## 3 Results and Discussion
### \(\mathrm{Al_{2}O_{3}}\)/Ag (Au)/\(\mathrm{Al_{2}O_{3}}\) Dry Etching Process  
The developed low-damage reactive ion etching of multilayer plasmonic waveguides with ultrathin noble metal films consists of multiple steps and uses tungsten hard masks. The whole process starts from hard mask etching which assumes the etching pause during the process to cooldown and protect the ultrathin metal film from thermal degradation. Next, the first 240 nm thick \(\mathrm{Al_{2}O_{3}}\) layer (\(\mathrm{Al_{2}O_{3}}\)/Au(Ag)/\(\mathrm{Al_{2}O_{3}}\)) is selectively etched using the \(\mathrm{BCl_{3}}\)-based process for the depth of approximately 225 nm. The second step allows for both eliminating metal degradation due to chemical reactions of metal with etching gasses and providing an opportunity to selectively etch the major part of the hard \(\mathrm{Al_{2}O_{3}}\)/Ag (Au)/\(\mathrm{Al_{2}O_{3}}\) stack. As mentioned above, thinner tungsten hard mask (compared to silicon oxide and PMMA) and etch recipe optimization allows for fabricating high-quality waveguide topologies with the typical 100 nm sized features (Figure 10).  
Figure 9: Fabrication process flow issues of \(\mathrm{Al_{2}O_{3}}\)/Au(Ag)/\(\mathrm{Al_{2}O_{3}}\) plasmonic LR-DLSPP waveguides on a quartz substrate (**a**) Cross section of \(\mathrm{Al2O3}\)/Ag/\(\mathrm{Al2O3}\) waveguides after top \(\mathrm{Al2O3}\) layer etching, (**b**,**c**) Adhesion failure after vapor HF silicon oxide hard mask removal.  
In the next process step, the tungsten hard mask is removed using the standard CF4-based dry etching high-selectivity (>10:1) process, eliminating the possibility of Al2O3 waveguide structures damage. Finally, the residue Al2O3/Ag(Au)/Al2O3 waveguide stack, including the ultrathin plasmonic metal layer, is etched using the physical Ar-sputtering process (without tungsten hard mask) (Figure 11).  
Figure 11: SEM images of Al2O3/Au/Al2O3 plasmonic LR-DLSPP waveguides fabricated using the proposed low-damage reactive ion etching: (**a**) input/output diffraction grating and the part of the waveguide, (**b**) input/output diffraction grating (zoom in).  
Figure 10: SEM images cross-section of a three-layer waveguide structure etched through the tungsten (**a**) and silicon oxide (**b**) hard mask: (**a**) Dependence of the etch profile angle on inductively coupled plasma (ICP) generator power, (**b**) Dependence of the etch profile angle on RF generator power.  
In order to take into account the top Al\({}_{2}\)O\({}_{3}\) oxide layer thinning during the Ar-sputtering process, we deposit a thicker top layer of the initial three layer Al\({}_{2}\)O\({}_{3}\)/Ag (Au)/Al\({}_{2}\)O\({}_{3}\) stack. At the final process stage, all the chlorine and fluorine residues have to be removed from the fabricated waveguide nanostructures and the substrate open areas. It can be effectively done by treating the samples in deionized water from 1 to 2 minutes immediately after the etching process [27]. The proposed process allows for eliminating ultrathin plasmonic films degradation, which usually actively interact with aggressive chlorine-based gasses and ensure a high quality LR-DLSPP waveguide nanostructures fabrication of on quartz substrates (Figure 11).