## 4 Conclusions  
In conclusion, we developed design and fabrication technology of the long-range dielectric-loaded surface plasmon-polariton waveguides. First, we considered and numerically optimized various multilayer nanoplasmonic waveguides with silver and gold ultrathin films in between two Al\({}_{2}\)O\({}_{3}\) dielectric layers on a quartz substrate. For the optimized thicknesses of the three layers on the substrate, the waveguide topology was optimized to maximize the long-range waveguide plasmonic mode propagation length while keeping the waveguide to be a single mode. We numerically demonstrated that the Al\({}_{2}\)O\({}_{3}\)/Ag/Al\({}_{2}\)O\({}_{3}\) LR-DLSPP waveguide calculated long-range plasmon propagation length could achieve L\({}_{\text{prop}}\) = 0.27 mm for the 785 nm wavelength. Then, the waveguide input/output diffraction gratings design was developed, and its parameters were numerically optimized, providing the calculated gratings efficiency over 13%. In addition, it was numerically shown that the intersection of such waveguides in the plane at right angles ensures the negligible signals crosstalk (less than 0.01%), that allows planar wiring of a complex integrated optical scheme (see Figure 1) without using multilayer wiring. Unless it has very high and time-stable properties, the Al\({}_{2}\)O\({}_{3}\) is a well-known natural, hardly treated material, in particular, in combination with ultrathin metal films in stack. In the paper we proposed the fabrication process flow (Figure 5) for the Al\({}_{2}\)O\({}_{3}\)/Au(Ag)/Al\({}_{2}\)O\({}_{3}\) waveguides with high aspect ratio nanostructures which ensure the non-damage patterning of multilayer stacks with ultrathin plasmonic metal films. The process begins with substrate surface wet cleaning using megasound DIW and 80 \({}^{\circ}\)C Piranha solution. Next, a three layer thin film stack (Al\({}_{2}\)O\({}_{3}\)/Au(Ag)/Al\({}_{2}\)O\({}_{3}\)) is e-beam evaporated with both in-situ quartz and optical thickness monitors. Then, the tungsten hard mask is deposited on top of the three-layer thin film stack. Then, it is coated with a conventional e-beam resist and mask-less e-beam lithography is employed to pattern the waveguide topology. These topology elements serve as the mask during the hard mask ICP dry etch process, followed by a second ICP dry etch (Figure 5d) process that transfers the hard mask topology into the Al\({}_{2}\)O\({}_{3}\)/Au(Ag)/Al\({}_{2}\)O\({}_{3}\) plasmonic LR-DLSPP waveguides. One of the key technology issues of such a waveguides nanofabrication is a dry low-damage etching of multi-layer dielectric-metal-dielectric stack with extremely sensitive optical-quality ultrathin metal films. Here, we demonstrated the multistep low-damage reactive ion etching fabricating technology of such plasmonic waveguides. For the first (top) Al\({}_{2}\)O\({}_{3}\) layer pattering, we developed the dry-etch BCl\({}_{3}\)-based process, using the tungsten hard mask. We compared three types of masks for this task: the PMMA resist mask, and tungsten and silicon oxide hard masks. We found that the PMMA resist masks selectivity (ranging from 1:10 to 1:3) is insufficient. We also observed waveguide functional layers degradation during SiO2 hard mask residues final removal step. During the etch process of the first layer, alumina radicals react with the ultrathin silver layer, and micro- and nano-particles of non-volatile reaction products are formed on the surface of the etched areas. In order to prevent the contact of radicals of the gas mixture, we developed a selective high-aspect ratio etch process of the first layer of Al\({}_{2}\)O\({}_{3}\) with the under-etch, leaving 10-25 nm of alumina above the metal layer. For the silver-based waveguides, we proposed an ultrathin silver film etching process based on the argon sputtering followed by the DIW rinse.