## 4 Preparation of Plasmonic Thin Film Sensors  
### Thin Film Deposition Methods  
The huge diversity of thin film materials available, and still to discover, implies the existence of several wet/dry, chemical/physical, and vacuum/atmospheric processing and fabrication techniques [128, 129]. Regarding the scope of this work, the wet chemical deposition techniques are not discussed here; their overview can be found elsewhere, including the sol-gel [130, 131], chemical bath [131, 132], spray pyrolysis [133, 134], electrophoretic deposition [135], and electroplating [136, 137, 138] techniques. Considering only dry vacuum deposition methods, it is possible to classify these techniques into two groups, physical and chemical processes [139]. In chemical methods, chemical reactions are needed to obtain the final thin film (Figure 10a,b), while physical methods cover the deposition techniques that depend on the ejection of the material from a source by evaporation or sputtering (Figure 10c-e). Figure 9: (a) Illustration of a TiO\({}_{2}\) thin film sensor along with AFM topography of the TiO\({}_{2}\) thin film surface. Plots showing, at the top, the sensor resistance, as a function of the top electrode width, for 0 and 10,000 ppm of H\({}_{2}\) at room temperature, 50, and 100 \({}^{\circ}\)C and, at the bottom, the corresponding response of the sensors at room temperature (adapted from [PERSON] et al. [109], copyright 2015 Elsevier). (b) Optical microscopy image of a microheater coated in the centre with a sensing p-type CuO semiconducting layer. Comparison of the response obtained under dynamic tests in the full temperature range (20–500 \({}^{\circ}\)C, 0 to 55 mW heating) (adapted from [PERSON] et al. [110], copyright 2017 MDPI). Chemical vapour deposition (CVD) can be defined as a material synthesis method in which particles in the vapour phase react together to form a solid film in an energy-activated surface. The chemical reaction is a crucial characteristic of this method; therefore, besides the control of the usual deposition process variables, the related reactions must be well understood. Chemical reactions such as reduction, oxidation, and hydrolysis, among others, may occur. The activation energy may come from different sources, for example, heat, in conventional CVD (Figure 10a), or a plasma, in plasma enhanced CVD (PE-CVD) [140] (Figure 10b). When plasmas are used in CVD reactors to activate and partially decompose the reactants, the deposition will be possible at a temperature lower than conventional CVD. These plasmas are generated by using radiofrequency (RF) generators, as can be seen in Figure 10b. CVD processes are widely used in industry due to their versatility for depositing a large variety of elements and compounds covering a wide range of materials. Among these materials, metal oxides may be produced with a high degree of perfection and purity. The advantages of physical methods are mostly related to dry processing, high purity and cleanliness, and compatibility with semiconductor integrated circuit processing, while being considered as green processes. However, there are certain disadvantages such as low deposition rates, difficult stoichiometry control, and high initial investments [140]. In physical vapour deposition (PVD) techniques, (i) the solid material to be deposited is physically transformed to a vapour phase, (ii) this vapour phase is transported through a low pressure volume from the source to the substrate, and (iii) the vapour condenses on the substrate to form the thin film [141]. The first stage (solid to vapour) is carried out by physical removal of surface atoms by particle-to-particle momentum transfer, in sputter deposition (Figure 10c), or by heating in evaporation deposition systems (Figure 10d,e). A large number of materials can be evaporated, and if the evaporation is undertaken in a vacuum system, then the evaporation temperature will be considerably reduced, and the impurities in the growing film will be minimized [140].