Low-drift micro flow sensors

Abstract

The emerging fields of micro total-analysis systems (micro-TAS), micro-reactors and bio-MEMS drives the need for further miniaturisation of sensors measuring quantities such as pressure, temperature and flow. The research described in this thesis concerns the development of low-drift micro flow sensors for accurate measurement of minute amount of liquid flow in the nl⋅min-1 range. Miniaturisation means that flow channel dimensions and flow rates become smaller. This requires thermally-isolated flow channels, where the complete fluid can be heated in order to obtain maximum sensitivity. A microchannel fabrication concept (Chap. 3) was developed, based on buried channel technology (BCT), allowing for easy fluidic interfacing and integration of transducer materials in close proximity to the fluid. This is achieved by the reliable fabrication of completely sealed microchannels directly below the substrate surface. The channel technology has found application in the fabrication of resonant flow sensors and low-drift micro flow sensors (Chap. 4-8) in the nl⋅min-1 range. Additionally, the technology has been extended by the possibility to integrate nanochannels using fluidic vias. This has been used in the fabrication of nano-nozzle electrospray emitters (Chap. 9). In current micromachined thermal flow sensors the elements for temperature sensing are made by thin films. The problem is that thin films reproduce poorly and that practically all materials properties are subject to drift. This translates directly into the accuracy of thermal micro flow sensors. In this thesis low-drift micro flow sensors were investigated, using two heaters and a thermopile in order to eliminate material drift (Chap. 5). The low offset drift of thermopiles has been exploited in a feedback loop controlling the dissipated powers in the heater resistors, minimising inevitable influences of resistance drift, mismatch of thin-film metal resistors and thermopile material drift. The control system cancels the flow-induced temperature difference across the thermopile by controlling a power difference between both heater resistors, thereby giving a measure of the flow rate. Alternatively, a sensor resistor and heat waves can be used to provide for a low offset-drift error signal (Chap. 8). It was demonstrated that material drift can largely be compensated. Sensitivity can be increased by designing flow sensors with a large number of integrated thermocouple junctions (Chap. 6), however it was observed that externally applied temperature gradients over the chip can still lead to drift of the sensor output signal. A special meandering microchannel layout was used to create a fully symmetrical flow sensor, where the arrangement of thermopile junctions has resulted in low-drift micro thermal flow sensors for liquids in the nl⋅min-1 range, with compensation for external temperature gradients (Chap. 7).