Modeling of large area hot embossing

Today, hot embossing and injection molding belong to the established plastic molding processes in microengineering. Based on experimental findings, a variety of microstructures have been replicated so far using the processes. However, with increasing requirements regarding the embossing surface and the simultaneous decrease of the structure size down into the nanorange, increasing know-how is needed to adapt hot embossing to industrial standards. To reach this objective, a German–Canadian cooperation project has been launched to study hot embossing theoretically by a process simulation and experimentally. The present publication shall report about the first results of the simulation—the modeling and simulation of large area replication based on an eight in. microstructured mold.     

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.     

Design and application of a hybrid alignment platform for a double-sided hot embossing process

Hot embossing is a novel technique for the cost-effective production of micro-parts. Misalignment between the top and bottom surfaces of a product that has double-sided microstructures reduces its functions. Most solutions involve repairing or reproducing the embossing mold, which increases the cost for the molding process. This study proposes a hybrid alignment platform that allows accurate positioning and angular alignment for the embossing processes. The hybrid alignment platform includes two units. The upper unit allows angular adjustment using a precision rotary table. The lower unit allows positional adjustment using two orthogonal micrometers. Calibration tests are conducted for a series of adjustment steps along the X, Y and C axis. Only one alignment step is necessary for each axis. The alignment platform allows rapid positioning and increases replicability. The hybrid mold adjustment platform is used to resolve a problem with gear misalignment and the problem of eccentricity in the production of micro-gears is effectively resolved.

     

Rapid polymer microchannel fabrication by hot roller embossing process

Using thermoplastic polymers as substrate material is an attractive approach to develop low-cost, disposable microfluidic devices. This study investigates a simple and rapid polymer replication method of fabricating microchannels by a hot roller embossing process. The hot roller embosser used in this study was modified from a commercially available film laminator, and the roller micromold was fabricated by spin coating an SU-8 layer on a flexible copper sheet. A straight microchannel measuring 5?cm long, 200?μm wide, and 41.4?μm deep was used to evaluate the imprinting performance on cyclic olefin copolymer and polyvinylchloride film. This study also investigates the effects of hot roller embossing temperature, rolling speed, and embossing pressure on the microchannel depth and geometry transfer efficiency.     

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.     

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.     

A roller embossing process for rapid fabrication of microlens arrays on glass substrates

This paper reports an innovative technique for rapid fabrication of polymeric microlens arrays based on UV roller embossing process. In this method, a thin flat mold is fabricated by electroforming of nickel against a microlens master. The thin Ni mold with microlens cavities is then wrapped onto cylinder to form the roller. During rolling operation, the roller pressing and dragging the UV-curable photopolymer layer on the glass substrate through the rolling zone, the microlens array is formed. At the same time, the microlens array is cured by the UV light radiation while traveling through the rolling zone. The technique can be developed to an effective roll-to-roll process at room temperature and with low pressure. In this study, a roller embossing facility with UV exposure capacity has been designed, constructed and tested. Under the proper processing conditions, the 100×100 arrays of polymeric microlens, with a diameter of 100 μm, a pitch of 200 μm and a sag height of 21 μm can be successfully fabricated.     

Gas pressurized hot embossing for transcription of micro-features

Hot embossing has proven productive for the parallel replication of precision micro-features onto thermoplastic substrates at low cost. During conventional hot embossing, the substrate and the stamp are brought into contact and are compressed directly by the hot plates of the machine. The accuracy and area of replication are limited due to the inherent non-uniform pressure distribution. Si-wafers are too brittle to be used as embossing tools with the conventional hot embossing operation. This paper describes an innovative pressurizing method for hot embossing using gas as pressure media. The film/stamper/substrate stack is placed in a closed chamber. After heating, the gas is blown in at high pressure to pressurize the stack. Micro patterns in the stamp can be successfully replicated onto the substrate. Perfectly uniform embossing pressure throughout the whole area can be achieved. Glass or wafers with micro-features on their surfaces can be used as stampers directly.The authors would like to thank National Taiwan University for help with lab set-up, Professor Ping-Hei Chen and Mr. Jian Ying Tsai for providing the embossing stamper, and their coworkers at Grace Laboratory for stimulating discussions and experimental assistance.     

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.     

A combinative technique to fabricate hot embossing master for PMMA tunneling sensors

A combinative approach of anisotropic bulk etching and modified plasma etching has been successfully employed in a single wafer to fabricate silicon masters for the hot embossing process. The masters hold both pyramid pits and positive profile sidewalls with smooth surfaces and steep angles. The SiO₂ layer is utilized as a etching mask with the aid of photoresist in three steps of photolithography patterning. The first polymethyl-methacrylate (PMMA)-based tunneling transducer with polymer membrane structures is fabricated by hot embossing replication with the silicon master. Consequently, the exponential relations between tunneling currents and applied deflection voltages are also reported.This work is partially supported by grants NSF/LEQSF (2001–04)-RII-02, DARPA DAAD19–02–1-0338, and NASA (2002)-Stennis-22.     

