MTL ANNUAL RESEARCH REPORT September 2006 56 Understanding Uniformity and Manufacturability in MEMS Embossing H.K. Taylor, D.S. Boning Sponsorship: Singapore-MIT Alliance The hot embossing of thermoplastic materials, such as polymethylmethacrylate (PMMA) or cyclo- olefn copolymer (COC), is a promising way to manufacture microfuidic channels and networks [1]. Hot embossing potentially offers lower per-area cost than the micromachining of quartz or silicon and easier scaling-up of production than soft lithography using polydimethylsiloxane [2]. In hot embossing, a microfabricated mold (typically of silicon or nickel) is pressed into a fat sample of polymeric material that has been softened by heating it above its glass-transition temperature. We are particularly interested in how the spatial distribution of mold features—their diameters, shapes, and areal densities—may infuence the quality of embossed patterns. We are developing a simulation approach whose building-block is a simple model in which, for given embossing conditions, a feature-sized disk of viscous polymer is compressed at a rate inversely proportional to the square of the radius of the disk [3] (Figure 1). Such a model implies that the mold will sink into the substrate at a spatially uniform rate when the product of the areal density of mold features and the square of their average radius remains constant across the mold. We aim to construct a reliable model that is computationally effcient and that can predict the combination of embossing pressure and duration required by any mold design. We are investigating the measurement of birefringence of embossed samples [4] as a way of monitoring the embossing process (Figure 2). We are also pursuing a technique for the bonding of polymer surfaces that promises minimal deformation of pre-embossed features: the polymer surfaces are exposed to an oxygen plasma for ~1 minute and then pressed together [5]. Figure 1: Proposed model for the spatially non-uniform flling of embossing mold features with heated thermoplastic material (blue). Arrows indicate the displacement of material. Regions of the mold with higher areal densities of protruding features, e.g., on the left in this fgure, are expected to be flled more quickly (a), and to “coagulate” into effectively larger features (b). Eventually all features would be flled and the polymeric substrate may continue to be compressed as one large disk (c). t Figure 2: Light transmitted by each of two embossed PMMA samples sandwiched between perpendicular polarizers. The two samples were embossed under ~1 MPa for equal lengths of time. At 110 °C (a), material within about 1 mm of the corners of embossed, 30 µm-deep rectangular channels exhibits substantially higher birefringence than the rest of the sample, implying concentrations of residual stress there. At 150 °C (b), feature-scale birefringence becomes less important than sample-scale birefringence. Samples fabricated by Wang Qi. t REFERENCES [1] A. Pépin, P. youinou, A. Lebib, and y. Chen, “Nanoimprint lithography for the fabrication of DNA electrophoresis chips,” Microelectronic Eng., vol. 61–62, pp. 927–932, Jul. 2002. [2] J.R. Anderson, D.T. Chiu, R.J. Jackman, O. Cherniavskaya, J.C. McDonald, H. Wu, S.H. Whitesides, and G.M. Whitesides, “Fabrication of topologically complex three-dimensional microfuidic systems in pdms by rapid prototyping,” Anal. Chem., vol. 72, no. 14, pp. 3158–3164, Jul. 2000. [3] T. Hoffmann, “Viscoelastic properties of polymers,” Alternative Lithography, Ed. C.M. Sotomayor Torres, Ny: Kluwer, 2003. [4] H.S. Lee, S.K. Lee, T.H. Kwon, and S.S. Lee, “Microlenses array fabrication by hot embossing process,” IEEE/LEOS Intl. Conf. on Optical MEMS, 2002, pp. 73–74. [5] J. Mizuno, S. Farrens, H. Ishida, V. Dragoi, H. Shinohara, T. Suzuki, M. Ishizuka, T. Glinsner, and S. Shoji, “Cyclo-olefn polymer direct bonding using low temperature plasma activation bonding,” Int’l. Conf. MEMS, Nano and Smart Systems, 2005, pp. 1346–1349.