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

High-Throughput, Continuous Nanopatterning Technologies for Display and Energy Applications.

The motivation of this work is to enable continuous patterning of nanostructures on flexible substrates to push nanoscale lithography to an entirely new level with drastically increased throughput. The Roll-to-Roll Nanoimprint Lithography (R2RNIL) technology presented in this work retains the high-resolution feature capabilities of traditional NIL, but with an increase in throughput by at least one or two orders of magnitude. We demonstrated large-area (4” wide) continuous imprinting of nanogratings by using a newly developed apparatus capable of roll-to-roll imprinting on flexible substrates (R2RNIL) and roll-to-plate imprinting on rigid substrates (R2PNIL). In addition, analytical models were developed to predict the residual layer thickness in dynamic R2RNIL. As a potential application, high-performance metal wire-grid polarizers have also been fabricated utilizing R2RNIL. Another research focus involved Direct Metal Imprinting (DMI) to create discrete nano-scale metal gratings. DMI uses a polymer cushion layer between a thin metal layer and a hard substrate, which enables room-temperature nanoimprinting of the metal by overcoming troublesome hard-to-hard surface contact issues while preserving the Si mold. We also introduced a novel nanofabrication technique, Dynamic Nano-Inscribing (DNI) for creating truly continuous nanograting patterns by using the sharp edge of a tilted Si mold on a variety of metals or polymer materials, creating linewidths down to 50 nm at extremely high speeds (~100 mm/sec) under ambient conditions. Additionally, a new nanograting fabrication method, Localized Dynamic Wrinkling (LDW) has been developed. LDW enables the continuous formation of micro/nano-scale gratings by simply sliding a flat edge of a cleaved Si wafer over the metal film. LDW shares the same basic principle as the buckling (wrinkling) phenomenon but the moving edge of the tilted Si wafer exerts stress on a metal coated polymer and sequentially generates localized winkles in the metal film in a dynamic fashion. The period in LDW can be controlled by several processing parameters and shows good agreement with a theoretical model. Finally, we developed a Dynamic Nano-Cutting (DNC) process using high-frequency indentations on a moving substrate to sequentially create nanograting patterns. DNC provides perfectly straight lines with real-time period modulation, which is difficult to achieve by other nanomanufacturing techniques.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/76015/1/happyash_1.pd

Scaleable nanomanufacturing of metasurfaces using microsphere photolithography

“The cost-effective manufacturing of metasurfaces over large areas is a critical issue that limits their implementations. Microsphere photolithography (MPL) uses a scalable self-assembled microsphere array as an optical element to focus collimated light to nanoscale photonic jets in a photoresist layer. This dissertation investigates the fabrication capabilities, process control, and potential applications of MPL. First, the MPL concept is applied to the fabrication of metasurfaces with engineered IR absorption (e.g. perfect absorption with multiband/broadband and wavelength/polarization dependences). Improving the patterning of the photoresist requires a fundamental understanding the photochemical photonic jet interactions. The dissertation presents a model of the MPL process with a cellular automata algorithm to simulate the development process. The model accurately predicts the size and shape of the features generated from MPL. It enables the identification of fabrication conditions to improve the resolution for the MPL process. Finally, the dissertation discusses the potential for a reusable microsphere array. Control of the contact forces is critical for minimizing the gap in between the microsphere array and the substrate and maintaining the consistent performance. Overall, the dissertation provides a foundation for understanding the process-structure-performance relationships for the fabrication of metasurfaces using microsphere photolithography. The use of the MPL for the fabrication of metasurfaces, with application such as sensing and thermal management, is novel as is the modeling of the MPL process”--Abstract, page iv

