Pinhole diffraction holography for fabrication of high-resolution Fresnel zone plates.

Fresnel zone plates (FZPs) play an essential role in high spatial resolution x-ray imaging and analysis of materials in many fields. These diffractive lenses are commonly made by serial writing techniques such as electron beam or focused ion beam lithography. Here we show that pinhole diffraction holography has potential to generate FZP patterns that are free from aberrations and imperfections that may be present in alternative fabrication techniques. In this presented method, FZPs are fabricated by recording interference pattern of a spherical wave generated by diffraction through a pinhole, illuminated with coherent plane wave at extreme ultraviolet (EUV) wavelength. Fundamental and practical issues involved in formation and recording of the interference pattern are considered. It is found that resolution of the produced FZP is directly related to the diameter of the pinhole used and the pinhole size cannot be made arbitrarily small as the transmission of EUV or x-ray light through small pinholes diminishes due to poor refractive index contrast found between materials in these spectral ranges. We also find that the practical restrictions on exposure time due to the light intensity available from current sources directly imposes a limit on the number of zones that can be printed with this method. Therefore a trade-off between the resolution and the FZP diameter exists. Overall, we find that this method can be used to fabricate aberration free FZPs down to a resolution of about 10 nm.

Element-specific hysteresis loop measurements on Individual 35 nm islands with scanning transmission X-ray microscopy.

Using scanning transmission X-ray microscopy combined with X-ray magnetic circular dichroism, element-specific hysteresis loops with a 25 nm X-ray probe are obtained on 35 nm Fe/Gd multilayer nanoislands fabricated by extreme ultra-violet interference lithography. Local hysteresis loops measured for the individual islands and the antidot film between the islands display similar behavior resulting from the lateral confinement. Line scan measurements confirm ferrimagnetic coupling between Fe and Gd in the patterned region. The ability to measure magnetization reversal with X-rays at high spatial resolution will provide an important tool for future characterization of sub-50 nm nanostructures.

Displacement Talbot lithography: a new method for high-resolution patterning of large areas.

Periodic micro and nano-structures can be lithographically produced using the Talbot effect. However, the limited depth-of-field of the self-images has effectively prevented its practical use, especially for high-resolution structures with periods less than 1 micrometer. In this article we show that by integrating the diffraction field transmitted by a grating mask over a distance of one Talbot period, one can obtain an effective image that is independent of the absolute distance from the mask. In this way high resolution periodic patterns can be printed without the depth-of-field limitation of Talbot self-images. For one-dimensional patterns the image obtained is shown to be related to the convolution of the mask transmission function with itself. This technique, which we call Displacement Talbot Lithography (DTL), enables high-resolution photolithography without the need for complex and expensive projection optics for the production of periodic structures like diffraction gratings or photonic crystals. Experimental results showing the printing of linear gratings and an array of holes on a hexagonal lattice are presented.

High-resolution Fresnel zone plate fabrication by achromatic spatial frequency multiplication with extreme ultraviolet radiation.

We used an approach based on the self-imaging property of gratings to fabricate high-resolution Fresnel zone plates (FZPs). Under certain conditions, the illumination of a parent ZP with a wideband EUV beam produces a radially oscillating intensity distribution with double the spatial frequency of the ZP. This intensity distribution is observed in a certain distance range, given by the local zone width, the focal length of the ZP, and the spectral bandwidth of the illuminating beam. This phenomenon has been used to lithographically record daughter ZPs that have approximately half the zone width, thus twice the resolution, of the parent ZP. FZPs with zone widths as low as 30 nm have been fabricated in this way. Use of this technique in the extreme ultraviolet (EUV) region has the potential for high throughput production of FZPs and similar high-resolution diffraction optics with variable spatial frequency for the EUV and x-ray regions.

Epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates.

Parallel processes for patterning densely packed nanometre-scale structures are critical for many diverse areas of nanotechnology. Thin films of diblock copolymers can self-assemble into ordered periodic structures at the molecular scale (approximately 5 to 50 nm), and have been used as templates to fabricate quantum dots, nanowires, magnetic storage media, nanopores and silicon capacitors. Unfortunately, perfect periodic domain ordering can only be achieved over micrometre-scale areas at best and defects exist at the edges of grain boundaries. These limitations preclude the use of block-copolymer lithography for many advanced applications. Graphoepitaxy, in-plane electric fields, temperature gradients, and directional solidification have also been demonstrated to induce orientation or long-range order with varying degrees of success. Here we demonstrate the integration of thin films of block copolymer with advanced lithographic techniques to induce epitaxial self-assembly of domains. The resulting patterns are defect-free, are oriented and registered with the underlying substrate and can be created over arbitrarily large areas. These structures are determined by the size and quality of the lithographically defined surface pattern rather than by the inherent limitations of the self-assembly process. Our results illustrate how hybrid strategies to nanofabrication allow for molecular level control in existing manufacturing processes.

