## 3 Results and Discussion
### LR-DLSPPW Fabrication Process Flow and Key Issues  
The general fabrication process flow for \(\mathrm{Al_{2}O_{3}/Au(Ag)/Al_{2}O_{3}}\) plasmonic LR-DLSPP waveguides on a quartz substrate is illustrated in Figure 5. The process begins with substrate surface wet cleaning using megasound particle removal in DIW, followed by 80 \({}^{\circ}\)C Piranha solution and drying with nitrogen (Figure 5a). Next, a three-layer thin film stack \(\mathrm{Al_{2}O_{3}/Au(Ag)/Al_{2}O_{3}}\) is e-beam evaporated in the same high vacuum cycle (base vacuum \(3\times 10^{-8}\) Torr.) with both in-situ quartz and optical thickness monitors to ensure accurate and reproductive film thicknesses (Figure 5b). If hard masks are used for following the dry etch steps, then hard mask thin film is deposited together with the main three-layer thin film stack. Then, it is coated with a conventional e-beam resist and mask-less e-beam lithography is employed to expose a latent image of the waveguide topology in the resist layer. The e-beam lithography is completed (Figure 5c) with a wet development that reduced the resist to a uniform topology of waveguides (interconnects or plasmonic nanocircuitry). In case of the resist mask, these topology elements serve as a mask during an ICP dry etch process that transfers the photoresist image into the \(\mathrm{Al_{2}O_{3}/Au(Ag)/Al_{2}O_{3}}\) plasmonic LR-DLSPP waveguides. In case of a hard mask, these topology elements serve as a mask during a hard mask ICP dry etch process followed by a second ICP dry etch (Figure 5d) process that transfers the hard mask topology into the \(\mathrm{Al_{2}O_{3}/Au(Ag)/Al_{2}O_{3}}\) plasmonic LR-DLSPP waveguides. A final stage cleaning process (Figure 5e) removes any residual photoresist, hard mask or contaminants from the etch plasma. Figure 4: Electric field amplitude distribution at the crossing of two long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs). One of the most challenging processes of the proposed fabrication process flow is a low-damage reactive ion etching of a multilayer dielectric/metal/dielectric stack with ultrathin noble metal films, especially in cases of ultra-hard dielectrics like alumina due to a number of contradicting technological requirements. On the one hand, dry etching of a waveguide stack top alumina layer requires aggressive chlorine-based etching gasses and high process power as the result of natural alumina hardness and low etching speed. On the other hand, chlorine-based etching chemistry usually leads to intensive reaction (plasma/thermally stimulated) with complete degradation of ultrathin noble metal films. The widely used BCl\({}_{3}\)-based alumina etching process was chosen to ensure high selectivity patterning of plasmonic LR-DLSPP waveguides nanostructures (due to efficient oxygen extraction in the form of BCl\({}_{x}\)O\({}_{y}\) components [26], which then are decomposed by plasma and chemically react with both components of Al2O3 oxide):  
\[2\text{BCl}_{3}+3\text{O}\rightarrow\text{B}_{2}\text{O}_{3}+6\text{Cl} \tag{2}\]  
\[2\text{Cl}+\text{Al}\rightarrow\text{AlCl}_{2} \tag{3}\]  
Two main gas phase and volatile reaction products (AlCl\({}_{2}\) and BOCI) are finally formed, which could be easily removed from the etching reaction zone during the process. To provide high selectivity and etching speed, the process is usually carried out in inductively coupled plasma (ICP) etching mode at relatively high ICP generator power (~1000-2000 W). However, such high-energy processes with aggressive (corrosive) etching gasses in most cases result in underlying ultrathin metal films damage because of both thermal and chemical influence (Figure 6). Figure 6a, b demonstrates the Al\({}_{2}\)O\({}_{3}\)/Ag/Al\({}_{2}\)O\({}_{3}\) waveguide cross-sections with completely destructed silver ultrathin film inside because of overheating, only when the upper Al\({}_{2}\)O\({}_{3}\) layer has been partly etched (70% of its thickness) and there was no interaction of etching gasses with silver films.