Fabrication of Ultrafine Structures and Functional Surfaces Over Large Areas

Micro-/nanofabrication finds wide use in the preparation of patterned surfaces, which are of vital importance in the applications of functional surfaces, electronics, optics, and sensors. This dissertation will present the preparation of ultrafine structures and functional surfaces over large areas utilizing a combination of top-down lithography technique and bottom-up wrinkling method. First, a simple, scalable and cost-effective spacer lithography approach utilizing polydopamine coating technique was developed for the fabrication of well-controlled nanopatterns with feature size in the sub-20 nm scale. Briefly, a thin layer of polydopamine (PDA) was conformally deposited on the sidewalls of pre-patterned poly (methyl methacrylate) (PMMA) features to form a spacer layer. After etching the PDA film on the horizontal surfaces and removing the PMMA resist, only the PDA spacer was left on the substrate, forming a new pattern. The pattern density of the new features was doubled and the feature size was well tuned to a sub-20 nm scale. We also utilized a bottom-up wrinkling technique to spontaneously pattern and functionalize polymer films. Through a reactive silane-infusion induced surface instability, wrinkled patterns with tunable wavelengths were easily produced over large areas and the surface chemical functionality of the wrinkles was well-tuned by the infusion of different functional silanes. Hierarchically wrinkled patterns with micro/nano structure were achieved by combining wrinkling with nanoimprint lithography. The hierarchically wrinkled surfaces exhibited superhydrophobicity with water contact angles higher than 160°and water sliding angle lower than 5°. To scale up these patterned functional surfaces over large areas, roll-to-roll nanoimprint lithography technique was investigated for the continuous fabrication of superhydrophobic surfaces and lubricant infused patterned surfaces based on hierarchically wrinkled surfaces. These functionally patterned surfaces displayed self-cleaning properties to a variety of liquid contaminations and anti-biofouling properties when challenged with Escherichia coli bacteria. This study suggested a potential transformation of artificial biomimetic structures from small, lab scale coupons to low cost, large area platforms

Nanowire Alignment: Techniques, Quantification, and Applications in Large-Area Devices

Nanowire alignment is essential for their integration into large-area devices, as well as to obtain certain functionality such as the ability to polarize light and increase surface-enhanced Raman scattering. Various nanowire alignment methods have been developed, however, major drawbacks have limited their application such as complex processing, high cost, limited compatible nanowire materials, and limited scalability. In addition, the methods used to quantify the quality of nanowire alignment are lacking in accuracy, speed, and applicability to all kinds of nanowires. In this thesis, two simple and large-area alignment methods are studied that are applicable for nanowires synthesized by any method and compatible with large-area electronic device fabrication processes. The first method is accomplished by depositing nanowires on polyvinyl alcohol films followed by film stretching, which achieves high-quality alignment (with an order parameter S=0.93). Nanowire breakage, which is commonly encountered in similar techniques, is minimized and the average length of nanowires after alignment is nearly the same (~99.3%) as before alignment. The second alignment method is accomplished directly during rod-coating deposition of the nanowires, without the need of any additional step. Two image processing methods based on edge-detection and skeletonication are presented to recognize nanowires from microscopy images. Then an order parameter and an orientational distribution function are used for alignment quantification. Compared with previously reported studies, these methods are fast and automated, reliable without bias, generally applicable, easy to implement, and computationally efficient. The alignment methods described above are applied in two applications. Firstly, the electrical and optical anisotropy of slightly aligned silver nanowire films, which can be used as transparent electrodes, are investigated. Their transparency to polarized light is increased by 7.3 percentage points compared to typical randomly oriented silver nanowire films, which may benefit end uses such as liquid crystal displays and the touch sensors on top of them. Secondly, a crossed film structure consisting of semiconductor nanowires aligned in one direction and metal nanowires orthogonally aligned is designed. The metal nanowires are intended to act as interconnects to substantially reduce semiconductor nanowire-nanowire junction resistances while avoiding lithographically-defined metal pads, the latter which can have poor mechanically flexibility and involve fabrication processes not desired for large-area electronics. Such a device structure can be developed further for use in large-area flexible devices such as light, strain and chemical sensors and energy generators

Plasmonic Waveguide Lithography for Patterning Nanostructures with High Aspect-Ratio and Large-Area Uniformity

