Analysis of the demolding forces during hot embossing

Hot embossing is one of the main processing techniques for polymer microfabrication, which helps the LIGA (UV-LIGA) technology to achieve low cost mass production. When hot embossing of high aspect ratio microstructures, the deformation of microstructures usually occurs due to the demolding forces between the sidewall of mold inserts and the thermoplastic (PMMA). The study of the demolding process plays a key role in commercial manufacturing of polymer replicas. In this paper, the demolding behavior was analyzed by Finite element method using ABAQUS/Standard. Simulation identified the friction force caused by interface adhesion and thermal stress due to shrinkage between the mold and the polymer as the main sources of the demolding forces. Simulation also showed that the friction force made a greater contribution to the deformation than thermal stress, which is explained in the accompanying theoretical analysis. To minimize the friction force the optimized experiment was performed using PTFE (Teflon) as anti-adhesive films and using Ni-PTFE compound material mold inserts. Both lowered the surface adhesion energy and friction coefficient. Typical defects like pull-up and damaged edges can be reduced.     

Hot roller embossing of multi-dimensional microstructures using elastomeric molds

Microsystem Technologies - In this work, we investigated how elastomeric mold properties could affect the final replication accuracy in hot roller embossing. Amorphous polyethylene terephthalate...     

Hot embossing - The molding technique for plastic microstructures

 Hot embossing is the technique to fabricate high precision and high quality plastic microstructures. Industrial fabrication of plastics components is normally achieved by injection molding. Hot embossing is actually used only for a few optical applications where high precision and high quality are important. The advantages of hot embossing are low material flow, avoiding internal stress which induces e.g. scattering centers infavorable for optical applications, and low flow rates, so more delicate structures can be fabricated, such as free standing thin columns or narrow oblong walls. The development of modular molding equipment, orientated on industrial standards has opened the door to the fabrication of plastic microcomponents in great numbers (for example LIGA-UV/VIS-spectrometers). Hot embossing has the potential of increasing production rates and therefore decreasing production costs by the enlargement of the molding surface and automatization of the molding process. Received: 25 August 1997/Accepted: 22 September 1997     

Roll-to-roll hot embossing of microstructures

In this paper we present a new roll-to-roll embossing process allowing the replication of micro patterns with feature sizes down to 0.5 μm. The embossing process can be run in ‘continuous mode’ as well as in ‘discontinuous mode’. Continuous hot embossing is suitable for the continuous output of micro patterned structures. Discontinuous hot embossing has the advantage that it is not accompanied by waste produced during the initial hot embossing phase. This is because in ‘discontinuous mode’, embossing does not start before the foil has reached the target temperature. The foil rests between two parallel heating plates and foil movement and embossing starts only after the part of the foil resting between the heating plates has reached a thermal steady state. A new type of embossing master is used which is based on flexible silicon substrates. The embossing pattern with sub-μm topographic resolution is prepared on silicon wafers by state of the art lithography and dry etching techniques. The wafers are thinned down to a thickness of 40 μm, which guarantees the mechanical flexibility of the embossing masters. Up to 20 individual chips with a size of 20 × 20 mm2 were assembled on a roller. Embossing experiments with COC foils showed a good replication of the silicon master structures in the foil. The maximum depth of the embossed holes was about 70% of the master height.     

Numerical analyses of peel demolding for UV embossing of high aspect ratio micro-patterning

Ultraviolet (UV) embossing, involving molding against micro-structured molds, is a quick and efficient method to mass produce high aspect ratio micro-features. A crucial challenge to the repeatability and large-scale application of this technique is successful demolding, which escalates in difficulty with increasing aspect ratio, due to increased polymer-mold mechanical interlocking. Some of the key factors affecting UV embossing include the crosslinked polymer shrinkage and material properties, interfacial strength between polymer to mold and the demolding method. This paper presents a new method to simulate the demolding of UV cured polymer from a nickel mold. Hyperelastic material model and rate-independent cohesive zone modeling were employed in the numerical simulation; linear elastic polymer response, although relatively easy to apply, was not adequate. Progressive shrinkage was implemented, leading to delamination of the polymer-mold interface. The subsequent peeling of the cured polymer from the mold was modeled with increasing prescribed displacement. The optimal shrinkage degree was found to increase from 0.92 to 1.9% with increasing mold aspect ratio (aspect ratio is defined as height to width ratio) from 5 to 10.     

