## 4 Patterning towards Devices: Conventional and Unconventional Methods
### Lithographic techniques
#### 4.1.1 Photolithographic techniques  
Light source lithography allows to transform masked irradiated areas of a liquid photoresist formulation in a hard materials by means of photo-polymerization reactions. Photosensitive resists consist of multi-functional monomers, oligomers and a photoinitiator, which promotes the photopolymerization after light activation. Among the two photolithography methods, UV-lithography is well known and typically used in microfabrication equipments; UV-light is typically adsorbed by photosensitive polymers [136]. On the contrary, X-rays are not absorbed by the organic material, and can deeply penetrate along the film thickness, allowing the production of high quality vertical sidewalls and high aspect ratio patterns. But, the difficult access to synchrotron radiation lines, the high cost of equipment and the typical constraint in terms of safety in the processing conditions, limit the use of X-rays and restrict their applications to thick films, thus making UV-lithography more versatile and common in machining protocols [136]. On the other hand, the applicability of UV-lithography to polymer based nanocomposites for fabricating micro- and nano-components is limited by the nanocomposite homogeneity. Such characteristics, in fact, determining the light penetration along the film thickness, finally affect the lithography resolution, aspect ratio and surface quality of the structures. Processes such as light diffraction, reflection and scattering from the embedded nanofillers or their aggregates either decrease the optical transparency [140] of the film, because light intensity drops along the thickness, or even hinder the cross linking of the matrix limiting the applicability of the manufacturing procedure [141,142]. As a consequence, the adhesion of the structures to the silicon wafer, as well as the resolution and aspect ratio decrease and the exposure dose necessary to obtain fully resolved structures, as well as the sidewall surface roughness, increase. In many works, photolithography has been applied for machining polymer nanocomposites on chemically amplified epoxy photoresist formulations and SU-8 formulations. SU-8 pre-polymer is a negative tone resist which possesses eight photoreactive epoxy moieties and thus a high sensitivity and high conversion efficiency [136]. The photopolymerization of SU-8 leads to highly cross-linked, hard permanent features having high mechanical, chemical and thermal stability, high aspect ratio, nearly vertical sidewalls and a sub-100 nm lithographic resolution. In particular, the mechanical stability of SU-8 allows for fabricating complex 3D architectures [136]. Such structural properties of SU-8 are attractive, but its applicability is however limited by the lack of inherent functionalities (_i.e.,_ electrical and thermal conductivity, luminescence, magnetism, piezoresistivity) of the resist and many efforts have been devoted to overcome this restriction by incorporating either nano- and micro-powders or colloidal NPs in the resist to convey the missing functionalities. Nano- and micro-powders are typically prepared as suspensions and are generally mechanically mixed in SU-8. Ag micropowders 0.2-2.5 \(\upmu\)m in size have been added to SU-8 for manufacturing conductive nanocomposite structures having a resolution down to 5 \(\upmu\)m and aspect ratio of 5. A conductivity of 105 S cm-1 has been obtained with a loading up to 20 vol% and micrometer-sizedelectrode arrays have been manufactured combining UV-lithography to an UV-LIGA (Lithographie, Galvanoformung, Abformung) process [143]. Piezoresistive chips on SU-8 and carbon NPs of tens of nanometers have been fabricated. The nanocomposite shows stable [139, 140, 141, 142, 143, 144] Gauge factors [144], thus representing an interesting structural material for cantilever sensors with piezoresistive read-out. Magnetically-actuated micromirrors have been manufactured by using a suspension of 80-150 nm diameter Ni powders in SU-8 [145].