Introducing surface functionality on thermoformed polymeric films

We demonstrated an economical way of fabricating gel–film-based devices by combining nanoimprint lithography (NIL) and a sol–gel technique. A novel imprinting procedure, new mold surface passivation, and an effective surfactant added to sol were developed. Gratings with 300 nm pitch and 80 nm linewidth and waveguide gratings with varying periods were imprinted in a single step and with excellent uniformity into the gel films coated on a quarter of 4 in. wafers, respectively. Surface roughness measurements of waveguide gratings by atomic force microscope showed smooth profiles with root mean square roughness less than 6 nm. NIL is an excellent patterning technology for gel–film-based optical devices.

This review concentrates on the applications of nanoimprint lithography (NIL) and hot embossing for the fabrications of nanolectronic devices, nanophotonic metamaterials and other nanostructures. Technical challenges and solutions in NIL such as nanofabrication of templates, removal of residual resist, pattern displacement in thermal NIL arising from thermal expansion are first discussed. In the nanofabrication of templates, dry etch in plasma for the formation of multi-step structures and ultra-sharp tip arrays in silicon, nanophotonic chiral structures with high aspect ratio in SiC are demonstrated. A bilayer technique for nondestructive removal of residual resist in thermal NIL is described. This process is successfully applied for the fabrication of T-shape gates and functional high electron mobility transistors. However, pattern displacement intrinsically existing in thermal NIL/hot embossing owing to different thermal expansions in the template and substrate, respectively, limits its further development and scale-up. Low temperature even room temperature NIL (RTNIL) was then proposed on HSQ, trying to eliminate the pattern distortion by avoiding a thermal loop in the imprint. But, considerable pressure needed in RTNIL turned the major attentions to the development of UV-curing NIL in UV-curable monomers at low temperature. A big variety of applications by low-temperature UV-curing NIL in SU-8 are described, including high-aspect-ratio phase gratings, tagging technology by nanobarcode for DNA sequencing, nanofluidic channels, nanophotonic metamaterials and biosensors. Hot embossing, as a parallel technique to NIL, was also developed, and its applications on ferroelectric polymers as well as metals are reviewed. Therefore, it is necessary to emphasize that this review is mainly attempted to review the applications of NIL/embossing instead of NIL technique advances.

Nanoimprint lithography (NIL), as a non-conventional pattern transfer technique, presents unique competitiveness in the fabrication of semiconductor structures for medium-to-low resolution, cost-sensitive, and high-volume applications, owing to its advantages in high resolution, high throughput, low cost, and scalability. Compared with conventional lithography, NIL achieves high-precision pattern transfer beyond the optical diffraction limitation by directly imprinting and transferring patterns through mechanical contact, without relying on complex optical systems, thereby significantly reducing equipment and operational costs. This article systematically reviews the fundamental principles of NIL and its major process types, including thermal nanoimprint lithography (T-NIL), ultraviolet nanoimprint lithography (UV-NIL), and roller nanoimprint lithography (R-NIL), elaborating on the characteristics and applicable scenarios of each in terms of resolution, process conditions, substrate compatibility, throughput, and cost. Furthermore, a multi-dimensional comparison is conducted to analyze NILs competitive positioning within the current universal lithography technology landscape, while potential future development directions are outlined. Although NIL still faces challenges such as limited mold lifetime and pattern flexibility, advancements in material engineering, process optimization, and technology integration are expected to broaden its industrialization prospects.

