Two integrated thermal gas flow sensors have been developed for the simultaneous measurement of the velocity and the direction of gas flow. The sensors were fabricated using a commercial CMOS process followed by silicon micromachining. The first sensor consists of two crossed dielectric microbridges, while the second device is based on a dielectric membrane. Both sensors contain integrated heaters and two orthogonal pairs of integrated thermopiles, which constitute a vector probe for the two components of the velocity of gas flows. In contrast to previous designs, the reported sensors show a linear output as a function of flow velocity, with sensitivities of 0.19 and 1.9 V W-1 m-1 s, respectively. Gas flow direction measurements with average errors of 1.7 degrees and 0.6 degrees , respectively, are achieved. We also demonstrate gas flow velocity and direction measurements using the rear of the membrane devices, which opens new perspectives for the packaging of integrated flow sensors. In this configuration a sensitivity of 0.67 V W-l(m/s)-l and an angular resolution of 3.1 degrees were obtained.

https://iopscience.iop.org/article/10.1088/0960-1317/5/3/007/pdf

Two-dimensional integrated gas flow sensors by CMOS IC technology

In this paper we present a comprehensive discussion of silicon thermal anemometry. Its possibilities and the current state of the art are discussed in detail. Based on forced convective heat transfer, it is seen how silicon technology can be used to make thermal flow sensors. The mechanism is based on development of a thermal hydrodynamic boundary layer across a heated silicon chip due to a forced flow. It is shown how this mechanism is converted into an electrical signal. Important parameters such as possible sensing elements and geometries are discussed. Different biasing modes to operate thermal sensors are presented. One of the very basic issues in silicon thermal anemometry is drift. The performance can be expressed in terms of a quality factor , where S is the desired signal, D is the drift and N is the contribution of noise. The control of this drift term plays a crucial role in the accuracy of silicon thermal flow sensors. A novel method to eliminate drift for thermal anemometry, namely the alternating direction method, is presented. In particular this method plays a crucial role when extremely low and long-term volume measurements must be performed. Examples of measurements are given. With regard to its applications, the type of packaging is a major issue. Dependent on its application, also the frequency behaviour is important. It is discussed briefly. Finally, applications and future developments of silicon thermal anemometry are discussed.

https://iopscience.iop.org/article/10.1088/0957-0233/7/10/007/pdf

Silicon thermal anemometry : developments and applications

For the first time a packaged single-chip anemometry microsystem is reported The system includes a thermal CMOS flow sensor with on-chip power management, signal conditioning, and A/D conversion It is fabricated using an industrial IC process followed by post-CMOS micromachining The system is packaged on a flexible substrate using flip-chip interconnection technology The measurement of wind speeds is demonstrated in the range from 0 to 38 m/s (0 to 12 Beaufort), with a dynamic range of 65 dB Total power consumption is 3 mW

/pdf/single-chip-cmos-anemometer-3s2kio23e8.pdf

Single-chip CMOS anemometer

New robust three-dimensional silicon structures with integrated heaters for dynamic actuation have been fabricated and tested. The structures can be used in both static and dynamic modes for 3-D sensor and actuator applications. The proposed technique for bending 3-D structures out of the wafer plane is based on thermal shrinkage of polyimide in V-grooves, producing static bending. This static angle can be chosen in a wide range by varying the curing temperature. The relatively large thermal expansion of polyimide makes it possible to use the structure in dynamic mode. A local power dissipation in the joint is achieved by using a current through an integrated heater. This makes it possible to control the bending angle of each individual structure. For the tested polyimide V-groove joints, static bending angles of 35 ° and dynamic angles of 3 ° per V-groove have been achieved.

Dynamic actuation of polyimide V-groove joints by electrical heating

To compensate for changes in ambient temperature, thermal flow sensors are usually heated to a constant temperature above ambient temperature. This operating mode, however, requires an extra ambient-temperature sensor. Using a simple model, it is shown that for a two-dimensional thermal flow sensor, similar compensation may be obtained by using a constant beating power, thus eliminating the need for an ambient-temperature sensor. The flow sensor's interface circuitry consists of two thermal sigma-delta modulators that heat the sensor with constant power and dynamically cancel the flow-induced temperature gradient. The modulator's bit-stream outputs are then a digital representation of the heat distribution in the sensor. Wind tunnel tests confirm the validity of the sensor model used and show that both flow speed and direction may be accurately determined from the bit-streams, with errors less than /spl plusmn/5% and /spl plusmn/3/spl deg/, respectively, for flow speeds between 1 and 25 m/s.

/pdf/constant-power-operation-of-a-two-dimensional-flow-sensor-tzc5us3lr1.pdf

B.W. van Oudheusden

Papers

Integrated silicon flow sensors