Extreme Ultraviolet Interference Lithography Research Articles

Optimization of a gas discharge plasma source for extreme ultraviolet interference lithography at a wavelength of 11 nm

In this work, we report about the optimization of the spectral emission characteristic of a gas discharge plasma source for high-resolution extreme ultraviolet (EUV) interference lithography based on achromatic Talbot self-imaging. The working parameters of the source are optimized to achieve a required narrowband emission spectrum and to fulfill the necessary coherence and intensity requirements. The intense 4f-4d transitions around 11 nm in a highly ionized (Xe8+–Xe12+) xenon plasma are chosen to provide the working wavelength. This allows us to increase the available radiation intensity in comparison with an in-band EUV xenon emission at 13.5 nm and opens up the possibility to strongly suppress the influence of the 5p-4d transitions at wavelengths between 12 and 16 nm utilizing a significant difference in conditions for optical thickness between 4f-4d and 5p-4d transitions. The effect is achieved by using the admixture of argon to the pinch plasma, which allows keeping the plasma parameters approximately constant while, at the same time, reducing the density of xenon emitters. It is demonstrated that with this approach it is possible to achieve a high intensity 11 nm EUV radiation with a bandwidth of 3%–4% without the use of multilayer mirrors or other additional spectral filters in the vicinity of the working wavelength. The achieved radiation parameters are sufficient for high-performance interference lithography based on the achromatic Talbot effect.

Patterning at the nanoscale: Atomic force microscopy and extreme ultraviolet interference lithography

We present our recent results achieved using atomic force microscopy nano-oxidation and nano-indentation of thin layers of polymers, and using extreme ultraviolet interference lithography. We focus our attention on the advantages and drawbacks of such lithographic techniques. In particular, we show the possibility to create patterns on polymers compatible with the conventional metal deposition and lift-off processes on submicrometer scale (EUV-IL and nano-indentation) and we experimentally confirm the theoretical prediction on the strong dependence of the oxidation process to the sample conductivity.

Template-directed self-assembled magnetic nanostructures for probe recording

We employ a template-directed self-assembly process to arrange nanospheres as small as 20 nm in a polymer resist template with regular hole arrays with periods down to 42 nm produced using extreme ultraviolet interference lithography. To demonstrate magnetic probe recording employing magnetic tips, an array of magnetic caps with perpendicular magnetic anisotropy was then created by depositing Co/Pt multilayer films on 60 nm nanospheres, arranged on a square lattice with 100 nm period. The magnetic nanocaps can be switched individually in a controlled fashion, with recognition of a successful switching event realized by measurement of a force-distance curve.

Surface plasmon resonance in near-field coupled gold cylinder arrays fabricated by EUV-interference lithography and hot embossing

Arrays of metal nanoparticles with nanometer-scale gaps between the particles is highly interesting for plasmonic field enhancement applications. We report a simple method to fabricate arrays of closely spaced Au particles with inter-particle separation down to 20 nm. We used extreme ultraviolet interference lithography (EUV-IL) and a mechanical press to fabricate two-dimensional arrays of Au nanoparticles. Lithographically produced particle arrays were modified by hot pressing in a nanoimprint machine and the gap was varied in a range from 50 nm to below 20 nm. Optical measurement shows two resonances at 520 nm and 620 nm, with the latter gaining strength as the gap is reduced. The experimental and theoretical investigations using a FDTD algorithm demonstrate that the low-energy resonance can be assigned to a collective surface plasmon resonance arising from the strong near-field coupling between the nanoparticles. Surface Enhanced Raman Spectroscopy (SERS) experiments performed on a model molecule (BPE) show a large gain in signal intensity as a result of the reduced gaps between the particles.

