Nickel Mold Research Articles

Fatigue of Electroformed Nickel

A 2–3 mm nickel mould, manufactured solely by electroforming, showed early cracking under thermal load cycles. Possible causes were attributed to material, design, or service deficiencies. The root cause of early fracture has been investigated thoroughly and is reported here. Monotonic and cyclic properties of electroformed nickel have been obtained at room and elevated temperature. Fatigue mechanisms were identified and compared to pure nickel. Finite element modeling of the mould under thermal cycles enabled the study of material, design, and in-service applications of the mould. It was found that over constraining the mould, while it was in service, caused excessive thermal stresses which accelerated crack initiation and propagation. Remedies to avoid failure are proposed.

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.

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.

Design of Ni Mold for Discrete Track Media

Nano imprint lithography (NIL) and NIL mold formation play a vital role in discrete track media (DTM) production. In this paper, we created a nickel (Ni) mold for DTM production using an R-theta electron beam mastering device to draw data groove and servo information on a silicon substrate master, and transferred the resulting pattern to a nickel mold via an electroplating process. The resulting Ni mold exhibits a 90-nm track pitch (282 kTPI equivalent), 30-nm line width, 60-nm groove width, 60-nm land height, and 7-nm line width roughness (LWR). Since variations in downtrack LWR and line edge roughness (LER) on the Ni mold are much smaller than variations in the size of HDD media magnetic clusters, we believe the Ni mold production method that is the subject of this study can be successfully applied to the production of DTM molds for next-generation HDD media.

초음파진동에너지를 이용한 고분자 마이크로구조물의 성형

A new polymer replication technology using ultrasonic vibration is proposed and demonstrated. A commercial ultrasonic welder has been used in this experiment. Two different types of nickel molds have been fabricated: pillar type and pore type microstructures. Polymethyl methacrlylate (PMMA) has been used as the replication material and the optimal molding time was 2 sec and 2.5 sec for pillar-type and pore-type micromolds, respectively. Compared with the conventional polymer micromolding techniques, the proposed ultrasonic micromolding technique has the shortest processing time. In addition, only contact area between micromold and polymer substrate is melted so that the thermal shrinkage can be minimized. The fabricated PMMA microstructures have been very accurately replicated without vacuum. The proposed ultrasonic molding technique is a good alternative for high volume production.

An efficient cell count method using a lattice molded on indents of a culture dish

Cell count is an important task for obtaining biological and medical information. In this paper, a novel cell count method is presented for improving the efficiency of the procedure as well as reducing microbial contamination compared to the conventional cell count method using a hemocytometer. The proposed method involves a lattice array consisting of a 50 μm × 50 μm square with lines of 2-μm width and 1.4-μm depth on a surface indented from a culture dish bottom. This configuration enables observation of cells at the same focus as the lattice. Therefore, an instant cell count during incubation is possible without the tedious, error-prone preparation steps such as harvesting and loading required in conventional methods. In addition, cells can be preserved with minimal contact with the external environment. These advantages become magnified with a periodic long-term cell count. A polystyrene culture dish, 35 mm in diameter, was fabricated by injection molding using a nickel mold, wherein indents of 3 mm × 3 mm in area and 1 mm in height were electroplated based on microfabrication technology. For easy separation from the nickel mold, the four sides of the indents in the mold are inclined at 54.74° via anisotropic silicon etching. The usefulness of the suggested method was verified using adhering HeLa (cervical carcinoma cell) cells and floating Jurkat cells. Both were placed in culture dishes and cultivated for 3 days in a carbon dioxide incubator (5% CO 2, 95% air) at 37 °C, and then successfully observed through the divided lattice using an inverted microscope. The dish was also assessed with a hemocytometer by counting HEK 293T (human embryonic kidney) cells and yeast cells.

Experimental characterization of transcription properties of microchannel geometry fabricated by injection molding based on Taguchi method

Microchannel is a fundamental structure in various microfluidic systems. Precise reproduction of microchannel is required to achieve reliable efficiency of microfluidic systems. In this paper, we present the parametric sensitivity study of the replication of a cross microchannel by injection molding with a nickel mold insert which was fabricated by the UV-LIGA process. The effects of processing parameters of the injection molding on the transcription properties of cross-sectional geometry of the microchannel were quantitatively characterized by means of design of experiments based on the Taguchi method by varying mold temperature, melt temperature, injection speed and packing pressure. To quantitatively characterize the transcription properties, a measure, relative error between the size of mold insert and that of injection molded microchannel was newly suggested. From the sensitivity analysis of the experimental results based on the relative error, it was identified that mold temperature is the most sensitive processing parameter in this study. And also, the important processing parameter was found to become mold temperature, injection speed, packing pressure and melt temperature in the order of sensitivity. The optimal and worst processing conditions were also found in this study.

