Rapid hot embossing of polymer microfeatures

Although the hot embossing process is gaining popularity in molding polymer micro/nano structures, it needs a breakthrough improvement in reducing cycle time before it can become a mass-production process. In this study, an embossing tool with a rapid heating and cooling capability was investigated to reduce the cycle time. During the process, the embossing tool was rapidly heated to above the polymer softening temperature (in less than a couple of seconds), pressed against a room-temperature polymer substrate, and subsequently rapidly cooled for mold separation. Different miniaturized features including microsquare and hexagonal wells, microcircular holes, and submicron surface features were successfully produced using the new process with a total embossing cycle time around 20 s. The fatigue test result indicated that the new embossing technology is durable and reliable for microscale feature replication.     

Nickel stamp fabrication and hot embossing for mass-production of micro/nano combined structures using anodic aluminum oxide

Recently, “micro/nano combined structure” has attracted many researchers’ attentions due to its high potential in various research fields and applications such as biomimetics, tissue engineering, micro systems for biochemical analysis and so forth. The present paper proposes a simple and promising method for mass-production of the micro/nano combined structure, in particular, nano dimple array with micro structures with cost-effective procedures. Three major procedures of (a) master template fabrication; (b) nickel electroforming onto the master template; (c) replication by hot embossing process, are employed: the master template is fabricated by utilizing an anodic aluminium oxide (AAO) process and UV lithography technique; nickel stamp is then obtained by means of electroforming onto the master template; finally, micro/nano combined structures are moulded on a polymethyl methacrylate (PMMA) substrate using the nickel stamp via hot embossing. So replicated micro/nano combined structures turns out to be quite successful according to experimental observation via scanning electron microscope (SEM) and atomic force microscope (AFM).     

UV nano embossing for polymer nano structures with non-transparent mold insert

In recent years, UV nano embossing (or imprinting) process has been widely used for mass replication of nano structures. Most of the UV embossing machines use a UV transparent mold insert (e.g. quartz or glass). However, when a master of nano structures of interest could be realized only in a UV non-transparent mold material, it would be desirable to have a UV embossing machine which could be operated with such a UV non-transparent mold insert. In this regard, we have designed and manufactured a new UV embossing system in such a way that UV non-transparent (e.g., metal or ceramic) mold inserts can also be used as the master for the mass replication of nano structures. For the new UV embossing system, we fabricate several metal mold inserts: a nickel electroformed mold insert having grating nano structures and two AAO (anodic aluminum oxide) mold inserts having dimple nano structures and high-aspect-ratio nanopores. Finally, corresponding nano structures are successfully replicated via the UV nano embossing machine developed in this study.     

Resin micromachining by roller hot embossing

The roller hot embossing is an efficient process of manufacture in which patterns are continuously transcribed on film, etc. Recently, the application of the embossing roll to the manufacturing processes of micro parts is paid attention. In this paper, we examined the development of the embossing roll with patterns of micron level and we tried to make the embossing roll mold by using the LIGA process. In this study, instead of producing embossing patterns directly on the roll surface, we fabricated a flexible thin mold with micro-patterns, which was then wrapped onto a cylinder to form an embossing roll, and tested the soft-mold roller hot embossing method. First, by optimizing UV exposure conditions of UV lithography, we prepared a resist pattern of numerous dots with a diameter of 10 μm, a sag height of 8 μm and a pitch of 20 μm. By Ni-electroforming this pattern, a 50 μm-thick thin mold was successfully fabricated. The 50 μm-thick mold was then wrapped onto a cylinder to form an embossing roll. In the roller hot embossing process, the 10 μm-diameter dot shape was successfully replicated on PET sheets.     

Replication of high-aspect-ratio nanopillar array for biomimetic gecko foot-hair prototype by UV nano embossing with anodic aluminum oxide mold

In this paper, we present a simple and cost-effective replication method of high-aspect-ratio polymer nanopillar array as a biomimetic gecko’s foot hair prototype. A UV nano embossing process was applied for the replication of polymer nanopillar arrays. Highly ordered straight nanoporous AAO (anodic aluminum oxide) templates were utilized as reusable master molds. Densely arranged high-aspect-ratio nanopillar arrays have been successfully fabricated by means of the UV nano embossing process with the AAO mold. Pull-off force measurements were carried out to characterize the adhesive force of the replicated nanopillar arrays on the polymer substrates based on the force–distance curves obtained from the atomic force microscope (AFM) with a modified AFM cantilever. The force measurement results showed that the larger diameter and the taller height of the nanopillars result in the larger adhesive force.     

