Lithography Templates Research Articles

Versatile nanosphere lithography technique combining multiple-exposure nanosphere lens lithography and nanosphere template lithography

A versatile nanosphere composite lithography (NSCL) combining both the advantages of multiple-exposure nanosphere lens lithography (MENSLL) and nanosphere template lithography (NSTL) is demonstrated. By well controlling the development, washing and the drying processes, the nanosphere monolayer can be well retained on the substrate after developing and washing. Thus the NSTL can be performed based on MENSLL to fabricate nanoring, nanocrescent and hierarchical multiple structures. The pattern size and the shape can be systemically tuned by shrinking nanospheres by using dry etching and adjusting the tilted angle. It is a natural nanopattern alignment process and possesses a great potential in the scope of nano-science due to its low cost, simplicity, and versatility for variuos nano-fabrications.

Scanning probe-based high-accuracy overlay alignment concept for lithography applications

Overlay alignment is a concern for nanolithography applications, in particular, for those using step and repeat techniques targeting next-generation lithographic applications. In this context, a new method and a proof of concept (POC) setup for accurately aligning a mask with a semiconductor wafer is presented. Utilizing active scanning probe technology, the method is employable for various lithographic techniques such as photolithography, electron- and ion-beam lithography, nanoimprint lithography (NIL). The developed method is demonstrated in the example of NIL. It employs compact highly integrated atomic force microscopes (mini-AFM), which are fixed on the lithographic template. The mini-AFM systems are applied for imaging of the surface relief marks on the semiconductor wafer to carry out the alignment process. In a next step, the obtained AFM section images are used to calculate the deviations and steer the bottom stage carrying the processed wafer in order to achieve the desired positioning accuracy. A POC test setup was built for emulation of the alignment procedure. Several measurement studies are addressed to evaluate the applicability of the overlay alignment method. As a result, it is shown that the implemented test setup is able to determine the positioning error of the bottom stage carrying the wafer with an accuracy of around 10 nm (without temperature compensation).

Low-cost fabrication of nanoimprint templates with tunable feature sizes at a constant pitch

Nanoimprint lithography (NIL) templates are in general costly, especially for large area and small feature sizes. With a simple shrinking technique using a serial of well-known technologies, it has been feasibly realized to produce high-quality soft NIL templates with widely tunable feature sizes at a constant pitch from a single master at very low-cost and in a short processing cycle. The master featuring at sub-micron range has been replicated to new silicon NIL templates with feature sizes at, but not limited to, sub-200nm. soft UV-NIL combined with a dry etch process, were employed to fabricate an intermediate template of nanocone patterns that is further used for an imprint on the final substrate. The feature size of the SiO2 etch mask on the final substrate can be adjusted by varying the duration of mask formation etching. The final patterning of the template is realized by cryogenic etching based on SF6/O2 chemistry. Silicon NIL templates featuring nanopillar patterns with diameters of 150nm, 200nm and 250nm, respectively, have been fabricated on wafer level from the same master with 450nm feature size. The presented process flow avoids the time-consuming and cost-intensive electron beam writings and gives more flexibility in the fabrication of nanopatterns. The fabrication cycle for such NIL working templates with tunable feature sizes is maintained short and at a low cost. Moreover, the technique allows the fabrication of wafer level products at a constant pitch, which is of importance as well for the stacked large-area imprintings with varying feature sizes.

Fabrication of self-aligned multilevel nanostructures

Multilevel three-dimensional nanostructures are essential in many integrated nanoelectronic and nanophotonic applications. With the continued shrinking of critical device dimensions, extremely precise nanoscale overlay is required between multiple individual levels of these integrated devices. Multilevel nanoimprint lithography has been proposed in the literature as a potential solution to this overlay problem. In this context, self-aligned (perfectly aligned) multilevel templates (SAMT) for multilevel nanoimprint lithography are proposed in this article. By combining nanolithography, atomic layer deposition, and highly selective reactive ion etch, SAMTs can enable the fabrication of sophisticated integrated devices. Four specific self-aligned multilevel fabrication techniques have been demonstrated that result in symmetric multilevel structures, bilaterally symmetric multilevel structures, tubular structures, and asymmetric multilevel structures, all in the sub-100nm scale. When used in conjunction with a nanoimprint lithography process, the SAMTs can enable high-throughput patterning of various nanoelectronic and nanophotonic devices using a single patterning step with perfect alignment and overlay. SAMTs further enable large area patterning, such as wafer-scale patterning and roll-to-roll patterning on flexible substrates, without compromising perfect overlay.