Biosensing by densely packed and optically coupled plasmonic particle arrays.

Densely packed plasmonic particle arrays are investigated for biosensing applications. Such particle arrays exhibit interparticle optical coupling creating a strong field between the particles, which is useful for sensing purposes. The sensor properties, such as bulk sensitivity, layer sensitivity, and the depth of sensitivity are investigated with the aid of a multiple multipole program. Sensitivity to the analyte with low concentration is also examined by a dynamic adsorption processes. The detectable concentration limit of streptavidin within 3000 s in the detection system is expected from the signal-to-noise to be less than 150 pM.

Fabrication of molecular nanotemplates in self-assembled monolayers by extreme-ultraviolet-induced chemical lithography.

Extreme-UV interference lithography (EUV-IL) is applied to create chemical nanopatterns in self-assembled monolayers (SAMs) of 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) on gold. X-ray photoelectron spectroscopy shows that EUV irradiation induces both the conversion of the terminal nitro groups of NBPT into amino groups and the lateral crosslinking of the underlying aromatic cores. Large-area ( approximately 2 mm(2)) nitro/amino chemical patterns with periods ranging from 2000 nm to 60 nm can be generated. Regions of pristine NBPT on the exposed samples are exchanged with protein-resistant thiol SAMs of polyethyleneglycol, resulting in the formation of molecular nanotemplates, which can serve as the basis of complex biomimetic surfaces.

Bilayer Al wire-grids as broadband and high-performance polarizers.

We have fabricated, characterized and theoretically analyzed the performance of bilayer (or stacked) metallic wire-grids. The samples with 100 nm period were fabricated with extreme-ultraviolet interference lithography. Transmission efficiency over 50% and extinction ratios higher than 40 dB were measured in the visible range with these devices. Simulations using a finite-difference time-domain algorithm are in agreement with the experimental results and show that the transmission spectra are governed by Fabry-Perot interference and nearfield coupling between the two layers of the structure. The simple fabrication method involves only a single lithographic step without any etching and guarantees precise alignment and separation of the two wire-grids with respect to each other.

Nanofabrication of broad-band antireflective surfaces using self-assembly of block copolymers.

We present a simple and cost-effective method for the fabrication of antireflective surfaces by self-assembly of block copolymers and subsequent plasma etching. The block copolymers create randomly oriented periodic patterns, which are further transferred into fused silica substrates. The reflection on the patterned fused silica surface is reduced to well below 1% in the ultraviolet, visible, and near-infrared ranges by exploiting subwavelength nanostructures with periodicities down to 48 nm. We show that by choosing the appropriate block copolymers and pattern transfer parameters the optical properties of the antireflective surface can be easily tuned, and the spectral measurements verify a significant reduction of the reflectivity by a factor of 10. The experiments, confirmed with simulations based on rigorous diffraction theory, also show that the tapered shape of the nanostructures gives rise to a graded index surface, resulting in a broad-band antireflective behavior.

Patterning of self-assembled pentacene nanolayers by extreme ultraviolet-induced three-dimensional polymerization.

Most researchers expect extreme ultraviolet lithography (EUVL) to be used to create patterns below 32 nm in semiconductor devices. An ultrathin EUV photoresist (PR) layer a few nanometers thick is required to further reduce the minimum feature size. Here, we show for the first time that pentacene molecular layers can be employed as a new EUV resist. Nanometer-scale dots and lines have been successfully realized using the new molecular resist. We clearly show the mechanism that forms the nanopatterns using a scanning photoemission microscope, EUV interference lithography, an atomic force microscope, and photoemission spectroscopy. The molecular PR has several advantages over traditional polymer EUV PRs. For example, it has high thermal/chemical stability, negligible outgassing, the ability to control the height and width on the nanometer scale, fewer residuals, no need for a chemical development process and thus a reduction of chemical waste when making nanopatterns. Besides, it can be applied to any substrate to which pentacene bonds chemically, such as SiO2, SiN, and SiON, which are important films in the semiconductor device industry.

Extraordinary optical transmission in the ultraviolet region through aluminum hole arrays.

