Novel interconnection technologies for integrated microfluidic systems

doi:10.1016/S0924-4247(99)00185-5

Sensors and Actuators A: Physical

Volume 77, Issue 1, 28 September 1999, Pages 57-65

https://doi.org/10.1016/S0924-4247(99)00185-5 Get rights and content

Abstract

A new approach to realize silicon based integrated microfluidic systems is presented. By using a combination of silicon fusion bonding (SFB) and deep reactive ion etching (DRIE), multi-level fluidic `circuit boards' are fabricated and used to integrate microfluidic components into hybrid systems. A multi-level laminating mixer and a manifold with multiple pressure sensors are presented as application examples. To interface the microfluidic system to the macroscopic world, three types of DRIE-fabricated, tight-fitting fluidic couplers for standard capillary tubes are described. One type of coupler is designed for minimal dead space, while another type reduces the risk of blocking capillaries with adhesive. A third design demonstrates for the first time a silicon/plastic coupler combining DRIE coupler technology with injection-molded press fittings.

Introduction

Integrated microfluidic systems have gained interest in recent years for many applications including chemical, medical, automotive, and industrial. A major reason is the need for accurate, reliable, and cost-effective liquid and gas handling systems with increasing complexity and reduced size. While a wide range of miniature fluidic devices such as valves, pumps, mixers, and flow sensors have been demonstrated [1], efficient interconnections between these devices and coupling to external fluidic structures are not yet available. However, it is primarily these and other packaging issues that will determine the commercial success of microfluidic devices and systems. There remains a need for a microfluidic interconnection technology that mimics the electronic printed circuit board in ease of use, reliability, and versatility, and can accommodate active components as well as coupling to other microfluidic circuits and macroscopic equipment.

A miniature hybrid fluidic circuit board based on bonding of plastic was demonstrated by Verlee et al. [2]. A reduction in size and increased integration level was achieved using epoxy printed circuit board technology [3]or gasketed stacked modules [4]. Further size reduction can be obtained with fully monolithic systems. However, the integration complexity of these systems is usually limited by wet silicon etch processes for the definition of channels and vias. The new approach presented in this paper uses deep reactive ion etching (DRIE) technology 5, 6to fabricate complex fluidic `circuit boards'. DRIE offers many advantages over wet silicon etch processes, in particular, higher density of fluidic interconnects, precise via holes with uniform cross-sections through the wafer thickness, and the ability to fabricate vias and channels of nearly arbitrary size and shape. The fabrication process includes a combination of DRIE, silicon fusion bonding (SFB), and anodic bonding to obtain multi-level fluidic substrates. These fluidic circuit boards can be used to build complex multi-level structures or can be combined with surface-mounted microfluidic devices.

Two application examples are presented. The first is a mixer structure that relies on multiple levels of flow for operation. The second is an integrated hybrid system for measuring pressure in a microchannel using surface-mounted micromachined pressure sensors.

A major issue in integrated microfluidic systems is the coupling of the fluidic circuits to the macroscopic world. Fluidic couplers consisting of a capillary tube glued into an insertion channel which is iso- or anisotropically etched into a silicon substrate have been previously demonstrated 7, 8. These couplers do not have accurately fitted insertion channels resulting in difficult handling and increased dead space. Our approach is to use successive DRIE steps to fabricate accurately sized cross-sections for the connecting capillaries. To circumvent the necessity of gluing the capillaries, a new coupler combining DRIE with injection-molded press fittings has been developed, allowing the capillaries to be exchanged.

Section snippets

Fabrication

Fig. 1 illustrates the fabrication process for multi-level fluidic circuit boards. The first deep etch, approximately 100 μm deep, is performed on a double-side polished silicon wafer to define the first channels. This is followed by a second DRIE step from the opposite side of the wafer to form interconnection vias, 50–100 μm in diameter, all the way through the 390-μm thick wafer. Photoresist is used to protect the wafer front with the first channels during the through-wafer etch and all

Multi-level laminating mixer

The process shown in Fig. 1 was used to fabricate a multi-level laminating mixer. Time-efficient mixing of two fluids is fundamental to the creation of many `on-chip' microfluidic processing systems. However, the channel sizes and flow rates associated with such processing systems usually imply low Reynolds numbers, precluding turbulence as a mixing mechanism. Mixing by diffusion can be very slow unless the contact area between the two fluids is increased. One well-known method to increase

Fluidic couplers

Fluidic couplers for standard fused silica capillary tubing obtained from Polymicro Technologies Inc., (Phoenix, AZ) were fabricated using the process sequence shown in Fig. 1. The left coupler in Fig. 1d illustrates the first design. DRIE was used to define circular holes matching the inside and outside diameter of the capillaries, thus eliminating any dead space. Fig. 8 shows an SEM photograph of the minimum dead space coupler. The capillary was inserted into the opening and fixed in place

Conclusions

A novel technology for the fabrication of fluidic circuit boards including couplers to standard capillary tubes has been presented. A multi-level laminating mixer was used to demonstrate the technology for making multi-level structures. Good agreement was found between simulated and measured mixing of differently dyed water streams at 60 μl/min. To demonstrate the technology for making fluidic circuit boards, the measurement of pressure in a microchannel was demonstrated with good agreement

Acknowledgements

Funding for this work was provided by the DARPA MicroFlumes Program (Contract Number: N66001-96-C-8631). The authors would like to thank R. Scimeca of Lucas NovaSensor, Fremont, CA, M.G. Giridharan of CFD Research Corporation, Huntsville, AL, and T. Callenbach of H. Weidmann AG, Plastic-Technologies, Rapperswil, Switzerland for their help and technical discussion with processing, CFD modeling, and heat-staking, respectively. Load cell measurements by G. Cornella, Department of Materials Science

