MICROMACHINED AND ENCAPSULATED CORIOLIS MASS FLOW SENSOR IN SILICON

Goal: To investigate if the encapsulation method, including excitation and detection, developed for a silicon density sensor also can be adopted for mass-flow detection with the same sensor structure. This will probably involve a redesign of parts of the encapsulation, mainly detection electrodes. The project is aimed at finally if possible realizing the fully encapsulated mass-flow sensor.
Description: A at the department developed tube structure in silicon have been used for measurement of fluid density, where a change of density of the fluid in the tube changes the resonance frequency of the tube structure and is thus a measure of the fluid density. The development of an excitation and detection technique has enabled the sensor structure to be fully encapsulated. The encapsulation consists of two glass wafers bonded on each side of silicon sensor. On the outside of these the detection and excitation electrodes are deposited. The structure was also used as the first silicon mass flow sensor based on the Coriolis effect. The measuring principle is that a liquid flow in the tube structure, which is oscillated in a resonant mode, results in a measured superimposed twisting of the structure which amplitude is proportional to the liquid mass flow. The amplitudes induced by the Coriolis forces are very small and thus an elaborate measurement have been used for the evaluation of the sensor structure. The excited bending angle-oscillation of a tube loop is aligned and detected along one axis of a lateral photodetector. The resulting twisting angle oscillations due to the Coriolis forces are detected along the other, orthogonal, detection axis of the photodetector. The measured amplitude ratios as a function of the mass flow velocity are plotted for different bending excitation modes. The measurement results show clearly the linear properties of the mass flow dependence. We now intend to use the integrated electrodes and the detection/excitation methods developed for the densitometer to further develop the sensor structure to also act as a fully workable mass flow sensor. Field of application: The Coriolis mass flow measurement principle can be applied to nearly any type off flow measurement with the inherent advantage of true mass flow sensing independent off flow profile etc.
Proposed project plan:
1. Literature study (10%) During a first stage, a literature survey has to be done in order to get an overview of the research domain and design possibilities. The student will learn more about micro machining and reach a deeper insight in especially resonant sensors.
2. Design (15%) Based on earlier results a design (i.e. encapsulation, the earlier developed silicon sensor structure will be used) should be theoretically evaluated analytically or by simulations (e.g. ANSYS). These will be used to determine the exact dimensions of the encapsulation and excitation/detection electrodes.
3. Device fabrication (45%) Once a certain encapsulation design has been chosen, the first test devices can be fabricated using micromachining technology. The masks, if new masks are needed, have to be drawn using a special CAD program (L-Edit). The fabrication then takes place in the cleanroom in Kista. Here the student has to learn to work with the specific state-of-the-art microfabrication techniques in a cleanroom environment.
4. Device testing (20%) The mass flow sensitivity will be evaluated on the encapsulated sensor. The results will be compared to the analytical/simulation results.