Large area micro hot embossing of Pyrex glass with GC mold machined by dicing

Micro/Nano imprinting or hot embossing is currently a target of interest for industrial production of micro and Nano devices for the low cost aspect. In Fluidic MEMS (Micro Electromechanical Systems) applications, polymer materials have been widely employed for their low cost to fabricate the economical products (Becker and Heim in Sens Acuators A 83:120–135, 2000; Becker and Gaertner in Mol Biotechnol 82:89–99, 2001). However glasses are much more suitable for the higher temperature applications or under the stronger chemical environments. Moreover UV absorption of glass materials is much less than that of polymers, which is the advantage for bio-analysis. In Optical MEMS as well, glasses are good candidate materials for the better optical properties, such as high refractive index, low UV absorption and others. Although wet etching of glasses is widely employed for fabrication of fluidic MEMS devices, the wet etching is not satisfactory for the low machining resolution, the isotropic etched profile and poor roughness of the fabricated structures. Dry etching of glasses is then an alternative for Micro/Nano structuring, but the etching rate is extremely low (order of 0.1 μm/min) and the cost is too high because of the expensive RIE (Reactive Ion Etching) facility. Above mentioned is the reason why we are interested in hot embossing or imprinting of glasses of Micro/Nano scale. In our previous study, Micro/Nano imprinting was developed for Pyrex glasses using GC (Glassy Carbon) mold prepared by FIB machining (Takahashi et al. in Symposium on DTIP 2004 pp 441–446, 2004). The disadvantage of FIB machining is limited area of etching. The typical area of FIB is less than several hundreds micrometer square. This is the reason why we tried the large area of embossing using GC mold fabricated by dicing machine. Micro hot embossed test structures were successfully demonstrated with good fidelity. Fabricated micro structures can be applied for fabrication of microchamber array for PCR (Akagi et al. in Sci Techol Adv Mater 5:343–349, 2004; Nagai et al. in Anal Chem 73:1043–1047, 2001).     

Development of fluid-based heating and pressing systems for micro hot embossing

This paper reports three innovative methods of rapid heating and uniform pressing for micro hot embossing. Fluids were used as heating and pressing media. With these three systems, the temperature of substrate rises rapidly and uniform pressure is exerted over the whole substrate. The working fluids used in this experiment included steam, gas, and oil. In addition, rapid heating through far infrared radiation (FIR) was also implemented with a gas pressurized hot embossing process. It was found that a 0.2 millimeter-thick PVC substrate can be heated from 25°C to 130°C in 30 seconds using steam heating, in only 25 seconds using FIR heating, and in 3.5 minutes using oil heating. The heating speeds of all three methods are much faster than those using conventional hot-plate heating, which takes more than 10 minutes. Successful replications of micro-features onto substrates have been achieved.The authors would like to thank National Taiwan University for help with lab set-up, the King May Chen Company for providing the embossing stamper, and their coworkers at Grace Laboratory for stimulating discussions and experimental assistance.     

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.     

Microfabrication by hot embossing and injection molding at LASTI

LIGA process includes three processes as X-ray lithography, electroforming to fabricate metalic molds and replication, and can be fabricated nano and micro parts for various devices that it is difficult to product by conventional machining methods. A key technology which gathers mass-production efficiency in the LIGA process is micro-replication technology. We choiced hot embossing and injection molding methods for replication. For a demonstration, two kinds of Ni molds, a mesh pattern within a line width of 100 m, and an aspect ratio of 1.0 and a mesh pattern within a line width of 40 m, and an aspect ratio of 2.5, were prepared. These were produced with X-ray lithography and nickel electrofoming technique. In hot embossing, an experiment of micro-replication using polymethyl methacrylate (PMMA) and polycarbonate (PC) sheets succeeded. At injection molding, it could not transfer well with PMMA and PC, but injection temperature was set up highly, and it succeeded by cycloolefin polymer. Furthermore, we measured sidewalls surface roughness of microstructures produced at each steppes of the LIGA process, and it checked that the LIGA process had processing accuracy higher than a conventional machining method.We would like to thank Ms. A. Kitajima and Dr. R. Maeda at National Institute of Advanced Industrial Science and Technology (AIST), Mr. M. Ohtomo at Ikegami Mold Engineering Co., Ltd., and Mr. Noriaki Sato at Juken Kogyo Co., Ltd. for their valuable collaboration and contributions. This research was the contract research from the New Industry Research Organization (NIRO) supported financially by the New Energy and Industrial Technology Development Organization (NEDO).     

