Nanoimprint processes

Fig. 8.7 Fabrication sequence of a polymer microring resonator (a) prepare a nanoimprint mold (b) spin coat a polymer thin film (c) perform nanoimprinting process (d) separate the sample from the mold (e) dry etch the residual layer (f) create pedestals by wet etch...

Here we describe two simple fabrication processes to modify the surface of the device to achieve a very high water contact angle. In the first approach, the device was first coated with a thin film of hydrophobic materials, fluoropolymer in this case, and then oxygen plasma was used to create superhydrophobic surfaces. However, only in some cases, the chemical properties of the hydrophobic materials could be altered by the oxygen plasma treatment [19]. Therefore, a second technique has been developed where the nanostructures can be created on the device surfaces by a nanoimprint process [20]. Both of these approaches are compatible with the micro-fabrication process. 

To create a superhydrophobic surface on the ITO glass by the nanoimprint process, a 1 pm thick layer of polymer (Teflon AF) was coated on the ITO glass. Then the nanoimprint stamp was pressed against the polymer coated ITO glass under 70 mbar pressure at 150°C for 30 min. After removing the stamp, nanostructures with desired dimension can be fabricated on the device surfaces. 

In a previous experiment, we had utilized nanosphere lithography to create well-ordered nanostructures with tunable hydrophobicity on the surface [27]. However, such process is not compatible with micro-fabrication process. We have modified this technique by transferring the pattern of nanostructure into the silicon stamp and the nanostructures can be replicated by nanoimprint process. In other experiments, we had demonstrated that it was possible to create nanoimprint stamp with different... 

In summary, we have developed two techniques to impart superhydrophobic property to the surfaces of devices. In the first approach, oxygen plasma treatment was used to roughen the Teflon coating whose surface water contact angle could be tuned form 120° to 168° by varying the oxygen plasma treatment time. However, the application of the oxygen plasma process is limited to fluoropolymers. In the second approach, nanoimprint process was used to create nanostructures on the... 

The nanoimprint process utilises a patterned, 3D mould (template or stamp) to define patterns by embossing a soft polymer or liquid material. Once the material has completely filled the template cavities, it is hardened, using either a thermal or photochemical process, and the template is removed. The hardened imprinted polymer is an inverse 3D replication of the template mould. NIL uses a stamp or template to imprint or emboss a pattern into a polymer. The 3D polymer structures themselves may be used to create the desired nanostructures or alternatively, the polymer structures may be used as a protective mask to selectively protect a substrate during a subsequent process e.g. etching or deposition. 

Common to all NIL techniques is the requirement for a master stamp or template. The master is usually fabricated on silicon, glass or other rigid wafer substrate. For a nanoimprint process, the master must be patterned with nanoscale features using electron beam lithography. 

A nanoimprint-based patterning technique that can be applied in a continuous roll-to roll process to drastically increase the patterning speed is thus an attractive proposition. Continuous roll-to-roll nanoimprint processes are capable of replicating 300 nm line width optical grating patterns on both hard glass and flexible plastic substrates have been demonstrated [60, 61]. 

In an effort to realize the roller-based nanoimprint process. Tan et al. have used a solid rod to apply pressure to a piece of Si mold in a thermal nanoimprint... 

Fig. 25 Vertical chain alignment of P3HT within nanostructures produced by nanoimprint lithography (NIL) [97] (a) the nanoimprinting process (b) the chain alignment process induced by NIL, caused by both the material flow and interactions between the alkyl chains of the polymer chain and the hydrophobic surface of the mold cavities and (c, d) ideal chain orientation within...

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