Harun H. Solak

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 (∼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.

Nanolithography with coherent extreme ultraviolet light

Extreme ultraviolet interference lithography (EUV-IL) is a newly developed technique for the production of periodic nano-structures with resolution below 20 nm. The technique is based on coherent radiation that is obtained from undulators at synchrotron radiation laboratories. The high resolution is afforded by small wavelength and practical absence of the proximity effect at this energy. The throughput of this parallel exposing method is much higher than that of the serial electron-beam lithography. Interference schemes based on both reflection (mirrors) and diffraction (gratings) optics have been realized. Both one-dimensional and two-dimensional patterns such as arrays of dots have been achieved. Achromatic interference schemes have been developed to make efficient use of the beam power available from the wideband sources in the extreme ultraviolet region. EUV-IL is used in a growing number of applications; examples include fabrication of self-assembly templates, magnetic nanodot arrays and nano-optical components.

Sub-50 nm period patterns with EUV interference lithography

We have used transmission diffraction gratings in an interferometric setup to pattern one-and two-dimensional periodic patterns with periods near 50 nm. The diffraction gratings were written with e-beam lithography. The exposures were made at 13.4 nm wavelength with undulator radiation, which provides spatially coherent radiation. This technique offered a multiplication of pattern frequency by a factor of 2 and √2 in the one-and two-dimensional cases, respectively. Interference lithography with gratings offers a number of advantages, including achromaticity and insensitivity to misalignment. The demonstrated structures include line/space patterns with 45 nm period and a square array of holes with 56 nm period.

Plasmon resonances of aluminum nanoparticles and nanorods

We report experimental and theoretical analysis of the plasmonic resonances of Al nanoparticles and nanorods. Ordered nanoparticle arrays with well-defined shapes and narrow size distributions are fabricated on quartz substrates over large areas using extreme ultraviolet interference lithography. The structures, which have sizes down to 40 nm, exhibit strong and sharp particle plasmon resonances in the near and deep-UV ranges. A comprehensive theoretical analysis carried out using dipolar approximation and finite-difference time-domain methods shows good overall agreement with measurements while revealing the dependence of the optical response of Al structures on the fabrication conditions. The results demonstrate the suitability of using Al as a plasmonic material in the UV range and the feasibility of extending applications of plasmonics, such as surface-enhanced Raman spectroscopy, down to the deep-UV range.

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.

Fabrication of large-area ordered arrays of nanoparticles on patterned substrates

Many potential applications in nanotechnology require virtually defect-free arrays of nanometre-scale particles over large areas. Guided self-assembly of colloidal particles on patterned templates has been shown to produce ordered arrays of colloidal particles. However, there is a need to extend this technique to particles measuring much less than 50 nm in size and to develop robust fabrication techniques that would lead to defect-free, large-area arrays. We have investigated the use of the dip-coating technique to assemble one- and zero-dimensional arrays on patterned templates using particles with diameters in the 15–50 nm range. Substrates with high-resolution groove or hole patterns were prepared with extreme-ultraviolet interference lithography (EUV-IL). Particle arrays with low defect density were achieved by adapting the deposition conditions (particle concentration, pH, dip speed and orientation). The experimental findings are explained with a model that describes the relative influences of the contributing forces in the assembly process. The driving force behind the assembly is found to be the capillary forces that organize the particles with respect to the pattern on the substrate and each other. The process does not seem to impose any inherent limitations on the defect density or the size of the particle arrays.

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.

Fabrication of sub-10 nm gap arrays over large areas for plasmonic sensors

We report a high-throughput method for the fabrication of metallic nanogap arrays with high-accuracy over large areas. This method, based on shadow evaporation and interference lithography, achieves sub-10 nm gap sizes with a high accuracy of ±1.5 nm. Controlled fabrication is demonstrated over mm2 areas and for periods of 250 nm. Experiments complemented with numerical simulations indicate that the formation of nanogaps is a robust, self-limiting process that can be applied to wafer-scale substrates. Surface-enhanced Raman scattering (SERS) experiments illustrate the potential for plasmonic sensing with an exceptionally low standard-deviation of the SERS signal below 3% and average enhancement factors exceeding 1 × 106.

Extreme ultraviolet interference lithography at the Paul Scherrer Institut

We review the performance and applications of an extreme ultraviolet interference lithography (EUV-IL) system built at the Swiss Light Source of the Paul Scherrer Institut (Villigen, Switzerland). The interferometer uses fully coherent radiation from an undulator source. 1-D (line/space) and 2-D (dot/hole arrays) patterns are obtained with a transmission-diffraction-grating type of interferometer. Features with sizes in the range from one micrometer down to the 10-nm scale can be printed in a variety of resists. The highest resolution of 11-nm half-pitch line/space patterns obtained with this method represents a current record for photon based lithography. Thanks to the excellent performance of the system in terms of pattern resolution, uniformity, size of the patterned area, and the throughput, the system has been used in numerous applications. Here we demonstrate the versatility and effectiveness of this emerging nanolithography method through a review of some of the applications, namely, fabrication of metallic and magnetic nanodevice components, self-assembly of Si/Ge quantum dots, chemical patterning of self-assembled monolayers (SAM), and radiation grafting of polymers. (c) 2009 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3116559]

Arrays of nanoscale magnetic dots: Fabrication by x-ray interference lithography and characterization

X-ray interference lithography (XIL) was employed in combination with electrodeposition to fabricate arrays of nanoscale nickel dots which are uniform over 40μm and have periods down to 71nm. Using extreme-ultraviolet light, XIL has the potential to produce magnetic dot arrays over large areas with periods well below 50nm, and down to a theoretical limit of 6.5nm for a 13nm x-ray wavelength. In the nickel dot arrays, we observed the effect of interdot magnetic stray field interactions. Measuring the hysteresis loops using the magneto-optical Kerr effect, a double switching via the vortex state was observed in the nickel dots with diameters down to 44nm and large dot separations. As the dot separations are reduced to below around 50nm a single switching, occurring by collective rotation of the magnetic spins, is favored due to interdot magnetic stray field interactions. This results in magnetic flux closure through several dots which could be visualized with micromagnetic simulations. Further evidence of t...