rougher surfaces will result in higher scattering losses for propagating light. A scanning electron microscope (SEM) picture of an etched trench is shown in Fig. 3(a) for a width of w =0.8 $_{m}$m and a depth of d =1.0 $_{m}$m. Note that the vertical direction of all the SEM pictures in Fig. 3 appears to be smaller because the pictures were taken under an angle. Figure 3(a) also shows a slight positive taper, meaning that the width of the trench increases from bottom to top, which ensures a conformal deposition. Without tapering, the deposition is not fully conformal, and a gap between the silicon nitride and the trench wall appears, which increases the propagation losses as a consequence. SEM pictures of two completed waveguide cores are shown in Fig. 3(b) and 3(c) with a cross section of w = 0.83 $_{m}$m by d = 0.8 $_{m}$m and w = 0.9 $_{m}$m by d = 1.2 $_{m}$m, respectively, with and without a top cladding, respectively. To increase the contrast between the Si$_{3}$N$_{4}$ and the SiO$_{2}$ in the SEM pictures, the samples were etched, which resulted in the Si$_{3}$N$_{4}$ waveguide core sticking out from the SiO$_{2}$ because of a difference in etching rates of the materials. In Fig. 3(b) it can be seen that a homogeneous waveguide core with w = 0 . 83 $_{m}$m and d = 0 . 8 $_{m}$m is formed using this manufacturing method. By comparing the shape of the trench as shown in Fig. 3(a) and the waveguides in Figs. 3(b) and 3(c), the stress induced by the deposition of silicon nitride is evident. The large difference in material stress between the SiO$_{2}$ and Si$_{3}$N$_{4}$ layers leads to a change in both the shape of the waveguide as well as an inwards bend of the side walls. The rounded shape of the waveguides leads to a small ( ∼ 10 nm) blue-shift in the ZDW compared to the rectangular shape assumed in Fig.1. However, the variations in Fig. 3. SEM pictures: (a) A trench etched in thermally oxidized silicon with w = 0 . 8 $_{m}$m and d = 1 . 0 $_{m}$m. (b) and (c): the cross section of silicon nitride waveguide core with w = 0 . 83 $_{m}$m and d = 0 . 8 $_{m}$m with a layer PECVD silica on top (b) and w = 0 . 9 $_{m}$m and d = 1 . 2 $_{m}$m without a top layer (c). Note that all pictures are taken at an angle, making the vertical direction appear smaller. a) 400 nm  b) 400 nm  c) 400 nm 