Guest Editorial: Extreme Ultraviolet Interference Lithography

This PDF file contains the editorial “Guest Editorial: Extreme Ultraviolet Interference Lithography” for JM3 Vol. 8 Issue 02

Extreme ultraviolet interference lithography as applied to photoresist studies

Extreme ultraviolet interference lithography (EUV IL), especially in combination with tool-independent metrics for resist performance, is a powerful technique for judging progress with current resists, the potential of new materials and studying the fundamentals of resist performance. We provide an overview of how EUV IL is applied for resist testing and early material selections. Also discussed are examples of EUV IL being used to gain fundamental understanding for resist characterization under EUV imaging conditions.

Design considerations for a cascaded grating interferometer suitable for extreme ultraviolet interference lithography

Recently, extreme ultraviolet interference lithography using a single grating interferometer and a highly coherent synchrotron insertion device source has proven to be an extremely useful technique for producing patterns with feature sizes in the range of 10 nm. The high demand for these nanoscale patterns and the small number of suitable highly coherent extreme ultraviolet sources has created new interest in the cascaded grating interferometer because of its relaxed demands for spatial and temporal coherence. This work extends that of earlier researchers on such systems by providing a compact algebraic analysis of the effects on fringe contrast of source divergence, spectral bandpass, lack of parallelism of the grating rulings, grating period mismatch, defocus, and wavefront curvature. The results are applied to illustrate the feasibility of implementing the interferometer on a small bending magnet synchrotron source, but the analysis should be applicable to typical portable plasma sources as well.

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.

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.

Partially coherent extreme ultraviolet interference lithography for 16nm patterning research

Conditions are reported under which a partially coherent plasma source of 13.5nm wavelength radiation is found to be suitable for interference lithography. The predicted resolution exceeds the capability of present imaging systems and is comparable to synchrotron-based approaches. Methods borrowed from ray tracing are utilized for a partially coherent interference analysis, and a rigorous coupled wave theory is applied to optimize grating efficiency. The results suggest that a compact patterning tool with a resolution of 16nm is possible by a careful selection of the design parameters.

Electric and magnetic resonances in arrays of coupled gold nanoparticle in-tandem pairs

We present an experimental and theoretical study on the optical properties of arrays of gold nanoparticle in-tandem pairs (nanosandwiches). The well-ordered Au pairs with diameters down to 35 nm and separation distances down to 10 nm were fabricated using extreme ultraviolet (EUV) interference lithography. The strong near-field coupling of the nanoparticles leads to electric and magnetic resonances, which can be well reproduced by Finite-Difference Time-Domain (FDTD) calculations. The influence of the structural parameters, such as nanoparticle diameter and separation distance, on the hybridized modes is investigated. The energy and lifetimes of these modes are studied, providing valuable physical insight for the design of novel plasmonic structures and metamaterials.

Dual Grating Interferometric Lithography for 22-nm Node

An extreme ultraviolet (EUV) interference lithography beamline using a single grating has been constructed at the BL3 beamline in the NewSUBARU synchrotron radiation facility. Using a single grating, a 400-nm line and space (L&S) resist pattern was replicated on a wafer by single grating interferometric lithography system combined with a bending magnet as a light source. In addition, a dual grating interferometric lithography which is suitable for a bending magnet as a light source has been designed and constructed at the BL3 beamline in NewSUBARU. Dual grating interferometric lithography has a capability to replicate of a 28 nm L&S pattern on the basis of the interference-fringes calculation under conditions of a partial coherent light source such as a bending magnet. In the dual grating interference optical system, two transparent gratings was employed. In addition, the dual grating interference lithographic exposure method can be combined with a stand alone EUV source, such as a laser produced plasma or a discharge produced plasma. Therefore, this exposure system is a compact system for the evaluation of resolution and line edge roughness (LER) in a EUV resist.

Impact of template variations on shape and arrangement of Si∕Ge quantum dot arrays

Templated self-assembly allows the fabrication of quantum dot (QD) arrays for use in nanoelectronic devices. Here, we show the strong dependence of the shape and arrangement of QDs on the template structures. Arrays of etched pits are patterned on Si (100) substrates by extreme ultraviolet interference lithography on which Si∕Ge layers are grown in a molecular beam epitaxy system. Single Ge dome clusters or quantum molecules consisting of four Ge hut clusters are obtained by a change of the pit diameter. Both arrays exhibit a narrow size distribution and exact alignment of the dots. In addition, multiple stacking of these arrays is demonstrated.