Submicron polymer structures with X-ray lithography and hot embossing

This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiO _x membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λ _c ≈ 0.4 nm) at a dose of 4 kJ/cm³ using a Si₃N₄ membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.     

Induction heating of dual magnetic particles embedded PDMS molds for roller embossing applications

Microsystem Technologies - Roller embossing is an effective manufacturing process for replicating microstructures onto polymeric substrates, featuring high-volume production, rapid manufacturing,...     

Modeling and optimization of the hot embossing process for micro- and nanocomponent fabrication

Hot embossing, a polymer molding process conceived by Forschungszentrum Karlsruhe, is one of the established replication processes for microstructures The process is especially well suited for manufacturing small and medium series of microcomponents (SPIE Conference 1997; Polymer News 25:224–229, 2000; J Micromech Microeng 14:R1–14, 2004; Sensors Actuators 3:130–135, 2000). However, a wider application of the process currently is seriously hampered by the lack of adequate simulation tools for process optimization and part design. This situation is becoming more critical, as the dimension of the microstructures shrink from micron and submicron levels to the nanoscale and as productivity requirements dictate the enlargement of formats to process larger numbers of devices in parallel. Based on the current scientific work (Forschungszentrum Karlsruhe, FZKA-Bericht 7058 2003; DTIP Conference Montreux 2004; Microsystem Tech 10:432–437 2004), a German–Canadian cooperation has been started. The objective of this cooperation is to fill the gap mentioned above by developing reliable computer models and simulation tools for the hot embossing process and to incorporate these models in a user-friendly computer code. The present paper will give an overview of the activities in the project. The activities related to material characterization, especially the development of a viscoelastic material model, the characterization of friction between polymer and mold during demolding, the development of an 8-in. microstructured mold, and the fabrication of nanostructured molds will be discussed.

     

Modeling and optimization of the hot embossing process for micro- and nanocomponent fabrication

Hot embossing, a polymer molding process conceived by Forschungszentrum Karlsruhe, is one of the established replication processes for microstructures The process is especially well suited for manufacturing small and medium series of microcomponents (SPIE Conference 1997; Polymer News 25:224–229, 2000; J Micromech Microeng 14:R1–14, 2004; Sensors Actuators 3:130–135, 2000). However, a wider application of the process currently is seriously hampered by the lack of adequate simulation tools for process optimization and part design. This situation is becoming more critical, as the dimension of the microstructures shrink from micron and submicron levels to the nanoscale and as productivity requirements dictate the enlargement of formats to process larger numbers of devices in parallel. Based on the current scientific work (Forschungszentrum Karlsruhe, FZKA-Bericht 7058 2003; DTIP Conference Montreux 2004; Microsystem Tech 10:432–437 2004), a German–Canadian cooperation has been started. The objective of this cooperation is to fill the gap mentioned above by developing reliable computer models and simulation tools for the hot embossing process and to incorporate these models in a user-friendly computer code. The present paper will give an overview of the activities in the project. The activities related to material characterization, especially the development of a viscoelastic material model, the characterization of friction between polymer and mold during demolding, the development of an 8-in. microstructured mold, and the fabrication of nanostructured molds will be discussed.     

Fabrication of microneedle array using LIGA and hot embossing process

We demonstrate a novel fabrication technology of the microneedle array applied to painless drug delivery and minimal invasive blood extraction. The fabrication technology consists of a vertical deep X-ray exposure and a successive inclined deep X-ray exposure with a deep X-ray mask whose pattern has a hollow triangular array. The vertical exposure makes triangular column array with a needle conduit. With the successive inclined exposure, the column array shapes into the microneedle array without deep X-ray mask alignment. Changing the inclined angle and the gap between the mask and PMMA (PolyMethylMetaAcrylic) substrate, different types of microneedle array are fabricated in 750–1000 m shafts length, 15^o–20^o tapered tips angle, and 190–300 m bases area. The masks are designed to 400–600 m triangles length, 70–100 m conduits diameter, 25–60EA/5 mm² arrays density, and various tip shapes such as triangular, rounded, or arrow-like features. In the medical application, the fabricated PMMA microneedle array fulfills the structural requirements such as three-dimensional sharp tapered tip, HAR (High-Aspect-Ratio) shafts, small invasive surface area, and out-of-plane structure. In the skin test, the microneedle array penetrates back of the hand skin with minimum pain and without tip break and blood is drawn after puncturing the skin. Hot embossing process and mold fabrication process are also investigated with silicon and PDMS mold. The processed tetrahedral PMMA structures are fabricated into the microneedle array by the additional deep X-ray exposure. With these processes, the microneedle array can be utilized as the mold base for electroplating process.The author thanks the staff in 9 C LIGA beamline, Pohang Light Source (PLS), Korea for their assistance on the fabrication process.