## 4 Patterning towards Devices: Conventional and Unconventional Methods
### Lithographic techniques
#### 4.1.2 Electron beam lithography  
Electron-beam (e-beam) lithography (EBL) is a patterning technique based on the exposure of electron-sensitive resists to an electron beam. The electrons impacting the resist film generate secondary electrons with relatively low energy, which form reactive free radicals promoting chain scissions in the resist matrix. Due to the small wavelength of electrons (1 A), EBL allows for defining patterns with a nanometric resolution (>10 nm) and the small penetration depth of low-energy electrons leads to structures with thickness lower than those achieved with conventional photolithography [152].  
PMMA (poly(methyl 2-methylpropenoate)), an acrylic, transparent and thermoplastic polymer is the first photoresist for which electron beam sensitivity has been discovered and it is still now the most applied in EBL. PMMA presents an exceptional optical clarity, high strength and excellent mechanical stability and provides the development of positive images as lines and spaces with dimensions down to 50 nm and aspect ratios up to 5.  
Polymer nanocomposite materials typically processed by EBL are mainly prepared either by _ex situ_ incorporation of colloidal NCs in an electron-sensitive polymer matrix or by _in situ_ generation of NPs starting from an electron sensitive precursor.  
Figure 5: Tilted SEM images of the photostructured CdSe@ZnS NC modified resists, after UV photolithographic processing(Top panel). Fluorescence microscopy images and emission spectra (bottom panel) of the microstructures obtained from the CdSe@ZnS NC modified (a) and unmodified (b) epoxy resist [106].  
Waveguide photonic devices formed of CdSe(r)ZnS NCs embedded in PMMA have been fabricated. In particular, the incorporation of such NCs in PMMA increases the refractive index of the polymer from 1.49 up to 1.62. The nanocomposite has been used to fabricate a ridge cavity confined by two active blend/air distributed Bragg Reflectors (DBRs) with an e-beam energy of 20 keV and a distributed feedback (DBF) structure formed of periodic corrugations on top of waveguides by a beam energy of 1 keV [153]. Highly ordered triangular-shaped gold nanopatterns 200 nm in size on PMMA resist modified with CdSe(r)ZnS NCs and CdSe nanorods, respectively have been fabricated. The coupling between the surface plasmon resonance of the metallic nanostructures and the emission of the NCs results in a strong enhancement of the NC fluorescence promising for optical sensor and photonic devices [154]. Nano- and micro-sized multicolour pixels characterized by a high colour definition and high emission tuneability have been fabricated by applying UVL and EBL on nanocomposites formed of red and green CdSe(r)ZnS NCs incorporated in SU-8 and PMMA, respectively by using chlorobenzene as common solvent. The combination of both the lithography techniques in subsequent re-alignments steps, the control of both the NC concentration and geometry of patterns localized in contiguous regions have allowed for achieving a finely tuning of the pixel emission [155].  
Finally, many papers deal with the fabrication of conductive patterns based on electron structurable polymers and metal NPs generated _in situ_ under electron beam irradiation via reduction or decomposition of a precursor. Conductive patternable nanocomposites on Ag NPs embedded in PS have been fabricated for nanocircuitry and plasmonic devices [20]. In the work, an electron active Ag precursor, namely silver triphenylphosphine nitrate (Ag(PPh\({}_{3}\))\({}_{3}\)NO\({}_{3}\)) was used to generate Ag nano-objects. A moderate e-beam dose of 150 \(\upmu\)C cm\({}^{-2}\) and energies of 5, 10 and 30 KeV, respectively, have been used to convert the nanocomposite in conductive PS patterns filled with 5-50 nm diameter core-shell NPs formed of an Ag core and an amorphous organic carbon shell [156].