Nanoimprint Lithography

Introduction
Nanoimprint lithography (NIL) has over the last decade gone from being a new and exciting research topic, to be a technology used in the most advanced parts of various industries. Since the first test of nanoimprint was carried out, the production technology has been greatly improved. Today, nanoimprint can be used for fast and inexpensive production of large areas of nanostructures.

NIL Technology (NILT) has worked intensively with nanoimprint and optimization of the imprint process. We have the ability to imprint large areas with nanostructures with very low defect density and to transfer the imprinted pattern into substrates while preserving crittical pattern dimensions. 


Stamp / Template Fabrication 
Our stamps are produced using, state of the art e-beam lithography systems:
JEOL JBX9300FS (Gaussian beam) or Vistec SB352HR (Shaped beam). This ensures that parameters such as pitch and linewidth of the structures are very well-defined and uniform across the entire stamp.

The stamp is subsequently dry-etched in order to transfer the e-beam written pattern into the stamp material, typically being silicon or quartz. The dry-etching ensures a very good height uniformity of the etched structures and also enables good control of the sidewall angle of the structures. Finally, the stamp is normally coated with an anti-sticking layer, ensuring the release of stamp and substrate in the imprint process.


Imprint Process
A substrate is coated with a thermoplastic polymer – typically PMMA or similar. The polymer is heated to a temperature above its glass transition temperature and the stamp is pressed into the polymer. Thereafter, the polymer is cooled to below the glass transition temperature and the stamp and substrate are separated. Thereby, the inverse profile of the stamp is replicated in the polymer.

When a substrate has been imprinted the residual layer of resist needs to be removed. This is done using anisotropic dry-etching. Although the dry-etch is optimized for strong anisotropy in the vertical direction there will always be a small etching in the lateral direction as well. This will cause the imprinted structures to be widened and the linewidth of the imprint will therefore be larger than that of the stamp. To avoid this problem the imprint process is optimized to produce very thin and uniform residual layers. In this way, a very short dry-etch is enough to completely remove the residual layer on the entire imprint. Thus, minimizing the widening of the structures.

The imprinted substrates can be further processed after the residual layer has been removed. The imprint resist can be used as a soft etch mask for direct pattern transfer into the substrate. Alternatively, the imprint resist can be used as a lift-off resist for metal liftoff.



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