Nanoscale perpendicular magnetic island arrays fabricated by extreme ultraviolet interference lithography

Magnetic island arrays with a period of 50nm and uniform over 20×20μm2 have been fabricated by depositing Co∕Pd multilayer films on prepatterned SiOx pillars produced by extreme ultraviolet interference lithography. Scanning electron microscopy and magnetic force microscopy measurements made on the same islands give a direct, island-by-island comparison of the size and remanent switching field. The results demonstrate that the switching field distribution (SFD) is not primarily due to magnetostatic interactions, and a strong dependence of SFD on size is also not observed, indicating that a distribution of material properties is likely to be responsible for the SFD.

Large area arrays of metal nanowires

We present novel modified lift-off processes for the fabrication of large-area, uniform metal nanowire arrays. In all processes, dense line arrays with periods in the 50–100nm range were obtained in photoresist films with Extreme Ultraviolet Interference Lithography (EUV-IL). The critical problem of preparing lift-off masks with a negative resist profile is solved by the use of either a bilayer resist stack of HSQ/PMMA or deposition of a metal layer at oblique angles on top of the patterned resist lines. As an added benefit, the metal deposition step enables fine-tuning of the width of the nanowires. Using the developed processes, Au and Cr nanowires with 8nm–70nm linewidth were obtained.

Nanostructured substrates for high density protein arrays

Nearly defect-free arrays of several 104 gold dots of 12±3nm in diameter inside 10nm deep cavities were fabricated for immobilization of proteins. Extreme ultraviolet interference lithography (EUV-IL) at the XIL-beamline of the Swiss Light Source was used to produce 140nm period arrays of 50nm holes in a 40nm thick PMMA layer on oxidized silicon wafers. The size of the openings was reduced by glancing angle deposition (GLAD) of metals such as chromium and silver. Reactive ion etching of the underlying substrate, followed by deposition of a few nanometers of gold, lift-off and thermal annealing resulted in perfectly ordered arrays of small gold nanoparticles with well-defined size distribution. The combination of passivation of the silica surface with polyethylene glycol (PEG) derivatives and functionalization of gold with thiols enables the preparation of large area arrays of well separated functional protein molecules.

Pattern Replication in EUV Interference Lithography

Extreme ultraviolet interference lithography (EUVIL) beamline which employed a single grating was constructed at the BL3 beamline in NewSUBARU synchrotron radiation facility. Bending magnet was attempted as a light source, and Ta layer was employed as an absorber layer of 0th order light in transparent grating. Using this system, 400-nm L&S resist pattern was replicated on a wafer, which shows possibility of EUV interference lithographic technology employing bending magnet as a light source.

In-Situ Electrical Addressing of One-Dimensional Gold Nanoparticle Assemblies

Substrates with 1-dimensional nanosize grooves were prepared using extreme-ultraviolet interference lithography (EUV-IL), wherein gold nanoparticles were self-assembled to form 1-dimensional structures. To measure the electrical properties of gold nanoparticle chains we introduce a novel in-situ measuring method based on nanomanipulator system in a scanning electron microscope. This method comprises enormous versatility for the precisely electrical addressing of low-dimensional nanoscale structures and may even be applied to routinely addressing of structures in the sub-10 nm range.

Bit-array patterns with density over 1Tbit∕in.2 fabricated by extreme ultraviolet interference lithography

The use of patterned magnetic media in future data storage devices requires the fabrication of two-dimensional periodic patterns with resolution well below 35nm in terms of the pattern period. The authors have used extreme ultraviolet interference lithography based on transmission diffraction gratings to demonstrate arrays of structures in negative resists calixarene and hydrogen silsesquioxane. The produced patterns have density exceeding 1Tbit∕in.2. Analysis of structure size and position on one of the patterns indicates variations of subnanometer magnitude.

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.