We investigated the extraordinary optical transmission phenomenon in the UV range by fabricating large-area, free-standing aluminum hole arrays using extreme UV interference lithography and shadow thermal evaporation. Transmission spectra show strong peaks in the UV region resulting from both surface plasmon polariton and localized surface plasmon excitations. The results indicate that the high plasmon frequency of Al is directly responsible for the presence of strong resonance peaks in the UV region, which supports the role of plasmonic phenomena in the extraordinary transmission. The simple fabrication method enables large-area production of such structures for research and industrial production purposes.

Three-dimensional Si/Ge quantum dot crystals.

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of lambda = 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 x 10(7) dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally (+/-10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.

Nanostructured bio-functional polymer brushes.

Structured poly(glycidyl methracrylate) (poly-GMA) brushes have been grafted onto flexible fluoro-polymer films using a radiation grafting process. The reactive epoxide of poly-GMA provides the basis for a versatile biofunctionalization of the grafted brushes. Structure definition by extreme ultraviolet (EUV) exposure allowed nanometer-scale resolution of periodic patterns. By variation of the exposure dose the height of the grafted structures can be adapted in a wide range. Derivatization of the grafted brushes included reaction with various amines with different side chains, hydrolysis of the epoxide to diols to increase protein resistance and introduction of ionic groups to yield poly-electrolytes. As an example for biofunctionalization, biotin was linked to the grafted brush and biofunctionality was demonstrated in a competitive biotin-streptavidin assay. In this article we also present a brief review of other approaches to obtain structured biofunctional polymer brushes.

Phase behavior of symmetric ternary block copolymer-homopolymer blends in thin films and on chemically patterned surfaces.

The phase diagram of symmetric ternary blends of diblock copolymers and homopolymers in thin films was determined as a function of increasing volume fraction of homopolymer (phi(H)) and was similar to that for these materials in the bulk. Blends with compositions in the lamellar region of the diagram (phi(H)< or =0.4) could be directed to assemble into ordered lamellar arrays on chemically striped surfaces if the characteristic blend dimension (L(B)) and the period of the stripes (L(S)) were commensurate such that L(S)=L(B)+/-0.10L(B). Blends with compositions in the microemulsion region of the diagram (phi(H) approximately 0.6) assembled into defect-free lamellar phases on patterned surfaces with L(S)> or =L(B), but formed coexisting lamellar (with period L(S)) and homopolymer-rich phases when L(S)

Fabrication of complex three-dimensional nanostructures from self-assembling block copolymer materials on two-dimensional chemically patterned templates with mismatched symmetry.

A study is presented of the self-assembly of a lamella-forming blend of a diblock copolymer and its respective homopolymers on periodically patterned substrates consisting of square arrays of spots, that preferentially attract one component, as a function of pattern dimensions and film thickness. The blend morphology follows the pattern at the substrate and forms a single quadratically perforated lamella (QPL). At intermediate film thicknesses necks connect this QPL to the film surface, resulting in a bicontinuous morphology. The necks do not register with the underlying square lattice but exhibit a substantial amount of hexagonal short-range order. For thicker films we observe bicontinuous morphologies consisting of parallel lamellae with disordered perforations. These results demonstrate a promising strategy for the fabrication of complex interfacial nanostructures from two-dimensional chemically patterned templates.

Directed assembly of block copolymer blends into nonregular device-oriented structures.

Self-assembly is an effective strategy for the creation of periodic structures at the nanoscale. However, because microelectronic devices use free-form design principles, the insertion point of self-assembling materials into existing nanomanufacturing processes is unclear. We directed ternary blends of diblock copolymers and homopolymers that naturally form periodic arrays to assemble into nonregular device-oriented structures on chemically nanopatterned substrates. Redistribution of homopolymer facilitates the defect-free assembly in locations where the domain dimensions deviate substantially from those formed in the bulk. The ability to pattern nonregular structures using self-assembling materials creates new opportunities for nanoscale manufacturing.

Preparation of micro- and nanopatterns of polymer chains grafted onto flexible polymer substrates.

In this work a simple novel method for preparing micro- and nanoscale patterns of polymer chains grafted onto flexible polymer substrates is described. A combination of the two techniques of radiation grafting and "grafting-from" has been made. This combination makes it possible to prepare grafted structures having micro- or nanoscale lateral dimensions that are determined by the electron beam or X-ray irradiation patterns used. The height of the grafted features can be controlled by the irradiation dose or such grafting reaction conditions as time, temperature, or monomer concentration. Our first results for nanopatterned samples demonstrate resolution comparable to those of other polymer-based lithography processes.