Lithography represents the core-technology for the realization of devices for both electronic and optical applications. The requirement for faster and cheaper in the modern technology represents a key issue for the implementation and the development of new scientific prototype methods for the realization of micro and nano-features onto smart, soft, and functional materials. Although currently optical lithography is still dominant in the micro- and opto-electronics fields, important efforts have been done towards the development of techniques able to overcome the disadvantages of the photolithographic process, such as the optical diffraction-induced limit to achievable resolution, the high costs and the very poor flexibility in terms of employable materials.

To this aim, we have specifically developed a platform of alternative, low cost, non photolithographic techniques applicable to a wide range of organic and biological materials. The Soft Lithography approach consists of techniques based on Replica Molding and the self assembly of organic compounds. In Fig. 1, we schematise some methods. All the approaches still require one step of conventional fabrication technique (photo- or electron beam lithography depending on the resolution of the features to be reproduced) for obtaining of a rigid patterned master structure. Once the master is available, many copies of the pattern can be produced.

Soft Lithography is based on the use of an elastomeric mold (replica), realized by casting the liquid precursor of an elastomer against the master. Typically, the replicas are produced in poly(dimethylsiloxane) (PDMS), which exhibits low Young’s modulus, chemical inertia, biocompatibility, optical transparency in the visible and ultraviolet. After thermal curing, the replica is peeled off from the master and it can be employed over a wide class of materials for different applications, from biology to optics. In all the soft lithographic methods, the key feature is the conformal contact between the mold and the target substrate. Once the conformal contact is established, the final lithographic processes can be driven by capillary forces such as in Soft Molding and Microfluidic methods, or by physico-chemical reactions among organic ligands reactive towards proper surfaces, such as in Microcontact Printing.

 

Fig. 1. Schematic diagram of the lithographies based on Replica Molding: Soft Molding, Microfluidic Lithography and Microcontact Printing (features not in scale).

 

Soft Molding or Capillary Force Lithography joins the advantages of Nanoimprint Lithography (NIL ) and replica-assisted techniques. The use of an elastomeric replica is here combined with the working principle of NIL, i.e. the molding of a polymer melt. The use of a PDMS stamp allows one to solve different problems of NIL, such as the difficult stamp release, and the pressure-induce transport limitations. Besides routinely working with conjugated polymers, thermoplastics, and composites, by means of soft molding, we were able to reproduce faithful copies of complex structures. Fig. 2 shows the pattern transfer onto organic compounds of 150 nm ripple structure formed by self-organization of nanoscale structures on a glass template by irradiating the surface with a defocused, low energy Ar ion beam (in collaboration with Dr. A. Toma, Dr. F. Buatier deMongeot, Dr. R. Buzio, Dr. C. Boragno, Dr. G. Firpo, Dr. V. Mussi and prof. U. Valbusa at University of Genoa).

 

Fig. 2 Atomic Force Microscopy two-dimensional view (5×5 µm2) of a nanostructured polymer.

 

 

The Microfluidic Lithography is one of the most interesting lithographic technique for biological applications and for the realization of analytical devices. The pattern formation is the result of the motion of a liquid driven by capillarity into micro- or nanochannels. An elastomeric network of hydraulically connected capillaries is cut to expose the micro/nano-channels, and placed in contact with the target substrate under its own weight. A liquid drop of the target compound is deposited at the edge of the mold, in order to make the channels spontaneously filled by capillary action. We employed Microfluidic Lithography for the realization of complex pattern geometries by photocurable polymers and proteins embedded into hydrogel matrices.

The Microcontact Printing is a powerful technique for transferring molecules from a stamp to target surfaces. The stamp is inked with a solution of molecules, then dried and pressed onto the surface to be patterned. Molecules are therefore transferred directly from the stamp to the surface in the areas where the PDMS stamp makes conformal contact with the surface.

We focused our attention also on the development of Multilevel Soft Lithographies that are of significant importance for photonics and for the fabrication of three-dimensional structures and circuital elements in microfluidics. High registration accuracy in subsequent lithographic steps has been accomplished by multilevel alignments with a suitably designed, state-of-the-art alignment system (inset of Fig. 3). Fig. 3 shows a sample (single dot diameter of about 1 µm) realized by soft molding lithography assisted by UV curing.