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SEM micrograph showing a close-up of a prong.

In research projects sponsored by the Swedish Research Council for Engineering Sciences (TFR) an interdisciplinary cooperation with the department of Thermo- and Fluid Dynamics at Chalmers has been established. The object of the research has been to develop micromachined silicon sensors specially designed for the measurement of pressure and gas flow fluctuations in a turbulent gas flow. The project will continue to include control of boundary layer thickness and flow gradient measurements.

Hot-wires have been used for over a century and are still very popular. Since smaller wires provide higher spatial and temporal resolution, MEMS has been an evident advantage. A drawback of MEMS-based hot-wires however, is that they most often use polysilicon wires and thus (due to their high resistance) need special electronics compared to standard metal hot-wires.

Hybrid assembly. Left: The sensing chip glued and wire bonded to the tip of a PCB is inserted in the KOH-etched hole in the holding chip. Middle: After insertion the "sensor wall" is in-plane with the holding chip wall. Right: SEM micrograph of the front of an assembled sensor.

MEMS hot-wires lend themselves well to measurement of turbulent flows due to their small size and low time constant. However, a problem arises when one wants to measure close to a surface since flow interference and thermal cross talk with the surface make the measurements unreliable. Therefore, specialized hot-wires are needed for this application. The solution is to have the sensor attached to the wall, which can greatly reduce the flow interference. Two major difficulties need to be considered: 1) to control the inevitable thermal influence from the wall that occurs at the distances we are interested in (<250mm.) 2) to place the contact leads to the sensor outside of the measured flow. This can be done either by placing the contact leads far downstream, or by wafer-through vias.

A micromachined hot-wire anemometer has been designed and fabricated. The wire is made of aluminium, enabling it to be used with existing CTA equipment. The largest discontinuity introduced in the system, excluding the prongs, was below 10µm, therefore, allowing measurements to be made without interfering with the flow. We were able to manufacture devices with wire dimensions down to 1µm x 2µm x 200µm, which is the size needed to obtain the required temporal and spatial resolution for the described turbulences. The hybrid assembly method developed permits easy through-wafer contacting without compromising the flatness of the surface exposed to the airflow. Experiments in the windtunnel have verified the feasibility of the hot-wire anemometer for near-wall turbulence measurements.

Project Sponsor

• Swedish Research Council for Engineering Sciences (TFR)

Project members

Sjoerd Haasl

Göran Stemme