Sub-100nm Resolution Research Articles

Coherent diffraction imaging with focused hard x-rays achieves sub-10nm resolution

Coherent diffraction imaging with focused hard x-rays achieves sub-10nm resolution

Polycarbonate electron beam resist using solvent developer

Polycarbonate is a popular membrane material fabricated by ion track etching method and used for filtration, thus it is a sort of ion beam resist. Here we show that it can also be used as a positive electron beam resist using solvent development. Compared to the popular resist PMMA, polycarbonate is more chemically and thermally stable, and is more resistant to plasma dry etching. Various solvents, including cyclopentanone, xylene, pentyl acetate and methyl isobutyl ketone, were found to be suitable developers for polycarbonate when diluted properly with 2-propanol. The resist showed a low contrast between 0.5 and 1.0 when using those solvent developers, and thus it is not a good resist for defining high resolution dense patterns, yet is ideal for grayscale lithography to generate quasi-three dimensional structures like Fresnel zone-plate lens. Nevertheless, we achieved sub-50nm resolution for sparse line array pattern.

A novel ToF-SIMS operation mode for improved accuracy and lateral resolution of oxygen isotope measurements on oxides

Oxygen isotope exchange with subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a highly valuable tool for determining oxygen diffusion coefficients in oxides. Since ToF-SIMS analysis enables an elemental and chemical mapping, it can also be used to visualize oxygen exchange-active zones by determining the local oxygen isotopic fraction. However, measuring accurate isotopic fractions can be a challenging analytical task owing to secondary ion interaction and signal saturation, particularly when dealing with high secondary ion intensities as commonly found when analyzing oxygen ions from oxides. It is shown that in many cases the calculated 18O− fraction erroneously shifts to higher values and can lead to systematic errors in the determination of diffusion coefficients. A novel ToF-SIMS operation mode, called “Collimated Burst Alignment” (CBA) mode, is therefore introduced to enable a more accurate determination of oxygen isotopic fractions with an optimized lateral resolution of sub-100 nm. Both improvements are rendered possible by a modified beam guidance in the primary ion gun. This modification reduces detector dead time effects and ion interactions to a minimum and secondary ion intensities can be obtained more accurately. The result of this optimization is demonstrated in measurements of the natural isotope abundance of several different oxides including SrTiO3 and Sr-doped LaCoO3.

Measuring oxygen reduction/evolution reactions on the nanoscale

The efficiency of fuel cells and metal-air batteries is significantly limited by the activation of oxygen reduction and evolution reactions. Despite the well-recognized role of oxygen reaction kinetics on the viability of energy technologies, the governing mechanisms remain elusive and until now have been addressable only by macroscopic studies. This lack of nanoscale understanding precludes optimization of material architecture. Here, we report direct measurements of oxygen reduction/evolution reactions and oxygen vacancy diffusion on oxygen-ion conductive solid surfaces with sub-10nm resolution. In electrochemical strain microscopy, the biased scanning probe microscopy tip acts as a moving, electrocatalytically active probe exploring local electrochemical activity. The probe concentrates an electric field in a nanometre-scale volume of material, and bias-induced, picometre-level surface displacements provide information on local electrochemical processes. Systematic mapping of oxygen activity on bare and platinum-functionalized yttria-stabilized zirconia surfaces is demonstrated. This approach allows direct visualization of the oxygen reduction/evolution reaction activation process at the triple-phase boundary, and can be extended to a broad spectrum of oxygen-conductive and electrocatalytic materials.

Using Synchrotron X-Ray Nano-CT to Characterize SOFC Electrode Microstructures in Three-Dimensions at Operating Temperature

In recent years, developments in tomography tools have provided unprecedented insight into the microstructure of electrodes for solid oxide fuel cells, enabling researchers to establish direct links between electrode microstructure and electrochemical performance. Here we present results of high resolution, synchrotron X-ray nano computed tomography experiments, which have enabled microstructural characterisation of a mixed ionic electronic conducting lanthanum strontium cobalt iron oxide (LSCF) cathode with sub-50nm resolution at operating temperature. Using the uniquely non-destructive nano-CT platform, it is possible to characterise microstructural evolution processes associated with heating and operation in-situ. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3615824] All rights reserved.