The rapid development of the semiconductor industry in the past decades has driven advances in nano-manufacturing technologies towards higher resolution, higher throughput, better large-area uniformity, and lower manufacturing cost. Along with these advancements, as the size of the devices approaches tens of nanometers, challenges in patterning technology due to limitations in physics, equipment and cost have quickly arisen. To solve these problems, unconventional lithography systems have attracted considerable interest as promising candidates to overcome the diffraction limit. One recently evolved technology, plasmonic lithography, can generate subwavelength features utilizing surface plasmon polaritons (SPPs). Evanescent waves generated by the subwavelength features can be transmitted to the photoresist (PR) using plasmonic materials. Another approach of plasmonic lithography involves the use of hyperbolic metamaterial (HMM) structures, which have been studied intensively because of their unique electromagnetic properties. Specifically, epsilon near zero (ENZ) HMMs offer the potential to produce extremely small features due to their high optical anisotropy. Despite the advancements in plasmonic lithography, several key issues impede progress towards more practical application, which includes shallow pattern depth (due to the evanescent nature of SPPs), non-uniformity over a large area (due to the interference of multiple diffraction orders) and high sensitivity of the roughness on the films and defects on the mask. The light intensity in the PR is very weak which results in an extremely long exposure time. To this end, this dissertation is dedicated to plasmonic lithography systems based on SPP waveguides and ENZ HMMs for patterning nanostructures with high aspect-ratio and large-area uniformity. New schemes are exploited in this thesis to address these challenges. Lithography systems based on a specially designed waveguide and an ENZ HMM are demonstrated. By employing the spatial filtering properties of the waveguide and the ENZ HMM, the period, linewidth and height of the patterns can be well controlled according to various design purposes. Periodic structures were achieved in both systems with a half-pitch of approximately 50 ~ 60 nm, which is 1/6 of the exposure wavelength of 405 nm. The thickness of the PR layer is around 100 ~ 250 nm, which gives an aspect-ratio higher than 2:1. The subwavelength patterns are uniform in cm2 areas. In addition to the design principle, various numerical simulations, fabrication conditions and corresponding results are discussed. The design principle can be generalized to other materials, structures and wavelengths. The real-world performance of the lithography system considering non-idealities such as line edge roughness and single point defect is analyzed. Comparisons between the plasmonic systems based on different design rules are also carried out, and the advantages of the spatial frequency selection principle is verified. The plasmonic waveguide lithography systems developed in this dissertation provide a technique to make deep subwavelength features with high aspect-ratio, large-area uniformity, high light intensity distribution, and low line-edge-roughness for practical applications. Compared with the previously reported results, the performance of plasmonic lithography is drastically improved. A plasmonic roller system combining the photo-roller system and plasmonic lithography is also developed. This plasmonic roller system can support a continuous patterning with a high throughput for cost sensitive applications. Several potential applications of the plasmonic materials including near field spin Hall effects and a particle based Lidar design are explored. Other advances towards plasmonic functional devices including silicon (Si) nanowire (NW) arrays, light-thermal converters and plasmonic lasers are also reported.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144126/1/sxichen_1.pd

Hybrid Nanomaterials

Two of the hottest research topics today are hybrid nanomaterials and flexible electronics. As such, this book covers both topics with chapters written by experts from across the globe. Chapters address hybrid nanomaterials, electronic transport in black phosphorus, three-dimensional nanocarbon hybrids, hybrid ion exchangers, pressure-sensitive adhesives for flexible electronics, simulation and modeling of transistors, smart manufacturing technologies, and inorganic semiconductors

Functional optical surfaces by colloidal self-assembly: Colloid-to-film coupled cavities and colloidal lattices

