Imprinted polymer stamps for UV-NIL

Thermal step and stamp nanoimprint lithography (SSIL) offers an alternative to fabricate transparent polymer stamps for UV-imprinting. The fabrication process does not require any other subsequent steps, e.g. dry etching or anti adhesive coating.In this work, we have manufactured UV-stamp by combining patterns of two different silicon masters. The patterns of the silicon masters were transferred into resin coated quartz plate by sequential imprinting. The first master consisted gratings with 50 nm features and the second master consisted dot arrays of 350 nm diameter features. The novel idea is the ability to create a large UV-stamp using a combination of small masters. Thus fabricated UV-stamps were used for demonstrating step and repeat UV-imprinting. The quality of the UV-stamps and imprints were analyzed by AFM. High fidelity patterns were achieved in respect to patterns in the original silicon master.     

Fabrication of carbon nanotube arrays for field emission and sensor devices by nanoimprint lithography

This work brings forth the idea of incorporating insulation in the resist used for ultraviolet (UV) curing nanoimprint lithography (NIL). Carbon nanotubes (CNTs) are grown in the space between two insulated resist patterns on the conductive substrate to make CNTs arrays. Two imprinting processes, soft UV curing NIL with DRPPR process and novel NIL without cured residual resist, are presented to achieve the insulation patterns. First the fabricating process is performed using a polydimethylsiloxane (PDMS) stamp. Subsequently, inductively coupled plasma (ICP) is essential to wipe off the residual resist film. To avoid the ICP process, a novel UV curing NIL is presented. Its special hard quartz stamp with chrome shelter can protect the residual resist film out of curing during the UV exposure process, and the uncured resist can be easily removed by ultrasonic vibration in organic solutions. The CNT arrays are prepared on the patterned substrates by the pyrolysis of iron phthalocyanine (FePc). Field emission experiments reveal that the turn-on field of those CNTs arrays is low to 1.3 V/um.     

Nanoimprint in PDMS on glass with two-level hybrid stamp

In this article we examine the use of two-level hybrid-material stamps and nanoimprint lithography (NIL) of poly(dimethylsiloxane) (PDMS) on glass substrates. A silicon/SU8 stamp manufacturing process has been developed, in order to combine nanometer and micrometer structures, thus avoiding complex deep etching processes. The stamp has been test printed in polymethyl methacrylate (PMMA) to demonstrate functionality. We describe polymer flow problems for imprinting large structures and identify optimized parameters, in accordance with previously published findings. The use of PDMS as imprint polymer was examined. Imprinting works well, however, large recovery after separation shrinks the micrometer channels substantially and renders the nanochannels useless. Glass substrates in combination with silicon stamps were used, evaluated and showed to work well at low temperature.     

Fabrication and application of silicon-reinforced PDMS masters

A new molding process is developed in this work to generate a silicon (Si)-reinforced polydimethylsiloxane (PDMS) master of a 4 in wafer size using an SU-8 mold. The reinforced PDMS master is applied to pattern a conducting polymer, poly-3-hexylthiophene (P3HT), which is normally dissolved by a non-polar solvent. PDMS is usually patterned by a molding process, in which PDMS is first coated on and then peeled off from a rigid mold. However, in the new molding process, the Si-reinforced PDMS master is rigid but the SU-8 mold is flexible, and the SU-8 mold is first placed on and then peeled off from the rigid PDMS master. In such a way, a reinforced PDMS master of a size as large as a 4 in wafer can be produced. Meanwhile, a new way of obtaining free-standing, large SU-8 structures is presented. PDMS swells when it gets exposed to non-polar solvents. This swelling makes PDMS not suitable for patterning materials, which are usually dissolved by non-polar solvents, e.g., P3HT. In this work, we demonstrate that, with the reinforcement of a Si plate, the swelling effect in generating this specific type of materials is much reduced, and good patterns can be produced.     

Negative UV–NIL (NUV–NIL) – A mix-and-match NIL and UV strategy for realisation of nano- and micrometre structures

This paper presents a novel strategy for aligning patterns created with nano-imprint lithography (NIL) and UV lithography, similar to a mix-and-match process, which allows for the fabrication of large and small features in a single layer of resist. The resin used to demonstrate this new imprinting scheme is SU-8, a very widely used negative photoresist. Rapid stamp manufacturing using ma-N 2405 photoresist is also demonstrated. The processing scheme is a promising candidate for patterning of sensors featuring nanometre sized electrodes.     