A simple and cost effective method to fabricate anti-reflective (AR) and hydrophobic (HP) polymeric films was attempted by developing micro-, nano- and micro/nano-porous Al/ anodic aluminum oxide (AAO) imprinting molds and then replicating the surface morphology of the molds on poly methyl methacrylate (PMMA) and polycarbonate (PC) polymeric films by using thermal nanoimprint lithography (NIL). A micro-porous structure was fabricated by using UV-light lithography followed by etching, and a nano-porous structure using repetitions of anodizing and subsequent etching. As a result, micro-, nano- and micro/nano-porous Al/AAO imprints and micro-, nanoand micro/nano-pillared PMMA and PC replica films in which the pores and the pillars were uniformly well-arranged in a hexagonal array, were successfully fabricated. An AR and HP PMMA replica film with an array of nano-pillars revealing a transmittance of 94.3% and a water contact angle (WCA) of 124°, could be fabricated. By incorporating nano-pillars on a micro-pillared structure, the WCA of the PC film was increased from 128 to 143°, but that of the PMMA film remained almost the same. The incorporation of the smooth micro-pillar structure on both the bare and the nano-pillared PMMA and PC films was found to reduce significantly the visible light transmittance, but a high transmittance was found only in the nano-pillared films.

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications.

Versatile fabrication method for multiscale hierarchical structured polymer masters using a combination of photo- and nanoimprint lithography

Repair of a cracked aircraft structure by an adhesively bonded composite patch has gained lot of importance in extending the fatigue life as well as improving the structural integrity of it. In the present work, an experimental study is carried out using digital image correlation (DIC) technique to analyze the behavior of single and double-sided adhesively bonded patch repair of an inclined center cracked aluminium panel subjected to uniaxial tensile loading. Further, the shear strain distribution in the adhesive layer is also estimated using the DIC technique. Shear strain concentration at the overlap edge in the adhesive layer is very high leading to patch debonding. Further, optimum patch dimension and patch thickness for the given cracked panel are arrived using a genetic algorithm-based optimization technique in conjunction with finite element analysis (FEA). With the optimum patch configuration, a 3-D FEA is further carried out and the obtained results are compared with the experimental prediction from DIC. The DIC prediction correlates with FEA contours on overall basis.

Measuring small-magnitude strain fields using a digital image correlation (DIC) technique is challenging, due to the noise-signal ratio in strain maps. Here, we determined the level of accuracy achievable in measuring small-magnitude (<0.1%) homogeneous strain fields. We investigated different sets of parameters for image processing and imaging pre-selection, based on single-image noise level. The trueness of DIC was assessed by comparison of Young’s modulus (E) and Poisson’s ratio (ν) with values obtained from strain gauge measurements. Repeatability was improved, on average, by 20–25% with experimentally-determined optimal parameters and image pre-selection. Despite this, the intra- and inter-specimen repeatability of strain gauge measurements was 5 and 2.5 times better than DIC, respectively. Moreover, although trueness was also improved, on average, by 30–45%, DIC consistently overestimated the two material parameters by 1.8% and 3.2% for E and ν, respectively. DIC is a suitable option to measure small-magnitude homogeneous strain fields, bearing in mind the limitations in achievable accuracy.

To follow Moore’s law and the trend of devices to keep shrinking, the nanotechnology industry is challenged in finding a suitable technique for the mass production of integrated circuits with critical dimension in the sub-10 nm range. At the time this project started, EUV lithography, that employs light with a wavelength of 13.5 nm and thus requires the scanner to be kept under vacuum, was encountering difficulties in making its way into high volume manufacturing. Therefore different technologies were being explored as alternative to EUVL. The European project Single Nanometer Manufacturing for beyond CMOS devices (SNM) [1] aims at developing a manufacturing platform that routinely provides sub-10 nm resolution. To achieve that, fabrication processes based on Nano Imprint Lithography (NIL) are investigated. NIL is a low cost, high resolution and high throughput patterning technique, suitable for the mass production of devices. In NIL, a UV-transparent stamp is pressed on top of a substrate covered with polymer, called the NIL resist. The features on the stamp are imprinted in this polymer, that hardens under the exposure of UV light. The features on the polymer are then transferred into the underlying substrate using etching processes. To fabricate these stamps, we propose to use electron beam induced deposition (EBID)...

