|
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.
|