Hot embossing of high performance polymers

Hot embossing and injection moulding belong to the established plastic moulding processes in microengineering. Based on experimental and theoretical findings, a variety of microstructures have been replicated using these processes. However, beside the technology also the moulding materials their physical and chemical properties determines the process and their parameters. Especially high temperature semicrystalline polymers like PEEK and LCP are well suited for many applications in microfluidics. The moulding of these polymers by hot embossing requires a precise setting of temperatures for successful moulding. The present publication describes the moulding of high performance polymers by hot embossing and thermoforming. The focus is set to the process parameters and the required heating conditions for the replication of micro- and nanostructured mould inserts. In detail, the process conditions for hot embossing of LCP, PEEK, FEP and PSU will be discussed.     

Hot embossing of thermoplastic multilayered stacks

The combination of different polymer materials during replication offers additional opportunities for fabrication and functionality of microsystems. Different surface and structural properties of polymers allow for improvements in microsystems for example by means of hydrophilic and hydrophobic combinations in microfluidic devices. Due to its high flexibility and precision hot embossing as one of the established micro replication processes facilitates processing of several polymer layers in one single process step. By this multi-component process micro structured systems consisting of thin layers of different polymers with adapted surface properties are fabricated. In this paper we describe the challenge of molding different types of polymers and some applications for multi-component micro systems.     

Ultraviolet embossing for patterning high aspect ratio polymeric microstructures

In ultraviolet (UV) embossing, a substrate with a coating of liquid or semi-solid UV curable resin mix is pressed against a patterned embossing mold. The resin mix is irradiated with UV before demolding of the hardened microstructures. UV embossing can be done at room temperature and low pressure. However, demolding of UV embossed high aspect ratio microstructures from a metallic mold is typically difficult since there is no differential thermal contraction between the mold and the embossing. Several factors have been identified to influence demolding of UV embossed microstructures: (1) Roughness of mold, (2) Taper angle of microstructures of mold, (3) Chemical interaction between mold and embossing, (4) Tensile and crosslinking shrinkage properties of the irradiated resin and (5) Uniformity of crosslinking process through the thickness of the molded microstructures. By controlling these five parameters, a microarray with an aspect ratio of 5 was demonstrated using a Formulation containing epoxy acrylate, Irgacure® 651, silicone acrylate and other acrylates. The embossed microstructures replicated the features of the mold very well. It was also shown that by controlling the amount of irradiation, the tensile modulus of the UV formulation increased whilst the elongation decreased. An optimum irradiation is needed for clean demolding from the mold without microcracking.This research was supported by a Strategic Development Scheme fund (SDS 15/2001) from the Nanyang Technological University. The authors also acknowledge the kind contributions of chemicals by UCB Chemicals, Sartomer, Henkel (Singapore), Dupont (Singapore) and Ciba Chemicals and a microstructured mold by Dr R. C. Liang of SiPix Imaging (CA, USA). The second author acknowledges the financial support of Nanyang Technological University through a Research Scholarship.     

Hot roller embossing for microfluidics: process and challenges

We report an initial study on hot roller embossing as a potential process for the mass production of polymer based microfluidic chips. Measurements conducted on 100 μm features showed that the lateral dimensions could be replicated to within 2% tolerance, while over 85% of mould depth was embossed. Feature sizes down to 50 μm and feature depths up to 30 μm had been achieved. Results revealed that the embossing depth increased with an increase in the nip force or a decrease in the rolling speed. There was an optimum temperature for achieving a high embossing depth; this was due to the reflow effect seen at higher temperatures. One observation included an asymmetric pile up of polymer material outside the embossed regions as a result of the orientation of the microchannel with respect to the rolling direction. This directional effect could be due to the dynamics of the roller setup configuration.     

