Low drift TBA micro flow sensors

The general trend in miniaturisation systems in chemistry, pharmacy and biology asks for the miniaturisation of the instruments for the control. In these fields, the accurate and reliable measurement of the amount of fluid (liquids and gases) flowing through pipes is one of the key issues. Basically, in these micro flow sensors the fluid is locally heated and the temperature distribution at and around the heater(s) is measured. Any flow will change the temperature distribution, and its measurement provides information on the flow. The elements for sensing in current micro flow sensors are made by thin films of metal. These films have the notorious problem of drift of their properties. This causes unreliable measurement results, which is extremely problematic if the sensors need to be very sensitive. In this project we propose a solution to this problem by combining two innovations.

One innovation is related to the measurement of the temperature by using thermocouples. The other one is related to a control scheme of the sensors. The idea is to control the heaters so that their temperature difference is zero, and the control power provides the information on the flow. The importance of thermo elements lays in the fact the only temperature difference cause any signals. According to simple models of this sensors system the drift can be eliminated completely.

The research program consists of micro fabrication of the sensor elements, modelling of their interaction (heat transfer) with the flowing medium, and an in-depth study of the performance of the sensors. A successful project will result in demonstrators ready for industrial tests.

First, a sacrificial microchannel based flow sensor was investigated for DI water flow rates in the order of nl·min-1 (figure 1). Pt sensor elements were deposited across the microchannel. Problems were encountered with step-coverage of Pt elements and the sacrificial microchannel required long etching time to be released. This made the sensor structure unsuitable for integration of Al/poly-Si thermopiles, required for the low-drift flow sensor to be constructed. Still, it was demonstrated that the sensor could measure nl·min-1 DI water flow using a dynamic sensing method.

Figure 1: Structure of a fabricated surface microchannel flow sensor with Pt sensing elements.

To solve the step-coverage problems, a surface channel fabrication concept was developed, which allows for direct deposition of transducer structures on freely suspended microchannels. The technology allows for pattern transfer by lithography and deposition of transducer materials on a planar wafer surface, while sacrificial layer etching is not required, therefore thermopile materials can be integrated without problem (figure 2).

Figure 2: Surface microchannel concept.

Using the surface microchannel technology a flow sensor was constructed with two Pt sensor resistors around a heater resistor positioned in the middle of a freely suspended microchannel, released by KOH etching (figure 3). It was demonstrated that this flow sensor measures DI water flow down to 20 nl·min‑1.

Figure 3: Surface microchannel flowsensor.

Research now focuses on the integration of Al/poly-Si thermopiles and the implementation of the power control scheme for the heating elements.