Capacitive Micromachined Ultrasonic Transducers for Gas Sensing

Gas sensors are used in many fields for a wide range of applications from monitoring the indoor air quality control to detecting tiny amounts of illegal drugs. Different sensing methods exist, but in this thesis the focus is on gravimetric detection, that is measuring a perturbation (typically a resonance frequency shift) due to the addition of a mass on the sensor. Gravimetric sensors have a number of advantages over the other sensor types: small device footprint, low power consumption, and low cost. Capacitive micromachined ultrasonic transducer (CMUT) gas sensors are a subcategory of gravimetric gas sensors that offer a high sensitivity, low limit of detection per area, simple fabrication process, and an electrostatic actuation and detection scheme. Consequently, CMUTs are good candidates for being used as gas sensors. The aim of this thesis is to develop, fabricate, and apply state of the art CMUT gas sensors.
The theoretical background of CMUTs is given by presenting both a static and a dynamic model of the CMUT. Furthermore, design rules are developed for optimizing the mass sensitivity and limit of detection. Four generations of CMUT chips are designed, fabricated and characterized. The design space was investigated which resulted in an optimal choice of materials and geometry for CMUTs with small radii (radii < 5 μm). The best CMUTs showed a record high distributed mass sensitivity of 0:83 Hz=ag and a record low limit of detection of 1:16 ag, which in part is due to an optimized design and fabrication process. The CMUTs were used for detection of a precursor molecule for the synthesis of (meth)amphetamine called Benzyl Methyl Ketone (BMK). This was done in conjunction with a colorimetric chip which showed a high selectivity towards BMK. Finally, an alternative method of reading out the resonance frequency from the CMUT was presented. The top electrode of the CMUT is split in three: one for actuating the CMUT and two for detecting the capacitance change caused by the plate deflection. Hence, the actuating and detection electrode and signals are decoupled and common mode noise can be suppressed by making a differential measurement of the signals from the two sensing electrodes.