SOI wafer mold with high-aspect-ratio microstructures for hot embossing process

This paper reports using a Silicon oil insulator (SOI) wafer as a mold insert for the hot embossing process on high-aspect-ratio microstructures to overcome two drawbacks of Inductive Coupled Etching (ICP) process, the area dependent etching and the micrograss. A thin sacrificial wall to eliminate the undercut in the big open area during ICP etching is also described. A good result of final embossed structure on PMMA with aspect ratio of 12 : 1, uniform thickness, and smooth surface is presented.This work is partially supported by grants NSF/LEQSF (2001-04)-RII-02, DARPA DAAD19-02-1-0338, and NASA (2002)-Stennis-22.     

Hot embossing of polymer nanochannels using PMMA moulds

A novel hot embossing method is developed to fabricate polymer nanochannels. The pattern on the silicon nanomould is transferred to polymethylmethacrylate (PMMA) plates, and then polyethylene terephthalate (PET) nanochannels are embossed by using the PMMA mould. The use of the PMMA intermediate mould can extremely increase the device yield of the expensive silicon nanomould. To avoid the use of nanolithography, a method based on UV-lithography techniques for fabricating silicon nanomoulds with sub-micrometer width was put forward. 1 PMMA mould can be used to repeatedly emboss at least 30 PET substrates without damage and obvious deformation. Good pattern fidelity of PET nanochannels was obtained at the optimized embossing temperature of 90 °C. For an 808 nm-wide and 195 nm-deep nanochannel, the variations in width and depth between PET nanochannels and PMMA moulds were 1.8 and 2.5 %, respectively. The reproducibility was also evaluated, and the relative standard deviations in width and depth of 5 PET nanochannels were 5.1 and 7.3 %, respectively.     

Hot embossing of transparent high aspect ratio micro parts

Accurate replications of complex, high aspect ratio nano and micro structured parts are still challenging due to the comparatively high surface-volume ratio. The critical process step of automated, undistorted demoulding in high precision replication techniques like hot embossing or thermal nano imprint require good process control at elevated temperatures just below the solidification of the thermoplastic material and higher adhesive forces of the polymer part to the substrate plate than to the structured tool insert. The required increase in interfacial surface to the substrate plate is typically done by a rough substrate plate which results in milky, in-transparent residual layer. We demonstrate a process modification which keeps the advantage of precise automated demoulding and allows for replication of micro structured parts with optically transparent residual layer even for high aspect ratio structures resulting in high demoulding forces.     

Reactive ion etching for the production of metal microstructures by hot embossing

A reactive ion etching process which has been developed in order to remove the residual polymer layer, that remains at the bottom of microstructures made by hot embossing, is described. The influence of parameters and structures is explained. Received: 4 December 1998/Accepted: 20 January 1999     

Replication of nanostructures on microstructures by intermediate film mold inserted hot embossing process

The present study proposes a simple method to replicate nano/micro combined multiscale structures using an intermediate film mold and anodic aluminum oxide (AAO) nanomold in hot embossing process. The proposed method is simply to add an intermediate film mold with microscale thru-hole patterns to the ordinary mold system, on which nanostructures are patterned, in the hot embossing process. The intermediate film mold is inserted between polymer substrate and AAO nanomold. During the hot embossing process, the polymer first fills microscale thru-hole patterns in the intermediate film mold and subsequently fills nanopores in AAO nanomold, resulting in the nano/micro combined structures. The intermediate film molds, which have microscale thru-hole patterns were fabricated by micro-milling, laser ablation, etching methods and/or LIGA process. The nano/micro combined structures were successfully replicated by the proposed method.     

Hot embossing of microstructure with moving induction heating and gas-assisted pressuring

A new apparatus for a moving induction heating and gas-assisted hot embossing apparatus has been developed. A mechanism was designed and implemented to move the platform in and out the wrapped coil, on which the sealed box for substrate/mold was placed. A chamber of 195 mm diameter and 221 mm length was machined. The movable platform, the sealed box with substrate/mold stack, wrapped coil and cooling fan were all implemented in the high pressure chamber. The nine-point thermocouples attached on the mold, thus, a temperature history of the moving induction heating can be obtained and study the influence of the moving path and power on the heating rate and temperature distribution. The micro V-cut structure hot embossing experiment were performed to prove the potential of this moving induction heating and gas-assisted pressuring hot embossing for fast fabrication of microstructure onto polymeric substrates. As a results, replication rates were all above 95% at 200 °C and 5 kgf/cm² and the cycle time was less than 4 min and the optic measurement shows the replicated V-cut film can enhance the 36.8% illuminance. The experiment results show the manufacturing potential of this apparatus.

