Dynamics of a Suspended Microchannel Resonator

Abstract

Micro-Electro-Mechanical-System (MEMS) mass sensing devices have a wide variety of applications in healthcare diagnostics and biosciences. Typically, mass sensing on a MEMS device works by physisorption and chemisorption methods. The mass to be sensed is carried in a media surrounding the MEMS structure, which dampens the resonator and results in reduced measurement resolution. A Suspended Microchannel Resonator (SMR) is an unconventional MEMS sensor with an enclosed microchannel in the overhanging resonator structure. The microchannel facilitates the flow of analytes, and frequency shifts are registered as mass traverses along the cantilever length. SMRs have better resolution and sensitivity compared to conventional mass sensors as they can be vacuum-packaged, and the internal flow of fluid does not diminish the Quality factor, unlike the case of external flow. This device has a variety of applications, such as biomolecular sensing, density sensing, viscometers, cell division studies, and more, most of which are based on frequency shift measurements. Fabrication of SMRs with large hydraulic diameters is very complex, and no standard processes have emerged. Furthermore, the fluid-structure interaction in these devices is non-trivial, and there is a lot of room for improving our understanding from both modelling and experimental point of view. In this study, we present two fabrication techniques for SMRs. The first method relies on wafer bonding and enables the building of SiO2-based SMRs with 350nm thick walls with very large channel cross-sections of 35um x 50um. We first studied the mass sensing capability of these devices and confirmed that the expected sensitivity was achieved. Next, we studied the fluid-structure interaction by flowing gasses such as N2, CO2 and He through the microchannel and simultaneously carrying out vibration studies. We observed a new phenomenon in which the gas flow changes the resonant frequencies of various modes of the SMR device. We developed a semi-analytical model for the same, which largely agrees with the observed data. To validate the model accurately, considerable energy was devoted to measuring the exact flow through the SMRs. We next explored a second fabrication method that relies on standard surface micromachining techniques and enables capacitive actuation while getting rid of some of the deficiencies of the previously presented fabrication process. We successfully fabricated these devices and characterized the quality factor of the device as a function of flow rate. It was observed that the quality factor of the first two modes has an opposite trend dependence on fluid flow. One of the key problems of the fluidic system interface with the SMR chip was leakage through the PDMS-based port. We created a more robust solution for the same and also customized a measurement setup in a vacuum environment, which enabled much higher flow rates through SMRs. In so doing, we observed a self-resonant behavior that is induced purely by fluid flow. It was observed that the resonant behavior once the flow crosses a minimum flow rate. This study reveals the fluid-structure interaction of SMR sensors and demonstrates the effect of fluid flow velocity inside the microchannel on the resonator frequency. This aspect of SMRs has led us to find its application as a gas flow rate sensor. Our fabrication method for SMRs provides exceptional design and relatively easier process flow. A simplified model is proposed to address the observed frequency shifts and has been extended for an application in high-resolution mass flow rate sensing. A Mathew equation-based model was developed, and the phenomenon is understood in this framework.