Nanoimprint lithography

Nanoimprint lithography is a method of fabricating nanometer scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting. Adhesion between the resist and the template is controlled to allow proper release.

The term "Nanoimprint Lithography" (NIL) was coined in the scientific literature in 1996, when Prof. Stephen Chou and his students published a report in Science, although hot embossing (now taken as a synonym of NIL) of thermoplastics had been appearing in the patent literature for a few years already. Soon after the Science paper, many researchers developed different variations and implementations. At this point, nanoimprint lithography has been added to the International Technology Roadmap for Semiconductors (ITRS) for the 32 and 22 nm nodes.

There are many different types of nanoimprint lithography, but three of them are most important: thermoplastic nanoimprint lithography, photo nanoimprint lithography and resist-free direct thermal nanoimprint lithography.

Thermoplastic nanoimprint lithography (T-NIL) is the earliest nanoimprint lithography developed by Prof. Stephen Chou's group. In a standard T-NIL process, a thin layer of imprint resist (thermoplastic polymer) is spin coated onto the sample substrate. Then the mold, which has predefined topological patterns, is brought into contact with the sample and they are pressed together under certain pressure. When heated up above the glass transition temperature of the polymer, the pattern on the mold is pressed into the softened polymer film. After being cooled down, the mold is separated from the sample and the pattern resist is left on the substrate. A pattern transfer process (reactive ion etching, normally) can be used to transfer the pattern in the resist to the underneath substrate.

Alternatively, cold welding between two metal surfaces could also transfer low-dimensional nanostructured metal without heating (especially for critical sizes less than ~10 nm),. Three-dimensional structures can be fabricated by repeating this procedure. The cold welding approach has the advantage of reducing surface contact contamination or defect due to no heating process, which is a main problem in the latest development and fabrication of organic electronic devices as well as novel solar cells.

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