To explore the application of miniaturised polarisers into the detection of the skylight polarisation angle, a fabrication process for single-layer metallic nano-gratings, which was treated as polarisers in the polarised light testing instrument, was proposed. With the help of the combination of two efficient fabrication methods, the nanoimprint lithography and the lift-off method, different directions of the metallic gratings with 200 nm period, 110 nm linewidth and 60 nm height were fabricated simultaneously on the substrate. An interval ultrasound method is proposed to avoid grating structure damage during the lift-off process. Additionally, the effect of the SiO2 layer addition during the etching process on the transmission performance of metallic gratings is presented by a brief theory analysis and experiment results. The fabricated grating is installed on the designed testing instrument to measure the angle of polarised light and the final angle output error is within ±0.25°, which demonstrates that the fabricated metallic gratings have a good prospect as a polaroid applied in the polarisation navigation.

This paper presents a study of micro-scale deformation of materials utilizing scanning electron microscopy (SEM) images and the digital image correlation (DIC) technique. A loading stage was integrated into the SEM imaging system. During the experiment, a series of SEM images of the specimen were acquired in situ. The DIC technique was then applied to these SEM images to calculate the displacement and strain field at the area of interest. Additional surface preparation may be needed in order to have an effective pattern for DIC analysis. Two applications are presented in the paper. Using small tensile specimens, the mechanical properties of electrodeposited nickel-based LIGA (an acronym from German words for lithography, electroplating, and moulding) specimens were characterized. In this case, the natural microstructural feature of the specimen surface was used directly as the pattern for DIC analysis. This method was also applied to study the strain concentration around the crack tip during the ductile fracture test of Al 6061-T651. In contrast to the previous case, the DIC patterns were generated by sputtering a thin layer of gold film on to the specimen surface through the copper mesh grid.

Thin-walled metal tubes are extensively used in aircraft and automobile industries as energy absorbers during collision. When a thin wall tube is subjected to an axial compressive load, lobes are formed sequentially and each lobe undergoes a large plastic deformation without any cracking; this is referred to as progressive buckling. The focus of this paper is to study the displacement and strain fields near the buckled zone of a thin-walled square tube both experimentally and numerically. Quasi-static test was conducted on thin-walled square tubes made of aluminum alloy AA-6063 and the displacement and full-field strain was measured using Digital Image Correlation (DIC) technique. It is noted that while one face of the tube undergoes tensile deformation, the adjacent faces undergo compressive deformation. The strain levels exceed the fracture strain obtained during a tensile test. Strain estimated from DIC was found to be in good agreement with the strain gauge measurements at far field. Further, strain estimations obtained through numerical simulations showed a reasonable agreement with DIC measurements.

The development of high-sensitive, and cost-effective novel biosensors have been strongly desired for future medical diagnostics. To develop novel biosensor, the authors focused on the specific optical characteristics of photonic crystal. In this study, a label-free optical biosensor, polymer-based two-dimensional photonic crystal (2D-PhC) film fabricated using nanoimprint lithography (NIL), was developed for detection of C-reactive protein (CRP) in human serum. The nano-hole array constructed NIL-based 2D-PhC (hole diameter: 230 nm, distance: 230, depth: 200 nm) was fabricated on a cyclo-olefin polymer (COP) film (100 µm) using thermal NIL and required surface modifications to reduce nonspecific adsorption of target proteins. Antigen-antibody reactions on the NIL-based 2D-PhC caused changes to the surrounding refractive index, which was monitored as reflection spectrum changes in the visible region. By using surface modified 2D-PhC, the calculated detection limit for CRP was 12.24 pg/mL at an extremely short reaction time (5 min) without the need for additional labeling procedures and secondary antibody. Furthermore, using the dual-functional random copolymer, CRP could be detected in a pooled blood serum diluted 100× with dramatic reduction of nonspecific adsorption. From these results, the NIL-based 2D-PhC film has great potential for development of an on-site, high-sensitivity, cost-effective, label-free biosensor for medical diagnostics applications.

A comprehensive experimental study on bi-adhesive single lap joints using DIC technique

Magnetron sputter deposition of heat-resistant coatings on high temperature alloys: Beregovsky, V.V., Shergin, G.G., Shesterkin, N.G., Kuznetsov, E.V. and Anisimova, M.A. Surf. Coat. Technol. Oct. 1991 48 (1) 13–18