Future developments in nanophotonics require facile, inexpensive and parallelizable fabrication methods and need a fundamental understanding of the spectroscopic properties of such nanostructures. These challenges can be met through colloidal self-assembly where pre-synthesized colloids are arranged over large areas at reasonable cost. As so-called colloidal building blocks, plasmonic nanoparticles and quantum dots are used because of their localized light confinement and localized light emission, respectively. These nanoscopic colloids acquires new hybrid spectroscopic properties through their structural arrangement. To explore the energy transfer between these nanoscopic building blocks, concepts from physical optics are used and implemented with the colloidal self-assembly approach from physical chemistry. Through an established synthesis, the nanocrystals are now available in large quantities, any they receive the tailored spectroscopic properties through directed self-assembly. Moreover, the tailored properties of the colloids and the use of stimuli-responsive polymers allow a functionality that goes beyond current developments. The basics developed in this habilitation thesis can lead to novel functional devices in the field of smart sensors, dynamic light modulators, and large-area quantum devices.:1 Abstract 2 2 State of the art 4 2.1 Metallic and semiconductive nanocrystals as colloidal building blocks 4 2.2 Concept of large-scale colloidal self-assembly 7 2.3 Functional optical nanomaterials by colloidal self-assembly 9 2.4 Scope 13 2.5 References 14 3 Single colloidal cavities 20 3.1 Nanorattles with tailored electric field enhancement 20 4 Colloidal -to-film coupled cavities 31 4.1 Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces 31 4.2 Single particle spectroscopy of radiative processes in colloid-to-film-coupled nanoantennas 50 4.3 Active plasmonic colloid-to-film coupled cavities for tailored light-matter interactions 65 5 Colloidal polymers 74 5.1 Direct observation of plasmon band formation and delocalization in quasi-infinite nanoparticle chains 74 6 Colloidal lattice 84 6.1 Hybridized guided-node resonances via colloidal plasmonic self-assembled grating 84 6.2 Mechanotunable surface lattice resonances in the visible optical range by soft lithography templates and directed self-assembly 94 6.3 Tunable Circular Dichroism by Photoluminescent Moiré Gratings 103 7 Conclusion and perspective 112 8 Appendix 113 8.1 Further publications during the habilitation period 113 8.2 Curriculum vitae of the author 116 9 Acknowledgments 117 10 Declaration 118Zukünftige Entwicklungen in der Nanophotonik erfordern einfache, kostengünstige und parallelisierbare Herstellungsmethoden und benötigen ein grundlegendes Verständnis der spektroskopischen Eigenschaften solcher Nanostrukturen. Diese Herausforderungen können durch kolloidale Selbstorganisation erfüllt werden, bei der kostengünstige und zuvor synthetisierte Kolloide großflächig angeordnet werden. Als sogenannte kolloide Bausteine werden wegen ihrer lokalisierten Lichtfokussierung unterhalb der Beugungsbegrenzung plasmonische Nanopartikel sowie wegen ihrer lokalisierten Lichtemission Quantenpunkte verwendet. Diese nanoskopischen Kolloide werden in dieser Habilitationsschrift verwendet und durch Selbstanordnung in ihre gewünschte Nanostruktur gebracht, die neue hybride Eigenschaften aufweist. Um den Energietransfer zwischen diesen nanoskopischen Bausteinen zu untersuchen, werden Konzepte aus der physikalischen Optik verwendet und mit dem kolloidalen Selbstorganisationskonzept aus der physikalischen Chemie großflächig umgesetzt. Durch eine etablierte Synthese sind die Nanokristalle nun in großen Mengen verfügbar, wobei sie durch gerichtete Selbstorganisation die gewünschten spektroskopischen Eigenschaften erhalten. Darüber hinaus ermöglicht die Verwendung von stimulierbaren Polymeren eine Funktionalität, die über die bisherigen Entwicklungen hinausgeht. Die in dieser Habilitationsschrift entwickelten Grundlagen können bei der Entwicklung neuartiger Funktionsgeräte im Bereich für intelligente Sensorik, dynamischer Lichtmodulatoren und großflächiger Quantengeräte genutzt werden.:1 Abstract 2 2 State of the art 4 2.1 Metallic and semiconductive nanocrystals as colloidal building blocks 4 2.2 Concept of large-scale colloidal self-assembly 7 2.3 Functional optical nanomaterials by colloidal self-assembly 9 2.4 Scope 13 2.5 References 14 3 Single colloidal cavities 20 3.1 Nanorattles with tailored electric field enhancement 20 4 Colloidal -to-film coupled cavities 31 4.1 Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces 31 4.2 Single particle spectroscopy of radiative processes in colloid-to-film-coupled nanoantennas 50 4.3 Active plasmonic colloid-to-film coupled cavities for tailored light-matter interactions 65 5 Colloidal polymers 74 5.1 Direct observation of plasmon band formation and delocalization in quasi-infinite nanoparticle chains 74 6 Colloidal lattice 84 6.1 Hybridized guided-node resonances via colloidal plasmonic self-assembled grating 84 6.2 Mechanotunable surface lattice resonances in the visible optical range by soft lithography templates and directed self-assembly 94 6.3 Tunable Circular Dichroism by Photoluminescent Moiré Gratings 103 7 Conclusion and perspective 112 8 Appendix 113 8.1 Further publications during the habilitation period 113 8.2 Curriculum vitae of the author 116 9 Acknowledgments 117 10 Declaration 11

Nanoimprint Lithography Technology and Applications

Nanoimprint Lithography (NIL) has been an interesting and growing field in recent years since its beginnings in the mid-1990s. During that time, nanoimprinting has undergone significant changes and developments and nowadays is a technology used in R&D labs and industrial production processes around the world. One of the exciting things about nanoimprinting process is its remarkable versatility and the broad range of applications. This reprint includes ten articles, which represent a small glimpse of the challenges and possibilities of this technology. Six contributions deal with nanoimprint processes aiming at specific applications, while the other four papers focus on more general aspects of nanoimprint processes or present novel materials. Several different types of nanoimprint processes are used: plate-to-plate, roll-to-plate, and roll-to-roll. Plate-to-plate NIL here also includes the use of soft and flexible stamps. The application fields in this reprint are broad and can be identified as plasmonics, superhydrophibicity, biomimetics, optics/datacom, and life sciences, showing the broad applicability of nanoimprinting. The sections on the nanoimprint process discuss filling and wetting aspects during nanoimprinting as well as materials for stamps and imprinting