Nanoimprint Lithography:A Processing Technique for Nanofabrication Advancement

Nanoimprint lithography(NIL) is an emerging micro/nano-patterning technique,which is a high-resolution,high-throughput and yet simple fabrication process.According to International Technology Roadmap for Semiconductor(ITRS),NIL has emerged as the next generation lithography candidate for the22 nm and 16 nm technological nodes.In this paper,we present an overview of nanoimprint lithography.The classfication,research focus,critical issues,and the future of nanoimprint lithography are intensively elaborated.A pattern as small as 2.4 nm has been demonstrated.Full-wafer nanoimprint lithography has been completed on a 12-inch wafer.Recently,12.5 nm pattern resolution through soft molecular scale nanoimprint lithography has been achieved by EV Group,a leading nanoimprint lithography technology supplier.

NANOIMPRINT LITHOGRAPHY TECHNOLOGY WITH AUTOMATIC ALIGNMENT

Nanoimprint lithography （NIL） is recognized as one of the most promising candidates for the next generation lithography （NGL） to obtain sub-100 nm patterns because of its simplicity, high-throughput and low-cost. While substantial effort has been expending on NIL for producing smaller and smaller feature sizes, considerably less effort has been devoted to the equally important issue—alignment between template and substrate. A homemade prototype nanoimprint lithography tool with a high precision automatic alignment system based on Moiré signals is presented. Coarse and fine pitch gratings are adopted to produce Moiré signals to control macro and micro actuators and enable the substrate to move towards the desired position automatically. Linear motors with 300 mm travel range and 1 μm step resolution are used as macro actuators, and piezoelectric translators with 50 μm travel range and 1 nm step resolution are used as micro actuators. In addition, the prototype provides one translation （z displacement） and two tilting motion（α and β ） to automatically bring uniform intact contact between the template and substrate surfaces by using a flexure stage. As a result, 10 μm coarse alignment accuracy and 20 nm fine alignment accuracy can be achieved. Finally, some results of nanostructures and micro devices such as nanoscale trenches and holes, gratings and microlens array fabricated using the prototype tool are presented, and hot embossing lithography, one typical NIL technology, are depicted by taking nanoscale gratings fabrication as an example.

Strategies to obtain pattern fidelity in nanowire growth from large-area surfaces patterned using nanoimprint lithography

Position controlled nanowire growth is important for nanowire-based optoelectronic components which rely on light emission or light absorption. For solar energy harvesting applications, dense arrays of nanowires are needed; however, a major obstacle to obtaining dense nanowire arrays is seed particle displacement and coalescing during the annealing stage prior to nanowire growth. Here, we explore three different strategies to improve pattern preservation of large-area catalyst particle arrays defined by nanoimprint lithography for nanowire growth. First, we see that heat treating the growth substrate prior to nanoimprint lithography improves pattern preservation. Second, we explore the possibility of improving pattern preservation by fixing the seed particles in place prior to annealing by modifying the growth procedure. And third, we show that a SiNx growth mask can fully prevent seed particle displacement. We show how these strategies allow us to greatly improve the pattern fidelity of grown InP nanowire arrays with dimensions suitable for solar cell applications, ultimately achieving 100% pattern preservation over the sampled area. The generic nature of these strategies is supported through the synthesis of GaAs and GaP nanowires.

Soft thermal nanoimprint lithography using a nanocomposite mold

Soft nanoimprint lithography has been limited to ultraviolet （UV） curable resists. Here, we introduce a novel approach for soft thermal nanoimprinting. This unprecedented combination of the terms ＂soft＂ and ＂thermal＂ for nanoimprinting became possible thanks to an innovative nanocomposite mold consisting of a flexible polydimethylsiloxane （PDMS） substrate with chemically attached rigid relief features. We used soft thermal nanoimprinting to produce high-resolution nanopatterns with a sub-100 nm feature size. Furthermore, we demonstrate the applicability of our nanoimprint approach for the nanofabrication of thermally imprinted nanopattems on non-planar surfaces such as lenses. Our new nanofabrication strategy paves the way to numerous applications that require the direct fabrication of functional nanostructures on unconventional substrates.

High-refractive index acrylate polymers for applications in nanoimprint lithography

The development of polymeric optical materials with a higher refractive index,transparency in the visible spectrum region and easier processability is increasingly desirable for advanced optical applications such as microlenses,image sensors,and organic light-emitting diodes.Most acrylates have a low refractive index(around 1.50)which does not meet the high perfo rmance requirements of advanced optical materials.In this research,three novel acrylates were synthesized via a facile one-step approach and used to fabricate optical transparent polymers.All of the polymers reveal good optical properties including high transparency(≥90%)in the visible spectrum region and high refractive index values(1.6363)at 550 nm.Moreover,nanostructures of these acrylate polymers with various feature sizes including nanogratings and photonic crystals were successfully fabricated using nanoimprint lithography.These results indicate that these acrylates can be used in a wide range of optical and optoelectronic devices where nanopatterned films with high refractive index and transparency are required.

