High-resolution Nanopatterning Research Articles

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. Thisunprecedented combination of the terms “soft” and “thermal” for nanoimprinting became possible thanks to an innovative nanocomposite mold consisting of aflexible polydimethylsiloxane (PDMS) substrate with chemically attached rigidrelief 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 thermallyimprinted nanopatterns 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.

Enhanced mechanical properties of the high-resolution EUVL patterns of hybrid photoresists containing hexafluoroantimonate

The present study investigates the mechanical properties of hybrid photoresists in the context of their pattern-collapse behaviors. The mechanical properties such as the DMT (Derjaguin, Muller, Toropov) modulus and tip-sample adhesion forces of the high-resolution patterns obtained from two hybrid EUV photo-resists, 1.5% & 2.15%-MAPDSA-MAPDST bearing hexafluoroantimonate and triflate counter ion moieties have been investigated using peak force quantitative nano-mechanical mapping (PF-QNM) technique. The mechanical properties of the well resolved high-resolution 90–20 nm (L/5S) line patterns, 20–32 nm (L/2S-L/5S) lines patterns and nano-features such as line-elbow connections have been investigated and analyzed against the differences in their SbF6− composition. For the 1.5%- MAPDSA-MAPDST resist case the DMT modulus and tip-sample adhesion forces are found strong dependence on the resist line width and line spacing, as compared to 2.15%- MAPDSA-MAPDST resist. There is a significant improvement in the modulus value of 5 GPa for 2.15%-MAPDSA-MAPDST resist in contrary to the ~2.7 GPa for the 1.5%-MAPDSA-MAPDST resist for the 20 nm (L/2S) Line patterns. Similarly, the tip sample adhesion forces on resist surface are also found dependent on patterns aspect ratio as well as on SbF6− content in the resist composition. These studies revealed that an increase of the SbF6− content in the resist formulation, imparts cascading effects to the mechanical properties of their high-resolution nanopatterns, which in turns helps to reduce the pattern collapse resulting in superior patterning performances.

Proximity effect correction in electron-beam lithography based on computation of critical-development time with swarm intelligence

Electron-beam lithography (EBL) is an important technique in manufacturing high-resolution nanopatterns for broad applications. However, the proximity effect in EBL can degrade the pattern quality and, thus, impact the performance of the applications greatly. The conventional proximity effect correction (PEC) methods, which employ computationally intensive cell or path removal method for development simulation, are very computational lengthy, especially for complex and large-area patterns. Here, the authors propose a novel short-range PEC method by transforming the evaluation of pattern feasibility into the shortest path problem based on the concept of critical-development time. The authors combine this evaluation algorithm with the swarm intelligence which mimics the natural collective behavior of animals to optimize the design of electron dose distribution in EBL. The PEC algorithm is applied for pattern fabrication for U-shaped split-ring resonator and produces optimized exposure pattern that shows excellent agreement with the targeted objectives. Our work on the PEC strategy reduces the computational cost significantly and is particularly suitable for the design of complex pattern with various constraints.

High-aspect ratio nanopatterning via combined thermal scanning probe lithography and dry etching

Thermal scanning probe lithography is an emerging nanofabrication technique for rapid prototyping of arbitrary topographies in thermally sensitive resist. This feature, paired to the recent advances in dry plasma etching techniques, allows the fabrication of high-resolution nanopatterns in hard substrates. Here, we investigate the key process parameters allowing the fabrication of high aspect ratio nanopatterns in silicon. By a combination of resist heat treatment, the use of a hard mask and optimized etch parameters during pattern transfer, we amplified the shallow resist patterns by a factor of 100 into the silicon substrate. Low surface roughness and vertical sidewalls are thereby maintained. We demonstrate the fabrication of 240nm wide lines and 4μm deep single crystal silicon patterns.

High-resolution nanopatterning of biodegradable polylactide by thermal nanoimprint lithography using gas permeable mold

We report high-resolution (150 nm) nanopatterning of biodegradable polylactide by thermal nanoimprint lithography using dichloromethane as a volatile solvent for improving the liquidity and a porous cyclodextrin-based gas-permeable mold. This study demonstrates the high-resolution patterning of polylactic acid and other non-liquid functional materials with poor fluidity by thermal nanoimprinting. Such a patterning is expected to expand the utility of thermal nanoimprint lithography and fabricate non-liquid functional materials suitable for eco-friendly and biomedical applications.

Laser-Assisted Focused He+ Ion Beam Induced Etching with and without XeF2 Gas Assist.