Compact LED based nanoimprinter for UV-NIL

UV-based nanoimprint lithography (UV-NIL) is a cheap and fast way to imprint patterns ranging from nanometres to micrometres. However, commonly used equipment can be expensive and require a clean room infrastructure. Here we present the design and testing of a simple UV-NIL system based on a light emitting diode. The current design permits imprints of 10 × 10 mm² in size using a 25 × 25 mm² master. This printer can be used in a semi-clean environment such as a laminar flow bench. The imprinter was used to imprint photoresists as well as UV sensitised hydrogels. The best results were obtained using SU-8 photoresist with features down to 50 nm in size, only limited by the imprint master. Patterns in SU-8 resist were also transferred into silicon substrates by reactive ion etching demonstrating its full potential as a lithographic tool.     

UV nanoimprint lithography of sub-100 nm nanostructures using a novel UV curable epoxy siloxane polymer

We reported the replication of sub-100 nm nanostructures by an ultraviolet (UV) nanoimprint lithography (NIL) technique. We used a novel UV curable epoxy siloxane polymer as the NIL resist to achieve features as small as 50 nm. The polymeric soft molds for the NIL were fabricated by casting toluene diluted poly(dimethyl-siloxane) (PDMS) on the hydrogensilsesquioxane (HSQ) hard mold. The NIL results were characterized by using a scanning electron microscope and an atomic force microscope. Our results illustrate that, with the epoxy siloxane resist, the 50 nm HSQ features on the hard mold can be successfully replicated using PDMS soft molds.     

Comparison of quartz and silicon as a master mold substrate for patterned media UV-NIL replica process

Nanoimprint lithography (NIL) is a promising candidate technology to fabricate patterned media for the next generation hard disk drives (HDD). The requirement of pattern pitch for the HDD or discrete-track recording (DTR) media will be as small as from 40 to 50 nm by 2011 or 2012. However not only to create such fine pitch but also long e-beam writing time such as 1 week with conventional high resolution resist ZEP520A are critical. This paper addresses the fabrication processes to combine silicon substrate and a new chemically amplified resist (CAR) for the master molds of this NIL. The e-beam writing speed with this new CAR was achieved over 3-times faster while 50 nm fine DTR patterns were demonstrated with rotary stage e-beam writer. Furthermore, the replication with J-FIL from the master mold into quartz working mold was also demonstrated.     

Direct soft UV-NIL with resist incorporating carbon nanotubes

As a potential candidate for the next generation of nanolithography, nanoimprint lithography (NIL) has drawn ever-increasing worldwide attention. It involves physical contact to overcome the optical limits occurring in sub-100 nm photolithography. Affordable tool cost is one of major attractive points of NIL. This work proposes the idea of incorporating carbon nanotubes (CNTs) in the resin used for ultraviolet nanoimprinting (UV-NIL). CNTs can make the resin electrically conductive when mixed with it. Patterns imprinted in the CNT-mixed resist can then be used to replace conductive metal structures directly. This enhances the productivity of basic UV-NIL where the imprinted patterns are used as sacrificial etch masks. In this work, several types of CNTs were purified chemically and dispersed before being mixed with UV-NIL resin using ultrasonic vibration. On drops of CNT-mixed resin, soft UV-NIL was performed using a polydimethylsiloxane (PDMS) stamp with a minimum feature size in the range of 200 nm. Even with increased resin viscosity due to the addition of CNTs, UV imprinting down to 200 nm was successfully done with moderate pattern fidelity. The loading rate of nanotubes should be minimized to prevent the increased viscosity from degrading the pattern transfer resolution. The electrical conductivity of CNT-mixed resist increases with the loading of CNTs. Therefore, the trade-off between the electrical properties and pattern transfer resolution needs to be optimized carefully.     

Fabrication of ordered micro- and nano-scale patterns based on optical discs and nanoimprint

A simple method to fabricate one-dimensional(1-D) and two-dimensional(2-D) ordered micro- and nano-scale patterns is developed based on the original masters from optical discs, using nanoimprint technology and soft stamps. Polydimethylsiloxane(PDMS) was used to replicate the negative image of the 1-D grating pattern on the masters of CD-R, DVD-R and BD-R optical discs, respectively, and then the 1-D pattern on one of the PDMS stamps was transferred to a blank polycarbonate(PC) substrate by nanoimprint. The 2-D ordered patterns were fabricated by the second imprinting using another PDMS stamp. Different 2-D periodic patterns were obtained depending on the PDMS stamps and the angle between the two times of imprints. This method may provide a way for the fabrication of complex 2-D patterns using simple 1-D masters.     