Discretely-supported nanoimprint lithography for patterning the high-spatial-frequency stepped surface

Non-planar morphology is a common feature of devices applied in various physical fields,such as light or fluid,which pose a great challenge for surface nano-patterning to improve their performance.The present study proposes a discretely-supported nanoimprint.lithography(NIL)technique to fabricate nanostructures on the extremely non-planar surface,namely high-spatial-frequency stepped surface.The designed discretely imprinting template implanted a discretely-supported intermediate buffer layer made of sparse pillars arrays.This allowed the simultaneous generation of air-cushion-like buffer and reliable support to the thin structured layer in the template.The resulting low bending stiffness and distributed concentrated load of the template jointly overcome the contact difficulty with a stepped surface,and enable the template to encase the stepped protrusion as tight as possible.Based on the proposed discretely-supported NIL,nanostructures were fabricated on the luminous interface of light emitting diodes chips that covered with micrometer step electrodes pad.About 96%of the utilized indium tin oxide transparent current spreading layer surface on top of the light emitting diode(LED)chips was coated with nanoholes array,with an increase by more than 40%in the optical output power.The excellent ability of nanopatterning a non-planar substrate could potentially lead innovate design and development of high performance device based on discretely-supported NIL.

Key process study in nanoimprint lithography

Nanoimprint lithography（NIL） is widely used in the fabrication of nano-scale semiconductor devices for its advantages of high resolution,low cost,and high throughput.However,traditional hard stamp imprinting has some drawbacks such as short stamp lifetime,bad uniformity in big areas,and large particle influence.In this paper, a flexible intermediate polymer stamp（IPS） is proposed to solve the drawbacks mentioned above.Meanwhile, we use a method of temperature-pressure variation imprinting to improve the resist liquidity in the process of imprinting,and eventually we achieve high quality patterns.This method combined with IPS has been used to fabricate a high quality grating whose half pitch is 50 nm.

Flexible Broadband Light Absorbers with a Superhydrophobic Surface Fabricated by Ultraviolet-assisted Nanoimprint Lithography

We present a simple approach to fabricate a kind of composite films with a superhydrophobic and broadband light absorbing surface by ultraviolet-assisted nanoimprinting over a gradiently deposited composite matrix.The wettability and optical property of the resultant surfaces are tunable by the deposition time before polymerization(T_(s))and mold’s topography.Mechanically robust and elastomeric films exhibiting high sunlight absorptivity up to 98.13%and contact angle of their surfaces up to 150°are prepared under optimized conditions,as using a mold with a small pattern size(hexagonal periodic mold with cylinder diameter of ca.37μm)under T_(s)=10 min for imprinting the crosslinked poly[di(ethylene glycol)ethyl ether acrylate]and poly(isobornyl acrylate)in the presence of polypyrrole(PPy)nanoparticles.Such dual functions are found related to the hierarchical architecture of the surface,arising from the synergetic effects of the periodical patterned polymer substrate and spontaneously assembled PPy microstructures on the patterns.The current strategy based on the combination of ultraviolet-assisted nanoimprint lithography and hierarchical assembly of gradiently deposited black nano-fillers offers a new insight into the design of robust superhydrophobic and black surfaces,which is helpful to deepen our understanding of the relationship between liquid/light manipulation and micro/nanostructured surfaces.

Spatially-separated and photo-enhanced semiconductor corrosion processes for high-efficient and contamination-free electrochemical nanoimprint lithography

Free of any thermoplastic or photocuring resists, electrochemical nanoimprint lithography(ECNL) has emerged as an alternative nanoimprint way to fabricate three-dimensional micro/nano-structures(3D-MNSs) directly on a semiconductor wafer by a spatially-confined corrosion reaction induced by the metal/semiconductor contact potential. However, the consumption of electron acceptors in the ultrathin electrolyte between imprint mold and semiconductor wafer will slow down or even cease the corrosion rate. To solve this problem, we change the short-circuited corrosion cell into a spatially-separated primary cell: the imprint mold compacted gallium arsenide(GaAs) wafer in the anodic chamber while the platinum(Pt) plate connected to the imprint mold in the cathodic chamber. Thus, the GaAs corrosion rate will be stabilized in its limiting steady-state current density because of the abundant source of electron acceptors in the catholic chamber. The corrosion processes can be photo-enhanced by white-light illumination. Consequently, both the accuracy and the efficiency are promoted dramatically, which are demonstrated by the excellent performance of the fabricated binary optical elements. Moreover, the contamination problem caused by the electron acceptors is totally avoided. All the results prove that this novel ECNL mode is competitive and prospective in imprinting 3D-MNSs directly on semiconductor wafer.

