. The study indicates that moisture in UHP gases, rather than the gases themselves (such as argon or helium), primarily causes the observed negative frequency shifts. The research includes analytical and finite element method (FEM) simulations to understand the adsorption-desorption kinetics influenced by moisture content, gas pulse width, and temperature. These simulations offer a detailed understanding of the interactions between mechanical resonators and UHP gases, including their impurities. The thesis also explores dynamic microcantilever-based hydrogen sensors, discussing their fabrication, experimental setup, and response analysis. These microcantilevers, fabricated with advanced surface micromachining techniques, show exceptional sensitivity. The setup uses electrostatic actuation and a Laser Doppler Vibrometer, coupled with Zurich Instruments' Ultra-High Frequency Lock-In (UHFLI) amplifier with Phase-Lock Loop (PLL), allowing real-time frequency shift monitoring. A key part of the research is the investigation of the squeeze film-damping effect on microcantilevers with varying hydrogen gas concentrations. This study provides insight into the sensing behavior of microcantilevers, taking into account the squeeze film-damping effect. The frequency stability of the device is assessed using Allan deviation measurement, demonstrating its capacity to detect very low hydrogen concentrations (down to 0.002%). This research is crucial for advancing gas sensing technologies using mechanical resonators, offering insights into their behavior and potential applications in sensitive gas detection and mass measurements. It also demonstrates the capability of these devices in array configurations, using PLL circuits for simultaneous measurements. This array approach can enable the detection of gas traces in the parts per billion range from a gas mixture with high certainty. Enabled by ultra-sensitive micro/nano cantilevers, this technology stands to benefit immensely from emerging fields like artificial intelligence and machine learning, marking a new frontier in gas sensing technology.