Fabrication of Three-Dimensional Microfluidic Systems by Soft Lithography
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Fabrication of
Three-Dimensional Microfluidic Systems by Soft Lithography
J. Christopher Love, Janelle R. Anderson, and George M. Whitesides Introduction Two-dimensional (2D) methods for transferring patterns to planar substrates have enabled the technological revolution in microfabrication that has marked the last 40 years.1 The overall trend toward increased miniaturization has led to the development of new types of devices in areas unrelated to conventional microelectronics: analytical tools, chemical reactors, microelectromechanical systems (MEMS), optical systems, and sensors.2 The widespread use and high level of technological development associated with photolithography has also made the methodologies for microelectronics—patterning photosensitive polymers, etching and deposition of thin films, and liftoff—ubiquitous in the fabrication of these new classes of microsystems. These new systems have specialized requirements, however, and are not simple extensions of microelectronics technologies. They often require materials—especially organic polymers— that are not commonly used in microelectronic systems, they must have low cost, and they may need 3D structures in order to implement complex designs. These requirements have stimulated the development of new methods for microfabrication.
Mastering and Replication The process of microfabrication generally can be divided into two steps: (1) the generation of a master and (2) the use of the master to produce replicas. In photolithography, a mask writer is used to cre-
MRS BULLETIN/JULY 2001
ate a pattern of chromium on a glass or silica plate for each lithography step; the pattern subsequently is transferred onto a coating of photosensitive polymer on each wafer in production. The process used to generate the original mask is a serial, high-precision technique that uses a beam of photons or electrons to transfer a pattern from the computer into a resist layer. This process is expensive, but the high cost of the original master is offset if the number of replicas generated from the master is large and the value of the final device is high.
Soft Lithography Soft lithography is a collection of low-cost techniques for replicating patterns—from masters generated by photolithography, machining, or other methods—onto a range of substrates.3–5 The term “soft” is taken from condensed-matter physics, and we used it originally to refer to organic elastomers used as stamps or molds. We now use it more generally to refer to organic and organometallic materials; the “soft” component can refer to either the system used in pattern transfer or the materials being patterned, but particularly the former. In most applications, soft lithography uses an elastomeric, topographically patterned replica or stamp—typically fabricated from poly(dimethylsiloxane) (PDMS), an elastomeric organic polymer— to transfer the original pattern of a master by molding or printing. PDMS is inexpen-
sive, homogeneous, optically transparent, nontoxic, and commercially available. The flexibility of th
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