Aperture-edge scattering in MeV ion-beam lithography. II. Scattering from a rectangular aperture

The capability of collimators to define beams of MeV ions with sub-100nm dimensions has recently been demonstrated. Such nanometer beams have potential applications in MeV ion-beam lithography, which is the only maskless technique capable of producing extremely high aspect-ratio micro- and nanostructrures, as well as in high resolution MeV ion-beam imaging. Ion scattering from the collimator edges can be a resolution-restricting factor in these applications. Scattering processes at edges are difficult to study using conventional simulation codes because of the complicated geometry. In this part of our work, the authors used the GEANT4 toolkit as a simulation tool for studying the behavior of beams of 3MeV He ions with 0.2–1mrad divergence impinging onto the programmable proximity aperture comprising four 100-μm-thick Ta plates. The transmission and scattering from the aperture are asymmetric due to the aperture design. For a perfectly parallel beam, the fluence of ions scattered from the aperture edges is spread over large areas. The enhancement of the fluence from the edge-scattered ions is two to three orders of magnitude smaller compared to the incident-beam fluence. Therefore, the edge scattering is not a resolution-restricting factor, and the sub-100nm resolution is achievable. However, for diverging beams, the significant penumbra broadening of the beam spots restricts the attainable resolution. In lithography applications, the halo associated with penumbra broadening causes pattern edge roughening by producing single ion tracks in close vicinity to the pattern element edges.

A Compact Field Emission SEM for Low Voltage Imaging

Abstract A disruptive innovation in how scanning electron microscopes (SEMs) are designed and built has produced a low cost, but highperformance compact field emission SEM. By leveraging silicon processing technologies, Novelx has miniaturized the core technology inside a SEM. This miniaturization enabled the design of an all-electrostatic electron beam column that when coupled with a thermal field emission (TFE) electron source is optimized for lowvoltage imaging and sub-10nm resolution. In addition, the mySEM© has a quad-segmented microchannel plate (MCP) detector that has a topographic imaging mode that is capable of electron channeling contrast imaging (ECCI) at low imaging voltages. These capabilities were previously only available in high-end and much larger sized field emission SEMs outfitted with additional detectors. The Novelx mySEM show in Fig. 1 is now being used in material science and life science applications for characterizing a variety of energy sensitive nanomaterials, biomaterials and thin films.

Toward 1Tdot∕in.2 nanoimprint lithography for magnetic bit-patterned media: Opportunities and challenges

Nanoimprint lithography presents unique opportunities for patterned media applications due to its advantages of sub-10nm resolution capability, patterning of a whole disk in a single imprint step with reasonably high throughput, and the relatively low capital cost in comparison to other next generation lithography technologies. However, there are several critical issues that still remain very challenging. This article will briefly discuss these challenges in general and then focus on imprint lithography work including the fabrication of templates and demonstrate the imprinted results. In this work two types of polarities of high-density templates (pillar tone and hole tone) have been fabricated on fused silica substrates for the UV imprint process. The difficulties and limitations in each of the template fabrication processes will be discussed. The authors have successfully demonstrated template fabrication followed by imprinted results with a pitch of 24nm (1.1Tdots∕in.2) for both tones of templates. Initial imprinted results of dense dot patterns with a pitch as small as 18nm (2.0Tdots∕in.2) have been achieved. High-resolution scanning electron microscopy images are used as the primary metrology for both the dot size uniformity and the placement accuracy analysis. The difficulties and the limitations in template fabrication, the imprint process, and associated metrology will be discussed.

