Soft Ultraviolet Nanoimprint Lithography Research Articles

Thin crystalline silicon with double-sided nano-hole array fabricated by soft UV-NIL and RIE

The thin crystalline silicon (c-Si) is deemed to be an alternative material for solar cells, but it is too thin to effectively absorb light on a broad spectrum. Here we experimentally demonstrate, for the first time, that a double-sided nano-hole array on free-standing thin c-Si (20 µm) by combining soft ultra-violet nanoimprint lithography (soft UV-NIL) and reactive ion etching (RIE), which is simple and possible for mass production. This thin c-Si with double-sided nano-hole array proves to show 40% lower light reflectivity than flat silicon at long wavelength range, which is coincident with the simulated results. The thin c-Si with double-sided nano-hole array also has the advantages of good flexibility and uniform thickness, adding feasibility to apply the structure to photonic devices.

Large-Scale Uniform Silver Nanocave Array for Visible Light Refractive Index Sensing Using Soft UV Nanoimprint

In this paper, a wafer-scale uniform silver nanocave array is fabricated by soft ultraviolet nanoimprint lithography. We investigate its plasmonic effects using far-field and near-field experimental approaches and illuminate the physics inside by theoretical analysis and computational simulation. The array shows robust multispectral features for various surrounding media and possesses the sensitivity up to 514.7 nm/RIU in the visible range, which is promising for the mass production of high-performance plasmonic refractive index sensors.

Ambipolar light-emitting organic single-crystal transistors with a grating resonator.

Electrically driven organic lasers are among the best lasing devices due to their rich variety of emission colors as well as other advantages, including printability, flexibility, and stretchability. However, electrically driven lasing in organic materials has not yet been demonstrated because of serious luminescent efficiency roll-off under high current density. Recently, we found that the organic ambipolar single-crystal transistor is an excellent candidate for lasing devices because it exhibits less efficient roll-off, high current density, and high luminescent efficiency. Although a single-mode resonator combined with light-emitting transistors (LETs) is necessary for electrically driven lasing devices, the fragility of organic crystals has strictly limited the fabrication of resonators, and LETs with optical cavities have never been fabricated until now. To achieve this goal, we improved the soft ultraviolet-nanoimprint lithography method and demonstrated electroluminescence from a single-crystal LET with a grating resonator, which is a crucial milestone for future organic lasers.

Bloch surface plasmon enhanced blue emission from InGaN/GaN light-emitting diode structures with Al-coated GaN nanorods

InGaN/GaN light-emitting diode structures with Al-coated GaN nanorods were fabricated by using soft ultraviolet nanoimprint lithography. The intensity of light emission was found to be greatly enhanced due to the strong near-fields confined at the interface of Al/GaN and extended to the multiple quantum wells (MQWs) active region. The dynamics of carrier recombination and plasmon-enhanced Raman scattering were also investigated, providing a progressive view on the effective energy transfer between MQWs and surface plasmons.

Geometric phase analysis based on the windowed Fourier transform for the deformation field measurement

Abstract The geometric phase analysis (GPA), an important image-based deformation measurement method, has been used at both micro- and nano-scale. However, when a deformed image has apparent distortion, non-ignorable error in the obtained deformation field could occur by using this method. In this paper, the geometric phase analysis based on the windowed Fourier transform (WFT) is proposed to solve the above-mentioned issue, defined as the WFT–GPA method. In WFT–GPA, instead of the Fourier transform (FT), the WFT is utilized to extract the phase field block by block, and therefore more accurate local phase information can be acquired. The simulation tests, which include detailed discussion of influence factors for measurement accuracy such as window size and image noise, are conducted with digital deformed grids. The results verify that the WFT–GPA method not only keeps all advantages of traditional GPA method, but also owns a better accuracy for deformation measurement. Finally, the WFT–GPA method is applied to measure the machining distortion incurred in soft ultraviolet nanoimprint lithography (UV-NIL) process. The successful measurement shows the feasibility of this method and offers a full-field way for characterizing the replication quality of UV-NIL process.

Large-Area Anodized Alumina Nanopore Arrays Assisted by Soft Ultraviolet Nanoimprint Prepatterning

Aluminium anodization under optimized conditions can naturally generate close-packed and aligned nanopore arrays, but the spatial extent of such regular pore arrangement is generally limited. Here we demonstrated the use of soft ultraviolet nanoimprint lithography to guide the formation of nanopores at specific locations, using an elastomer negative mold for the process. By anodizing at voltages which naturally led to the formation of pores with matching averaged interpore separation, pre-patterned triangular holes (diameter 100 nm, periodicity 350 nm) on aluminium thin films induced conformal growth of nanopores at pre-defined positions. In addition, pores in geometries other than close-packed patterns were prepared, with square pore arrangement being demonstrated in this work. The influence of the anodization voltage on the final pore formation was also studied. Our results illustrated the possibility to fabricate well-organized nanopore arrays with conditions far less stringent than those reported in literature, which has the potential to be adopted for applications where regular pore alignment are critical.

