Micro coriolis mass flow sensor with large channel diameter by wet etching of silicon

The research was 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 for micro Coriolis mass flow sensing application. 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.