This thesis reports on the feasibility study of a new vacuum sensor based on microresonators focusing on the capacitive driving and sensing schemes. Low-pressure sensing devices nowadays have their shortcomings. They are either rely heavily on a reference pressure, like those in the membrane type pressure sensor or have a low resolution and a narrow dynamic range, like the Pirani sensor. A new vacuum sensing method based on microresonators is introduced. A microresonator is subjected to air damping force when vibrating in air. Energy is lost to squeeze the air out from the gap between the microresonator and a fixed wall. This energy loss is dependent on the pressure of the air in which the microresonator is vibrating. Hence, by exploiting this phenomenon, a microresonator could,...[ Read more ]

This thesis reports on the feasibility study of a new vacuum sensor based on microresonators focusing on the capacitive driving and sensing schemes. Low-pressure sensing devices nowadays have their shortcomings. They are either rely heavily on a reference pressure, like those in the membrane type pressure sensor or have a low resolution and a narrow dynamic range, like the Pirani sensor. A new vacuum sensing method based on microresonators is introduced. A microresonator is subjected to air damping force when vibrating in air. Energy is lost to squeeze the air out from the gap between the microresonator and a fixed wall. This energy loss is dependent on the pressure of the air in which the microresonator is vibrating. Hence, by exploiting this phenomenon, a microresonator could, in theory, be used as a vacuum sensor. The energy loss could be extracted from the vibration amplitude. Thus, by sensing the vibration amplitude, it is possible to measure the corresponding pressure. Capacitive sensing is simple to implement into the fabrication process and could be integrated easily with other microelectronic devices. However, feed through current is a major problem for capacitive sensing schemes especially when obtaining the motion induced current as the two currents are usually mixed up in the sensing electrode. A good sensing scheme is needed to separate the motion induced current. The energy loss in microresonator is measured using the quality factor. Accurately extracting the quality factor requires an accurate modeling of the microresonator.

In this thesis, a prototype of a microresonator was designed, fabricated and tested under various pressures to observe the change in vibration amplitude at resonance. A lumped nonlinear model was constructed to describe the dynamics of the vibrating microresonator and to correlate the vibration amplitude with the energy loss. The microresonator vibrates nonlinearly. Hence, by using the Duffing equation, the parameters in the model such as spring constants and quality factor were tuned to fit the experiment results in order to extract the true stiffness and the quality factor of the microresonator. Compared to the FEM model, the spring constants were less than the designed value due to the over etching of the supporting beams. The quality factor is identified also by fitting the model to the experiment result. The experiment results and the model agree with each other very well. The microresonator is feasible to be used as a vacuum sensing device.