Byoung Hee You

Simulation study on stress and deformation of polymeric patterns during the demolding process in thermal imprint lithography

Thermal imprint lithography or hot embossing is a processing technique using molding to produce surface patterns in polymer resist at micro- and nanoscales. While fast molding is important to improve the yield of the process, the process step that determines the success of imprinting high aspect ratio structures is demolding, a process to separate the mold insert from the patterned resist after conformal molding. In this paper the authors studied the stress and deformation behavior in polymer resist during the cooling and demolding process of thermal imprint lithography via finite element method. A simple model structure of the Si stamp/poly(methyl methacrylate) (PMMA) resist/Si substrate was used for the simulation, assuming that PMMA is viscoelastic. As demolding proceeds, Von Mises stress in the PMMA layer is highly localized in two locations, one at the transition corner zone between the residual layer and the replicated PMMA pattern and the other close to the contact region with the moving stamp edge...

Titer plate formatted continuous flow thermal reactors for high throughput applications: fabrication and testing

A high throughput, multi-well (96) polymerase chain reaction (PCR) platform, based on a continuous flow (CF) mode of operation, was developed. Each CFPCR device was confined to a footprint of 8 × 8 mm2, matching the footprint of a well on a standard micro-titer plate. While several CFPCR devices have been demonstrated, this is the first example of a high-throughput multi-well continuous flow thermal reactor configuration. Verification of the feasibility of the multi-well CFPCR device was carried out at each stage of development from manufacturing to demonstrating sample amplification. The multi-well CFPCR devices were fabricated by micro-replication in polymers, polycarbonate to accommodate the peak temperatures during thermal cycling in this case, using double-sided hot embossing. One side of the substrate contained the thermal reactors and the opposite side was patterned with structures to enhance thermal isolation of the closely packed constant temperature zones. A 99 bp target from a λ-DNA template was successfully amplified in a prototype multi-well CFPCR device with a total reaction time as low as ~5 min at a flow velocity of 3 mm s−1 (15.3 s cycle−1) and a relatively low amplification efficiency compared to a bench-top thermal cycler for a 20-cycle device; reducing the flow velocity to 1 mm s−1 (46.2 s cycle−1) gave a seven-fold improvement in amplification efficiency. Amplification efficiencies increased at all flow velocities for 25-cycle devices with the same configuration.

Passive micro-assembly of modular, hot embossed, polymer microfluidic devices using exact constraint design

Low-cost microfluidic platforms have the potential to change accepted practices in many fields, including biology and medicine, in the near future. Micro-assembly of molded polymer microfluidic devices is one approach to cost-effective mass production of modular, microfluidic instruments. Polymer, passive alignment structures were used to precisely assemble molded polymer components to prevent infinitesimal motions and minimize the misalignment between assembled components and devices. The motion and constraint of the assemblies were analyzed using screw theory to identify combinations of passive alignment structures that would provide exact constraint of all degrees of freedom of the two mating parts without over-constraint. One option identified by kinematic analysis was a set of three v-groove and hemisphere-tipped pin joints, which are well known from precision engineering and suitable for microfabrication. To validate the passive alignment scheme, brass mold inserts containing alignment structures were micro-milled and used to hot emboss components in polycarbonate (PC). Dimensional and location variations of prototype alignment structures were measured to quantify the difference between the as-designed and actual dimensions and the locations of the alignment structures. The dimensional variation was 0.2–3% less than the designed dimensions and the location variation was 0.7% less. The alignment accuracy of an assembly was characterized by measuring the mismatch and vertical variation between molded alignment standards embossed on each pair of mating plates. With molded, polymer alignment structures the mean mismatch and mean vertical variation were as low as 13 ± 3 µm in the lateral plane along the x- and y-axes and −6 ± 15 µm with respect to the nominal value of 107 µm. This micro-assembly technology is applicable to the integration of all microsystems including the interconnection of microfluidic devices, the assembly of hybrid microsystems and the parallel assembly of microdevices.

Injection molding analysis of a needle cover — Optimum filling for gate location design

Injection molding is one of the most popular manufacturing methods for the cost-effective mass production of the plastic parts. Filling analysis of the molten polymer provides useful information to investigate the process conditions to ensure successful replication. To validate appropriate gate location for a NEEDLE COVER, flow fronts and flow stresses for four different gating options and three different design options are analyzed and compared to the field results. Based on the results, the optimum gate location for the minimum flow stresses and uniform fill patterns appears to be at gate 3. Thus it also provides minimum possibility of part warpage throughout PORT HOUSING and NEEDLE HOUSING. The results of analysis on the increased wall thickness, lower melt temperature, and longer injection time indicated that higher melt temperatures were recommended to achieve successful molding. Injecting the polymer at a longer time (1.2 second) leads to a significant increase in flow stresses throughout the part and the increase of wall thickness achieve successful replication of the parts.

Influence of material transition and interfacial area changes on flow and concentration in electro-osmotic flows

This paper presents a numerical study to investigate the effect of geometrical and material transition on the flow and progression of a sample plug in electrokinetic flows. Three cases were investigated: (a) effect of sudden cross-sectional area change (geometrical transition or mismatch) at the interface, (b) effect of only material transition (i.e. varying ζ-potential), and (c) effect of combined material transition and cross-sectional area change at the interface. The geometric transition was quantified based on the ratio of reduced flow area A2 at the mismatch plane to the original cross-sectional area A1. Multiple simulations were performed for varying degrees of area reduction i.e. 0-75% reduction in the available flow area, and the effect of dispersion on the sample plug was quantified by standard metrics. Simulations showed that a 13% combined material and geometrical transition can be tolerated without significant loss of sample resolution. A 6.54% reduction in the flow rates was found between 0% and 75% combined material and geometrical transition.

