A robust and low-power 2-D thermal wind sensor based on a glass-in-silicon reflow process

In this paper, the design, fabrication, and characterization of a robust and low-power micro-machined two-dimensional (2-D) wind sensor based on a glass-in-silicon reflow process are presented for the first time. The four thermistors, which act simultaneously as heat sources and as temperature sensors, are placed on a low thermal conductivity glass substrate, and arranged in a Wheatstone bridge configuration supplied with constant voltage. In this self-heated mode, the total power consumption of the sensor could be reduced into the sub-milliwatt range, offering high initial sensitivity and wide measurement range, respectively. The embedded vertical silicon vias in the glass substrate are used to realize the electrical connections between the sensing elements and the electrode-pads, which are respectively placed on the front and the back surface of the chip. Then, the sensor and the external circuit are connected using the wire-bonding process through the electrode-pads on the back surface. The bonding wires at the backside is encapsulated by polyester paint, protecting the electrical connections of the sensor from the effect of the external environment. In addition, a passivation layer of nitride is deposited on the surface of the wind sensor to prevent direct exposure of the sensing elements to harsh media. The sensor was tested in a wind tunnel in constant voltage mode. Measurement results show that the thermal wind sensor can measure wind speeds up to 17.5 m/s, and the measured sensitivities of the sensor with different applied voltages (0.5, 1, 1.5 V) are, respectively 24.9, 148.3 and 440.61 mV/(m/s) at zero-flow point. The corresponding power consumption of the sensor with different voltages are respectively 4.81, 19.23 and 43.27 mW. Measurement results also show that wind direction in a full range of 360° with an err within 6° could be obtained. The proposed sensor can be used for many applications with a low power consumption and high reliability.