Nanoimprint Lithography (NIL) is a high throughput technique for producing features with size down to the sub-10 nm scale onto a polymeric layer over a large area, exploiting the glass transition phenomenology of organic materials. The operation principle of NIL is the deformation of the surface morphology of a thermoplastic film under the application of pressure and temperature, not relying on the modification of the chemical structure of resists upon irradiation by energetic beams, such as in photo- or electron beam lithography. The simple patterning procedure, whose high resolution is not limited by diffraction of e.m. rays, and the relatively low cost of the experimental setup are main advantages of NIL in comparison to the conventional lithographic approaches. In addition, this technique can be applied to a wide range of materials, both inert polymers and active compounds.

In the conventional imprint method, the morphology of a starting master is transferred into a polymeric layer by the application of external pressure and by heating the system above the glass transition temperature of the polymer. Because of the high values of temperature reached during the imprint process (up to 200 °C), the application of NIL for the direct patterning of organic active molecules requires the development of setups providing vacuum or nitrogen environments. In fact, the oxygen incorporation and substitution into the molecular backbones occurring at high temperature can determine the irreversible degradation of the optical and the electrical properties of active materials. The use of processing chambers with controlled atmosphere leads however to a reduction of the operational simplicity and an increment of the overall cost of production. We developed and implemented the Room-Temperature Nanoimprint Lithography (RT-NIL) technique as an alternative to conventional NIL for patterning light-emitting conjugated polymers in air, in order to avoid the drawbacks of hot embossing. Fig. 1 displays a comparison between the conventional hot embossing process and RT-NIL is displayed.

 

RT-NIL allows one both to carry out an easier mold release, avoiding many distortions of the imprinted structures that are possible by high temperature imprinting, and, importantly, to repeat sequentially the imprinting step on the same film for realizing more complex patterns by printing the same region of the substrate with the same master template for several times. By performing hot embossing twice, one would destroy the first printed pattern because of the redistribution of the polymer layer as a consequence of heating the sample above glass transition during the second lithographic step. The possibility of achieving two dimensional (2D) nanostructures by sequential imprinting, not possible by conventional NIL, is significantly important for the realization of organic based nanopatterned photonics, and particularly 2D PhCs, since the fabrication of 2D PhCs by electron-beam lithography (link) is much more time-consuming than that of 1D gratings. In Fig. 2a, the schematic representation of sequential RT-NIL processes is shown. Fig. 2b displays a 2D PhC realized onto an active polymeric film, obtained by realizing the first 1D grating by RT-NIL and, then, imprinting again at room temperature the same region by rotating the 1D master of 90°, in order to position the features of the mold perpendicular to the first pattern. In Fig. 2c, we show an example of a conjugated polymer patterned by RT-NIL with resolution as low as 100 nm.

 

Fig. 2 (a) Schematic diagram of the RT-NIL process for the realization of 1D and 2D PhC-like patterns (features not in scale). Atomic Force Microscopy planar topographic views of patterns realized on optically active polymer compound: (b) 2D pattern with 500 nm feature size; (c) 1D 200 nm period grating.

 

 For more information, please contact: Dr. Dario Pisignano ( This e-mail address is being protected from spam bots, you need JavaScript enabled to view it )

Publications:

[1] E. Mele, F. Di Benedetto, L. Persano, R. Cingolani, D. Pisignano
Nano Lett. 5(10), 1915-1919 (2005).

 

[2] E. Mele, A. Camposeo, R. Stabile, P. Del Carro, F. Di Benedetto, L. Persano, R. Cingolani, D. Pisignano
Appl. Phys. Lett. 89, 131109 (2006).
 

[3] P. Del Carro, A. Camposeo, R. Stabile, E. Mele, L. Persano, R. Cingolani, D. Pisignano
Appl. Phys. Lett. 89, 201105 (2006).

 

[4] E. Mele, A. Camposeo, P. Del Carro, F. Di Benedetto, R. Stabile, L. Persano, R. Cingolani, D. Pisignano
Mater. Sci. Engineer. C 27(5-8), 1428-1433 (2007).