Focused helium ion (He+) milling has been demonstrated as a high-resolution nanopatterning technique; however, it can be limited by its low sputter yield as well as the introduction of undesired subsurface damage. Here, we introduce pulsed laser- and gas-assisted processes to enhance the material removal rate and patterning fidelity. A pulsed laser-assisted He+ milling process is shown to enable high-resolution milling of titanium while reducing subsurface damage in situ. Gas-assisted focused ion beam induced etching (FIBIE) of Ti is also demonstrated in which the XeF2 precursor provides a chemical assist for enhanced material removal rate. Finally, a pulsed laser-assisted and gas-assisted FIBIE process is shown to increase the etch yield by ∼9× relative to the pure He+ sputtering process. These He+ induced nanopatterning techniques improve material removal rate, in comparison to standard He+ sputtering, while simultaneously decreasing subsurface damage, thus extending the applicability of the He+ probe as a nanopattering tool.

Towards a novel positive tone resist mr-PosEBR for high resolution electron-beam lithography

Herein, we present the results of a systematic material development study we carried out in order to obtain a new positive tone resist for high resolution electron-beam lithography. Several acrylic copolymer materials with different mass fractions of the comonomers, different molecular weights and similar molecular weight distributions were synthesized and – as resist solutions – evaluated in terms of electron-beam lithography performance. On the one hand, within the ranges investigated, it was shown that the lithographic sensitivity is significantly influenced by the composition rather than by the molecular weight or molecular weight distribution. On the other hand, the etch resistance of the materials remains unaffected by changes of these parameters. The resist material exhibiting the best combination of the desired properties, mr-PosEBR, is 2 times more sensitive than PMMA (495kDa) and performs comparably to the known high resolution resist ZEP520A. For example, a grating pattern with 29nm wide lines with a period of 100nm could be generated in films of mr-PosEBR with an area dose of 100μC/cm2. In terms of resolution, single lines of only 35nm width could be fabricated via metal lift-off using 100kV EBL. Furthermore, the dry etch stability of mr-PosEBR in a reactive-ion etching (RIE) process (etch gases: CF4/SF6) is similar to the one of ZEP520A (etch rates, mr-PosEBR: 190nm/min, ZEP520A: 150nm/min, silicon: 440nm/min). Moreover, high resolution nanopatterns in mr-PosEBR could be smoothly transferred into the underlying Si substrate by a RIE process.

High-Resolution Metal Nanopatterning by Means of Switchable Block Copolymer Templates.

In this review, recent developments in the fabrication of hexagonal and parallel ordered arrays of metallic nanodomains on a substrate are described. We focus on the nanopatterning approach by means of switchable block copolymer thin films. This approach is highly advantageous, because it can lead to extremely regular patterns with metal subunits of only a few nanometers in diameter and center-to-center distances of tens of nanometers. Hence, the resulting 1D or 2D periodic arrays of metal nanodots and nanowires on silicon substrates can be fabricated with extremely high unit densities and on very large areas. The templated deposition of presynthesized metal nanoparticles on functional block copolymers is described in detail. Current challenges are discussed and an outlook for further developments is given.

Defect/oxygen assisted direct write technique for nanopatterning graphene.

High resolution nanopatterning of graphene enables manipulation of electronic, optical and sensing properties of graphene. In this work we present a straightforward technique that does not require any lithographic mask to etch nanopatterns into graphene. The technique relies on the damaged graphene to be etched selectively in an oxygen rich environment with respect to non-damaged graphene. Sub-40 nm features were etched into graphene by selectively exposing it to a 100 keV electron beam and then etching the damaged areas away in a conventional oven. Raman spectroscopy was used to evaluate the extent of damage induced by the electron beam as well as the effects of the selective oxidative etching on the remaining graphene.

Application of thermal initiator for characteristic improvements of polymeric replica mold for UV nanoimprint lithography.

We report the improvement of the hardness and modulus properties in a silsesquioxane-based soft replica mold by adding thermal initiator, without deteriorating the UV transmittance at the wavelength of 365 nm. It is found that thermal initiator used for this work contributes to improving the hardness and modulus values up to 0.175 and 3.585 GPa, while the UV transmittance value is still above 75%. The optimized soft replica mold built on a flexible plastic substrate allows submicron-scale patterns to be transferred onto a rigid Si substrate by means of UV-NIL process. Consequently, we demonstrate that the developed soft replica mold can be a suitable replacement for typical hard molds, promising further use in mold-based nanolithography for the fabrication of high-resolution nanopatterns over large areas.

High-Resolution Resistless Nanopatterning on Polymer and Flexible Substrates for Plasmonic Biosensing Using Stencil Masks

The development of nanoscale lithographic methods on polymer materials is a key requirement to improve the spatial resolution and performance of flexible devices. Here, we report the fabrication of metallic nanostructures down to 20 and 50 nm in size on polymer materials such as polyimide, parylene, SU-8, and PDMS substrates without any resist processing using stencil lithography. Metallic nanodot array analysis of their localized surface plasmon spectra is included. We demonstrate plasmon resonance detection of biotin and streptavidin using a PDMS flexible film with gold nanodots. We also demonstrate the fabrication of metallic nanowires on polyimide substrates with their electrical characteristics showing an ohmic behavior. These results demonstrate high-resolution nanopatterning and device nanofabrication capability of stencil lithography on polymer and flexible substrates.

