Microfluidics and Microtechnology for Microreactor Systems

M. Matlosz et al. (eds.), Microreaction Technology © Springer-Verlag Berlin Heidelberg 2001
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Heater resistor Air chamber Flow sensor
Pump Chamber
Valve 2
Piezos Pyrex Silicon
Figure 3: Peristaltic micropump. From left to right: top view and cross section showing the basic design, and photograph of a realized pump. In the cross section the pump membranes are shown in the activated upward position.
Microfluidics and Microtechnology for Microreactor Systems
N.R. Tas, R.E. Oosterbroek, T.T. Veenstra, M. Elwenspoek and A. van den Berg MESA+ Research Institute, University of Twente, P.O. Box 217,7500 AE Enschede, The Netherlands. Fax: +31-53-4893343. E-mail: A.vandenberg@el.utwente.nl
Wafer Bonding Technology
To understand the conditions for fusion bonding, a fundamental study has been done into the bondability of wafers as a function of surface roughness, elastic and adhesive properties of the surfaces. The result is a model that predicts bondability with a single parameter, the dimensionless surface adhesion parameter [7]. With the knowledge of the relation between surface roughness and bondability, it is
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Figure 2: Thin {III} plate after fusion bonding to create a closed channel, magnification of the {Ill} plate, and 3D-structured 4: MFS demonstrator. It consists of two inlets, two flow sensors and two pumps, a mixing unit and a detection unit. The fluidic system is connected to the control electronics which are collected on the lower circuit board [11].
Robust Peristaltic Pump An improved peristaltic piezoelectrically driven micropump has been made using silicon-glass sandwich technology [9). The improvement, compared to previously presented peristaltic micropumps, is that it is self-priming and it can pump any gas bubbles present in the liquid. The design and fabrication are extremely simple: the process is robust and in principle needs only one lithographic step. In order for a pump to be bubble tolerant, this compression ratio should be larger than 0.08. In the design of the peristaltic pump, the compression ratio is chosen to be 0.5. This results in a maximum pressure of 500 mBar. The design of the micropump is shown in fig. 3. It also shows a picture of a realized pump. The membranes are created from a 220± 10 11m thick pyrex wafer, which is anodically bonded to the silicon substrate wafer. In order to have low-dead volume pumping chambers, a selective bonding method has been used, in which the membranes are coated with chromium to prevent bonding locally [10). The pump produces a maximum flow of 9 Ill/min, at a stroke frequency of about 400 Hz [9).
28 possible to obtain selective bonding: In regions with (tailored) increased surface roughness bonding can be prevented [8]. This technique can be used for example to prevent bonding of a bossed type valve seat to the counter surface [8].
1. INTRODUCTION In this paper we outline the evolution of the silicon-based technology for microchemical systems in the MESA Research Institute. There is a tendency toward the handling of smaller volumes, which leads to the development of fully integrated micro-fluidic systems, based mainly on thin film technology. Several examples of microdevices and systems will be discussed. 2. STACKING OF SILICON AND GLASS WAFERS The history of micro-liquid handling systems at Twente University goes back to the eighties, with the development of the piezoelectrically driven micropumps of Smits et al. [1] and van Lintel et al. [2]. Both pumps were based on stacking of glass-silicon-glass wafers, to create closed chambers and channels. An important step in system integration was the development of a micro-liquid dosing system [3], consisting of a pump and a flowsensors (fig. 1), fabricated simultaneously in glass-silicon-glass technology [3]. In order to support this bulk micromachining technology, our micromechanics group investigates etching of silicon, and both anodic and fusion bonding on a more fundamental level. In collaboration with the solid state chemistry group of the University of Nijmegen, physical-chemical theories of anisotropic and isotropic wet-chemical etching of silicon are developed. The results of this project include a simulation tool for anisotropic wet chemical etching [4], as well as an explanation for the formation and stabilization of pyramidal etch hillocks on Si{100} [5]. Another result of the more fundamental look at KOH etching is a design method to create thin {Ill} plates in <100> oriented silicon (fig. 2). Although the processing involves doublesided etching, precision alignment is only required at one side of the wafer [6]