Thermal tuning of surface plasmon resonance: Ag gratings on barium strontium titanate thin films

Surface plasmon tuning via thermally induced refractive index changes in ferroelectrics is investigated. Epitaxial (Ba0.7Sr0.3)TiO3 (BST) thin films were deposited on MgO (001) substrates by pulsed laser deposition. The refractive index of BST thin films measured by the prism-coupling technique was found to increase from 2.3932 (TE)/1.9945 (TM) at room temperature to 2.3949 (TE)/1.9965 (TM) at 66°C. Then 30-nm-Ag gratings with periodicity 750 nm and width 300 nm were fabricated on BST by soft ultraviolet nanoimprint lithography and subsequent lift-off process. The reflection spectra from 500 to 1000 nm with incident angle from 5° to 60° were measured at room temperature and 66°C, with a collimated and p-polarized light incident perpendicularly to the grating direction. Several modes were observed from the spectra. At 66°C, a red shift of a dip at about 850 nm by 2 nm was obtained at an incident angle of 15°. Calculations confirmed that the observed modes belong to the (−1), (2), (−2) and (3) surface plasmon modes from the Ag and BST interfaces and localized mode; the red shift by thermal tuning is also confirmed. The results indicate the feasibility of active modulation in surface plasmon resonance in solid-state structures.

Sub-200nm gap electrodes by soft UV nanoimprint lithography using polydimethylsiloxane mold without external pressure

In this article, the authors present an alternative approach to electron beam lithography allowing the fabrication of gap electrodes with dimensions less than 200nm. This approach consists of the soft ultraviolet nanoimprint lithography on nonstandard sample sizes, using polydimethylsiloxane (PDMS) as a flexible mold material. By simple deposit of the PDMS mold on the substrate, they succeeded in imprinting gaps in Amonil and the transfer of patterns of polymethylmethacrylate/Amonil bilayer was achieved by etching the substrate with a suitable reactive ion etching process. The imprint pressure was ensured by only the weight of the PDMS mold.

Fabrication of nano-scaled patterns on ceramic thin films and silicon substrates by soft ultraviolet nanoimprint lithography

Soft ultraviolet nanoimprint lithography is a cost-effective and versatile technique for the transfer of nano-scaled patterns to various surfaces. Here, we report on the fabrication of sub-micron square pillar arrays in epitaxial Ba 0.7Sr 0.3TiO 3 ceramic films, using a combination of nanoimprint lithography and inductively coupled plasma etching techniques. Based on a similar approach we have also succeeded in preparing positive (direct) and negative (inverse) replicas of silicon master molds. Such a generic process could find application in various materials.

Effects of exposure time on defects and demolding force in soft ultraviolet nanoimprint lithography

Based on a contact process, the ultraviolet (UV) nanoimprint process can overcome the linewidth limit of conventional photolithography caused by the diffraction effect. However, the contact between the mold and resist may introduce various defects to the two components. Some researchers have investigated the defects that emerge in the imprint process, such as particle contamination and curing shrinkage of the resist. However, the curing process of the resist itself should be studied because the curable resist is the carrier of the pattern transfer and the replication is accomplished by its shape remodeling. In this article, the authors focus on the effects of exposure time on the defects and demolding force in the soft UV imprint process. The replications are made by a soft polydimethylsiloxane mold under the same process parameters except that the curing time is different. Moreover, the curing degree of the resist and the value of the demolding force under various curing doses are measured. A mechanical model is established to analyze the behavior of the resist, which transitions from a liquid to solid stage by stage, and will help the authors to understand the above effects. By analyzing the relationships among the model of the resist, curing degree, demolding force, and resultant defects in the experiments, they propose a curing window, within which the curing time in the imprint process helps to remove defects in the shaped patterns and soft mold. Finally, using the optimized curing time, the best replicated patterns with 100 nm features are obtained.

The fabrication of a flexible mold for high resolution soft ultraviolet nanoimprintlithography

One key issue for all nanoimprint techniques is an appropriate method for thefabrication of desirable molds. We report on a novel flexible mold fabricationprocess—pressure-assisted molding (PAM)—for high resolution soft ultravioletnanoimprint lithography (soft UV-NIL). In PAM, enhanced master filling is achievedby applying an external pressure during the mold fabrication process. Flexiblemolds, fabricated with PAM using different pressures in the range of 10–90 kPa, arecompared to determine the role of pressures applied in the imprint performance.

Fabrication of Photonic Ring Resonator Device in Silicon Waveguide Technology Using Soft UV-Nanoimprint Lithography

We report on an innovative fabrication method for high-quality resonant photonic devices on silicon-on-insulator substrates with quality (Q)-factor of 47 700 and propagation losses of 3.5 dB/cm. The definition of a resonator device using soft ultraviolet-nanoimprint lithography is detailed followed by description of an adapted structure transfer process. Spectral response measurements and propagation loss analysis were carried out evidencing the functionality of the fabricated resonator.

Replication of photonic crystals by soft ultraviolet-nanoimprint lithography

Nanoimprint lithography assisted by ultraviolet photopolymerization through a soft elastomer-based mold is applied to the fabrication of silicon-on-insulator slab photonic crystals for optical wavelengths. Variable angular reflectance is used to measure the dispersion of the photonic leaky modes. Experimental results are in good agreement with both theoretical calculations and previous results obtained by standard nanoimprint lithography as well as conventional nanofabrication techniques such as electron-beam lithography.