Micromachining of electroformed nickel mold using thick photoresist microstructure for imprint technology

In order to fabricate polymer-based microstructures with feature sizes on the order of micrometers, we have been developing a microimprint technology with a fine nickel (Ni) mold instead of a conventional photolithography technique. The Ni mold was successfully fabricated by electroforming using a positive thick photoresist microstructure patterned on a silicon substrate as a replication master. The photoresist microstructure with excellent edge quality can be obtained under irradiation with single wavelength (g line) selected from a high-pressure mercury lamp. In addition, its sidewall angle in the range of 65° to 84° can be controlled precisely by varying the distance between a photomask and a photoresist surface. On the structured photoresist master, Ni was electroplated up to a thickness of about 110 μm, and then removed from the master. In this process, two-step electroplating at different current densities was carried out in order to prevent deformation of the photoresist master due to stress generated in a Ni electrodeposit. With the Ni mold, fine patterns with a width of 10 or 30 μm and a depth of 24 μm were almost completely transferred to polymetric materials (PMMA). The geometrical dimensions of the fabricated PMMA microstructures were found to be only about 10% reduction against the Ni mold.

Fabrication of Large-Area Imprint Mold with High-Aspect-Ratio Nanotip Arrays of Sub-Micron Diameter

Many research works have been focusing on nanoimprint technology due to the recent potential mass production for the nanostructure applications. For optical or display application, a nanoimprint mold of large area becomes one of the thorniest techniques since it takes much time to fabricate the whole mold with nanostructure and it may make the beginning nanostructures inconsistent with the final ones. In order to fabricate the nanostructure mold of large area in a short time, the plasma process forming nanostructures on silicon substrate and the electroforming process are explored in the current study. Well-aligned nanotip arrays of 4 inch silicon were fabricated by electron cyclotron resonance (ECR) plasma process using gas mixtures of silane, methane, argon, and hydrogen. The resultant tips have nano-scale apexes, approximately ~1 nm, with high aspect ratios, nearly ~15, which were achieved by simultaneous SiC nano-mask formation and dry etching during ECR plasma process. Next, the nickel mold of nanostructures is made from silicon nanostructures through the electroforming process by using Nickel Sulfamate. The total thickness of the nickel mold is 120 μm after a 10-hour-long electroforming process. The nanostructures of 100 nm diameter holes are successfully obtained. Nanoimprint process is proceeded by the nickel mold and the reflectance of the PMMA after imprinting at 160 °C has the lowest value, 0.2 %, compared with the other results for the incident optical wavelength of 550 nm. The large-area imprint mold with high-aspect-ratio nanotip arrays of sub-micron diameter is fabricated and is proofed by the optical application.

Control of Parameters Influencing the Thermal Imprint of Parylene/Silicon

The study aims to investigate the possible defects that may occur during imprinting of poly(chloro- p-xylylene) (parylene-C) film (thermal oxidation, delamination, thermal cracking and insufficient filling at the periphery) and to overcome them by modifying the process conditions and mold design. X-ray diffraction (XRD) analyses results for the parylene-C films indicated that higher deposition pressure leads to a lower crystallinity of parylene-C film. By tuning the process conditions and mold design, patterned fields (composed of arrays of 25-µm-high, 10-µm-wide and 1-mm-long lines with 10 µm spacing) in 0.4-mm-thick and 20-mm-sized nickel molds could be successfully replicated on 60-µm-thick parylene-C films deposited at both 25 and 45 mTorr. Complete filling over the whole imprint area could be achieved at <270 °C with the press force at 2 kN and the press hold time of 900 s with the aid of an implemented dummy pattern. Both thermal cracking and delamination could be avoided, even at 270 °C, under the established process conditions and mold design with the help of an adhesion promotion treatment of silicon substrates (SF6 plasma etching for 2 min and spin-coating of KBM-503-based solution). Furthermore, the molds used for paryelne imprinting could be cleaned by dipping in chloronaphthalene solution at >175 °C, followed by an oxygen plasma etching.

미세 패턴 성형용 판형 금형의 급속 가열을 위한 유도가열기구

Hot embossing, one of Nanoimprint Lithography(NIL) techniques, has been getting attention as an alternative candidate of next generation patterning technologies by the advantages of simplicity and low cost compared to conventional photolithographies. A typical hot embossing usually, however, takes more than ten minutes for one cycle of the process because of a long thermal cycling. Over the last few years a number of studies have been made to reduce the cycle time for hot embossing or similar patterning processes. The target of this research is to develop an induction heating apparatus for heating a metallic micro patterning mold at very high speed with the large-area uniformity of temperature distribution. It was found that a 0.5 mm-thick nickel mold can be heated from <TEX>$25^{\circ}C\;to\;150^{\circ}C$</TEX> within 1.5 seconds with the temperature variation of <TEX>${\pm}5^{\circ}C$</TEX> in 4-inch diameter area, using the induction heating apparatus.

