## 4 Preparation of Plasmonic Thin Film Sensors
### Glancing Angle Deposition (GLAD) to Tailor the Micro/Nano Structure  
Nanostructured thin films can be achieved by glancing angle deposition (GLAD) in reactive magnetron sputtering. The fundamentals of GLAD, also known as oblique angle deposition (OAD), (Figure 15a) rely on the self-shadowing effect (Figure 15b) that appears just after the deposition of the first atoms, as long as the atom mobility remains low [181]. In conventional sputtering, the direction of the atom flux is perpendicular to the substrate (\(\alpha=0^{\circ}\)), so the atoms condense on the nearest sites of nucleation or on the surrounding area to form the typical vertical columnar growth [141]. On the other hand, when the flux is not perpendicular to the surface (\(0^{\circ}<\alpha\leq 90^{\circ}\)), the atoms in the vapour phase condense on the substrate to form some nucleation sites, and, depending on several parameters (e.g., substrate condition and temperature or energy and nature of the atoms in the vapour phase), they may intercept the incoming atoms to produce a \"shadow\" on the substrate or on the material already deposited (Figure 15b). Due to this self-shadowing and although the atoms in the vapour phase reach the substrate at an angle \(\alpha\), the inclined columns grow at a lower angle \(\beta\) (Figure 15b). The effective column tilt angle \(\beta\) depends on several factors, namely material used, sputtered particle angle of incidence, substrate rotation speed, and gas pressure [182], and it can be roughly estimated, as a guideline, using [PERSON]'s rule [183]. The self-shadowing process is also be responsible for producing more porous thin films, which, depending on the angle of incidence and on the material, can lead to nanostructured materials with only 10% of the bulk material's density [184]. For an inclined columnar structure, only the angle of incidence (\(\alpha\)) must be set, but by using another rotation axis \(\varphi\) (perpendicular to the substrate, Figure 15a). GLAD allows the development of layers with a zigzag structure (Figure 15c), spirals, isolated columns, and other configurations, only by combining different rotation speeds and protocols [185]. For example, to prepare zigzag structures, after an incidence angle (\(\alpha\)) is selected, \(\varphi\) must be shifted by 180\({}^{\circ}\) at each half zigzag cycle, as depicted in Figure 15c. For spirals, an incidence angle (\(\alpha\)) must be selected, and then \(\varphi\) must be changed slowly and continuously, and each time \(\varphi\) rotates 360\({}^{\circ}\) one spiral is produced. By changing the speed of rotation for \(\varphi\) angle,  
Figure 15: Simplified schematics of (**a**) sputtering and GLAD sample holder position and \(\alpha/\varphi\) motion, relative to the sputtering target, for the production of, e.g., zigzag thin films, and (**b**) growth of inclined columns on a substrate when the sputtered particles arrive at an angle \(\alpha\) from the normal to the substrate. A growth competition occurs between columns due to the shadowing effect created by the columns themselves (self-shadowing), thus creating spaced columns tilted at an angle \(\beta\). (**c**) Example of a GLAD deposition process for the production of one zigzag cycle. the shape of the spiral is changed, and if the speed is sufficiently high, instead of spirals, isolated columns may be produced. In this last configuration, the distance between each column will depend on the selected \(\alpha\) angle [184] (see examples in Figure 16). The discovery that the properties of thin films deposited by GLAD are different from conventional thin films was made at the end of the 1950s [186; 187; 188], and soon several theories developed for the explanation of the self-shadowing effect, the columnar growth according to \(\beta\) angle, and to account the different deposition rates, some of them already supported by computer simulations [181].