Three dimensional silicon triple-hot-wire anemometer based on polyimide joints

The first three dimensional (3D) flow sensor probe based on polyimide joints has been fabricated and successfully tested. The new 3D sensor, which is specially designed for turbulent gas flow measurements, is based on the anemometer principle, i.e. gas cooling of electrically heated hot-wires. The sensor probe consists of three perpendicular 500 μm×5 μm×2 μm polysilicon hot-wires giving a measuring volume sufficiently small to resolve the small eddies in a turbulent flow. A bulk micromachining process in combination with sacrificial etching is used to form the hot-wire probes. The hot-wires are connected to 30 μm thick bulk silicon beams which are defined by double sided KOH etching. Two wires (x and y) are located in the wafer plane and the third z-wire is rotated out of the plane using a new robust micro-joint. The self-assembly micro-joint with small bending radius is based on thermal shrinkage of polyimide in V-grooves. High flow sensitivity for the anemometric hot-wires has been measured. Time constants of 120 and 330 μs were measured for the cooling and heating of the hot-wires, respectively