Microfabrication of cavities in polydimethylsiloxane using DRIE silicon molds

Abstract

We present a novel method to create cavities in PDMS that is simple and exhibits wide process latitude allowing control over the radius of curvature to form shallow concave pits or deep spherical cavities.Polydimethylsiloxane (PDMS) is a silicon-based polymer exhibiting tunable mechanical, chemical and optical properties that are widely leveraged in microfluidic and biotechnology applications. Its clear optical properties are especially advantageous for spectroscopic analysis through lab-on-a-chip devices. 1-4 Its compliant properties are exploited to perform large area lithography on non-planar surfaces 5 and to form variable-focus microlens arrays. 6-8 PDMS is inert and non-toxic, making it well suited for cell culture and tissue engineering applications. 9,10 PDMS stamps are used to spatially pattern antibodies, enzymes, protein ligands and cells onto sensors and other material surfaces using the micro-contact printing technique. 11-15 PDMS stamps are typically fabricated by reverse replication of features formed in a silicon wafer mold.The most widely used PDMS elastomer is Dow Corning Sylgard 184 (1 part catalyst : 10 parts base by weight) which is degassed prior to filling the mold. This material has a high elongation at break (160%) and a relatively low modulus (1.8 MPa). 16 Features molded in PDMS are typically rectilinear forms, including squares, rectangles, cylinders and composite geometries combining these. More complex shapes (spheres and ellipsoids) that are needed to form, for example, microlens arrays require innovative process development. Some recent examples include photoresist thermal reflow processing, 17,18 gel trapping, 8 mechanical deformation of elastomeric mold, 19 liquid-filled cavities 20 and surface tension extrusion. 21In this communication we report a new method to create smooth concave cavities in PDMS that allows control over the radius of curvature to form shallow pits or deep spherical cavities. While these structure are useful for microlens molding, our interest is to create microbioreactor arrays. Leveraging novel fluidic flows and affinity capture methods, cells can be sorted into spatially distinct reactors of different sizes, where they can be maintained on-chip for research purposes or until needed for therapeutic use. 10,22,23Our method, outlined in Fig. 1, utilizes standard lithographic processing to create a silicon wafer mold comprised of deep reactive ion etched (DRIE) trenches (Fig. 1A). An SEM image of a typical wafer mold containing linear arrays of DRIE trenches measuring 95 × 95 × 350 μm is illustrated in Fig. 2A. These trenches were fabricated using the Bosch process that NIH Public Access consists of sequential SF 6 etching and C 4 F 8 passivation steps. 24 The passivation step deposits a hydrophobic fluoropolymer coating that protects against side wall erosion as etching proceeds deeper into the silicon wafer. Normally, the hydrophobic passivation layer is removed prior to subsequent processing steps using oxygen plasma. Our method for molding cavitie...

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