Fabrication of a conductive nanoscale electrode for functional devices using nanoimprint lithography with printable metallic nanoink

This paper proposes two metal patterning processes. In each process, nanoimprint lithography (NIL) is used with commercialized particle-based silver nanoink which has appropriate properties for NIL. One process is a direct NIL process with a polydimethylsiloxane (PDMS) stamp; the other is a combined NIL and lift-off process. The direct NIL process is executed by using a xylene-absorbed PDMS stamp to decrease the curing time and minimize the residuals. A flexible PDMS stamp can also be wrapped around a quartz cylinder and used as a roll stamp to enlarge the patterned area. The direct NIL process successfully produced silver line patterns in the range of 200–300 nm, and the combined NIL and lift-off process successfully produced silver line patterns in the range of 15–60 nm.     

Fabrication of nickel stamp with improved sidewall roughness for optical devices

We proposed the simple and attractive fabrication method of nickel stamp with improved sidewall roughness for polymeric optical devices. For this, the imprinted optical devices patterns under optimum imprinting conditions were annealed to improve the sidewall roughness generated by the DRIE process in the silicon stamp fabrication. The annealed sidewall roughness is reduced to 24.6 nm, nearly decreasing by 76% compared with the result before the annealing. Then, low cost and durable nickel stamp with improved sidewall roughness was fabricated by the annealed polymeric patterns being used as original master for electroforming process. And, we verified the superiority of the improved nickel stamp by comparing the optical propagation losses for optical waveguides to be fabricated, respectively, using the nickel stamp and original silicon stamp. The optical waveguides fabricated by the imprint lithography using the improved nickel stamp was demonstrated that their optical losses were reduced as 0.21 dB/cm, which was less than the propagation loss for polymeric waveguides using the conventional original silicon stamp. This result could show the effectiveness of the fabricated nickel stamp with improved sidewall roughness. Furthermore, we were able to successfully fabricate a polymeric 1 × 8 beam splitter device using the improved nickel stamp. And, the insertion loss for eight channels obtained to be from 10.02 dB to 10.91 dB.     

Novel Process to Improve Defect Problems for Thermal Nanoimprint Lithography

The reliability of imprint patterns molded by stamps for industrial application of nanoimprint lithography (NIL) is an important issue. Usually, defects can be produced by incomplete filling of negative patterns and the shrinkage phenomenon of polymers in conventional NIL. In this paper, the patterns that undergo a varied temperature or varied pressure period during the thermal NIL process have been investigated, with the goal of resolving the shrinkage and defective filling problems of polymers. This paper also studies the effects on the formation of polymer patterns in several profiles of imprint processes. Consequently, it is observed that more precise patterns are formed by varied temperature (VT-NIL) and varied pressure (VP-NIL). The NIL (VT-NIL and VP-NIL) process has a free space compensation effect on the polymers in stamp cavities. From the results of the experiments, the polymer's filling capability can be improved. VT-NIL is merged with the VP-NIL, resulting in a better filling property. The patterns that have been imprinted in merged NIL are compared with the results of conventional NIL. This study achieves improvement in the reliability of the results of thermal NIL     

软印刷技术

软印刷技术是基于弹性体印章/模具来转移图形结构的微纳加工技术。详细介绍了软印刷技术中转移图形结构的多种方式,并探讨软印刷技术在微纳电子学、光学、传感器、生物等领域的广泛应用。对软印刷技术的弹性体印章/模具制备、聚二甲基硅氧烷的属性、理论研究等进行了探讨。     

Soft UV-nanoimprint lithography on non-planar surfaces

We report on a simple and effective process that allows direct UV-imprinting of micro- and nanostructures on non-planar surfaces, even at sharp edges such as step surfaces. The key for the process is the use of a thin flexible polymer stamp, which was fabricated by spin-coating poly(dimethylsiloxane) (PDMS) on a pre-patterned Si or poly(methyl methacrylate) (PMMA) master and releasing the thin PDMS layer after curing. The thin PDMS stamp was used to conformally mold a UV resist layer coated on various non-planar substrates with different radii of curvature. With this method, we have successfully demonstrated micro- and nanopatterns down to 63 nm on curved surfaces as well as sharp step-like structures. The process so developed will improve the versatility and applicability of molding technologies in many applications that require patterning non-planar substrates, considering that most molding technologies allow for patterning only on planar substrates or surfaces with large curvature radii.     

Fabrication of 50 nm patterned nickel stamp with hot embossing and electroforming process

Fabrication of imprint stamp is a key issue of nanoimprint lithography. In this study, we attempt to fabricate the nickel imprint stamp using hot embossing lithography and electroforming processes. As small as 50 nm sized patterns of original silicon master were faithfully transferred to polyvinyl chloride (PVC) film. By electroforming on hot embossed PVC film, nickel stamp, which has the same patterns of original silicon master stamp, was successfully fabricated.     