Rich Variety of Three-Dimensional Nanostructures Enabled by Geometrically Constraining Star-like Block Copolymers.

The influence of star-like architecture on phase behavior of star-like block copolymer under cylindrical confinement differs largely from the bulk (i.e., nonconfinement). A set of intriguing self-assembled morphologies and the corresponding phase diagrams of star-like (AB)f diblock copolymers with different numbers of arms f (i.e., f = 3, 9, 15, and 21) in four scenarios (ϕA = 0.3 and V0 > 0; ϕA = 0.3 and V0 < 0; ϕA = 0.7 and V0 > 0; and ϕA = 0.7 and V0 < 0 (where ϕA is the volume fraction of A block) and V0 < 0 and V0 > 0 represent that the pore wall of cylindrical confinement prefers the inner A block (i.e., A-preferential) and B block (i.e., B-preferential), respectively) were for the first time scrutinized by employing the pseudospectral method of self-consistent mean-field theory. Surprisingly, a new nanoscopic phase, that is, perforated-lamellae-on-cylinder (denoted PC), was observed in star-like (AB)3 diblock copolymer at ϕA = 0.3 and V0 > 0. With a further increase in f, a single lamellae (denoted L1) was found to possess a larger phase region. Under the confinement of A-preferential wall (i.e., V0 < 0) at ϕA = 0.3, PC phase became metastable and its free energy increased as f increased. Quite intriguingly, when ϕA = 0.7 and V0 > 0, where an inverted cylinder was formed in bulk, the PC phase became stable, and its free energy decreased as f increased, suggesting the propensity to form PC phase under this condition. Moreover, in stark contrast to the phase transition of C1 → L1 → PC (C1, a single cylindrical microdmain) at ϕA = 0.3 and V0 > 0, when subjected to the A-preferential wall (ϕA = 0.7), a different phase transition sequence (i.e., C1 → PC → L1) was identified due to the formation of a double-layer structure. On the basis of our calculations, the influence of star-like architecture on (AB)f diblock copolymer under the imposed cylindrical confinement, particularly the shift of the phase boundaries as a function of f, was thoroughly understood. These self-assembled nanostructures may hold the promise for applications as lithographic templates for nanowires, photonic crystals, and nanotechnology.

Block copolymer thin films: Characterizing nanostructure evolution with in situ X-ray and neutron scattering

Block copolymer (BCP) thin films have attracted significant attention as lithographic templates, separation membranes, and organic photovoltaic active layers for emerging nanotechnologies due to their ability to self-assembly into nanoscale features. To direct the self-assembly of BCP thin film nanostructures, a suite of annealing techniques has been developed (e.g. thermal annealing, solvent vapor annealing, magnetic/electrical field alignment), each with its own set of controllable parameters and mechanisms for nanostructure reorganization. In this Review, we discuss the importance of in situ X-ray and neutron scattering for the study of BCP thin films subjected to different annealing protocols. These scattering approaches have become vital for understanding the complex nanostructure reorganization processes inherent in thin film fabrication and for establishing more consistent control over the morphology, ordering, and orientation. A major advantage of in situ X-ray and neutron scattering characterization is the ability to link the thermodynamic and kinetic pathways of nanostructure evolution over macroscopic (several cm2) areas during annealing or processing. This feature has made in situ X-ray and neutron scattering ideal for refining annealing techniques, fostering robust assembly protocols, and developing the next-generation of directed assemblies. As the toolbox of viable processing methods continues to grow, we highlight potential opportunities to enhance current X-ray and neutron scattering capabilities through the improvement of scattering facilities, techniques, sample chambers, scattering/annealing protocols, and model development to establish universal control over BCP thin film self-assembly.

