Thermal NIL Research Articles

Vapour-assisted roll-to-roll nanoimprinting of micropillars on nanocellulose films

Here we demonstrate a straightforward vapour-assisted roll-to-roll nanoimprinting lithography (R2RVANIL) patterning method to biobased nanocellulosic films with the aim to fabricate microstructured films with diffraction effect. In the R2RVANIL technique a roll with pillar structures is softly pressed against nanocellulose film in the presence of hot water vapour replicating the pattern onto the film. The printed pillar structures are compared with the structures produced using traditional thermal R2RNIL. The manufacturing method is demonstrated using optically transparent TEMPO-oxidised cellulose nanofibrils (TEMPO CNF). TEMPO CNF films possess high tensile strength and modulus whereas the tensile strain is low, they are very hygroscopic - a property which improves material processing at elevated humidities, thus overcoming a lack of a clear softening point. In this context, the water molecules are acting as plasticising molecules thereby softening the relative brittle nanocellulose film structures. Patterned pillars were achieved due to the swelling and softening of the film which was contacted with moist air. R2RVANIL method appears to be more gentle and surface properties such as roughness are different from the ones achieved via conventional thermal NIL method. The diffraction grating is demonstrated using laser beam compared to calculated diffraction patterns. Water vapour assisted patterning methods are beneficial, for example, in the future optics and electronics where films cannot be processed under high pressure and, on the other hand, high production volumes are needed.

Fabrication and replication of re-entrant structures by nanoimprint lithography methods

In this work, the authors present and demonstrate a simple method to fabricate and mass replicate re-entrant structures. The method consists of the direct imprinting of polymer mushroomlike microstructures produced by a combination of photolithography and nickel up-plating process. In particular, they have studied the conditions to generate highly robust mushroomlike topographies and their topographical impact on the replication process. They discuss all the imprinting conditions suitable to replicate such topographies using both ultraviolet light assisted nanoimprint lithography (UV-NIL) and thermal NIL methods in two polymer films, poly(methyl methacrylate) and polypropylene, and a hybrid (organic–inorganic) UV light curable photoresist, namely, Ormocomp. Re-entrant topographies have been widely studied for liquid/oil repelling and dry adhesive properties, whereas in their experiments, they have proved evidence for their amphiphobic potential.

Nanoimprint Lithography - the Past, the Present and the Future

Background: Nanoimprinting lithography technique uses a very simple concept of transferring pattern of nanoscale features from a mold to a target substrate. In the past two decades, this technique has successfully broken through the barrier of laboratory scale production and become an industrial scale production technique. The aim of this paper is to introduce to readers to the basic working principle, applications, analysis the technological limitations. It will also point out future research direction of this useful nanofabrication technique. Methods: We adopted a systematic approach to give a comprehensive review of the work principle, hardware and analysis of advantaged and disadvantages of major nanoimprint lithography techniques. Moreover, a technical comparison of these methods is carried out to provide future research direction. Results: 87 papers were reviewed. Four techniques including thermal NIL, ultraviolet light NIL, laser-assisted direct imprint and nanoelectrode lithography have been identified as main stream of NIL techniques. These techniques possess certain advantages and disadvantages in terms of cost, throughput, attainable resolution. Lack of flexibility is the common limitation of current NIL techniques. NIL has gained wide applications in the fabrication of optoelectronics devices, solar cells, memory devices, nanoscale cells, hydrophobic surfaces and bio-sensors. The potential applications of NIL in biochips, artificial organs, diagnostic system, and fundamental research in cell biology will demand large scale 3D fabrication capability with resolution towards 10nm or less. Conclusions: The findings of this review confirm that NIL is one of the most employed commercial platforms for nanofabrication which offers high throughput and cost-effectiveness. One of the disadvantages of NIL over other nanofabrication techniques is the flexibility of patterning. Integrating NIL with other existing nanofabrication techniques can be helpful to overcome such issue. The potential applications of NIL in biochips, artificial organs, diagnostic system, and fundamental research in cell biology will attract researchers to push nanoimprint lithography forward at a resolution of 10 nm or less in the future.