Fabrication of Metallic Crescent-shaped Nanohole Arrays by Combination of Nanoimprint Lithography and Nanotransfer Printing

Molecular detection techniques based on localized surface plasmon resonances shift, surfaced-enhanced Raman spectroscopy, surface-enhanced fluorescence, and plasmon resonance energy transfer are all highly dependent on the intensity of localized electromagnetic fields. Many different nano- structures were fabricated for sensing. Ebbesen and his co-workers discovered that hole-based ＂hot spots＂ could act as optical antennae, which could concentrate the electromagnetic fields into extremely small regions. Many efforts have been devoted to understanding this unique transmission phenomenon in the past decade. The most widely used methods for hole array fabrication are e-beam lithography（EBL） and focused ion beam. The serial nature of these techniques allows only small regions to be patterned, and it is difficult to integrate such structures into integrated sensing architecture. To improve the fabrication efficiency, several other methods have been deve- loped. Wu et al. presented a process to fabricate 2D arrays via a self-assembled monolayer of hexagonally close packed silica and polystyrene microspheres. Li et al. reported a technique based on a combination of colloidal lithography and parallel imprinting for fabricating crescent-shaped nanohole structures. Through soft interference lithography, Henzie et al.

Development and Implementation of a Rotating Nanoimprint Lithography Tool for Orthogonal Imprinting on Edges of Curved Surfaces

Uniform molding and demolding of structures on highly curved surfaces through conformal contact is a crucial yet often-overlooked aspect of nanoimprint lithography(NIL).This study describes the development of a NIL tool and its integration into a nanopositioning and nanomeasuring machine to achieve high-precision orthogonal molding and demolding for soft ultraviolet-assisted NIL(soft UV-NIL).The process was implemented primarily on the edges of highly curved plano-convex substrates to demonstrate structure uniformity on the edges.High-resolution nanostructures of sub-200-nm lateral dimension and microstructures in the range of tens of microns were imprinted.However,the nanostructures on the edges of the large,curved substrates were difficult to characterize precisely.Therefore,microstructures were used to measure the structure fidelity and were characterized using profilometry,white light interferometry,and confocal laser scanning microscopy.Regardless of the restricted imaging capabilities at high inclinations for high-resolution nanostructures,the scanning electron microscope(SEM)imaging of the structures on top of the lens substrate and at an inclination of 45°was performed.The micro and nanostructures were successfully imprinted on the edges of the plano-convex lens at angles of 45°,60°,and 90°from the center of rotation of the rotating NIL tool.The method enables precise imprinting at high inclinations,thereby presenting a different approach to soft UV-NIL on curved surfaces.

Flexible nanoimprint lithography enables high-throughput manufacturing of bioinspired microstructures on warped substrates for efficient III-nitride optoelectronic devices

III-nitride materials are of great importance in the development of modern optoelectronics,but they have been limited over years by low light utilization rate and high dislocation densities in heteroepitaxial films grown on foreign substrate with limited refractive index contrast and large lattice mismatches.Here,we demonstrate a paradigm of high-throughput manufacturing bioinspired microstructures on warped substrates by flexible nanoimprint lithography for promoting the light extraction capability.We design a flexible nanoimprinting mold of copolymer and a two-step etching process that enable high-efficiency fabrication of nanoimprinted compound-eye-like Al2O3 microstructure(NCAM)and nanoimprinted compound-eye-like SiO_(2)microstructure(NCSM)template,achieving a 6.4-fold increase in throughput and 25%savings in economic costs over stepper projection lithography.Compared to NCAM template,we find that the NCSM template can not only improve the light extraction capability,but also modulate the morphology of AlN nucleation layer and reduce the formation of misoriented GaN grains on the inclined sidewall of microstructures,which suppresses the dislocations generated during coalescence,resulting in 40%reduction in dislocation density.This study provides a low-cost,high-quality,and high-throughput solution for manufacturing microstructures on warped surfaces of III-nitride optoelectronic devices.

High-aspect-ratio nanoimprint process chains

Different methods capable of developing complex structures and building elements with high-aspect-ratio nanostructures combined with microstructures,which are of interest in nanophotonics,are presented.As originals for subsequent replication steps,two families of masters were developed:(i)3.2μm deep,180 nm wide trenches were fabricated by silicon cryo-etching and(ii)9.8μm high,350 nm wide ridges were fabricated using 2-photon polymerization direct laser writing.Both emerging technologies enable the vertical smooth sidewalls needed for a successful imprint into thin layers of polymers with aspect ratios exceeding 15.Nanoridges with high aspect ratios of up to 28 and no residual layer were produced in Ormocers using the micromoulding into capillaries(MIMIC)process with subsequent ultraviolet-curing.This work presents and balances the different fabrication routes and the subsequent generation of working tools from masters with inverted tones and the combination of hard and soft materials.This provides these techniques with a proof of concept for their compatibility with high volume manufacturing of complex micro-and nanostructures.

Plasmonic nanostructure characterized by deep-neuralnetwork-assisted spectroscopy[Invited]

The lateral geometry and material property of plasmonic nanostructures are critical parameters for tailoring their optical resonance for sensing applications.While lateral geometry can be easily observed by a scanning electron microscope or an atomic force microscope,characterizing materials properties of plasmonic devices is not straightforward and requires delicate examination of material composition,cross-sectional thickness,and refractive index.In this study,a deep neural network is adopted to characterize these parameters of unknown plasmonic nanostructures through simple transmission spectra.The network architecture is established based on simulated data to achieve accurate identification of both geometric and material parameters.We then demonstrate that the network training by a mixture of simulated and experimental data can result in correct material property recognition.Our work may indicate a simple and intelligent characterization approach to plasmonic nanostructures by spectroscopic techniques.