Highly Sensitive Sensor for Flow Velocity and Flow Direction Measurement Franz Keplinger Jochen Kuntner Artur Jachimowicz Institute for Sensor and Actuator Systems Vienna University of Technology Vienna, Austria Email: franz.keplinger@tuwien.ac.at Franz Kohl Research Unit for Integrated Sensor Systems Austrian Academy of Sciences Wiener Neustadt, Austria Bernhard Jakoby Institute for Microelectronics Johannes Kepler University Linz, Austria Abstract— Miniaturized sensors for flow velocity and flow direction measurement based on thin-film germanium thermis- tors (TCR = -1.8% /K) offering extremely high sensitivity were developed. The thermistors are placed on a silicon nitride diaphragm (1.3 μm thick) which is carried by a silicon frame. To resolve the direction of the flow, eight thermistors are arranged circularly on the diaphragm. Two orthogonal pairs of diametrically opposed thermistors (e. g., N-S and E-W) feature a directional sensitivity of 152 μV/deg at a flow velocity of 1 m/s. An increase of the sensitivity of about 50% can be gained by analyzing the difference signal of two 90° rotated thermistors (e. g., N-E), which are in the downstream position. The measurable gas flow rate ranges from 0.025 m/s to about 3 m/s for the constant power mode. The sensor has a high sensitivity to flow direction of 30 mVs/m at low flow rates from 0.025 m/s to 0.2 m/s. I. I NTRODUCTION Measuring velocity and direction of fluid flows are very important tasks in various applications. Especially miniaturized sensors are suitable for the investigation of flows with high spatial resolution which are not accessible with mechanical anemometers. The miniaturized cutting-edge devices are based on the calorimetric principle allowing the simultaneous measurement of flow direction and velocity [1, 2]. The high resistive temperature coefficient (TCR) of amorphous germanium thermistors, which are already applied for flow sensor applications [3, 4], initiated the design of extremely sensitive flow direction sensors. Moreover, a further increase of sensitivity was expected by the use of four additional thermistors. II. EXPERIMENTAL A. Sensor The sensors are realized on 〈100〉-Si wafers, which are passivated on both sides with 250 nm thermal silicon oxide and 70 nm LPCVD (low-pressure chemical vapor deposition) silicon nitride. The heater consists of a chromium meander with a width of 5 μm, a thickness of 130 nm, and a mean Fig. 1. Photomicrograph of the micromachined flow sensor. ϕ denotes the direction of the flow with respect to the N-S-direction. The different shading of diaphragm and leads is caused by the buckling of the diaphragm. length of 310 μm. Its resistance amounts to about 580 Ω. The thermistors are made of a amorphous germanium layer with the extensions of 100 μm × 35 μm and a thickness of 250 nm, which is contacted by an interdigital sandwich structure con- sisting of 50 nm titanium (at the germanium side), 100 nm gold and 30 nm chromium. This forms a Ge-resistor with a cross- section of 75 μm 2 and a length of 5 μm. The resistance of the Ge-thermistors is typically 320 kΩ. Amorphous germanium was chosen as thermistor material because its resistivity is highly sensitive to temperature changes [5]. It exhibits a TCR of about -1.8 %/K being almost five times higher than the corresponding value of platinum. The metal layers and the germanium layer have been evaporated and patterned with image reversal photo resists. The sensor structure is passivated by a 1 μm thick LPCVD silicon nitride layer. Afterwards, the diaphragm is created by an anisotropic etching process applying 30 wt% potassium hydroxide solution at 80 °C from the backside of the wafer. The etching process is finished, when the solution reaches the silicon oxide layer, which acts as an etch stop. The resulting diaphragm consists of two silicon nitride layers and one silicon dioxide layer and features an overall thickness of about 1.3 μm and an area of 1.2 × 1.2 mm 2 . It is characteristic