Howon Jung

Plasmonic nano lithography with a high scan speed contact probe

We demonstrate plasmonic lithography with an optical contact probe to achieve high speed patterning without external gap distance control between the probe and the photoresist. The bottom surface of the probe is covered with a 10 nm thickness silica glass film for the gap distance control and coated with self-assembled monolayer (SAM) to reduce friction between the probe and the photoresist. We achieve a patterning resolution of ~50 nm and a patterning speed of ~10 mm/s. We obtain the quality of line patterning comparable to that in conventional optical lithography.

Resolution Limit in Plasmonic Lithography for Practical Applications beyond 2x-nm Half Pitch

A theoretical model is introduced to evaluate the ultimate resolution of plasmonic lithography using a ridge aperture. The calculated and experimental results of the line array pattern depth are compared for various half pitches. The theoretical analysis predicts that the resolution of plasmonic lithography strongly depends on the ridge gap, achieving values under 1x nm with a ridge gap smaller than 10 nm. A micrometer-scale circular contact probe is fabricated for high speed patterning with high positioning accuracy, which can be extended to a high-density probe array. Using the circular contact probe, high-density line array patterns are recorded with a half pitch up to 22 nm and good agreement is obtained between the theoretical model and experiment. To record the high density line array patterns, the line edge roughness (LER) is reduced to ≈17 nm from 29 nm using a well-controlled developing process with a smaller molecular weight KOH-based developer at a temperature below 10°C.

Accurate near-field lithography modeling and quantitative mapping of the near-field distribution of a plasmonic nanoaperture in a metal

In nanolithography using optical near-field sources to push the critical dimension below the diffraction limit, optimization of process parameters is of utmost importance. Herein we present a simple analytic model to predict photoresist profiles with a localized evanescent exposure that decays exponentially in a photoresist of finite contrast. We introduce the concept of nominal developing thickness (NDT) to determine the proper developing process that yields the best topography of the exposure profile fitting to the isointensity contour. Based on this model, we experimentally investigated the NDT and obtained exposure profiles produced by the near-field distribution of a bowtie-shaped nanoaperture. The profiles were properly fit to the calculated results obtained by the finite differential time domain method. Using the threshold exposure dose of a photoresist, we can determine the absolute intensity of the intensity distribution of the near field and analyze the difference in decay rates of the near field distributions obtained via experiment and calculation. For maximum depth of 41 nm, we estimate the uncertainties in the measurements of profile and intensity to be less than 6% and about 1%, respectively. We expect this method will be useful in detecting the absolute value of the near-field distribution produced by nano-scale devices.

Plasmonic lithography for fabricating nanoimprint masters with multi-scale patterns

We successfully demonstrate the practical application of plasmonic lithography to fabricate nanoimprint masters. Using the properties of a non-propagating near-field, we achieve high-speed multi-scale patterning with different exposure time during the scanning. We modulate the width of the line patterns using a pulse light source with different duty cycles during the scanning of the probe. For practical application in plasmonic lithography, we apply a deep reactive ion etching process to transfer an arbitrary fluidic channel into a silicon substrate and fabricate a high-aspect-ratio imprint master. Subsequently, we carry out the imprint process to replicate the fluidic channel with an aspect ratio of 7.2. For pattern width below 100 nm, we adopt a three-layer structure of photoresist, hard layer, and polymer to record the near field and form a hard mask and transfer mask. Using the multilayer structure, we fabricate high-resolution nanoimprint masters in a silicon substrate with an aspect ratio greater than 1.

Characterization of three-dimensional field distribution of bowtie aperture using quasi-spherical waves and surface plasmon polaritons

We present an analytical formula to predict the three-dimensional field distribution of a nanoscale bowtie aperture using quasi-spherical waves (QSWs) and surface plasmon polaritons, which are excited by the fundamental waveguide mode and local plasmons of the aperture, respectively. Assuming two separate bowtie apertures in a metal film, we analysed the decay characteristics of QSWs using a finite difference time-domain method. To verify the formula, we recorded the spot patterns of the bowtie aperture on a photoresist film using various exposure times, and fit the patterns to the analytical formula in terms of the width and depth of the patterns. In addition, it was found that the formula successfully represented the dipole characteristics of the spot patterns, which were in agreement with the surface geometry, with a root-mean-square error of 9.4%. We expect that our theoretical formula will extend the potential applications of nanoscale bowtie apertures to plasmonic device fabrication, three-dimensional plasmonic lithography, and other technologies.