High-resolution nanopatterning by achromatic spatial frequency multiplication with electroplated grating structures

The authors demonstrate generation of high-resolution nanostructures using achromatic spatial frequency multiplication in the extreme ultraviolet wavelength region. The technique based on the achromatic Talbot effect is used for periodic transmission gratings under wideband illumination, enabling one- and two-dimensional nanopatterns with sub-20 nm feature sizes. The transmission masks with desired properties are fabricated with electron-beam lithography followed by electroplating of gold. Features sizes down to 12 nm are obtained. The presented technique provides high-throughput and large-area nanopatterning with great flexibility in tuning pattern parameters such as linewidth and dot size.

Nanoimprint lithography with ≤60 nm overlay precision

Nanoimprint lithography is a high-resolution, high-throughput and cost-effective nanopatterning technology. However, the overlay accuracy is lagging behind the resolution because of the high cost of mechanical precision. We have built an inexpensive stand-alone machine based on the wafer bowing nanoimprint process, and demonstrated single-point overlay of two transferred pattern layers with an accuracy of ≤60 nm.

Rapid nanopatterning of zirconium dioxide via nanoprinting and microwave-assisted annealing

High resolution nanopatterning of metal oxide materials using low cost and high throughput processes is necessary for the ongoing development of electronic and optical devices. In this work, we demonstrate the fabrication of a sub-50 nm ZrO2 nanomesh structure by size reduction induced volume shrinkage that accompanies microwave-assisted annealing. The effect of microwave annealing time on pattern dimension variations such as hole size, wall thickness, and pattern depth is investigated. The evolution of the crystallinity as well as the optical properties of the ZrO2 thin films, including the refractive index and transmittance, are studied as a function of annealing time. Interestingly, the pure tetragonal crystalline phase of ZrO2 can be obtained within minutes by rapid microwave-assisted annealing. We believe that the nanopatterning of inorganic oxide materials using the size reduction effect and rapid crystallization capability of microwave annealing is very useful in the field of nanoscale patterning and for applications involving nanoscale oxide materials.

Sub-10 nm Nanofabrication via Nanoimprint Directed Self-Assembly of Block Copolymers

Directed self-assembly (DSA) of block copolymers (BCPs), either by selective wetting of surface chemical prepatterns or by graphoepitaxial alignment with surface topography, has ushered in a new era for high-resolution nanopatterning. These pioneering approaches, while effective, require expensive and time-consuming lithographic patterning of each substrate to direct the assembly. To overcome this shortcoming, nanoimprint molds--attainable via low-cost optical lithography--were investigated for their potential to be reusable and efficiently template the assembly of block copolymers (BCPs) while under complete confinement. Nanoimprint directed self-assembly conveniently avoids repetitive and expensive chemical or topographical prepatterning of substrates. To demonstrate this technique for high-resolution nanofabrication, we aligned sub-10 nm resolution nanopatterns using a cylinder-forming, organic-inorganic hybrid block copolymer, polystyrene-block-polydimethylsiloxane (PS-b-PDMS). Nanopatterns derived from oxidized PDMS microdomains were successfully transferred into the underlying substrate using plasma etching. In the development phase of this procedure, we investigated the role of mold treatments and pattern geometries as DSA of BCPs are driven by interfacial chemistry and physics. In the optimized route, silicon molds treated with PDMS surface brushes promoted rapid BCP alignment and reliable mold release while appropriate mold geometries provided a single layer of cylinders and negligible residual layers as required for pattern transfer. Molds thus produced were reusable to the same efficacy between nanoimprints. We also demonstrated that shear flow during the nanoimprint process enhanced the alignment of the BCP near open edges, which may be engineered in future schemes to control the BCP microdomain alignment kinetics during DSA.

All-Optical High-Resolution Nanopatterning and 3D Suspending of Graphene

We introduce a laser-based technique capable of both imaging and patterning graphene with high spatial resolution. Both tasks are performed in situ using the same confocal microscope. Imaging graphene is based on the recombination of a laser-created electron-hole plasma yielding to a broadband up- and down-converted fluorescence. Patterning is due to burning graphene by local heating causing oxidation and conversion into CO(2). By shaping the laser beam profile using 1D phase-shifting plates and 2D vortex plates we can produce graphene dots below 100 nm in diameter and graphene nanoribbons down to 20 nm in width. Additionally, we demonstrate that this technique can also be applied to freely suspended graphene resulting in freely suspended graphene nanoribbons. We further present a way of freely hanging graphene vertically and imaging it in 3D. Taking advantage of having vertically hanging graphene for the first time, we measure the out-of-plane anisotropy of the upconversion fluorescence.

