Variants of nano imprint lithography

Variants of nano imprint lithography

In addition to the above mentioned thermal and UV-nano imprint lithography, there a big number of variety of processes about nanoimprint that have flourished in recent years, such as laser assisted direct imprint, Sub-10 nm NIL, combined thermal and UV nanoimprint, electrical field-assisted NIL, soft UV-NIL, reverse NIL, Jet and Flash imprint lithography process

Reverse imprint lithography process Using the reverse nanoimprint process, instead of having the polymer spin coated on the substrate, we have polymer coated on the mold and then transferred into the substrate due to the different surface energy between the mold and the substrate that allows an easy transfer of polymer from the mold to the substrate.

Using this process, it is possible to create 3d structures relatively easier than with other methods, but the downside is that the mold gets easily dirty and the process can not therefore be used for large scale production.

Applications of the reverse imprint lithography process are: metallic nanowires, optical devices and microchannels.

Roll-to-roll imprint lithography process

In order to overcome one of the major limitations of traditional nano imprint lithography, that is the inability to imprint large areas at low cost, a new technology called roll-to-roll imprint has been developed.

Roll-to-roll imprint is a continuous and simple process that can allow patterning of large areas at a relatively low cost.

The process is relatively simple and involves a roller mold that imprints the flat substrate by rolling over it in a continuous and uniform way: with roll-to-roll imprint lithography features down to 50nm and lower have been demonstrated for large areas.

Nanoimprint on large areas

Large area imprint lithography can be done in other ways other than roll-to-roll imprint and step-and-repeat imprint.

In detail, preparing a large area mold that completely covers the substrate to be imprinted in a one-shot imprint is something not easy but somehow possible in some cases. In order to be able to perform a uniform imprint over the whole wafer surface and to overcome issues of flatness and uniformity several techniques have been pioneered, such as the Air Cushion Press that is used to ensure the pressure and pattern uniformities.

The technique is interesting but still far from commercial utilization due to the issues above: so far results have been achieved for patterns down to 200nm and with wafers up to 200mm

Laser-assisted direct nanoimprint

This technique is used to perform the nanoimprint directly on the surface without the use of a polymer film to transfer the patter from the mold to the substrate.

A laser melts the surface of the substrate so that the mold can directly pattern the substrate.

Usually the mold is made by hard and resistant metals such as nickel and the substrate needs to be made of a material which can be melt quite easily.

The advantage of the technique is that does not require a step of etching of the substrate and that flatness requirements become less stringent due to the fact that the surface of the substrate is melt.

The speed of the process is also a plus and patterns down to 10nm have been produced with this technique

One of the main drawbacks of the technique is that the process is hardly appropriate for full production due to the dirt that inevitably gets stuck into the mold after several imprints

Thermal and UV-NIL combined nanoimprint

This technique has been pioneered by Obducat and allows to combine UV-nanoimprint patterning with the use of heat to achieve better results

Substrate conformal imprint lithography

Substrate conformal imprint lithography bridges the gap between large area lithography using a soft stamp and small area lithography using a small stamp with high resolution: the technique aims at achieving the advantages of both techniques (large area and high resolution) minimizing the drawbacks of the soft stamp, which is the inability to reach high resolutions due to problems of thermal expansion.

Using this technique, patterns down to 50nm have been achieved on substrate that did not meet stringent planarity requirements.

Nanoelectrode lithography

This technique combines the use of an electrode with nanoimprint

The mold has to be conductive and the patterning process is combined with an electrochemical reaction that allows the oxide pattern to get transferred directly into the substrate surface

While still called as “nanoimprint”, the technique is quite different from traditional nanoimprint where the pattern is mechanically transferred to the surface while the process here happens by electrochemical reaction

Note:
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