Effect of nanoimprint on the elastic modulus of PMMA: Comparison between standard and ultrafast thermal NIL

This paper is focused on the understanding of the effect of the nanoimprint lithography process on the elastic modulus of thin, thermoplastic films. In particular, we present the comparison between the standard and an ultrafast thermal NIL technology as well as the way both processes affect the top surface of poly(methyl methacrylate) (PMMA). The PeakForce QNM™ (Quantitative Nanomechanical Property Mapping) scanning probe technique was used to determine the Young's modulus of PMMA by comparison with a polystyrene standard. We demonstrate that imprinted PMMA, regardless of the used method, shows a 9-fold increase of Young's modulus compared to non-imprinted PMMA at least in the top 3–5nm thick surface layer. This important finding proves that the ultrafast process with much higher temperatures, but also with much shorter process times, leads to elastic surface properties that are comparable to those of PMMA imprinted with the standard process. We have confirmed that annealing alone does not significantly influence the Young's modulus.

Sub-100 μs nanoimprint lithography at wafer scale

We present here an ultrafast thermal NIL technology, which enables the patterning of full wafers on the 100μs time-scale. This technique makes use of stamps with a heating layer integrated beneath their nanostructured surfaces. Injecting a single, short (<100μs), intense current pulse into the heating layer causes the surface temperature of the stamp to raise suddenly by hundreds of degrees°C, resulting in the melting of the thermoplastic resist film pressed against it and the swift indentation of the nanostructures. This paper introduces the main aspects of this technology, namely the process concept, the stamp structure, and the main features of the equipment by which the process at the wafer scale was implemented.

Legitimate domain of a Newtonian behavior for thermal nanoimprint lithography

• Imprint of a simple line into a polymeric layer is modeled with no-slip boundary condition and constant viscosity. • The legitimate domain of the newtonian behviour is expressed for a thermoplastic polymer. • Usual non-dimensional analysis is performed to express the output forces and maximum shear rate for thermal NIL. • Numerical simulations based on the natural element method allow to compute a master curve for a given geometry. Nanoimprint lithography is an efficient way to reproduce nanostructures down to 20 nm in sub-micrometer polymeric films. To optimize this process, simulation using a Newtonian behavior is a cheap and efficient way to predict the polymer flow in micro and nano size cavities. This behavior is nevertheless limited to flows with shear rates below a critical value that can be determined with standard rheology measurements. We have investigated the validity domain of this behavior to simulate thermal NIL. This domain of validity is composed of two uncoupled functions, one for the material properties and the mean pressure applied to the pattern, and one for the geometry considered. The latter function has been determined with numerical simulations using the natural element method. It is demonstrated that knowing the mean applied pressure, the critical shear rate, and the viscosity of the material we are able to determine, depending on stamp geometry, if shear-thinning may or may not occur during an imprinting process.

Fabrication of flexible metallic wire grid polarizer using thermal NIL and lift-off processes

Graphical abstractDisplay Omitted Highlights Double layer metallic/dielectric gratings were fabricated for WGP components. Thermal NIL, aluminum lift-off, and dielectric etching were adopted in exp...

Thermal nanoimprint resist for the fabrication of high-aspect-ratio patterns

Graphical abstractDisplay Omitted Highlights? We present a new thermal NIL resist for high-aspect-ratio patterns via a bilayer approach. ? The resist properties are tailored by free radical polymerization of specific comonomers. ? The resist shows excellent flowability and demoulding characteristics. ? A high oxygen RIE resistance demonstrates the suitability of the resist as a masking layer. We report a newly developed thermal nanoimprint (T-NIL) resist specifically engineered for the implementation in a bilayer system rendering the fabrication of high-aspect-ratio patterns. The synthesis of the T-NIL resist was accomplished by applying a free radical copolymerization of adequate comonomers. Due to the modular architecture of the terpolymer, the rather different material properties are directly adjusted by varying the amount of the comonomers and also by tuning the polymerization conditions like temperature and the amount of initiator. Following this synthetic strategy, the Si content for a sufficient oxygen plasma resistance as well as an excellent flowability behavior for a significant reduction of the overall processing cycle time could be easily achieved. Moreover, the already good mould release properties of the pure T-NIL resist can be further improved by the addition of a small amount of a fluorinated additive leading to a very low defect rate of the imprinted structures.