Effect of Bridging Position on the Two-Photon Polymerization Initiating Efficiencies of Novel Coumarin/Benzylidene Cyclopentanone Dyes

One- and two-photon photophysical and photochemical properties of dyes 3-DAC and 4-DAC containing coumarin and benzylidene cyclopentanone moieties were studied. Their ground state configurations were optimized using both Hartree-Fock and density functional theory (B3LYP functional) methods, and excited-state properties were calculated using time-dependent density functional theory. These two dyes share the same formula and possess similar structures, except for different bridging positions between the benzylidene cyclopentanone and coumarin moieties. The bridging position was found to have a significant effect on the electronic structure and photophysical and photochemical properties of the dyes. 3-DAC shows higher conjugation and is more planar than 4-DAC, and it exhibits a larger TPA cross section. In contrast, 4-DAC has a twisted conformation, exhibits a lower electron transfer free energy with initiator and shows higher sensitizing efficiencies in one-photon polymerization. Furthermore, the application potential of 3-DAC and 4-DAC in two-photon polymerization (TPP) was studied. Both dyes could be used directly as initiators in TPP. The TPP threshold energies of the corresponding resins were as low as the best reported results. High-resolution 2D and 3D nanopatterns containing low amounts of small molecule residue were successfully fabricated by TPP, demonstrating the extensive application prospects of these dyes in the fabrication of micromachines, microsensor arrays and biomedical devices.

Atomic-Scale Templates Patterned by Ultrahigh Vacuum Scanning Tunneling Microscopy on Silicon

The ultrahigh vacuum (UHV) scanning tunneling microscope (STM) enables patterning and characterization of the physical, chemical, and electronic properties of nanostructures on surfaces with atomic precision. On hydrogen-passivated Si(100) surfaces, selective nanopatterning with the STM probe allows the creation of atomic-scale templates of dangling bonds surrounded by a robust hydrogen resist. Feedback-controlled lithography, which can remove a single hydrogen atom from the Si(100):H surface, demonstrates high-resolution nanopatterning. The resulting patterns can be used as templates for a variety of materials to form hybrid silicon nanostructures while maintaining a pristine background resist. The versatility of this UHV-STM nanolithography approach has led to its use on a variety of other substrates, including alternative hydrogen-passivated semiconductor surfaces, molecular resists, and native oxide resists. This review discusses the mechanisms of STM-induced hydrogen desorption, the postpatterning deposition of molecules and materials, and the implications for nanoscale device fabrication.

APPLICATIONS OF NANOIMPRINT LITHOGRAPHY FOR BIOCHEMICAL AND NANOPHOTONIC STRUCTURES USING SU-8

Nanoimprint lithography (NIL) technology has aroused great interests in both academia and industry due to its high resolution, low-cost, and high-volume nanopatterning capability. And as an expoxy resin-based negative amplified photoresist, SU-8 is an ideal candidate for NIL because of its low-glass-transition temperature, low-volume shrinkage coefficient, and good optical properties. In this reviewing paper, we highlight the major technical achievements in NIL on epoxy resin and its applications for bio- and nanophotonic structures. NIL was also applied for the duplication of imprint templates, originally fabricated by e-beam lithography (EBL) followed by reactive ion etch (RIE), using a SU-8/ SiO 2/PMMA tri-layer technique. And nanoimprint properties were systematically investigated for optimization. The developed nanoimprint process for different applications indicates promising industrial potentials in the next generation lithography resolution.

High-Resolution Poly(ethylene terephthalate) (PET) Hot Embossing at Low Temperature: Thermal, Mechanical, and Optical Analysis of Nanopatterned Films

In this work we present controlled, low-damage nanotopographic surface modification of poly(ethylene terephthalate) (PET). High-resolution nanopatterning over macroscopic areas was performed by " low-temperature" hot embossing lithography (HEL). While for standard HEL the temperature is typically raised up to many tens of Celsius degrees above the polymer glass transition temperature (Tg), we demonstrate optimal results at a temperature very close to the bulk Tg of PET (72 degrees C). Nanopits and nanobarcodes were transferred onto the surface of PET commercial sheets, demonstrating reliable sub-100 nm resolution over macroscopic areas. Sample optical, mechanical, and thermal characteristics were systematically analyzed before and after embossing at low (75 degrees C) and high (150 degrees C) temperature by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, tensile tests, and differential scanning calorimetry (DSC). We show that, while conventional high-temperature HEL can lead to dramatic degradation of the polymer in terms of transparency, flexibility, and crystallinity content, our low-temperature process fully maintains original surface and bulk substrate properties.