Wafer level encapsulation of micromachined pressure sensors to realize a liquid flow sensor

A differential pressure flow sensor capable of measuring low flow rates 0.01 ml/hr to 100 ml/hr has been developed, using wafer level encapsulation. Glass frit was used as the bonding material to bond the cap and device wafers. The flow channel was created by combination of seal glass material and etch step carried out in the cap wafer fabrication process. This paper summarizes the research work carried out to realize a flow sensor with multiple sensing capabilities by utilizing the wafer level bonding technology. The realization of the flow channel was carried out in two phases in order to create a more uniform rectangular channel compared to the initial phase. This research activity demonstrates a very simple cap fabrication process that enabled the realization of a flow sensor with multiple sensing capabilities. Surface micromachined process was employed to fabricate the pressure sensors as well as the cap that defines the fluid channel. The flow sensor developed was capable of measuring flow rate, fluid type, fluid pressure, fluid presence, and flow direction. The sensor was configured in a way to successfully demonstrate two differential pressure sensing modes (i.e. sensors within the cavity and sensors in the flow channel). Both flow rate and viscosity measurements are based on the differential pressure sensing principle. Fluid presence is determined using an interdigitated structure which can also sense the fluid type based on permittivity of the fluid. The flow rate measured successfully has been as low as 0.01 ml/hr and showed very good linearity when compared to the theoretical model. The diameter of the pressure diaphragm utilized was as low as 100 microns and depends on the flow rate required to measure. The height and width of the fluidic channel realized was 12 microns and 380 microns respectively.