Nanoscale patterning with the double-layered soft cylindrical stamps by means of UV-nanoimprint lithography

In nanoscale stamp fabrication, the overheads of time, cost and patterning area are soaring. Furthermore, the lifetime of a stamp is affected by the process conditions, particularly the number of times, resist parameters, pressure, and temperature. The fabrication of cylindrical stamps is also rather difficult with some nanoscale patterns. However, if a cylindrical stamp can be fabricated with a nanoscale pattern, improvements can possibly be made with regard to the size of the area, the minimization of costs, and the protection of the master stamp. This paper proposes a process of fabricating a cylindrical double-layered stamp with two different properties. The stamps are verified through the application of a roller-type ultraviolet-nanoimprint lithographical process on the ANT-6R developed by the Korea Institute of Machinery and Materials.     

Capillary Force Lithography: Fabrication of Functional Polymer Templates as Versatile Tools for Nanolithography

The implementation of high‐resolution polymer templates fabricated by capillary force lithography (CFL) is explored both in nanoimprint lithography (NIL) and in the wet‐etching of metals. Several different thermoplastic and UV‐curable polymers and types of substrates are incorporated into the general CFL procedure to meet the diverging requirements of these two applications. The mechanical stability of UV‐curable templates for imprinting in polymers, as examined by atomic force microscopy (AFM), and their anti‐adhesive properties are excellent for application in NIL. The conditions for curing the UV‐curable polymer are optimized in order to obtain high‐stability polymer templates. Gold patterns on silicon with a lateral resolution of 150 nm are fabricated by subsequent lift‐off in acetone. Similar patterns with a lateral resolution of 100 nm are fabricated using templates of thermoplastic polymers on gold layers on silicon as an etch mask. The transfer of stamp residues during CFL with these polymer templates is proven by X‐ray photoelectron spectroscopy (XPS) and AFM friction analysis. For poly(methylmethacrylate) (PMMA), the presence of large amounts of silicon‐containing residues is found to compromise the processability of the resulting template in subsequent O₂ reactive‐ion etching (RIE) treatment. The extent of silicon contamination is up to six times less for polystyrene (PS). At this level, the etch performance of the PS etch mask is not affected, as was the case for PMMA. Accurate downscaling of the lateral dimensions of the resulting metal patterns by several factors with respect to the dimensions of the PS etch mask is achieved by over‐etching of the gold. Overall, the results in this paper demonstrate the potential of CFL templates as tools for high‐resolution soft lithography.     

Soft‐Contact Optical Lithography Using Transparent Elastomeric Stamps and Application to Nanopatterned Organic Light‐Emitting Devices

Conventional photolithography uses rigid photomasks of fused quartz and high‐purity silica glass plates covered with patterned microstructures of an opaque material. We introduce new, transparent, elastomeric molds (or stamps) of poly(dimethylsiloxane) (PDMS) that can be employed as photomasks to produce the same resist pattern as the pattern of the recessed (or non‐contact) regions of the stamps, in contrast to other reports in the literature^[1] of using PDMS masks to generate edge patterns. The exposure dose of the non‐contact regions with the photoresist through the PDMS is lower than that of the contact regions. Therefore, we employ a difference in the effective exposure dose between the contact and the non‐contact regions through the PDMS stamp to generate the same pattern as the PDMS photomask. The photomasking capability of the PDMS stamps, which is similar to rigid photomasks in conventional photolithography, widens the application boundaries of soft‐contact optical lithography and makes the photolithography process and equipment very simple. This soft‐contact optical lithography process can be widely used to perform photolithography on flexible substrates, avoiding metal or resist cracks, as it uses soft, conformable, intimate contact with the photoresist without any external pressure. To this end, we demonstrate soft‐contact optical lithography on a gold‐coated PDMS substrate and utilized the patterned Au/PDMS substrate with feature sizes into the nanometer regime as a top electrode in organic light‐emitting diodes that are formed by soft‐contact lamination.     

Microstructuring of SU-8 photoresist by UV-assisted thermal imprinting with non-transparent mold

In this study, we explored a rapid and low-cost process for patterning in a SU-8 photoresist by thermal imprinting with a non-transparent mold such as Ni mold. One of major obstacles in the process is that the extremely good formability of uncured SU-8 even near room temperature causes the collapse of imprinted patterns during and after de-molding because a sample cannot be exposed to UV light during imprinting owing to the non-transparency of a mold. To overcome this problem, un-cured SU-8 resists were pre-treated with UV light, heat, and O₂ plasma for controlling their formability, and applied to thermal imprint tests to be compared each other in terms of the replication fidelity. As a result, a SU-8 sample pre-treated with UV light for 8 s resulted in the best replication quality for given imprint conditions and mold dimensions, and we could successfully replicate micro patterns in SU-8 resist without a quartz mold. As compared with conventional UV-imprint processes, this process has potential merits such as a lower mold cost, an easier mold release and a less air-entrapment.