Proc. SPIE. 8323, Alternative Lithographic Technologies IV
KEYWORDS: Nondestructive evaluation, Photoresist materials, Near field, Lithography, Plasmonics, Process modeling, Finite-difference time-domain method, Photoresist developing, Near field scanning optical microscopy, 3D modeling
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.
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 10<sup>3</sup> 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.
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.
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.