Metallic glasses possess unique mechanical properties which make them attractive materials for fabricating components for a variety of applications. For example, the commercial Zr-based metallic glasses possess high tensile strengths (≈2.0 GPa), good fracture toughnesses (≈10-50 MPa √ m) and good wear and corrosion resistances. A particularly important characteristic of metallic glasses is their intrinsic homogeneity to the nanoscale because of the absence of grain boundaries. This characteristic, coupled with their unique mechanical properties, makes them ideal materials for fabricating micron-scale components, or high-aspect-ratio micro-patterned surfaces, which may in turn be used as dies for the hot-embossing of polymeric microfluidic devices. In this paper we consider a commercially available Zr-based metallic glass which has a glass transition temperature of T g ≈ 350 • C and describe the thermoplastic forming of a tool made from this material, which has the (negative) microchannel pattern for a simple microfluidic device. This tool was successfully used to produce the microchannel pattern by micro-hot-embossing of the amorphous polymers poly(methyl methacrylate) (T g ≈ 115 • C) and Zeonex-690R (T g ≈ 136 • C) above their glass transition temperatures. The metallic glass tool was found to be very robust, and it was used to produce hundreds of high-fidelity micron-scale embossed patterns without degradation or failure.
Scaling contact lithography (microcontact printing, microflexography, and nanoimprint lithography) to large roll-to-roll platforms will enable high speed, low cost lithographic patterning of surfaces. However, many details of robust implementations at the roll-to-roll scale remain an engineering challenge, including precise regulation of printing pressures and the stamp-substrate interaction. This paper introduces a method for precise control of contact pressure that can accommodate large dimensional variations, i.e. varying stamp and substrate thicknesses. This control algorithm is implemented on a simply supported roll positioning stage. Experimental results for microcontact printing and microflexography are shown both with in situ contact measurements on a pseudo substrate and with 5 um silver nanoparticle prints. Ultimately, this approach enables robust printing despite sensitive stamp patterns and large dimensional variations (> 10 μm) in substrates, stamps, and roll equipment.
Users of hot micro-embossing often wish to simulate numerically the topographies produced by the process. We have previously demonstrated a fast simulation technique that encapsulates the embossed layer's viscoelastic properties using the response of its surface topography to a mechanical impulse applied at a single location. The simulated topography is the convolution of this impulse response with an iteratively found stamp-polymer contact-pressure distribution. Here, we show how the simulation speed can be radically increased by abstracting feature-rich embossing-stamp designs. The stamp is divided into a grid of regions, each characterized by feature shape, pitch and areal density. The simulation finds a contact-pressure distribution at the resolution of the grid, from which the completeness of pattern replication is predicted. For a 25 mm square device design containing microfluidic features down to 5 μm diameter, simulation can be completed within 10 s, as opposed to the 10 4 s expected if each stamp feature were represented individually. We verify the accuracy of our simulation procedure by comparison with embossing experiments. We also describe a way of abstracting designs at multiple levels of spatial resolution, further accelerating the simulation of patterns whose detail is contained in a small proportion of their area.
Hot embossing is an effective technology for reproducing micro-scale features in polymeric materials, but large-scale adoption of this method is hindered by high capital costs and low cycle times relative to other technologies, and a general lack of manufacturing equipment. This work details a hot embossing machine design strategy motivated by maximum production speed and quality with minimal capital cost. The approach is to "right-size" the machine for specific product needs while making the design flexible and scalable. Toward this end, a minimal number of components were used, commercially available off-the-shelf components were chosen where possible, system layout was designed to be modular, and system size was scaled for the intended products (in this case microfluidic devices). Innovative design aspects include the use of new ceramic substrate heaters for electrical heating, use of a moveable heat sink to minimize heat load during the heating cycle, and the careful design of the thermal elements to minimize the heating and cooling cycle times. The capital cost and the cost per part produced with this machine are estimated to be an order of magnitude less than currently available hot embossing manufacturing options. The hot embossing machine has been tested extensively to characterize the process variability. The minimum cycle time is two minutes, and microstructures are replicated within a maximum of a 25mm by 75mm area with very low relative variance in dimensions.
Polymeric substrates have significant advantages over silicon and glass for use in microfluidics. However, before polymer microfluidic devices can be mass produced, it must be shown that the manufacturing method used to create these devices is robust and repeatable. For this paper, a polymer manufacturing process, hot embossing, was used to produce microsized features in polymethylmethacrylate (PMMA) chips. A design of experiments that varied two factors during the hot embossing process (temperature and pressure), was conducted to determine the robustness of hot embossing microsized channels in PMMA. The channel height and width were measured at three sites on each chip, and the results were analyzed in two ways: response surface modeling (RSM) and nested variance analysis. For the RSM analysis, two separate ANOVA tests and regressions were performed on both channel width and channel height to obtain the response surface models between temperature, pressure and the channel width and height. Furthermore, the variance of channel width and height at each design point was determined and then two ANOVA tests and two separate regressions were performed to obtain the response surface models between temperature, pressure and the variance of channel height and channel width. This analysis was used to determine if hot embossing microfluidic devices is a robust process capable of producing quality parts at different operating conditions. The nested variance analysis was used to determine the primary source of the variation in channel height and width. For the nested variance analysis, two separate calculations were performed in order to determine whether the variance of channel width and height is mostly caused by within-chip variance or chip-to-chip variance. The analysis showed that the channel widths and heights were statistically equal across the four different operating points used (the low-temperature, low-pressure point was omitted). The variance of channel width and the variance of channel height remained constant in the desired operating region. Based on this analysis, it was concluded that hot embossing is a robust process for features on the order of 50 μm. Furthermore, the nested variance analysis showed that the variance of channel width and height is mostly caused by site-to-site measurements on a chip rather than between-chip variance. Therefore, it was determined that hot embossing microfluidic devices are repeatable and consistent from chip-to-chip.
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