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Nanoimprint lithography

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Fig. 1 Illustrating the replication process using a PDMS stamp.

Nanoimprint Lithography (NIL) is a surface patterning technique first introduced in 1996 by Prof. Stephen Chou and his colleagues [1,2]. This new technology had such an important impact in so many research fields that it has been listed in 2003 by the Massachusetts Institute of Technology as one of 10 emerging technologies that will strongly impact the world [3]. Indeed beside its ultrahigh resolution reproduction capability (up to 5 nm) this technology is also very cost effective, easy to process and therefore makes it a serious candidate for the implementation of up-scaling applications and large scale industrial fabrication.

The principle of NIL (sometimes also called “hot embossing”) is very simple. A template or mold (e.g. a silicon wafer textured with nanoscale features) is pressed with a controlled pressure and temperature onto a substrate coated with a defined layer of polymeric material. After the mold removal an inverse reproduction of the features will then be directly imprinted onto the substrate.  Recently new UV-curable polymer materials (also called resists) have appeared on the market and have been used for nanoimprinting technology. This UV-NIL process makes it even simpler and faster than the traditional hot embossing technique because it can be done at room temperature. The only requirement is then to have a transparent mold which allows the UV light to pass through for the curing of the resist. The most common process is to make first a so-called transparent stamp of the original template and then use this stamp for the replication process of the resist dispersed onto the substrate. Poly(dimethyl siloxane) block polymer (PDMS) is commonly used for the fabrication of the transparent stamp. This material offers a very high resolution patterning and is also easy to separate from the mold due to its low surface energy.

enlarged view

Fig 2 PDMS stamp replication of three fields (6x8 mm2) with a hexagonal lattice structure with periods ranging from 600 to 800 nanometers.

In the Nano-SIPPE group we are working with UV-NIL technology in order to imprint silica nanostructures directly onto glass substrates. A UV-curable hybrid Sol-Gel resist is patterned using PDMS stamps. This process allows us to have a high degree of control over the different features of our structure like the periodicity, the diameter (in the case of nanopillar or nanohole structures) or even the height by varying the thickness of the resist on the substrate. These silica textured substrates are then used as templates for the investigation and development of silicon based new light trapping nanostructures and photonic crystals.