Nanostructured coatings for MEMS chemical sensors

Microelectromechanical (MEMS) chemical sensors have gained a great amount of interest over the years, due to their small size, low power consumption and their ability to be batch fabricated and integrated with electronic IC chips. Potential applications are anywhere that small portable and cheap sensors are needed, including industrial quality control, home and workplace safety and homeland security. Resonant MEMS chemical sensors detect a chemical via detecting the change of mass due to the adsorption on the sensor surface. The change of sensor mass causes a shift in resonance frequency, which can be tracked. In this thesis, a method is presented which applies the extremely high surface area to mass ratio of tin oxide nanowires to increase the sensitivity of the micro sensor by one order of magnitude.;First, the thesis work investigates how nanowires improve the responsivity of these sensors without further downsizing the resonator. Different methods of integrating nanostructures into MEMS are demonstrated. A fabrication approach is presented in which the nanowires are synthesized in-situ, directly on the resonator surface. The nanowires are synthesized with a chemical vapor deposition method; they have diameters of less than 100 nm and act as an adsorbent for many gas molecules.;Second, the sensor performance is investigated in response to various chemicals. Carbon monoxide can be detected at low concentrations. Here, real-time observations of the interactions between carbon monoxide, oxygen and the tin oxide nanowires are presented. The high sensitivity of the MEMS sensor allows for the quantitative measurement of the adsorption and desorption process. Interestingly, the removal of oxygen atoms from the nanowire lattice structure is also observed.;Third, the sensor performance is shown for a model application. Trimethy-lamine is an important volatile chemical in the quality control of food; for example in order to monitor product deterioration. It is commonly known as the smell of rotting fish. The thesis work demonstrates how silane chemistry can be applied to functionalize the nanowire surface and change its selectivity. The ability to identify the trimethylamine vapor from water vapor is presented.