Tuning Localized Surface Plasmon Resonance Wavelengths of Silver Nanoparticles by Mechanical Deformation.

We describe a simple technique to alter the shape of silver nanoparticles (AgNPs) by rolling a glass tube over them to mechanically compress them. The resulting shape change in turn induces a red-shift in the localized surface plasmon resonance (LSPR) scattering spectrum and exposes new surface area. The flattened particles were characterized by optical and electron microscopy, single nanoparticle scattering spectroscopy, and surface enhanced Raman spectroscopy (SERS). AFM and SEM images show that the AgNPs deform into discs; increasing the applied load from 0 to 100 N increases the AgNP diameter and decreases the height. This deformation caused a dramatic red shift in the nanoparticle scattering spectrum and also generated new surface area to which thiolated molecules could attach as evident from SERS measurements. The simple technique employed here requires no lithographic templates and has potential for rapid, reproducible, inexpensive and scalable tuning of nanoparticle shape, surface area, and resonance while preserving particle volume.

Aluminum Nanocrescent Plasmonic Antennas Fabricated by Copper Mask Nanosphere Template Lithography

Aluminum plasmonic nanocrescent antennas were fabricated over large substrate areas by incorporating a sacrificial copper layer in nanosphere template lithography (NTL). The addition of the copper mask eliminates the argon ion milling step in the NTL process that is challenging to implement for aluminum nanostructures because of the robust native oxide layer. The aluminum nanocrescents exhibit polarization-dependent localized surface plasmon responses that can be tuned from the near-infrared into the ultraviolet, a region that is difficult to access with more typically used gold and silver. Finite-difference time-domain simulations predict the observed multimodal plasmonic behavior of these aluminum structures. This simple fabrication process will facilitate the implementation of the aluminum nanocrescent antennas in long and short wavelength surface-enhanced spectroscopies, fluorescence lifetime reduction processes, and photocatalysis.

Three-Dimensional Multilayered Nanostructures from Crosslinkable Block Copolymers.

Block copolymer (BCP) lithography has generally been synonymous to one- or two-dimensional single layered lithographic templates as a means to fabricate simple nanoscaled structures. Recently, the rapidly increasing demand for complex nanostructures and the corresponding evolution in BCP lithography have led to three-dimensional (3D) BCP nanostructures, which can be fabricated in various ways such as directed self-assembly or stacking of cross-linked BCP patterns. This review covers the recent advances in the 3D multilayered structures from cross-linkable BCPs, which provide an easy and robust means for integrating various BCP structures into one scaffold. In this case, wetting-optimized adjustment of BCP microdomains at the layer interface plays a critical role in the formation of well-defined 3D multilayer nanostructures.

Development of nanoimprint lithography templates toward high-volume manufacturing

Development of nanoimprint lithography (NIL) templates is discussed. The template fabrication process and its performance are presented with consideration of the requirements of NIL for high-volume manufacturing. Defectivity, image placement, and critical dimension uniformity are the three major performance parameters of the templates, and their current status is shown.

Alternative nano-structured thin-film materials used as durable thermal nanoimprint lithography templates

Nanoimprint templates made of diamond-like carbon (DLC) and amorphous silicon carbide (SiC) thin films and fluorine-doped associated materials, i.e. F–DLC and F–SiC were investigated in the context of thermal nanoimprint lithography (NIL) with respect to their release properties. Their performances in terms of durability and stability were evaluated and compared to those of conventional silicon or silica molds coated with antisticking molecules applied as a self-assembled monolayer. Plasma-enhanced chemical vapor deposition parameters were firstly tuned to optimize mechanical and structural properties of the DLC and SiC thin films. The impact of the amount of fluorine dopant on the deposited thin films properties was then analyzed. A comparative analysis of DLC, F–DLC as well as SiC and F–SiC molds was then carried out over multiple imprints, performed into poly (methyl methacrylate) (PMMA) thermo-plastic resist. The release properties of un-patterned films were evaluated by the measurement of demolding energies and surface energies, associated with a systematic analysis of the mold surface contamination. These analyses showed that the developed materials behave as intrinsically easy-demolding and contamination-free molds over series of up to 40 imprints. To our knowledge, it is the first time that such a large number of imprints has been considered within an exhaustive comparative study of materials for NIL. Finally, the developed materials went through standard e-beam lithography and plasma etching processes to obtain nanoscale-patterned templates. The replicas of those patterned molds, imprinted into PMMA, were shown to be of high fidelity and good stability after several imprints.

