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Surface micromachined polyimide-based valveless micropump
Keywords: Microfluidics, micropump, MEMS
MEMS-based microfluidic pumps have been under development for over three decades, mainly for biomedical applications such as drug delivery, microTAS and Lab-On-Chip applications where transport and control of extremely small quantities of fluids is required [1]. Other applications for MEMs pumps include chemical analysis, environmental sensing, and micro cooling systems. Mechanical-displacement diaphragm pumps are the most common type of pump implemented at the micro- and chip-scale [2]. These devices in general consist of a diaphragm or membrane suspended over a substrate, and which is moved up and down by an actuator, PZT being the most commonly used actuation mechanism. The pump also contains inlet and outlet fluid ports and either has valves (whether active or passive) or diffuser-nozzle elements in the case of valveless pumps, the latter relying on geometrical features to produce flow rectification [3]. Since micro-valves often fail mechanically, the advantage of valveless micropumps is increased reliability and simpler fabrication. Most diaphragm micropumps made using MEMS technology are made from aligning and bonding 2 or more wafers/chips vertically [2]. In this presentation we describe a surface micromachined valveless micropump, made from layers of polyimide on a silicon substrate and driven by an external commercial PZT stack actuator. The device has footprint of 5 mm x 5 mm, and total package height of 1.2 cm including the commercial PZT stack actuator. The diaphragm, pump cavity and substrate are monolithically fabricated at the wafer-level. Layers of polyimide are spun and photopatterned on the silicon wafer to define the diaphragm over a thin-film sacrificial layer, and as well as part of the diffuser-nozzle elements. DRIE of the silicon is used to complete the fabrication of the diffuser-nozzle elements. Finally a release etch and critical point drying removes the sacrificial layer underneath the diaphgragm. This results in the diaphragm being suspended over the substrate by a height of 1-10 microns over a total chip area of 5mm x 5 mm. This small suspended height results in a low dead volume of the pump chamber with minimal (up to about 6 microns) diaphragm displacement by the PZT actuator. This in turn allows the use of low-cost, low-voltage, small-footprint commercial PZT stack actuators. Assembly consists of attaching copper tubes to the inlet-outlet ports in the back of the silicon substrate to interface with other microfluidic components, and mounting the commercial PZT stack actuator to the diaphragm. These assembly procedures require coarse alignment and low-temperature epoxy-based bonding. We then demonstrate pumping of both nitrogen gas as well as water. The nitrogen flow rate was 30 uL/minute at 1 kHz, and the water flow rate was 600 nL/minute with liquid back-pressure of 0.3 kPa. We thus describe a surface micromachined polyimide valveless micropump capable of pumping gases and liquids and with the advantages of fabrication simplicity, low cost and high reliability.
Li-Anne Liew, Senior Research Associate
University of Colorado at Boulder
Boulder, CO

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