Evolution of the nanomorphology of photovoltaic polyfluorene blends: sub-100 nmresolution with x-ray spectromicroscopy

We investigate the influence of annealing on the morphology of intimately mixed blends of the conjugatedpolymers poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) (PFB) andpoly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT) with scanning transmission x-ray microscopy(STXM). Through the use of a zone plate with theoretical Rayleigh resolution of 30 nm, weare able to resolve sub-100 nm bulk structure in these films. Surprisingly, for unannealedfilms spin-coated from chloroform we observe features with an average diameter of85 nm. The high degree of photoluminescence quenching in these as-spun films(>95%)implies that there is significant intermixing within the 85 nm structures, indicating that a hierarchyof phase separation exists even on the length scale of less than 100 nm. With annealing up to160 °C, closeto the Tg of the components, there is little change in the feature sizes observed by STXM, althoughan increase in variation of the composition is observed. With annealing above160 °C the imaged features begin to evolve in size, increasing to 225 nm inextent, alongside large changes in composition with annealing to200 °C. Comparing the evolution of morphology imaged by STXM with the change inphotoluminescence quenching with annealing, we propose that phase separationfirst evolves via the evolution of relatively pure phases on the length scale of afew to tens of nanometres within the larger 85 nm structures. Once the lengthscale of compositional fluctuations exceeds 85 nm (for anneal temperatures above160 °C) the hierarchy of phase separation is lost and the subsequent morphological evolution isreadily imaged by STXM. Applying the results of an exciton diffusion and quenchingmodel, we find good agreement between the size of the domains measured by STXM (above180 °C) and the results of the model for an exciton diffusion length of 15 nm. The growth indomain size and towards purer structures has also been observed with resonant soft x-rayscattering.

Synchrotron-based imaging and tomography with hard X-rays

Hard X-ray imaging with synchrotron radiation is a powerful tool to study opaque materials on the micro- and nano-lengthscales. Different imaging methods are available with an instrument recently built at Sector 34 of the Advanced Photon Source. In-line phase contrast imaging is performed with micrometer resolution. Increased spatial resolution is achieved using cone-beam geometry. The almost parallel beam is focused with a Kirkpatrick–Baez mirror system. The focal spot serves as a diverging secondary source. An X-ray magnified image of the sample is projected on the detector. For imaging and tomography with sub-100nm resolution, an X-ray full-field microscope has been built. It uses a Kirkpatrick–Baez mirror (KB) as a condenser optic, followed by a micro-Fresnel zone plate (FZP) as an objective lens. The zone plates presently provide 50–85nm spatial resolution when operating the microscope with photon energy between 6 and 12keV. Tomography experiments have been performed with this new device.

The Identification of Particles in a Polymer Film Using Nano-Thermal Analysis

Abstract Nano-TA is a local thermal analysis technique that combines the high spatial resolution imaging capabilities of atomic force microscopy (AFM) with the ability to obtain understanding of the thermal behaviour of materials with a spatial resolution of sub-100nm. This breakthrough in spatial resolution of thermal analysis, which is ~50× better than previously reported, has profound implications for the fields of polymers and pharmaceuticals where local thermal understanding is key. The conventional AFM tip is replaced by a special nano-TA probe that has an embedded miniature heater and is controlled by the specially designed nano-TA hardware and software. The nano-TA probe enables a surface to be visualised at nanoscale resolution using the routine imaging modes of AFM. The user is able to select the spatial locations for the investigation of the thermal properties of the surface. Heat is supplied locally via the probe tip and the thermomechanical response is measured.

Apertureless scanning near field optical microscope with sub-10nm resolution

We report on the implementation of a versatile dynamic mode apertureless scanning near field optical microscope (aSNOM) for nanoscopic investigations of optical properties at surfaces and interfaces. The newly developed modular aSNOM optomechanical unit is essentially integrable with a multitude of laser sources, homemade scanning probe microscopes (SPMs) as well as commercially available SPMs as demonstrated here. The instrument is especially designed to image opaque surfaces without a restriction to transparent substrates. In the description of the instrument we draw frequent attention to various possible artifact mechanisms, how to overcome them, and we present effective checks to ensure true near field optical contrast. Lateral optical contrast in optical amplitude and phase images below 10nm is demonstrated.