Thermal Imprint Process of Parylene for MEMS Applications

This study investigated a thermal imprint technique to pattern parylene microstructures over an area of 2525 mm2. A nickel mold having arrays of 25 m-high, 10 m-wide and 1 mm-long lines with 10 m spacing was fabricated using the deep RIE silicon etching followed by the electroplating process. Imprint tests were then carried out under different conditions of temperature, imprint-hold time and applied pressure to investigate a thermal imprint condition for the complete filling of parylene. Good release results without damage or deformation in parylene microstructures were achieved by the help of a release agent in the imprint temperature range of 160 oC to 250oC. With increasing temperature, the depths of imprinted structures increased and their distribution came to be homogeneous. Complete filling was obtained under the imprint temperature of 250oC, applied load of 195 kgf (3 MPa) and imprint hold time of 1800 s.

Collapse-free thermal bonding technique for large area microchambers in plastic lab-on-a-chip applications

Bonding is an essential step to form microchannels or microchambers in lab-on-a-chip applications. In this paper, we present a novel plastic thermal bonding technique to seal and form large area microchambers (planar characteristic width and length on the order of 1 mm and characteristic thickness on the order of 10–100 μm) without collapse by introducing a holed pressure equalizing plate (HPEP) that includes holes of the same size and shape as the microchambers. To demonstrate the proposed technique, two types of large area microchambers [(1) 20 × 10 mm and 40 μm thick and (2) 12 × 2.5 mm and 120 μm thick] with microchannels were designed and replicated on plastic substrates by means of hot embossing and injection molding processes with prepared two nickel mold inserts. The replicated large area microchambers as well as the microchannels in the plastic lab-on-a-chip were successfully sealed (i.e., no leakage) and formed without any collapse by the proposed thermal bonding technique with the help of the HPEP.

A replication process of metallic micro-mold by using parylene embossing and electroplating

This study demonstrated a replication process for metallic micro-mold that combines the parylene-C (poly-chloro- p-xylylene C) hot-embossing and electroplating techniques. A nickel original master was fabricated using the deep RIE silicon etching followed by the electroplating process. Then, the patterned fields composed of arrays of 25 μm-high, 10 μm-wide and 1 mm-long lines with 10 μm spacing in nickel molds were successfully replicated on the 60 μm-thick parylene-C films by the hot-emboss process. Under complete filling conditions, the deviation of the replicated micropattern was less than 2.4%. The electroplated copper successfully filled parylene-C replica master patterns with the aspect ratio of 2.5 without the void formation by both adding organic addictives and controlling the seed layer thickness. After electroplating, the copper micro-mold could be successfully separated from the parylene-C replica master.

Effects of intrinsic hydrophobicity on wettability of polymer replicas of a superhydrophobic lotus leaf

The present work suggests a mass-producible and large-scale fabrication method of superhydrophobic polymeric surfaces by means of material processing equipments which can maximize productivity and cost effectiveness. We fabricated two types of polymeric lotus leaf replicas using a nickel mold, i.e. R1 from intrinsically hydrophobic polydimethylsiloxane by means of polymer casting (PC) and R2 from an intrinsically hydrophilic UV-curable photopolymer by means of UV-nanoimprint lithography (UV-NIL). In the case of R1 from PC, although the nano-scaled structures were not well reproduced, the contact angle (CA) was remarkably high and the sliding angle (SA) was also close to that of the original lotus leaf, resulting in a superhydrophobic surface. In contrast to R1, in the case of R2 from UV-NIL, the nano-scaled structures as well as micro-scaled structures were also relatively well reproduced and the CA was increased noticeably by around 99° in comparison to a flat photopolymer. However, unexpectedly, the SA of R2 was much higher than that of R1. This work provides useful tips of polymeric material selection for the industrial mass production of the superhydrophobic polymer surface.

