Fig. 2. Steps of waveguide manufacturing. (a): A trench with depth d and width w is etched into an 8 $_{m}$m layer of thermally oxidized silicon (SiO$_{2}$ , blue) on a silicon wafer. (b): 250 nm of Si$_{3}$N$_{4}$ (red) is deposited homogeneously over all exposed surfaces using LPCVD. (c): The trench is completely filled with Si$_{3}$N$_{4}$ and a homogeneous Si$_{3}$N$_{4}$ layer over the whole trench width of up to 0.9 $_{m}$m is formed. (d): The residual Si$_{3}$N$_{4}$ on the top surface of the wafer is removed from the wafer using chemical-assisted mechanical polishing and dry etching. The remaining Si$_{3}$N$_{4}$ forms the waveguide core with the dimensions of the etched trench. A top cladding of SiO$_{2}$ can be deposited optionally on top of the waveguide cores. until the the two layers at the side walls merge in the middle of the trench, such that the trench is completely filled, as shown in Fig. 2(c). For the manufactured waveguides no more than two deposition steps were required. The final waveguide core is obtained by removing the remaining Si$_{3}$N$_{4}$ from the top of the wafer using chemical-assisted mechanical polishing and dry etching, as shown in Fig. 2(d). To ensure that etching is stopped when the Si$_{3}$N$_{4}$ from the top of the wafer is removed an etch rate monitor is used. If required, this last processing step can also be applied between deposition steps, to thin the Si$_{3}$N$_{4}$ layer on top of the wafer to prevent cracks. To complete the waveguide cladding, first a layer of tetraethyl orthosilicate (Si(OC$_{2}$H$_{5}$)$_{4}$, TEOS) is de- posited by LPCVD, which will form SiO$_{2}$ after annealing. Second, a layer of SiO$_{2}$ is grown to a thickness of 8 $_{m}$m by plasma-enhanced chemical vapor deposition (PECVD) (both not shown in Fig. 2). To convert the TEOS layer into SiO$_{2}$ and to reduce the absorption in the near infra-red caused by dangling Si-H bonds, the wafers are annealed after deposition at 1150 $^{◦}$C. Using this procedure, several TriPleX-based Si$_{3}$N$_{4}$ waveguides were manufactured with their width, w , ranging from 0.6 to 0.9 $_{m}$m, and depths, d , of 0.8, 1.0, and 1.2 $_{m}$m. To ensure that these waveguides have low propagation losses, it is important that the trenches have a low surface roughness at the bottom as well as at the side walls, since SiO$_{2}$ a) w d Si$_{3}$N$_{4}$ b) c) d) x y