## 3 Results and Discussion
### LR-DLSPPW Fabrication Process Flow and Key Issues
Furthermore, gold etching based on H\({}_{2}\), He and Ar gasses results in weak gold film etching rates, which are 3-5 times lower compared to chlorine-based processes [28]. The multi-step high-precision dry etching process is developed to provide high-quality waveguide stack etching, eliminate reaction products formation and redeposition, and prevent overheating of ultrathin metal films. At the first stage of the process, 90% of the top alumina layer is selectively etched in chlorine gasses, leaving around 10-25 nm of alumina above the metal layer. This process ensures to eliminate the contact of the metal film with chlorine gasses. At the same time, it provides selective etching of the main thickness of the top thick alumina layer and ensures perfect quality of waveguide nanostructures with a high aspect ratio. Then, the residue alumina 10-25 nm layer and metal thin film have to be etched. We observed that during the etching process, due to surface bombardment and ongoing chemical reactions, the surface temperature of the samples rises above 75 \({}^{\circ}\)C, which is one of the reasons for structural changes in metal layers. To prevent overheating of metal films, we tested two methods: pre-cooling the samples before the etching process and stopping the etching in the middle of the process for intermediate cooling. It can be noted that prior cooling of the samples to minus 20 \({}^{\circ}\)C before the etching process can significantly reduce overheating and structural changes in the metal film (Figure 7b), but it does not completely solve this problem. The second technological issue in the fabrication of the proposed waveguides design in is low-damage reactive ion etching of Al\({}_{2}\)O\({}_{3}\)/Ag(Au)/Al\({}_{2}\)O\({}_{3}\) high aspect ratio nanostructures. The Al\({}_{2}\)O\({}_{3}\) etch selectivity to majority standard electron resists (PMMA-like) in chlorine gasses is usually from 1:10 to 1:1 [29]. In the proposed waveguide design, the top alumina layer is around 250 nm thick, with the 100 nm sized topological elements critical dimensions. Taking into account Al\({}_{2}\)O\({}_{3}\)/PMMA etch selectivity, the 750-2500 nm thick resist mask is required to provide high-quality 100 nm features etching. In this case, the aspect ratio for the e-beam lithography process varies from 1:7.5 to 1.25, which is an extremely difficult task for standard PMMA-like resists and 50/100 kV e-beam tool. Moreover, dry plasma etching through the PMMA mask with such an aspect ratio is practically impossible due to  
Figure 4:0 SEM cross section images of Al2O\({}_{3}\)/Ag/Al2O\({}_{3}\) waveguides after top Al2O3 layer etching: (**a,b**) Standard process, (**c,d**) Precooling of the sample at –20 \({}^{\circ}\)C for 20 min, (**e,f**) 20 minutes pause after first half (of total time) of etching process to cool down the sample. extremely limited access of the etching reagents and further reaction products removal. That is why, in this paper, we compared the PMMA mask to silicon dioxide and tungsten hard masks (Figure 8), which have better selectivity and can be selectively removed after the etching process from the Al\({}_{2}\)O\({}_{3}\) surface. Figure 4:1 Fabrication process flow of Al\({}_{2}\)O\({}_{3}\)/Au(Ag)/Al\({}_{2}\)O\({}_{3}\) plasmonic LR-DLSPP waveguides on a quartz substrate using hard masks: (**a**) Deposition of a three-layer stack and a hard mask, (**b**) E-beam resist spin-coating, (**c**) E-beam lithography, (**d**) Etching a hard mask, (**e**) Removing the e-beam resist after etching the hard mask, (**f**) Etching of the first layer of Al\({}_{2}\)O\({}_{3}\) with residue alumina of 10 nm to silver, (**g**) Mask removal, (**h**) Etching of remaining silver. After the first set of experiments, we concluded that PMMA masks are not suitable for the proposed waveguide design fabrication due to very low PMMA selectivity in chlorine gasses. The experimentally measured etch selectivity of silicon oxide hard masks in \(\mathrm{BCl_{3}}\) gasses to alumina was from 1.5:1 to 1.7:1, depending on the process recipe.