A micro roller embossing process for structuring large-area substrates of laminated ceramic green tapes

This paper presents a micro roller embossing process for patterning large-area substrates of laminated green ceramic tapes. The aim of this research is to develop a large-area microstructure formation technique for green ceramic substrates using a thermal roller laminator, which is compatible with screen printing apparatus. A thin film nickel mold was developed via photolithographic patterning and nickel electroplating on a 75-μm-thick nickel film. The mold had an effective panel size of 150 mm × 150 mm with the height of plated protrusive patterns being about 38 μm. Formation of micro patterns was successfully demonstrated over the whole panel area on laminated green ceramic tapes using roller embossing. Micro patterns for inductors, heaters as well as interconnection with 50 μm line-width were embossed on green ceramic substrates. By means of tuning process parameters including roller temperature, applied pressure and feeding speed, we have demonstrated that micro roller embossing is a promising method for patterning large-area green ceramic substrates.     

Studies of polymer deformation and recovery in micro hot embossing

In large areas of micro hot embossing, process temperature plays a critical role to both the local-area fidelity and global uniformity of microstructure formation. Higher embossing temperature could improve structure fidelity, however, at the expense of demoulding easiness. Micro embossing at the lowest possible temperature with acceptable fidelity can improve global flatness after demoulding. This study focuses on polymer deformation and recovery in micro embossing when the process temperature is below the polymer glass transition temperature (Tg). PMMA (Polymethyl Methacrylate) substrates (Tg = 105°C) were employed with the process temperature ranging from 25°C to its Tg. At temperature below Tg −55°C, significant recovery occurred after processing, but permanent structures could still be formed with sufficiently high applied stress. With an increase in temperature, plastic deformation increased and was the dominant polymer deformation mode for permanent cavities formation. However, the formation of protrusive structures was not complete since there was little polymer flow. The polymer will lose its storage modulus at an even higher temperature and microstructures could be formed with high fidelity. A compromise between local fidelity of embossed patterns and global flatness of substrate has to be reached in micro hot embossing.     

Fabrication of various nano-structured nickel stamps using anodic aluminum oxide

This paper presents a fabrication method of various nickel stamps based on anodic aluminum oxide (AAO) and nickel electroforming (NE) processes. By AAO process, master templates which have closely-packed nano dimple or pore structure array are fabricated. Then nickel stamps having negative surface topology of the master templates are fabricated by NE process. Also positive nickel stamps are fabricated from the negative stamps by NE process with the help of nickel passivation technique. So achieved nickel stamps are employed in injection molding and hot embossing processes for replication of nano-structured surfaces.     

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.     

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.     

Study of Hot Embossing Using Nickel and Ni–PTFE LIGA Mold Inserts

Hot embossing is one of the main process techniques for polymer microfabrication, which helps X-ray lithography, electroplating, and molding (LIGA) to achieve low-cost mass production. Most problems in polymer micromolding are caused by demolding, especially for hot embossing of high-aspect-ratio microstructures. The demolding forces are related to the sidewall roughness of the mold insert, the interfacial adhesion, and the thermal shrinkage stress between the mold insert and the polymer. The incorporation of polytetrafluoroethylene (PTFE) particles into a nickel matrix can have the properties such as antiadhesiveness, low friction, good wear, etc. To minimize the demolding forces and to obtain high-quality polymer replicas, a Ni-PTFE composite microelectroforming has been developed, and the hot embossing process using Ni and Ni-PTFE LIGA mold inserts has been well studied in this paper. The morphologies, sidewall roughness, and friction coefficient have been explored in the fabricated Ni-PTFE LIGA mold insert. Finally, the comparison of embossed microstructures with various aspect ratios and the comparison of the embossing lifetimes of mold inserts have been carried out between Ni and Ni-PTFE mold inserts, which show a better performance of the Ni-PTFE mold and its potential applications.     

