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Electrons ease microfabrication Researchers have fabricated microfluidic structures in polymer resists by manipulating the dose and energy of electron beams.
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Studer of the City of Paris Industrial lower, PMMA layer became more soluomplicated structures could be easiPhysics and Chemistry Higher Educable. Sarkar states, “You have, in effect, er to fabricate with help from electional Institution (ESPCI) says the abilicreated a soft–hard structure where you trons. Aniruddh Sarkar, Prashant Mali, ty to make covered nanochannels in a and Rakesh Lal at the Indian Institute of dissolve the soft part out, again [creatsingle step is important. He explains, ing] an embedded structure through Technology, Bombay, have manipulated “One of the big problems of making which your liquid can be passed.” electron beam (e-beam) lithography so nanochannels is, how do you that polymeric microfluidic devices make closed channels, and how can be built in one step (Lab Chip can they sustain the pressure that 2006, 6, 310–315). you need? Here, [the investiga“There isn’t a mask. There isn’t tors] found a way to fabricate a mold. There isn’t any waiting nanochannels or microchannels around for various processes that with nanostructures that are are needed in other techniques, [for closed.” example] sacrificial layers that have 20 µm However, Studer says that the to be patterned and then etched,” approach has a potential limitasays Sarkar. The investigators have tion—access to an e-beam lithogfabricated covered microfluidic raphy machine or writer. “People channels for DNA electrophoresis, like us doing microfluidics fight channels with embedded micropilto have access to an e-beam writer lars, and a chaotic micromixer. With to [make] our channels,” he says. their scheme, Sarkar says, “You get “If you have the writer, it’s an to the structure directly from a easy way of doing [fabrication]. [computer-aided design] pattern.” But the question is, how many The investigators took advan20 µm people have access to one?” tage of the way in which negativeAnd Stroock points out that and positive-resist polymers absorb e-beam lithography could be electrons. A negative-resist poly(a) A microchannel with pillars and (b) a diaphragm suptime-consuming. “For better, an mer, like SU-8, becomes less solu- ported by beams were fabricated by e-beam lithography. e-beam directly follows the patble when an e-beam hits it; a posi- (Adapted with permission. Copyright 2006 Royal Society of tern you feed it. For worse, it has tive-resist polymer, such as PMMA, Chemistry.) to go through your entire pattern becomes more soluble. The investiAbraham Stroock of Cornell Universi- one step at a time,” he says. “If you gators manipulated the absorption of want to make a complicated multistep ty says the subtle ways in which the inelectrons by the resists in two ways. In microfluidic device, you might spend vestigators manipulate e-beam lithograthe first approach, they varied the enerphy have the potential “to make complex hours drawing it with your e-beam.” gy of the e-beam as it moved over a Sarkar says his group’s aim is to demicrofluidic systems in a clever way.” In layer of negative resist on a silicon subvelop “microtechnology which is not particular, Stroock points out that the strate. In some places, the e-beam hit method allows microfluidic structures to very complicated and hence can be used the entire thickness of the negative reto generate applications that are novel be built on top of traditional electronic sist. In other places, the beam penetratand useful.” The investigators are curcircuits made by the complementary ed only the top of the layer, so the rest rently pursuing the development of a metal oxide semiconductor (CMOS) of the resist dissolved away. The result variety of microfabricated instruments technique. In contrast, “if you go off was embedded structures, such as covincluding an immunosensor integrated and make your microfluidics [device] in ered microchannels. with microcantilever and optical sensing glass or PDMS and want to interface it In the second approach, the investicapabilities. They also plan to use their with a CMOS integrated circuit, that’s gators changed the dose of electrons e-beam fabrication schemes to make a not going to be trivial,” he says. striking a silicon substrate layered with single-cell electroporator, an acceleromThe electron dose variation techfirst PMMA and then SU-8. When the eter, and a bacterial cell sorter. a nique can provide sufficient resolution two layers were exposed to an e-beam, —Rajendrani Mukhopadhyay to build nanoscale structures. Vincent the top, SU-8 layer hardened while the 2878
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