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PhD Defence Qihui Yu | Micro Coriolis Mass Flow Sensor with Large Channel Diameter by Wet Etching of Silicon

Micro Coriolis Mass Flow Sensor with Large Channel Diameter by Wet Etching of Silicon

The PhD defence of Qihui Yu will take place in the Waaier Building of the University of Twente and can be followed by a live stream.
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Qihui Yu is a PhD student in the Department of Integrated Devices and Systems. Promotors are prof.dr.ir. J.C. Lötters and dr.ir. R.J. Wiegerink from the Faculty of Electrical Engineering, Mathematics and Computer Science.

A Coriolis mass flow sensor measures the mass flow through a secondary vibration induced by the Coriolis force generated by fluid flowing in a vibrating tube. This secondary vibration is a direct measure of the mass flow since the amplitude does not depend on the fluid’s properties. It has been decades since the first micro Coriolis mass flow sensor was introduced. After that, many studies on this topic were carried out. Different designs and fabrication processes were presented, which resulted in devices with different features. In our group, surface channel technology (SCT) was invented which allows the fabrication of suspended tubes with a thin silicon-rich silicon nitride (SiRN) tube wall in a silicon substrate. Based on this fabrication process, many papers about modelling and optimizing the design of the devices, using different actuation and readout methods to achieve better performances were published then. However, the exploration and optimization of the fabrication process to improve the performance of the sensor seemed lacking. All the fabricated sensors have a non-circular cross-section, which results in a pressure-dependent behavior of the sensor due to the deformation of the channel when fluids flow in it. Moreover, the cross-sectional area and the channel wall thickness, which affect the maximum flow range and the device sensitivity accordingly, are limited. Therefore, to overcome these drawbacks, a new fabrication method is needed to realize freely suspended tubes with a circular cross-section, a wide range of diameters, and relatively thin, chemically resistant channel walls.

The research first focused on finding or designing a preliminary fabrication process to realize a freely suspended tube with a circular cross-section, and relatively thin, chemically resistant channel wall. Several possible methods from the literature were studied and discussed. The conclusion was that buried channel technology (BCT) seemed to be a suitable one to achieve the goal. In the meantime, a different silicon isotropic etching technology, HNA etching, was studied and compared to the SF6-based silicon semi-isotropic etching method which was used in SCT. During this period, a buried channel with a circular cross-section and a width of 100 μm, although without a channel wall and not released from the silicon substrate, was fabricated by using BCT and HNA etching. However, many issues were encountered when going to the next step. By modifying the process and involving wafer bonding and thinning steps, finally, a freely suspended channel with an almost circular cross-section and a width of 300 μm was realized. The channel wall is made of a single SiRN layer with a thickness of only 1.5 μm, which resulted in the highest diameter-to-wall-thickness ratio for microfluidic channels. Unfortunately, due to the complexity of the process, no devices were realized and demonstrated. Further optimization of the process is necessary to complete this study.

Meanwhile, a demonstrator sensor was designed and fabricated based on existing SCT but using HNA etching, to realize the channel, which results in a freely suspended microfluidic channel with a relatively large cross-sectional area. Due to this large cross-sectional area of the channel, the flow range of the sensor is expanded. The channel has a semi-elliptical cross-section (200 μm wide, 70 μm deep) and a tube wall thickness of approximately 2.5 μm. The sensor has an obvious improvement in flow range (from 0 up to 50 g/h for water and up to 6 g/h for nitrogen gas under a pressure-drop of 1 bar). The sensor was also demonstrated to be able to measure fluid density.