Fabrication of flexible metallic wire grid polarizer using thermal NIL and lift-off processes

► Double layer metallic/dielectric gratings were fabricated for WGP components. ► Thermal NIL, aluminum lift-off, and dielectric etching were adopted in experiments. ► 192:1 Extinction ratio and 1.2× brightness gain were obtained with 240 nm pitch WGP. ► Designed optical performance and better process tolerance could be achieved. ► Low retardation COP films provide adequate optical performance. This work is dedicated to the fabrication of metallic/dielectric double layer gratings on polymeric films for wire grid polarizer (WGP) to obtain designed optical performance as well as better process tolerance. The fabrication process was based on thermal nanoimprint and aluminum lift-off techniques. Polymeric substrates PET (Polyethylene terephthalate) and COP (Cyclo-olefin polymer) were brought into comparison for the birefringence property. At the same time, by applying RCWA (Rigorous coupled-wave analysis) simulated optical designs, it would be also capable of minimizing difficulties of ultra-precision manufacturing. Experimental results revealed that using the 240 nm pitch stacked metallic/dielectric gratings performed 192:1 extinction ratio and 1.2× brightness enhancement, which allowed larger pitch replacing the smaller pitch of WGP gratings, but still possessed similar optical performance. Hence, these designs and process configuration not only make the continuous roll-to-roll process more feasible, but also relieve the difficulties of fabricating the imprint roller stamp.

Integration of rotated 3-D structures into pre-patterned PMMA substrate using step &amp; stamp nanoimprint lithography

In this work, we fabricated hybrid micron-scale blazed grating structures on top of linear nanoscale gratings by superimposing thermal nanoimprint process. A poly(methyl methacrylate) (PMMA) substrate was at first pre-patterned with a nanoscale binary line grating by thermal NIL. The second imprint was added into the linear surface-gratings by sequential thermal imprinting using a stamp with micron-scale blazed gratings. The rotation controlled patterning was realized by Step and Stamp Imprint Lithography (SSIL) using NPS300 nanoimprinting stepper with a rotation imprint head with ability to control X/Y positioning and angular orientation of the stamp. In this paper, we demonstrate the potential of the hybrid imprint process for fabrication of e.g. hybrid bio devices or optical elements with arbitrary orientation.

Study and development of polymer destabilization by Capillary NIL

Capillary NIL is an evolution of NanoImprint Lithography, which is based on the structuration of a polymer film by spontaneous destabilization. This technique is close to thermal NIL, but a soft contact is applied on the wafers, and it leads directly to a positive duplication of mold patterns. Using an un-patterned mold, micro-scale structurations are obtained and using a mold with nano-scale patterns, nano-structures could be obtained.This paper presents experimental results of Capillary NIL, and the influence of different parameters on this spontaneous destabilization such as applied temperature, film thickness, and contact time.

Nanostructures in protein chips: Effect of print buffer additive and wettability on immobilization and assay performance

Nanostructures of 1μm period and 180nm height were fabricated by thermal nanoimprint lithography (thermal NIL) and implemented in protein chips to enhance chip sensitivity. Three epoxy resins which are well characterized materials in lab-on-chip systems and (bio)MEMS and are able to bind proteins via their epoxy function are tested: SU-8 2000.5, EPIKOTE™ 157 and EPON™ 1002F. For improved wettability wetting agents like PEG 200, sorbitol, MCT and oleic acid were added to the protein printing solution. Best chip performance was achieved on EPON™ 1002F nanostructured surfaces, brightest signals when using 0.1% (w/v) PEG in the printing solution.

