# Low-Temperature and Low-Pressure Silicon Nitride Deposition by ECR-PECVD for Optical Waveguides
## 3 Characterization and Results
### Optical Loss  
In order to study the SiO\({}_{x}\)N\({}_{y}\):H\({}_{x}\) films as candidates for optical waveguides, a significant merit is a low optical loss. To quantify this, we used prism coupling to excite the transverse-electric (TE) polarized fundamental planar modes in the film and measure their decay as they propagate through the SiO\({}_{x}\)N\({}_{y}\):H\({}_{x}\) layer with a scanning fiber normal to the surface and coupled to a photodetector. This was carried out before annealing at four different wavelengths including 638, 847, 1310 and 1550 nm, as shown in Figure 2.  
The loss for Film 1 at 638 and 847 nm was too high to be measured on the system. This is anticipated because of the higher Si content in Film 1 as a result of a decreased N\({}_{2}\)/Ar:SiH\({}_{4}\)/Ar flow rate ratio. A similar trend is observed in Film 2, which exhibits high visible loss but promising losses in the 1310-1550 nm range. Films 3 and 4 demonstrate relatively low losses at all wavelengths and less than 0.5 dB/cm at 847 and 1310 nm, while Film 5 exhibits a higher loss at these wavelengths. Film 5 was deposited with the highest N\({}_{2}\)/Ar:SiH\({}_{4}\)/Ar flow rate ratio and the highest pre-deposition pressure. It is likely that this recipe leads to films rich in N-H bonds which is known to be an unwanted source of optical loss [2]. The increased loss at shorter wavelengths is anticipated to be a result of increased Si-H bonds in this deposition regime, potentially past the stoichiometric point of gas ratios. Figure 1: Refractive index and thickness of films deposited for 3.5 h at 140 \({}^{\circ}\)C with varying N\({}_{2}\)/Ar:SiH\({}_{4}\)/Ar flow rate ratios before and after annealing at 1000 \({}^{\circ}\)C for 1 h in ambient nitrogen (500 sccm). Connecting lines are to help guide the eye. In order to further investigate the films' optical quality, we measured the loss at a finer spacing of wavelengths from 1510 to 1600 nm, as shown in Figure 3a. The incorporation of hydrogen in the films is expected to lead to additional absorption in this wavelength regime [2]. Films 1-3 demonstrate higher loss around 1510 nm and a decrease in loss towards longer wavelengths past ~1540 nm, indicative of the presence of hydrogen in the films. Films 4 and 5, however, show consistent or increasing losses across this wavelength regime, indicative of other dominant loss mechanisms rather than hydrogen absorption, such as substrate interaction or intrinsic material loss. Film 2 was the only film which was measurable for loss after annealing, demonstrated as Film 2A in Figure 2a, showing a loss decrease from 1520 to 1550 nm. This was due to all other films deposited on SiO\({}_{2}\)-Si lifting off or cracking during the annealing step. To increase modal interaction with the planar waveguide layer, we also deposited a thicker film with the same recipe as Film 3, shown in Table 1 denoted as Film 6. This recipe was chosen as it demonstrated the lowest losses on average from visible to NIR telecommunication wavelengths. The loss results for this 650 nm-thick film are shown in Figure 3b from 638 to 1620 nm with finer 1480-1580 nm spacing shown in the inset. Figure 3: (**a**) Optical loss of various films determined via prism coupling from 1510 to 1600 nm. (**b**) Optical loss determined via prism coupling at various wavelengths in the visible to infrared for Film 6 and finer spaced loss measurements from 1480 to 1580 nm (inset). Connecting lines are to help guide the eye. Figure 2: Optical loss determined via prism coupling at various wavelengths in the visible to infrared for each SiO\({}_{\text{N}}\)\({}_{\text{y}}\):H\({}_{\text{z}}\) film. The films were deposited on 6 μm thermally grown SiO\({}_{2}\) layers on Si substrates. Connecting lines are to help guide the eye. Inset: red light streak from Film 3 visible during 638 nm loss measurement. Each data point represents a loss measurement derived from a fit to an exponential loss curve obtained from a normally scanning fiber during measurement.