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Bonnie L. Gray received the BS in Electrical Engineering from Rensselaer Polytechnic Institute in Troy, NY, in 1992. Also in 1992, she was awarded an NSF Graduate Student Fellowship. She received the MS in Electrical Engineering from the University of California at Berkeley in 1995, with a thesis on the design and operation of a surface-micromachined tactile sensor array for endoscopic manipulators. Since 1995, she has been an Electrical Engineering doctoral student at the University of California at Davis. She is currently interested in interconnection technologies for MEMS, including mechanical, optical, and especially fluidic interconnection for μTAS.Dominik Jaeggi received the MSc and PhD degrees in Electrical Engineering from ETH Zurich (Swiss Federal Institute of Technology), Zurich, Switzerland, in 1991 and 1996, respectively. At ETH Zurich, his research focused on the development of thermal microsensors using industrial IC processes in combination with postprocessing micromachining steps. Most of his PhD research was conducted at the CMOS EM Microelectronic Marin, Marin, Switzerland, where he continued his work on thermal sensor technology as a ETH Zurich Research Associate in 1996–1997. Since 1997, he has been working as a Post-doctoral Fellow at Stanford University, Stanford, CA, and Research Staff Scientist at Lucas NovaSensor, Fremont, CA. His current research interests include design, fabrication, and modeling of microfluidic devices.Nicholas J. Mourlas received the AB and BE degrees from Dartmouth College, Hanover, NH, in 1992. From 1992 to 1994, he was a Design Engineer at Reliable Power Products, Franklin Park, IL. He received the MS degree in Electrical Engineering from Case Western Reserve University, Cleveland, OH in 1996. In 1996, he worked as a research intern and consulted for Lucas NovaSensor. He is currently pursuing the PhD degree in the Department of Electrical Engineering at Stanford University. Since 1992, he has investigated MEMS devices for ink-jet printing, data storage, signal filtering, and chemical processing. His current research activities include interconnection and channel technologies for microfluidic systems.Bert van Drieënhuizen received the MSc with honors in 1991 and the PhD degree in 1996, both in Electrical Engineering from the University of Technology, Delft, The Netherlands. His doctoral research focused on the development of a surface micromachined RMS to DC converter. Since October 1996, he has been working for Lucas NovaSensor, Fremont, CA as a Research Staff Scientist.Kirt R. Williams received the BS degree with high honors with a double major in Electrical Engineering and Computer Sciences (EECS) and Materials Science and Engineering from the University of California at Berkeley in 1987. While pursuing this degree, he was employed at Eastman Kodak and Altera. After receiving the BS, he did digital and analog circuit design at Western Digital. Working in the Berkeley Sensor and Actuator Center at UC Berkeley, he earned the MS and PhD degrees in EECS in 1993 and 1997. His dissertation focused on the design, fabrication, and testing of micromachined hot-filament vacuum devices. He taught and revised the laboratory for the department's IC-fabrication class, receiving the EE Outstanding Graduate Student Instructor award in 1996. He is presently a Research Staff Scientist at Lucas NovaSensor, Fremont, CA.Nadim I. Maluf received the BE degree from the American University of Beirut, Lebanon, in 1984, the MS degree from the California Institute of Technology, Pasadena, CA, in 1985 and the PhD degree from Stanford University, Stanford, CA, in 1991, all in Electrical Engineering. From 1985 to 1988, he was employed at Siliconix, Santa Clara, CA, where he was responsible for the development of high-voltage integrated circuits. From 1991 to 1994, he was a Research Associate and Lecturer at Stanford University where he worked on micromachined transducers for medical applications and taught graduate courses on solid state physics and semiconductor devices. He is presently Manager of Advanced Technologies at Lucas NovaSensor, Fremont, CA, where his group is responsible for the development of advanced technologies and products for the automotive, medical, consumer, industrial and aerospace markets. He is also a Consulting Assistant Professor of Electrical Engineering at Stanford University and serves on the advisory boards of a number of organizations. He has authored or co-authored over 60 articles and patents in the areas of sensors and actuators, semiconductor technology and microinstrumentation.Gregory T.A. Kovacs received the BASc degree in Electrical Engineering from the University of British Columbia, Vancouver, BC, in 1984, the MS degree in Bioengineering from the University of California, Berkeley, in 1985, the PhD degree in Electrical Engineering from Stanford University, in 1990 and the MD degree from Stanford University in 1992. His industry experience includes the design of high-speed data acquisition systems, the design of instruments for GaAs device fabrication, commercial and consumer product design, extensive patent law consulting, and the co-founding of several companies, most recently Cepheid, in Sunnyvale, CA. In 1991, he joined Stanford University and is currently an Associate Professor of Electrical Engineering. He teaches courses in electronic circuits and micromachined transducers. He held the Robert N. Noyce Family Faculty Scholar Chair in 1992–1994, received an NSF Young Investigator Award in 1993, was appointed a Terman Fellow in 1994, was appointed to the Defense Sciences Research Council in 1995 and was appointed a University Fellow in 1996. His present research areas include solid-state sensors and actuators, micromachining, neural/electronic interfaces, integrated circuit fabrication, medical instruments, and biotechnology, all with emphasis on solving practical problems.

¹: Paper presented as part of the SSAW-98 Workshop.

²: Currently at the Micro Instruments and Systems Laboratory, Department of Electrical Engineering, EUII, University of California, Davis, CA 95616, USA.

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