Embossing (imprinting) has become one of the important polymer processing techniques in micro-/nano-fabrication. Compared to other techniques (e.g. injection molding, and silicone rubber casting), it provides several advantages such as relatively low costs for embossing tools, simple process, high replication accuracy for small features, flexibility in selection of substrate materials and feasibility in continuous mass production. The basic principle of embossing (imprinting) is that a polymer substrate is first heated above its glass transition temperature, T g (or softening temperature). A mold (or master) is then pressed against the substrate, fully transferring the pattern onto it (embossing). After a certain time of contact between the mold and the substrate, the system is cooled down below T g, followed by separating the mold and the substrate (de-embossing).
There are a variety of techniques to fabricate the mold inserts (or masters) such as traditional CNC-machining, diamond-based micro-milling or micro-drilling and excimer or femtosecond laser-based direct removal process, lithographic methods (i.e. surface machining) followed by either direct electroplating or wet/dry etching of silicon (which can either serve as a mold or subsequently be electroplated), LIGA and so on. Figure 1 shows how we produced nickel mold using lithographic methods followed by direct electroplating. Note that the whole mold insert is made of nickel and a proper draft angle is formed. Figure 2 shows mold inserts fabricated through various techniques.
n addition to conventional isothermal embossing technique, i.e. both mold and substrate are heated above T g, we also performed non-isothermal embossing by only heating the mold well above its T g. Figure 3 shows substantial different flow patterns of polymer deformation between isothermal and non-isothermal embossing. A commercial finite element code, DEFORM 2D and 3D, were used to simulate the embossing processes as shown from Figures 4 to 7. Recently, we have been developing a novel laser/IR-assisted embossing process. Through fast and local/surface heating on the polymer substrate, the embossing process can be completed within seconds. Figure 8 shows the schematics of the process and comparison between experimental and simulation results.
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