Characterization of adhesion property between fused silica and thermoplastic polymer film in thermal nanoimprint lithography using a novel pull-off test

A novel pull-off test that mimics the actual thermal NIL process was conducted to investigate the adhesion properties between a flat fused silica and thermoplastic polymer film used in thermal NIL process. The pull-off force was measured under various NIL conditions—such as use of various polymer materials, imprint pressures, and separation velocities—and the surfaces of the mold and polymer film were observed after the test. The anti-sticking layer (ASL) derived from (1H,1H,2H,2H-perfluorooctyl)trichlorosilane (F 13-OTS) was coated on the fused silica and its effects on the adhesion characteristics was also examined. In cases of the mold without ASL, the pull-off force varied significantly according to the process conditions and damage on the polymer film was observed in most of the tests. In cases of the mold coated with the ASL, on the other hands, the pull-off force was maintained at a lower level in the range of the imprint pressure from 2 to 10 MPa or separation velocity from 1 to 25 μm/s, and there was no damage to the polymer film due to adhesion.

Non-sticky silicate replica mold by phase conversion approach for nanoimprint lithography applications

We have developed a transparent, non-sticky silicate nano mold with high mechanical strength and excellent releasing properties through a simple phase conversion process of polyvinylsilazane (PVSZ) replica mold. This inexpensive inorganic polymer derived silicate mold does not require extra surface modification and could be used as an ideal mold with low adhesion force for nanoimprint lithographic applications. The silicate hard nano mold allows fabrication of sub-100 nm patterns down to 30 nm. The mold can be used for both UV and thermal NIL duplication processes in a repeated manner. The economic efficiency of the mold fabrication as well as the high durability and excellent releasing properties could be quite valuable to physical contact nanolithography for high-throughput fabrication of nano-devices.

Benchmarking of 50nm features in thermal nanoimprint

The objective of this benchmarking is to establish a comparison of several tools and processes used in thermal NIL with Si stamps at the nanoscale among the authors’ laboratories. The Si stamps have large arrays of 50nm dense lines and were imprinted in all these laboratories in a ∼100nm thick mr-18010E film. Other materials, such as mr-17010E, were also tested. Good patterns were obtained and some limitations were identified. Reducing the pressure to 15bars enables the printing of 50nm structures without pulling them off. At higher pressures, some bending effects resulting in pattern deformation were observed. It was proven that a pressure of 1.5bars is sufficient to imprint perfect 50nm lines. The influence of the antiadhesive layer and mold design has been characterized by the demonstration of pulled off lines in some cases. Moreover, it has been shown that the scatterometry method is particularly useful for the characterization of 50nm lines and that the residual layer thickness corresponds to the theoretical estimate as long as the lines are well defined. One process was demonstrated which combines high reproducibility with high throughput, achieving a cycle time of 2min.

Effects of mold geometries and imprinted polymer resist thickness on ultrasonic nanoimprint lithography

This paper investigated the effects of imprinted resist thickness and mold geometries on the polymer flow and the temperature distribution of ultrasonic nanoimprint lithography (U-NIL) through numerical simulations and experiments. In simulations, the velocity fields in the imprinting stage and the temperature distributions in ultrasonic vibrations are performed under variations of convexity width, cavity width and thickness of the imprinted polymer resist. The study of the velocity field, including the lateral and vertical velocity, is significant because the velocity field can directly describe the mode of the polymer deformation, which is the key role to determine the mechanism of nanoimprint forming. Though the velocity field is crucial for the polymer deformation, it is often ignored and is rarely found in the literature. In ultrasonic vibrations, the temperature distribution of U-NIL is different from that of the thermal NIL and deserves to be investigated. Moreover, the combined effects of the imprinting stage and ultrasonic vibrations in the U-NIL process are discussed. Furthermore, experiments were conducted to verify the results performed by the numerical simulations. In this work, the simulations and experiments provide a deep understanding of polymer flow affected by the velocity field and temperature distribution during U-NIL, which should be well applied to a thermal NIL.

Issues and Requirements of Polymers for Thermal NIL

Issues and Requirements of Polymers for Thermal NIL