PZE-transduced Suspended Microchannel Resonators for sensing applications

This PhD thesis aims at developing a system which can measure the mechanical properties of fluidic samples in the picoliter range. The ultimate goal is the characterization of cancer cells and viscoelastic fluids (i.e. biological fluids), in order to study their mass and stiffness for diagnostic applications. The core of this project is the design, fabrication and characterization of hollow micromechanical resonators containing embedded microfluidic channels and provided with integrated piezoelectric (PZE) transduction. The measurement strategy consists in monitoring the device resonance frequency over time, while the analyte flows through the microchannel. Arrays of singly-clamped and doubly-clamped resonators are made of low stress Silicon Nitride (ls-SiNx) and vibrate in a frequency range from tens of kHz to few MHz. Microfluidic channels span 25%, 60% and 100% of the cantilever length in order to compare the resonance frequency shift induced by the same particle at different positions, along identically-designed beams. The goal is to disentangle analyte mass and stiffness contribution from the suspended microchannel resonator (SMR) mechanical response. The cross section of the suspended fluidic channel, 6 um x 10 um, is designed to measure particles in the same size range (i.e. red blood cells or circulating tumor cells). SMR energy dissipation is studied via FEM simulation, as a function of fluidic properties and geometrical dimensions. A coupled fluid-structure interaction (FSI) problem, based on linear elasticity and linearized Navier Stokes equations, is studied. The model is found well in agreement with theory and experiments presented in literature, considering the viscosity range of interest in case of biosensing application. The fabrication of transparent and PZE-transduced suspended microchannel resonators is achieved via a 6-mask process flow. 250, 500, 750 and 1000 um long singly- and doubly-clamped SMRs are fabricated. Microfluidic channels are defined via the etching of high aspect ratio-trenches through a sacrificial layer of polysilicon, and filled with ls-SiNx. Discontinuous etch apertures are defined on top of the buried fluidic network and enable channel emptying in 25 minutes via KOH etching. PZE electrodes in platinum and aluminum nitride are deposited via sputter deposition and provide independent actuation and readout of each resonator, for the first time in SMR arrays. The development of an experimental platform enables fluidic and electrical connections, as well as temperature control during devices characterization. The mechanical response of fabricated sensors is measured in air and vacuum environment, while channels are filled with air, water and Isopropyl Alcohol. PZE electrodes demonstrate efficient actuation, in the order of 1.5 nm/V for 250 um- long singly-clamped SMRs. However, PZE detected signals result very low in amplitude, thus optical detection via laser Doppler vibrometer is preferred. Frequency stability of fabricated sensors is studied in order to estimate their sensing performances and characterize their physical limitations. 250 um-long SMRs filled with DI water exhibit a frequency stability of 30 ppb at 400 ms integration time, in air. This result is in agreement with values found in literature for SMRs sensors and translates in an estimated mass sensitivity of few femtograms.
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