Multi-functional silicone stamps for reactive release agent transfer in UV roll-to-roll nanoimprinting

A novel bi-layer silicone nanoimprint mold capable of dual-functionality as both a lithographic template and a release agent transfer vehicle.

Plant leaves as templates for soft lithography

Complex microvascular venation patterns of natural leaves are replicated into PDMS replicas, which allows for a leakage-tight seal with a flat substrate despite the surface topography.

Engineering 3D Nanoplasmonic Assemblies for High Performance Spectroscopic Sensing.

We demonstrate the fabrication of plasmonic sensors that comprise gold nanopillar arrays exhibiting high surface areas, and narrow gaps, through self-assembly of amphiphilic diblock copolymer micelles on silicon substrates. Silicon nanopillars with high integrity over arbitrary large areas are obtained using copolymer micelles as lithographic templates. The gaps between metal features are controlled by varying the thickness of the evaporated gold. The resulting gold metal nanopillar arrays exhibit an engineered surface topography, together with uniform and controlled separations down to sub-10 nm suitable for highly sensitive detection of molecular analytes by Surface Enhanced Raman Spectroscopy (SERS). The significance of the approach is demonstrated through the control exercised at each step, including template preparation and pattern-transfer steps. The approach is a promising means to address trade-offs between resolutions, throughput, and performance in the fabrication of nanoplasmonic assemblies for sensing applications.

Size-tuned ZnO nanocrucible arrays for magnetic nanodot synthesis via atomic layer deposition-assisted block polymer lithography.

Low-temperature atomic layer deposition of conformal ZnO on a self-assembled block polymer lithographic template comprising well-ordered, vertically aligned cylindrical pores within a poly(styrene) (PS) matrix was used to produce nanocrucible templates with pore diameters tunable via ZnO thickness. Starting from a PS template with a hexagonal array of 30 nm diameter pores on a 45 nm pitch, the ZnO thickness was progressively increased to narrow the pore diameter to as low as 14 nm. Upon removal of the PS by heat treatment in air at 500 °C to form an array of size-tunable ZnO nanocrucibles, permalloy (Ni80Fe20) was evaporated at normal incidence, filling the pores and creating an overlayer. Argon ion beam milling was then used to etch back the overlayer (a Damascene-type process), leaving a well-ordered array of isolated ZnO nanocrucibles filled with permalloy nanodots. Microscopy and temperature-dependent magnetometry verified the diameter reduction with increasing ZnO thickness. The largest diameter (30 nm) dots exhibit a ferromagnetic multidomain/vortex state at 300 K, with relatively weakly temperature-dependent coercivity. Reducing the diameter leads to a crossover to a single-domain state and eventually superparamagnetism at sufficiently high temperature, in quantitative agreement with expectations. We argue that this approach could render this form of block polymer lithography compatible with high-temperature processing (as required for technologically important high perpendicular anisotropy ordered alloys, for instance), in addition to enabling separation-dependent studies to probe interdot magnetostatic interactions.

Beryllium X-ray lithography templates

A simple technique for the production of lithography templates (lithography masks) with beryllium support membranes ∼50-100 μm in thickness with different types of masking coating (in the form of metallic layers produced through electroplating or in the form of a thick (∼0.13–1.10 mm) resistive mask made from, among other things, a sensitized X-ray resist) is described. Beryllium templates are characterized by their ease of fabrication; geometrical stability; almost complete lack of fluorescent radiation; low cost price; and high mechanical strength, radiation resistance, and X-ray transparency of the support membrane. Therefore, these templates may be used not only in the range of ultrashort X-ray wavelengths (0.5–3.0 A), but also in the 3–7 A range. Photographs of the first samples of such templates and diagrams illustrating their basic lithographic parameters are presented.

