A MICROMACHINED FLOW SENSOR FOR LIQUID AND GASEOUS FLUIDS

A thermal flow sensor based on high-resolution thin-film thermistors placed on SiN(X) microbridges is presented. A linear array of those bridges crossing a silicon micromachined flow channel forms the base of the investigated flow sensor. Each thermistor device consists of a vacuum-evaporated amorphous Ge (a-Ge) resistor passivated by silicon nitride layers that were prepared in a low temperature plasma-assisted CVD process. The top and bottom SiN(X) films serve as the mechanical support of the thermistor and as the mask for the micromachining etch process. The a-Ge thermistor can serve for high resolution temperature sensing or as a controlled temperature heater. The SiN(X) bridges are located in the centre plane of the flow channel yielding maximum thermal coupling between thermistor and fluid. A temperature difference between run-in fluid and heating thermistor of typically 5 K is sufficient for reliable operation. The corresponding heat flux causes an increase of fluid temperature of typically less than 1 K. The power per unit area required for convenient measurements amounts to 1 mW/mm2 for gaseous fluids and 4 mW/mm2 for liquids. Measuring ranges from 1 to 4000 ml/h for liquids and from 10 to 3000 sccm for gases are verified as useful. With a flow channel cross section of about 0.3 mm2 this corresponds to average fluid velocities at the upper limit of the flow ranges of about 4 m/s for liquid fluids and 180 m/s for gases, ignoring their compressibility. The upper limits of investigated flow rate are determined only by the available fluid supply equipment.