Micro-injection molding with the infrared assisted mold heating system

Mold temperature and packing pressure are the major factors that affect the process of micro-molding. The mold temperature was found to be the crucial factor that controls the success of the complete filling. In micro-injection molding, the mold temperature has to be close to the glass temperature to ensure a good replication of the micro-features. This study is to investigate the effect of infrared mold surface rapid heating system on the micro-injection molding. The micro-feature was defined by the UV lithography technology and transfer to the nickel mold insert by micro-electroforming. An infrared heating system is designed and built to heat the mold surface dynamically. The infrared heating system raises the mold cavity temperature locally to facilitate the replication of the micro-structure. The results show that mold cavity surface must be heated to above a critical temperature before molding. The critical temperature is close to the glass transition temperature and it decreases with the increase of the packing pressure. For mold temperature in 80 °C, heating more than 10 s can result in a complete filling of the micro-feature used in this study.

응집반응 검출을 위한 미세 유체 Lab on a chip의 사출성형 금형 인서트의 디자인 및 제작

Agglutination is one of the most commonly employed reactions in clinical diagnosis. In this paper, we have designed and fabricated nickel mold insert for injection molding of a microfluidic lab-on-a-chip for the purpose of the efficient detection of agglutination. In the presented microfluidic lab-on-a-chip, two inlets for sample blood and reagent, flow guiding microchannels, improved serpentine laminating micromixer(ISLM) and reaction microwells are fully integrated. The ISLM, recently developed by our group, can highly improve mixing of the sample blood and reagent in the microchannel, thereby enhancing reaction of agglutinogens and agglutinins. The reaction microwell was designed to contain large volume of about <TEX>$25{\mu}l$</TEX> of the mixture of sample blood and reagent. The result of agglutination in the reaction microwell could be determined by means of the level of the light transmission. To achieve the cost-effectiveness, the microfluidic lab-on-a-chip was realized by the injection molding of COC(cyclic olefin copolymer) and thermal bonding of two injection molded COC substrates. To define microfeatures in the microfluidic lab-on-a-chip precisely, the nickel mold inserts of lab-on-a-chip for the injection molding were fabricated by combining the UV photolithography with a negative photoresist SU-8 and the nickel electroplating process. The microfluidic lab-on-a-chip developed in this study could be applied to various clinical diagnosis based on agglutination.

PATTERNING OF TWO-DIMENSIONAL PHOTONIC CRYSTAL STRUCTURES BY NANOIMPRINT LITHOGRAPHY

We report on the process parameters of nanoimprint lithography for the fabrication of two-dimensional (2D) photonic crystals. The nickel mold with 2D photonic crystal patterns covering an area up to 20 mm2 is produced by electron-beam lithography and electroplating. Periodic pillars as high as 200 nm to 250 nm are produced on the mold with the diameters ranging from 180 nm to 500 nm. Optimization of process parameters is essential to generate high-quality nanoscale patterns and control the residual stress in the mold. The mold is employed for nanoimprinting on the poly-methyl-methacrylate (PMMA) layer spin-coated on the silicon substrate. Periodic air holes are formed in PMMA above its glass-transition temperature and the patterns on the mold are well transferred. This process can be utilized for commercial applications of photonic crystal devices.

Mass-producible replication of highly hydrophobic surfaces from plant leaves

Many plant leaves found in nature are known to exhibit a characteristic of superhydrophobicity(‘lotus leaf effect’). The present study proposes a mass-production method of highlyhydrophobic surfaces by simply replicating the highly hydrophobic plant leafsurfaces in two steps: the first step of making a nickel mould via electroformingand the second step of replication via a UV-nanoimprint lithography. Makinga nickel mould, either a plant leaf or its negative polymer replica is used as amandrel in electroforming, and final products become positive or negative polymerreplicas of a plant leaf, respectively. It is found that the nickel-mould makingusing the plant leaf as a mandrel is quite successful and the final products inthe form of a positive replica are better than those in the form of a negativereplica in terms of replication quality and hydrophobicity. Contact angle values ofthe positive replicas are less than those of the natural leaves’ surfaces by only2°–5°.

Fabrication of a polymeric tapered HARMs array utilizing a low-cost nickel electroplated mold insert

A simple low-cost technique has been developed to fabricate a mold insert for replicating polymeric tapered high aspect ratio microstructures. A backside exposure technique is used to first obtain a tapered sidewall structure as an electroplating mold in SU-8 photoresist on a glass wafer. Nickel electroplating is utilized to form the mold insert. The lowest average surface roughness of the nickel mold insert on the side that interfaces with the glass wafer during electroplating is measured to be 7.02 nm. A novel technique involving use of titanium putty is introduced here to reduce cost and effort required to fabricate the mold insert. Replication of tapered microstructures in polymeric materials utilizing the fabricated mold insert is demonstrated here in polydimethylsiloxane by a direct molding process and in polymethyl methacrylate by hot embossing. The fabrication details for the mold insert are described. Advantages and disadvantages of the use of titanium putty for achieving superior metal surface finish are given.