Generation of 3D nanopatterns with smooth surfaces

Ga implantation into Si and reactive ion etching has been previously identified as candidate techniques for the generation of 3D nanopatterns. However, the structures manufactured using these techniques exhibited impedingly high surface roughness. In this work, we investigate the source of roughness and introduce a new patterning process to solve this issue. The novel patterning process introduces an additional layer absorbing the implanted Ga, thus preventing the clustering of the implanted Ga observed with uncoated Si substrates. This process enables 3D nanopatterning with sub-100 nm lateral resolution in conjunction with smooth height transitions and surface roughness down to 4 nm root mean square. Such patterns are ideally suited for optical applications and enable the manufacturing of nanoimprint lithography templates for low-profile Fresnel lenses.

Magnetic assembly and annealing of colloidal lattices and superlattices.

The ability to assemble mesoscopic colloidal lattices above a surface is important for fundamental studies related with nucleation and crystallization but also for a variety of technological applications in photonics and microengineering. Current techniques based on particle sedimentation above a lithographic template are limited by a slow deposition process and by the use of static templates, which make difficult to implement fast annealing procedures. Here it is demonstrated a method to realize and anneal a series of colloidal lattices displaying triangular, honeycomb, or kagome-like symmetry above a structure magnetic substrate. By using a binary mixture of particles, superlattices can be realized increasing further the variety and complexity of the colloidal patterns which can be produced.

Microfluidic biosensor for cholera toxin detection in fecal samples.

Sample preparation and processing steps are the most critical assay aspects that require our attention in the development of diagnostic devices for analytes present in complex matrices. In the best scenarios, diagnostic devices should use only simple sample processing. We have therefore investigated minimal preparation of stool samples and their effect on our sensitive microfluidic immunosensor for the detection of cholera toxin. This biosensor was previously developed and tested in buffer solutions only, using either fluorescence or electrochemical detection strategies. The microfluidic devices were made from polydimethylsiloxane using soft lithography and silicon templates. Cholera toxin subunit B (CTB)-specific antibodies immobilized onto superparamagnetic beads and ganglioside GM1-containing liposomes were used for CTB recognition in the detection system. Quantification of CTB was tested by spiking it in human stool samples. Here, optimal minimal sample processing steps, including filtration and centrifugation, were optimized using a microtiter plate assay owing to its high-throughput capabilities. Subsequently, it was transferred to the microfluidic systems, enhancing the diagnostic characteristic of the biosensor. It was found that the debris removal obtained through simple centrifugation resulted in an acceptable removal of matrix effects for the fluorescence format, reaching a limit of detection of only 9.0ng/mL. However, the electron transfer in the electrochemical format was slightly negatively affected (limit of detection of 31.7ng/mL). Subsequently, cross-reactivity using the heat-labile Escherichia coli toxin was investigated using the electrochemical microfluidic immunosensors and was determined to be negligible. With minimal sample preparation required, these microfluidic liposome-based systems have demonstrated excellent analytical performance in a complex matrix and will thus be applicable to other sample matrices.

Cost-Effective Three-Dimensional Printing of Visibly Transparent Microchips within Minutes

One-step fabrication of transparent three-dimensional (3D) microfluidic to millifluidic devices was demonstrated using a commercial 3D printer costing $2300 with 500 mL of clear resin for $138. It employs dynamic mask projection stereolithography, allowing fast concept-to-chip time. The fully automated system allows fabrication of models of up to 43 mm × 27 mm × 180 mm (x × y × z) at printing speeds of 20 mm/h in height regardless of the design complexity. The minimal cross sectional area of 250 μm was achieved for monolithic microchannels and 200 μm for positive structures (templates for soft lithography). The colorless resin's good light transmittance (>60% transmission at wavelengths of >430 nm) allows for on-chip optical detection, while the electrically insulating material allows electrophoretic separations. To demonstrate its applicability in microfluidics, the printer was used for the fabrication of a micromixer, a gradient generator, a droplet extractor, and a device for isotachophoresis. The mixing and gradient formation units were incorporated into a device for analysis of nitrate in tap water with standard addition as a single run and multiple depth detection cells to provide an extended linear range.