In order to minimize the defects formation when using nano-imprinting process we investigated the efforts
applied on the resist during the release of the template. Lift-off release has already been characterized accurately
but for peeling studies are still lacking. However from experimental results it has been observed that peeling
offers better performances when it comes to limit the defects. Using finite element method we simulated
imprinting on PMMA resist by a silicon template and extracted the maximal release force and the induced stress
in the resist in regard to the template stiffness and the number of patterns imprinted. Compared to lift-off
method we found that maximal release force was much lower and decided to investigate the induced stress
behaviour. We observed that using peeling the maximal release force doesn’t increase linearly in function of the
template size as in lift-off but instead saturates beyond a certain template size, that saturating point depending
on the template stiffness, a low stiffness meaning a lower maximal release force. However we found an opposite
trend when we extracted the induced stress in the resist which decreases as the template stiffness increases,
theoretically resulting in fewer defects. This seems to be due to the smaller bending of the more rigid template
that put less constraint on the imprinted features during the releasing and thus avoid breaking them in the
process. Therefore according to these results, to minimize defects when peeling release method is employed we
should use a highly rigid template.
Novel photo-lithography is newly proposed named built-in lens mask lithography. The method emulates optical propagation plane in exposure system using binary transmittance and phase mask instead of projection lens. The performance of the built-in lens mask lithography is studied by numerical simulation and experimental study using conventional proximity exposure system. The result shows resolution enhancement in deep focus plane.
Cost effective micro lithography tool is demanded for fine micro devices. However, resolution of a conventional proximity exposure system is not sufficient below several micron feature size for deep focus depth. On the other hand, a reduction projection system is sufficient to resolve it but the cost of the tool is too much high compared to proximity exposure systems. To enhance the resolution of photolithography, there has been proposed a number of novel methods beside shorting of wave length. Some of them are utilized in current advanced lithography systems, for example, the immersion lithography<sup>1</sup> enhances effective NA and the phase shift mask<sup>2</sup> improves optical transmittance function. However, those advanced technology is mainly focused on improvement for advanced projection exposure systems for ultra-fine lithography. On the other hand, coherence holography pattering is recently proposed and expected for 3-dimentional pattering<sup>3-5</sup>. Also, Talbot lithography<sup>6-8</sup> is studied for periodical micro and nano pattering. Those novels pattering are based on wave propagation due to optical diffraction without using expensive optical lens systems. In this paper we newly propose novel optical lithography using built-in lens mask to enhance resolution and focus depth in conventional proximity exposure system for micro lithographic application without lens systems. The performance is confirmed by simulation and experimental works.
Molecular dynamics simulation is performed to study the yield stress and fracture mechanism of single crystalline silicon mold with notch-defect structures. From the stress distribution, it is found that the stress is concentrated near the notch defect and the notch acts as a trigger of the crucial mold fracture. The yield stress with a nano scale notch on the mold sidewall deteriorates more than 7.5 % compared to a defect-free mold. It is found that a surface damage such as notch defect is significant for strength deterioration of the mold. This result shows that the surface defects on the sidewall, which could be induced during the mold fabrication process such as dry etching process, causes serious failure.
Hybrid patterning by thermal and UV nanoimprint lithography is newly proposed to fabricate micro-nano mixture
structures. The SU-8 resist is thermally imprinted using the quartz mold, which has fine nano structures and micro Cr
blank patterns. After the thermal nanoimprint, UV is exposed keeping the mold on the resist through the mold. Then,
the mold is detached and the resist is developed to fabricate micro structures. Using this process, micro gratings having
40 μm in width and 20 μm in depth nano dots pattern, which has 200 nm feature size is successfully demonstrated.
Anti-reflection structure having sharpened corn shape is reproduced by nano casting method. Firstly, the master
quartz structure is transferred to UV curable resin to fabricate the replicated mold of the master structure using nano
casting method. Next, the anti-reflection structure is transferred to PMMA film using the replicated mold by the nano
casting method. To avoid the defects at the mold releasing, thin sacrifice layer is coated on the replicated mold.
Also, the molecular weight of the PMMA is optimized to improve the yield of the releasing process and transferred
pattern shape. Fine anti-reflection structure is fabricated by the proposed process using the nano casting method
without damages to the master structure.
A fine grating with high aspect rate pattern is one of the essential elements for advanced nano optical devices such as a quarter wave plate. To fabricate high aspect ratio pattern having sub wavelength feature size, nanoimprint lithography is applied. However, fatal defects caused by mechanical stress and friction between the mold and polymer are significant problems. To eliminate the defects, the process sequence, pressure and temperature conditions are optimized. Using Si based mold, sub wavelength grating having 200nm in width and over 1.7 micron in height is demonstrated using PMMA thin film on quartz substrate. This method is a promising technology for industrial production of advanced nano optical elements having high aspect ratio structure.