Yongwoo Kim

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.

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.

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.

Plasmonic lithography modeling and measurement of near-field distribution of plasmonic nano aperture

In plasmonic nano lithography, a photoresist responds to the localized electric field which decays evanescently in the direction of depth. A simple analytic model is suggested to predict profiles of exposed and finally developed pattern with a finite contrast of photoresist. In this model, the developing process is revisited by accounting the variation of dissolution rate with respect to expose dose distribution. We introduce the concept of nominal developing thickness (NDT) to determine the optimized developing process fitting to the isointensity profile. Based on this model, we obtained three dimensional distribution of near-field of bowtie shaped plasmonic nano aperture in a metal film from the near-field lithography pattern profile. For the near-field exposure, we fabricated a nano aperture in a aluminum metal film which is coated on the contact probe tip. By illuminating 405 nm diode laser source, the positive type photoresist is exposed by the localized electric field produced by nano aperture. The exposed photoresist is developed by the TMAH based solution with a optimum NDT, which leads the developing march encounters the isoexposure contour at threshold dose. From the measurement of developed pattern profile with a atomic force microscope (AFM), the three-dimensional isoexposure (or iso-intensity) surface at the very near region from the exit plane of an aperture (depth: 5 ~ 50 nm) is profiled. Using the threshold dose of photoresist and exposure time, the absolute intensity level is also measured. The experimental results are quantitatively compared with the calculation of FDTD (finite- difference time-domain) method. Concerning with the error in exposure time and threshold dose value, the error in measurement of profile and intensity are less than 6% and 1%, respectively. We expect the lithography model described in this presentation allows more elaborated expectation of developed pattern profile. Furthermore, a methodology of mapping is useful for the quantitative analysis of near-field distribution of nano-scale optical devices.

Nano patterning with a single high-transmission nano-metal aperture system

We design a C-shaped aperture which overcomes the diffraction limit of light to produce a high-brightness nano-size light spot. For optical nano lithography, we construct a nano patterning system using an optical probe which adopts a solid immersion lens (SIL), the 120 nm thickness aluminum film on the bottom surface of the SIL and the C-shaped aperture engraved in the metal film. Light source is a diode laser of 405nm wavelength to expose h-line photoresist(PR). A linear stage holding the optical probe makes the nano aperture contact with the PR coated on silicon wafer. Using this patterning system, we obtain sub 100nm array patterns and measure the system performance in various exposure conditions to verify the feasibility of plasmonic lithography.