# Si Photonics for Practical LiDAR Solutions
## 3 Si Photonics for LiDAR
### LiDAR System Design Considerations
#### 3.2.2 Waveguiding Material  
The implementation of most Si photonics products is fabricated on silicon-on-insulator (SOI) wafers, which was originally invented as a better substrate for low power CMOS and RF applications. The type of SOI wafers used in Si photonics is with relatively thick device Si and buried oxide (BOX) layers which is closer to that for RF-CMOS than that for partially or fully depleted transistor CMOS. The device Si can act as a waveguiding core as it has larger refractive index than its surrounding silicon dioxide (SiO\({}_{2}\)) or silicon nitride (SiN) materials. The material platform, most notably specified by the thickness of the device silicon, has gradually come to a consensus of around 220 nm [17; 18; 19; 20; 21] though some variations exist for better integration with transistors [22] or for more tolerable optical performance [23; 24]. SiN, commonly found in many CMOS processes, can also act as a waveguiding material in Si photonics. The thickness of SiN layers varies a lot, from a few hundred nanometers to few microns [25; 26; 27]. In this article we exclude sub-100 nm thick SiN (for ultra-low loss applications [28]) or few \(\upmu\)m thick Si (for better polarization and process tolerance management [29]) because they are currently lack of a comparable manufacturing maturity or with vast different design rules that complicates following quantitative analysis. We summarize and compare both waveguiding materials in Table 3 with some typical designs and properties relevant to our discussions. The SiN used for waveguides is generally not crystalline materials and its properties vary with deposition processes so some data exhibit a range. Si waveguide transmits light with wavelength above its optical bandgap at around 1.1 \(\upmu\)m while SiN is transparent in both visible and near infrared. In most manufacturable Si photonics processes, Si and SiN waveguides are surrounded by SiO\({}_{2}\) as cladding which exhibits absorption starting from about 3.7 \(\upmu\)m and effectively introduces the upper wavelength limit for both types of waveguides. LiDAR chips using Si waveguides often adopts a telecom wavelength band such as  
Figure 1: Calculated maximum permissible exposure (MPE) according to IEC 60825-1 (2014), (**a**) in J/cm\({}^{2}\) and (**b**) in W/cm\({}^{2}\), of point laser sources at wavelengths of 850 nm, 905 nm, 940 nm, 1064 nm, 1310 nm, and 1550 nm respectively as a function of exposure time. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Transparency** & **Thickness** & **Loss** & **Bending** & \(\beta_{\text{TO}}\)/K & \(n2\) cm\({}^{2}\)/GW & \(\beta_{\text{TPA}}\) \\  & \(\upmu\)m & \(\upmu\)m & dB/cm & \(\upmu\)m & \(\beta_{\text{TO}}\)/K & \(n2\) cm\({}^{2}\)/GW & \(\beta_{\text{TPA}}\) \\ \hline Si & 1.1–3.7 & 0.2–0.5 [1] & 1–3 [1] & 5–50 [1] & \(1.8\times 10^{-4.2}\) & \(5\times 10^{-5.2}\) & 0.5 [2] \\ SiN & 0.5–3.7 & 0.2–2 [1] & 0.2–2 [1] & 20–200 [1] & \(2\)–\(4\times 10^{-5.2}\) & 3–\(7\times 10^{-6.2}\) & neg. \\ \hline \hline \end{tabular}  
* These ranges are coarsely summarized from some but not all data in Ref. [17; 18; 19; 20; 21; 25; 26; 27]. [2] These data are obtained from Ref. [30; 31; 32; 33; 34; 35; 36; 37] at around 1550 nm wavelength.  
\end{table} 
Table 3: A comparison of typical designs and properties between Si and SiN waveguide with SiO\({}_{2}\) claddings. C-band (1530-1565 nm) or O-band (1260-1360 nm) to leverage the existing process design kit (PDK) of foundries, a mature supply chain, and product development experience. The transparency of SiN in the 800-1100 nm allows possible LiDAR designs in this particularly interesting range leveraging mature light sources, Si sensors and imaging optics. On the other hand, SiN is lack of an efficient phase tuning mechanism. Thermal optics tuning is currently the dominant method for both Si and SiN whereas SiN's thermo-optic coefficient (\(\beta_{\text{TO}}\)) is nearly an order of magnitude smaller than Si's which means greater power consumption and a larger phase shifter.