## 4 Preparation of Plasmonic Thin Film Sensors
### Reactive Magnetron Sputtering Deposition  
Currently, sputtering is used in various fields of applications, for example, in the deposition of thin films, for surface etching, cleaning or activation, and in chemical analysis [142]. The industrial importance of sputtering includes semiconductor processing, surface finishing, jewellery, cutlery and optics, among others [143]. Since no chemical reactants are used and no waste is produced, this PVD process is also considered environmental friendly [144]. During the sputtering process, two main events occur after the introduction of a working gas and the application of an electric field: plasma generation and ion bombardment. After applying a negative potential to the cathode or \"target\" and grounding the anode or \"sample holder\", an electric field (\(\overset{\rightarrow}{E}\)) is generated (Figure 11a). Afterwards, the few existent electrons/ions (generated by natural processes such as cosmic rays or radioactive decay) are accelerated, leading to the creation of more charge carriers within the gas [145; 146], i.e., the formation of a plasma (the concept of \"plasma\" was first described in 1928 by [PERSON] as the fourth state of matter [147]). Figure 11: Schematic figures of (**a**) basic sputtering configuration; (**b**) the target surface, showing the important species and processes that take place during sputtering; (**c**) magnetron sputtering configuration; and (**d**) target and the preferential erosion zone as a result of the magnetic field of the magnetron that causes a local electron confinement (adapted from [PERSON] et al. [148], copyright 2010 Elsevier). During the sputtering process, the ionized species that are accelerated in the electric field bombard the target. The bombardment causes the ejection of atoms and secondary electrons from the target surface, among other processes schematized in Figure 11b, such as photon emission, which makes the plasma visible. The secondary electrons that are ejected from the target play a fundamental role in maintaining the plasma. The electric field between the target and the sample holder induces an acceleration of the secondary electrons, and as they collide with neutral particles inside the vacuum chamber new charged carriers are created, and a steady-state plasma is developed [143]. The ejected target atoms are then deposited onto a substrate placed in front of the target, giving rise to a thin film [141]. Nevertheless, this basic sputtering process for thin film deposition initially only played a minor role compared to, e.g., thermal evaporation. Major drawbacks given were the low deposition rates of the process, the low ionization efficiency, inferior properties of the films, and substrate heating [149]. These limitations were overcome with the development of the magnetron [146]. The basic principle of magnetron sputtering is to impose a magnetic field (\(\overset{\rightarrow}{B}\)) near the target (Figure 11c). The magnetron is a system of permanent magnets, with a specific orientation, placed behind the target. The introduction of a magnetic field confines the electron motion to the vicinity of the target (Figure 11d), increasing the probability of electron-ion/atom collisions [142]. Thus, the ionization efficiency increases, causing a higher plasma density near the cathode, which enhances the process efficiency (higher deposition rates) and hence the quality of the deposited films [143]. However, the plasma over the target surface is not uniform, so that the sputtering phenomena are much localized to a \"preferential\" erosion zone (Figure 11d) [148], sometimes using only about 30% of the whole target material. As the target is sputtered, its characteristics also change, so the deposition conditions such as the layer thickness and the film morphology are expected to change during the lifetime of the target [142]. To overcome this hurdle, moving targets/magnet systems have been developed, with almost 80% usage for flat targets [150], and more than 90% in cylindrical rotating targets [151].