High throughput plasmonic lithography for sub 50nm patterning with a contact probe

We suggest near-field optical lithography that uses contact probe for high speed patterning. The contact probe contains high transmission metal nano aperture and cover-layer for gap distance formation without external feed-back control unit. For contact mode operation, lubricant layer is applied between probe and photoresist surface. Using this contact probe, we recorded 50nm width line pattern with 10mm/s which is 500 times faster than conventional near-field scanning optical microscope lithography. The various line patterns having are recorded as increasing exposure dose and pattern qualities such as line width roughness (LWR) and depth roughness (DR) are evaluated. We expect the contact probe could be extended array probe lithography system for high throughput plasmonic lithography for mass production.

Nanoscale 2.5-dimensional surface patterning with plasmonic lithography

We report an extension of plasmonic lithography to nanoscale 2.5-dimensional (2.5D) surface patterning. To obtain the impulse response of a plasmonic lithography system, we described the field distribution of a point dipole source generated by a metallic ridge aperture with a theoretical model using the concepts of quasi-spherical waves and surface plasmon–polaritons. We performed deconvolution to construct an exposure map of a target shape for patterning. For practical applications, we fabricated several nanoscale and microscale structures, such as a cone, microlens array, nanoneedle, and a multiscale structure using the plasmonic lithography system. We verified the possibility of applying plasmonic lithography to multiscale structuring from a few tens of nanometres to a few micrometres in the lateral dimension. We obtained a root-mean-square error of 4.7 nm between the target shape and the patterned shape, and a surface roughness of 11.5 nm.

Calibration of exposure dose for nanoscale plasmonic lithography with microsized far-field spot patterns

To improve the reliability of a plasmonic lithography system for nanoscale device fabrication, a rapid calibration process is essentially required. The calibration needs a time-consuming process using an atomic force microscope (AFM) to measure a number of nano-sized spot pattern widths recorded for the variation of the exposure dose. On the basis of the underlying mechanisms of a propagating field through a bowtie aperture, we conducted a theoretical study to derive a fitting equation to predict the widths of spot patterns in a near-field region compared with those in the far-field region. We obtained a calibration curve of the exposure dose to fit the width of spot pattern in the far-field region that is measureable using an optical microscope (OM). The validity of the rapid calibration process using an OM was verified by comparison between the calibration curves determined using AFM and OM, and the uncertainty between them was found to be 3.4%. The drift of the calibration curve was further explored to calculate the system stability of the plasmonic lithography technique, which was estimated to be >93%. Furthermore, we also demonstrated that the calibration curve is effective in the prediction of the exposure dose for nanoscale line patterning.

Design of a high positioning contact probe for plasmonic lithography

We suggest a geometrically modified probe to achieve high positioning accuracy for plasmonic lithography which can record nanometer scale features and has high throughput. Instead of a cantilever probe, we propose a circular probe which has arc-shaped arms that hold the tip at the center. The modified probe is based on the fixed-fixed beam in material mechanics. To calculate the tip displacement, we used a finite element method (FEM) for a circular probe and compared the results with cantilever probe. We considered a silicon-based micro-fabrication process to design the probe. The probe has a square outline boundary with a length of 50μm, four arms, and a pyramidal tip with a height of 5μm. The ratio of the lateral tip displacement to the vertical deflection was evaluated to indicate the positioning accuracy. The probe has higher accuracy by a factor of 103 and 10 in approach mode and scan mode, respectively, compared to a cantilever probe. We expect that a circular probe is appropriate for the applications that require high positioning accuracy, such as nanolithography with a contact probe and multiple-probe arrays.

High-resolution laser direct writing with a plasmonic contact probe

We developed a contact-probe-based laser direct writing technique with nanometer scale resolution. The probe uses a solid-immersion-lens (SIL) or a bowtie nano-aperture to enhance the resolution in laser direct writing method and scans sample surface in contact mode for high scan speed. The bowtie shaped nano-aperture is fabricated by focused ion beam (FIB) milling on the metal film coated on cantilever type probe tip and dielectric material (Diamond-like carbon) is covered the probe for surface protection. Using a plasmonic contact probe, we obtained an optical spot beyond the diffraction limit and the size of spot was less than 30 nm at 405 nm wavelength. The proposed probe is integrated with a conventional laser direct writing system and by getting rid of external gap control unit for near-field writing, we achieved high scan speed (~10 mm/s). The raster scan mode for the arbitrary patterning was developed for practical applications. Furthermore, we designed developing a parallel maskless writing system for high throughput with an array of contact probes.