## 3 Results and Discussion
### LR-DLSPPW Fabrication Process Flow and Key Issues
The examples of the complex chemical  
Figure 5: Fabrication process flow for Al\({}_{2}\)O\({}_{3}\)/Au(Ag)/Al\({}_{2}\)O\({}_{3}\) plasmonic LR-DLSPP waveguides fabrication on a quartz substrate: (**a**) Substrate wet cleaning, (**b**) Al\({}_{2}\)O\({}_{3}\)/Au(Ag)/Al\({}_{2}\)O\({}_{3}\) films stack e-beam evaporation, (**c**) E-beam nanolithography, (**d**) Low-damage inductively coupled plasma-reactive ion etching process (ICP-RIE) plasma etching, (**e**) Resist strip and final wet cleaning. reactions (with silver chloride products formation due to combined plasma/temperature action) of etching gasses and waveguide stack thin films are shown in Figure 6c-e, when the Al\({}_{2}\)O\({}_{3}\)/Ag/Al\({}_{2}\)O\({}_{3}\) stack was etched through to the substrate. It is worth noticing, that due to the micron overall dimensions, entire ultrathin metals inside waveguides are usually damaged (Figure 6e) during the etching process, leading to complete plasmonic properties degradation. Figure 6: Scanning electron microscope (SEM) cross-sections of Al2O3/Ag/Al2O3 plasmonic LR-DLSPP waveguides on a quartz substrate: (**a**) waveguide input/output grating and (**b**) waveguide cross-section after 70% thickness etching of top alumina layer. (**c–e**) Waveguide elements cross section after full Al\({}_{2}\)O\({}_{3}\)/Ag/Al\({}_{2}\)O\({}_{3}\) stack etching. (**f**) Waveguide cross section energy-dispersive X-ray spectroscopy (EDX)-analysis (Ag–yellow, Cl–orange, Al–pink). We experimentally observed the active interaction of BCl\({}_{3}\) with silver thin films during the top Al\({}_{2}\)O\({}_{3}\) layer etching with the non-volatile chemical products formation and its redeposition on the resistive mask (Figure 6c-f). Additionally, these redeposited products mask the main waveguide micro- and nano-structures, and result in an uncontrolled, nonreproducible etching process. Moreover, when silver and top alumina layers are etched in the same BCl\({}_{3}\)-based process, micro- and nano-particles reaction products (Figure 6c,d) are formed on the etched surfaces, which could not then be selectively removed. The chemical reactions of particle formation in BCl\({}_{3}\) plasma and a possible solid precipitation reaction are presented below:  
\[\text{e2^{-}}+\text{BCl}_{3}\text{\to BCl}_{2}+\text{Cl}^{\text{-}} \tag{4}\]  
\[\text{e^{-}}+\text{BCl}_{3}\text{\to BCl}+\text{Cl}_{2}^{\text{-}} \tag{5}\]  
\[\text{e^{-}}+\text{BCl}\text{\to B}+\text{Cl}^{\text{-}} \tag{6}\]  
\[\text{Ag}+\text{Cl}\text{\to AgCl}\text{(s)} \tag{7}\]  
We confirmed this hypothesis using SEM EDX analysis of waveguide cross-sections (Figure 6f), demonstrating Ag\({}_{\text{x}}\)Cl\({}_{\text{x}}\)- and Cl-based reaction products [27] in the degraded silver film. To completely eliminate chemical degradation of LR-DLSPP waveguides ultrathin silver films, the etching process should be carried out using chemically inactive gasses like Ar, He or H\({}_{2}\)[19]. In contrast to silver, gold thin films dry etching in chlorine gasses could be very effective due to stable volatile reaction products (Au\({}_{2}\)Cl\({}_{6}\)) formation, which allows efficient removal of reaction products from the etching area. Additionally, it allows for providing high etching rates (approximately 350 nm/min) and minimizing material redeposition on the sidewalls of the resistive mask. By using chlorine-based etching processes, a good balance between the etching rate and profile could be achieved by optimizing recipe parameters. When using neutral etching gasses such as H\({}_{2}\), He and Ar, the physical sputtering process predominates, which drastically reduces etching selectivity. It is worth noticing, that for gold etching with hydrogen, the chemical reaction also takes place, since AuH and AuH\({}_{2}\) reaction products are formed, but its contribution is insignificant. These reaction products have low thermodynamic stability, which can also lead to the formation of redeposition defects on the sidewalls of the resistive mask.