Method for polymer hot embossing process development

Molding technologies associated with fabricating macro scale polymer components such as injection molding and hot embossing have been adapted with considerable success for fabrication of polymer microparts. While the basic principles of the process remain the same, the precision with which the processing parameters need to be controlled especially in the case of molding high aspect ratio (HAR) polymer microparts into polymer sheets is much greater than in the case of macro scale parts. It is seen that the bulk effects of the mold insert fixture and molding machine have a dominant influence on the molding parameters and that differences in material parameters such as the glass transition temperature (T _g) of polymer sheets are critical for the success and typically differ from sheet to sheet. This makes it very challenging to establish standard processing parameters for hot embossing of sheet polymers. In the course of this paper, a methodology for developing a hot embossing process for HAR microstructures based on known material properties and considering the cumulative behavior of mold, material, and machine will be presented. Using this method force–temperature–deflection curves were measured with the intent of fine tuning the hot embossing process. Tests were carried out for different materials using a dummy mold insert yielding information that could be directly transferred to the actual mold insert with minimum development time and no risk of damage to the actual microstructures.     

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.     

Multilayer patterning technique for micro- and nanofluidic chip fabrication

Polymer micro- and nanofluidic chips become increasingly significant for medical and biological applications. However, it is difficult to fabricate micro- and nanochannels integrately into a polymer substrate due to the reflow and insufficient flow of the polymer. In the present paper, micro- and nanochannels were hot embossed into a multilayer substrate by micromold and nanomold, respectively. To replicate high replication precision nanochannels without damaging the fabricated microchannels, the embossing parameters were optimized by Taguchi and analytic hierarchy process methods. The fabricated micro- and nanochannels were fully sealed at bonding parameters optimized according to the bonding rate of the chip. The fluorescence image indicates that there is no blocking or leakage over the entire micro- and nanochannels. With presented fabrication method, low-cost polymer micro- and nanostructures can be fabricated, which allows for commercial manufacturing of micro- and nanofluidic chips.     

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.

     

The edge-effect on roll-to-roll thermal embossing of micro channels

Roll-to-roll thermal embossing is a rising trend for large area processing of micro structures. However, pile-ups are formed when micro channel patterns are fed perpendicularly to the feeding direction of the embossing. Thus, there is a need to study the pile-up formation in order to reduce it for quality embossed patterns. In this work, polymethyl methacrylate and polypropylene films are used as the polymer substrates and the pile-up formation at various embossing temperature points or under various embossing pressures in the roll-to-roll embossing process is investigated. It is discovered that for polypropylene, there is a transition temperature range. Before this range, the lead pile-up is higher than the lag pile-up, while after this temperature range, the lag pile-up is higher than the lead pile-up. For polymethyl methacrylate, the lead pile-up is higher than the lag pile-up. By using a high embossing temperature and a high embossing pressure, it is able to reduce the pile-up formation and achieve good embossed patterns.     

Rubber-assisted micro forming of polymer thin films

Polymer thin films patterned with microstructures at a characteristic size greater than the film thickness are difficult to fabricate using the standard hot embossing technology. This study investigated a rubber-assisted embossing process for structuring polymer thin films. The main advantages of a rubber support, instead of a hard counter-tool, include simplification of the embossing tool, protection of the embossing master, buildup of uniform embossing pressure, and ease of demolding. The testing pattern for rubber-assisted embossing was a microgroove pattern with a characteristic size of 100 μm on a 25-μm thick polystyrene film. Results showed that the uniformity and replicability of the embossed pattern were significantly affected by the embossing temperature, the rubber hardness, and the embossing pressure. With an embossing temperature about 20°C above the glass transition temperature and appropriate rubber hardness and embossing force, uniform microgroove patterns were successfully replicated.     

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.     

New aspects of simulation in hot embossing

Hot embossing is especially well suited for manufacturing small and medium-volume series. However, wider diffusion of this process currently is seriously hampered by the lack of adequate simulation tools for process optimization and part design. This lack of simulation tools is becoming critical, as the dimensions of the microstructures continuously shrink from micron and sub-micron to nano scales and as productivity requirements dictate the enlargement of formats to process larger numbers of devices in parallel. Having no macroscopic equivalent, the micro hot embossing process cannot be described by simple downscaling of existing software tools like in injection molding. In this paper a first survey is given of how numerical simulation can also be applied to the hot embossing process.     

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.