     

Low cost method for hot embossing of microstructures on PMMA by SU-8 masters

Polymer based microfabrication technologies are used extremely in Bio-MEMS, especially in Microfluidic devices in recent years. In this paper, a novel method for fabrication of microstructures on a polymeric material using hot embossing lithography process is presented. The proposed method involves usage of low cost materials and procedure with respect to previous methods and can be processed in a short time. The master is made from SU-8 on an inexpensive glass substrate which is patterned by standard lithography. The embossing pressure can be increased in our master as the glass substrate used in this paper is more robust than Silicon. Master robustness and SU-8 to glass adhesion is optimized by some substrate pretreatments and SU-8 baking time and temperatures. Microchannels are replicated on a Polymethylmetacrylate (PMMA) stamp which is a plexiglass sheet with thickness of 1 mm. Significant embossing parameters including temperature, pressure and time are discussed and optimum values are determined. Microchannels are imprinted by depth of 50 μm and minimum width of 15 μm and aspect ratio more than 3. The microchannels are sealed by a PMMA cap using thermal annealing bonding.     

Hot embossing of micro and sub-micro structured inserts for polymer replication

Today replication of microstructured parts is state of the art in laboratory and commercial use. Beside the process of injection molding hot embossing enables the accurate replication of polymer structures in a broad variety of thermoplastic polymers even in the nanometer range. Characteristic for the most replication processes dealing with thermoplastic polymers is the use of microstructured mold inserts based on metals. In this paper we describe an alternative to the established mold inserts––the use of so called interstage mold inserts. These interstage mold inserts are replicated in high performance polymers and technical thermoplastics and can be fabricated many times by a previous replication step from a master even in the sub-micro range. Aspects like suitable material combinations, demolding behaviour, long time stability, production rate, and the quality of structures will be discussed. Because of the high flexibility the process of hot embossing is used for the fabrication of the microstructured interstage mold inserts and their replications.     

Hot embossing at viscous state to enhance filling process for complex polymer structures

In this paper, a new hot embossing process, molding at the viscous state, for fabrication of complex polymer structures at the micro and millimeter scale is presented. Polymer deformability is enhanced due to its low viscosity and is increased by an inner pressure from confinement of the polymer flow. Various millimeter-scale polymer structures with high aspect ratios and complex features were hot embossed. In addition, typical microstructures were achieved. This new approach promises the advantages of a broad process capability and strong compatibility with conventional hot embossing processes.     

A photopatternable superparamagnetic nanocomposite: Material characterization and fabrication of microstructures

A superparamagnetic nanocomposite obtained by dispersing superparamagnetic magnetite nanoparticles in the epoxy SU-8 is used to fabricate microstructures by photolithography. The dispersion of the nanoparticles and the level of agglomerations are analyzed by optical microscopy, TEM (transmission electron microscope), SAXS (small-angle X-ray scattering), XDC (X-ray disc centrifuge) and XRD (X-ray diffraction). Two different phosphate-based dispersing agents are compared. In order to obtain a high-quality nanocomposite, the influence of particle concentration 1-10 vol.% (4-32 wt.%) on composite fabrication steps such as spin coating and UV exposure are systematically analyzed. Features with narrow widths (down to 1.3 μm) are obtained for composites with 5 vol.% particle concentration. Mechanical, magnetic and wetting properties of the nanocomposites are characterized. These nanocomposites exhibit superparamagnetic properties with a saturation magnetization up to 27.9 kA m⁻¹ for10 vol.%. All nanocomposites show no differences in surface polarity with respect to pure SU-8, and exhibit a moderate hydrophobic behavior (advancing dynamic contact angles approximately 81°). Microcantilevers with particle concentrations of 0-5 vol.% were successfully fabricated and were used to determine the dynamic Young's modulus of the composite. A slight increase of the Young's modulus with increased particle concentration from 4.1 GPa (pure SU-8) up to 5.1 GPa (for 5 vol.%) was observed.