While there are many different flavours of nanoimprinting processes, two main ones are:
- Thermal nanoimprinting (also called as hot embossing)
Thermal nanoimprint technology
Thermal nanoimprint technology has been the first kind of nanoimprinting process ever used and the original one adopted by Prof. Chuo back in 1996.
The thermal nanoimprint process is very easy to understand and straightforward: a layer of thermoplastic polymer is deposited on the surface of the substrate by spin coating or alternative method; then the substrate is placed inside the imprint machine along with the mold and the mold is pressed against the substrate at a set pressure.
The substrate received a precise amount of heat from the machine and the temperature of the polymer rises over the glass-transition temperature and becoming soft.
The mold is kept pressed against the substrate for an amount of time, after that the substrate is cooled down and the mold is released
After the pattern is transferred to the mold polymer layer, it is then transferred to the beneath silicon substrate surface by etching process
UV-nanoimprint technology process is different from thermal nanoimprint in two aspects: it does not use heat to soften the polymer layer but UV light and requires a UV-transparent mold.
With UV-nanoimprint technology, the polymer used on the substrate surface is a UV-curable photopolymer and not a thermoplastic one as in the case of thermal nanoimprint; this allows the process to be effective at considerably lower pressures and helps to avoid the issue of thermal expansion
As discussed, the main advantage of thermal nanoimprint over UV-nanoimprint is that molds of any material can be used while, in case of UV-nanoimprint technology, only UV-transparent materials can be used
However, since UV-nanoimprint uses mainly UV light to deform the polymer, considerably lower pressure levels on the substrate are needed to be able to transfer the pattern to the polymer
Moreover, the advantage of not needing much heat to deform the polymer brings other advantages such as much lower special deformation of the pattern due to applied heat and therefore a better alignment accuracy and the ability to imprint larger areas.
As for UV-nanoimprint, there are two sub-categories: hard-mold UV-nanoimprint and soft-mold UV-nanoimprint: as the name implies, hard-mold UV-nanoimprint is a process that uses a quartz or other rigid mold, while soft-mold nanoimprinting uses a soft mold usually made in polymer.
Hard-mold UV-nanoimprint technology has two main shortcomings if compared to soft-mold UV-nanoimprint: the mold is rigid and therefore there are sometimes issues while releasing the mold from the substrate; moreover, there are other issues when trying to imprint large areas due to the inherent issues of flatness of the mold and the substrate.
Lack of perfect flatness in either the mold or the substrate can lead to defectivity in the pattern or, in worst cases, to a breakage or the substrate to the mold.
Using a soft mold, however, leads to some shortcomings too: while issues of flatness between the mold and the surface are greatly resolved while using a soft mold and releasing is far easier, problems like thermal expansion and non-uniformities are worsened due to the elasticity of the mold itself
Deformation of the pattern due to even small quantities of heat can become a significant problem at lower resolutions and this may lead to additional issues when the surface to be patterned is large and therefore the thermal expansion rate of the mold polymer varies across the substrate surface
A variant of the UV-nanoimprint process is the so-called Step and Flash Â® process.
Using this process, initlally developed at the University of Texas, the polymer is deposited on the substrate not by spin coating or by any other machine, but by a pattern generator which is embedded in the imprint machine. After the polymer has been deposited, a soft mold is gently pressed over the substrate surface and the pattern exposed to a UV-lamp.
After the soft mold is detached, the patterned polymer is etched and the unnecessary features are removed.
Other further categorizations of the process are: single step or multiple step and single layer and multiple layer nanoimprint
Single step nanoimprinting involves the patterning as a one-time process while multiple step process (also called step-and repeat nanoimprint) involves multiple step patterning of usually large areas of substrate with smaller area molds.
Single layer nanoimprinting is the traditional process where one single layer of pattern is transferred into the substrate surface, multiple layer nanoimprinting instead requires thhe patterning of multiple layers of patterning over the same area.
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