Development of nanoelectromechanical systems functionalized by vacuum organic thermal evaporation and their gas sensing applications

Abstract

Nanoelectromechanical systems (NEMS) are an exquisite sensor technology which is also potentially low-cost since it can be measured electronically and large scale produced using semiconductor fabrication techniques. Since they are extremely sensitive due to their miniscule dimensions and respond very fast due to their high resonance frequencies, these systems can be used as novel mass sensors or gas detectors. However, a functional polymeric layer that is selective to the analyte of interest is required to allow recognition by these sensors and use them as gas concentration detectors. This work focuses on fabrication of high frequency NEMS resonators, their chemical functionalization and gas sensing applications. Nanomechanical resonators with fully integrated transduction capabilities are fabricated in collaboration with Sabanci University, by using electron-beam lithography and reactive ion etching techniques. Representative micro gas chromatography columns are fabricated by utilizing deep reactive ion etching process. By employing an electronic circuit and downmixing technique, resonant responses of these sensors are measured. First five mechanical modes are observed in open-loop sweeps and up to two modes were tracked simultaneously by implementing a phase locked loop circuitry. Sub-attogram (10-18 g) mass sensitivity is achieved under ambient conditions. Different approaches were adopted for depositing polymer on NEMS and vacuum organic thermal evaporation was chosen due to its repeatability. Effects of functionalization process and thickness of the polymeric film on nanomechanical resonators are analyzed and negligible downgrade on performance of nanomechanical sensors is observed with this technique. A gas bubbler based vapor generation system was constructed and a microliter chamber for nanomechanical resonators is developed to generate and deliver volatile organic compounds at various concentrations to NEMS sensors functionalized by vacuum organic thermal evaporation. Initially adsorption of water-vapor and subsequent desorption from the sensor surface are successfully observed by tracking the frequency shift via open and closed loops. Then, isopropanol alcohol is used as a test gas and detected by NEMS resonators functionalized with poly (4-vinylphenol). Linear relationship between gas concentration and resulting frequency shift is observed. Finally, possible improvements to enhance selectivity of the NEMS sensor and allow faster recognition are discussed.