Passive alignment structures in modular, polymer microfluidic devices

For connecting polymeric, modular microfluidic devices, precise, passive alignment structures can prevent infinitesimal motions between the devices and minimize misalignment of the devices. The motion and constraint of passive alignment structures were analyzed for the design of assembly features using screw theory. A combination of three v-groove and sphere joints constrained all degrees of freedom of the two mating plates without over-constraint. To validate the designed passive alignment scheme, hot embossing experiments were conducted using a micromilled brass mold insert, containing alignment features. Prototype alignment structures have dimensional variation. The alignment accuracy of the stacked polymeric plates was estimated by the mismatches between alignment marks of two plates. The mismatches ranged from 28 μm to 70 μm.Copyright

Mold filling analysis of an alignment structure in micro hot embossing

Hot embossing is one of the most popular fabrication methods to replicate polymer microdevices in the field of micro-fluidics and micro-optics. Numerical models for hot embossing were constructed to analyze the advance of flow front of the molten polymer using commercial software, DEFORM-2D. A hemisphere tipped post, used as an alignment structure in the assembly of micro devices, was modeled to demonstrate the flow behavior of the molten polymer in mold filling. Hot embossing experiments were performed to validate the feasibility of the numerical models. Most of the simulations showed agreement with experiments. The mold filling was estimated with the heights of the embossed posts in the analysis. No significant mold filling with the molten polymer was observed below the glass transition temperature of 105 °C. The mold cavity was completely filled with the polymer at the molding temperature of 137.5 °C and 150 °C while the embossing forces were 300, 600, and 900 N.

Replication of Reliable Assembly Features for Polymer Modular Microfluidic Systems

BioMEMS are compact devices which use microfabrication to miniaturize conventional benchtop instruments. The benefits of using micro devices are the need for less chemical reagents, faster processing, and portability. Realizing a powerful sample-to-answer micro system requires more than just microfabrication technology; thermal management, microfluidic control, and interfacing technology must also be considered. You et al. [1] applied kinematic constraint analysis using screw theory to design the assembly features for passive alignment structures for polymer, modular microfluidic devices. Three pairs of v-groove and hemispherical pin joints were chosen as kinematic pairs for mating two modules. Dimensional variation of the assembly features was one of principal contributions to the mismatch of 28 μm-75 μm between stacked plates in assembly that was reported. This was primarily a result of incomplete mold filling and deformation of the passive alignment structures during the demolding process. Hemisphere-tipped recesses, with additional annular walls as dummy structures, were used to designed to achieve better replication by improving polymer filling and reducing the deformation during demolding. In prototype devices, the posts with the annular dummy structures had a mean height of 922.2 μm – 924.1 μm while the original posts without dummy structures had mean heights of 865.3 μm – 891.2 μm in 10 samples under the same embossing conditions for a design height of 925 μm. Alignment accuracy of better than 10 μm was achieved in the assembly of two plates.Copyright

Optimization of Geometry for Continuous Flow PCR Devices in a Titer Plate-Based PCR Multi-Reactor Platform

A highly parallel, polymerase chain reaction (PCR) multireactor platform is in high demand to satisfy the high throughput requirements for exploiting the accumulated genetic information from the Human Genome Project. By incorporating continuous flow PCR (CFPCR) devices in a polymer 96-well titer plate format, DNA amplification can be performed with steady-state temperature control and faster reaction speed at lower cost. Prior to the realization of a PCR multi-reactor platform, consisting of a sample delivery chip, a PCR multireactor chip, and a thermal cycler, optimization of the geometry for CFPCR devices in a titer plate-based PCR multi-reactor chip based on manufacturing feasibility is necessary. A prototype PCR multi-reactor chip was designed in a 96-well titer plate format with twelve different CFPCR configurations. High quality metallic, large area mold inserts (LAMIs) were fabricated using an SU-8 based UV-LIGA technique by overplating nickel in SU-8 electroplating templates. Micro molding of polycarbonate (PC) was done using hot embossing, resulting in good replication fidelity over the large surface area. Thermal fusion bonding of the molded PC chips using a custom-made bonding jig yielded acceptable sealing results. The manufacturability investigation throughout the design and the process sequence suggested that the microchannel walls require a minimum width of at least 20 μm and an aspect ratio of 2 for structural rigidity. An optimal CFPCR device for use in a PCR multi-reactor chip can be selected with a series of amplification experiments with the development of a thermal cycler.© 2007 ASME

Assembly of Polymer Microfluidic Components and Modules: Validating Models of Passive Alignment Accuracy

Low-cost modular polymer microfluidic platforms integrating several different functional units may potentially reduce the cost of molecular and environmental analyses, and enable broader applications. Proper function of such systems depends on well-characterized assembly of the instruments. Passive alignment is one approach to obtaining such assemblies. Model modular devices containing passive alignment features, hemispherical pins in v-grooves, and integrated alignment standards for characterizing the accuracy of the assemblies were replicated in polycarbonate using doubled-sided injection molding. The dimensions and locations of the assembly features and alignment standards were measured. The assemblies had mismatches from 16 ± 4 to 20 ± 6 μm along the x-axis and from 103 ± 7 to 118 ± 11 μm along the y-axis. The vertical variation from the nominal value of 287 μm ranged from -10 ± 4 to 34 ± 7 μm. An assembly tolerance model was used to estimate the accuracy of the assemblies based on the manufacturing variations of the alignment structures. Variation of the alignment structure features were propagated through the assembly using Monte Carlo methods. The estimated distributions matched the measured experimental results well, with differences of 2%-13% due to unmodeled aspects of the variations Accurate assembly of advanced polymer microsystems is feasible and predictable in the design phase.