High-resolution three-dimensional reconstruction: A combined scanning electron microscope and focused ion-beam approach

The ability to obtain three-dimensional information has always been important to gain insight and understanding into material systems. Three-dimensional reconstruction often reveals information about the morphology and composition of a system that can otherwise be obscured or misinterpreted by two-dimensional images. In this article, we describe tomographic measurements with 10nm scale resolution, combining focused ion-beam processing with field-emission scanning electron microscopy to obtain a series of high-resolution two-dimensional cross-sectional images. The images were then concatenated in a computer and interpolated into three-dimensional space to assess and visualize the structure of the material. The results of this research demonstrate the use of tomographic reconstruction of Si–Si∕Ge and θ′ Al2Cu samples to reproduce the three-dimensional morphology with sub-10nm resolution.

Numerical simulation of nanolithography with the subwavelength metallic grating waveguide structure

Metallic waveguide theory has been used to design subwavelength metallic grating waveguide structure which can excite the waveguide modes, especially the low frequency coupled surface plasmons mode, to achieve sub-50nm resolution lithography pattern by using the light with 436nm wavelength. The Finite Difference Time Domain method has been performed to analyze the performance of lithography pattern generated by two possible schemes. One named metal-layer scheme utilizes three different modes (two coupled surface plasmons and one non-coupled surface plasmons) on the metal layer to generate the lithography patterns with different resolution and visibility. The other named metal-cladding scheme excites the coupled mode in the metal-cladding region, which utilizes multi-layer coupled effect to generate the field with higher resolution (~ 34nm) and approximately same visibility compared with the metal-layer scheme. The effectively deviated range of grating period is also analyzed to keep the output pattern effective for the lithography.

Electromechanical imaging of biological systems with sub-10nm resolution

Electromechanical imaging of tooth dentin and enamel has been performed with sub-10nm resolution using piezoresponse force microscopy. Characteristic piezoelectric domain size and local protein fiber ordering in dentin have been determined. The shape of a single protein fibril in enamel is visualized in real space and local hysteresis loops are measured. Because of the ubiquitous presence of piezoelectricity in biological systems, this approach is expected to find broad application in high-resolution studies of a wide range of biomaterials.

Patterned Transfer of Metallic Thin Film Nanostructures by Water-Soluble Polymer Templates

Water-soluble polymer templates are replicated from surface patterns by spin-casting a poly(vinyl alcohol) film-forming solution that undergoes a room-temperature evaporation process to solidify in less than 1 min with lateral resolution of sub-100 nm. The patterned polymer templates are deposited with metal and coupled to substrate surfaces by methods that may include photocurable materials, two-component reaction systems, or direct bonding without an intervening adhesion layer. Water is used to dissolve the flexible polymer templates at the conclusion of the transfer process.

A linear-field scanning-force microscope: Application to x-ray gabor holography

We have built a linear field Scanning Force Microscope to readout x-ray Gabor holograms recorded in photoresist. A field linearity yielding one half or less pixel registration error (for 10-50nm pixel sizes) across the scanned hologram is needed so that the resulting reconstruction is not significantly degraded. The desire for a large (75μm)2 and linear scanning field necessitated designing our own stage since these conditions could not be met commercially.Gabor holography is well positioned to exploit flash x-ray sources, such as x-ray lasers and free electron lasers, as well as ultra-bright third generation synchrotron sources as a sub-50nm resolution imaging technique for biological samples. Gabor holography requires no optics (except for a monochromator with λ/Δλ ∽ 500) and no sample prefocusing since the image is focused in the reconstruction process. Furthermore, sample alignment is eased since the area of illumination can be quite large. In fact, Gabor holography can be used to image large fields with multimode sources with the resulting numerical aperture, and hence resolution, being limited by the size of a single mode.

Design of an extended image field soft-x-ray projection system

A soft-x-ray projection system has been designed, which consists of spherical components to be coated with multilayer reflection coatings. In the design, a two-mirror system and a spherical reflection mask, the optical aberrations were minimized. The design enables a resolution of sub-100 nm over a circular image field with a diameter of 4 mm at a wavelength of 10.5 nm. The assembly tolerances of the system and the fabrication tolerances of the substrates have been calculated. The surface roughness determined from our superpolished quartz substrates amounts to 0.3 nm, which is sufficient to meet the required specifications.