Single and double clamped resonator sensors - Design, fabrication and testing

Abstract: Micro- and nanomechanical single and double clamped resonators are widely being used as mass and temperature sensors due to their high sensitivity, as bio sensors due to their real time label-free detection ability. These resonators have attracted considerable interest from applications like point of care diagnostics, homeland security and environmental monitoring in recent years. Resonant sensors utilize the fact that a vibrating structure experiences a shift in resonance frequency for an added mass on its surface or change in damping due to change in surrounding air pressure or because of the bimetallic effect. Cantilevers are beams clamped at one end and strings are pre stressed beams clamped at both ends. Cantilevers have already been used as mass, pressure and bio sensors. Strings have been used for mass sensing and their application may possibly be extended to bio and temperature sensing. This thesis presents design, fabrication and characterization of silicon nitride cantilevers and gold strings. The goal of this project has been to enhance the sensitivity of the cantilevers and to find the possibility of using strings as temperature and bio sensors. The major part of this project has been to fabricate cantilever and string arrays in the clean room environment. Process flow and corresponding process recipe have been documented in this report for future documentation. Analytical model for the flexural eigenmodes, bending and both internal and air damping models of a vibrating cantilever have been studied. Resonant frequency and quality factor of the fabricated silicon nitride cantilevers at different pressure regions have been measured. Damping models at different pressure regions have been calculated from the measurement. Particle mass sensitivity of the fabricated cantilevers have been compared with pre fabricated cantilevers for different bending modes. It has been shown in this thesis that cantilevers fabricated in this project have improved mass and pressure sensitivity. Analytical model for eigenmodes, mass and temperature sensitivity of a double clamped gold beam have been studied. Resonant frequency and quality factor have been measured. Particle mass sensitivity and temperature sensitivity of the strings have been calculated. Analytical bending model of the fabricated double clamped beam have been presented in this thesis. Simulation models by finite element method in Comsol Multiphysics™ 4.1 have been designed in order to verify the analytical solutions and experimental results. These models can be used to predict results for different device dimensions and materials in varying pressure and temperature. A laser optical detection system along with a vacuum chamber setup has been used to carry out experimental investigations of silicon nitride cantilevers and gold strings. A combination of a peltier element and thermistor has been used to measure the temperature sensitivity of the gold string. All the measurements, calculations and comparisons have been presented.