Effect of Annealing Treatment on the Optical Properties of Silicon Nitride Waveguides

Introduction. Silicon nitride is a highly promising material for fabrication of photonic integrated circuits (PICs). Plasma-enhanced chemical vapor deposition is a prospective method for large-scale industrial production of silicon nitride-based PICs. The disadvantage of this method, which limits its practical application, consists in high insertion losses in the telecommunication frequency band due to absorption on the Si–H and N–H bonds remaining from the film growth process. Thermal annealing is the most common method for breaking these bonds and reducing losses. Therefore, investigation of the impact of annealing on the optical properties of photonic integrated waveguides is an important research task.

Aim. To investigate the effect of annealing treatment on the optical properties of PICs based on the silicon nitride films with different thicknesses obtained by plasma-enhanced chemical vapor deposition.

Materials and methods. The work investigates the effect of annealing treatment on the optical properties of PICs based on the silicon nitride films with thicknesses of 200, 400 and 700 nm. To that end, the transmission characteristics of a set of test elements were measured using a high-definition component analyzer in the frequency range of 185…196 THz.

Results. Frequency dependencies of loss and coupling coefficients, as well as the group index before and after annealing were extracted from the measured transmission characteristics of the test elements. It was found that waveguides on a 200-nm-thick film exhibited higher losses in comparison with the waveguides on thicker films. The waveguides with cross sections of 900 × 400 and 900 × 700 nm2 demonstrate the losses below 5 dB in the frequency range of 185…190 THz. A rapid increase in losses due to absorption on the N–H bonds was observed at the frequencies above 190 THz. The work shows that thermal annealing reduces insertion losses across the frequency range from 185 to 196 THz. The adequacy of extracted optical parameters is confirmed by comparing theoretical and experimental transmission characteristics of the ring resonator.

Conclusion. The obtained results demonstrate that silicon nitride waveguides fabricated by the method of plasma-enhanced chemical vapor deposition require the stage of thermal annealing. Vacuum annealing at 600 °C for 30 min reduces insertion losses in the waveguides with cross sections of 900 × 400 and 900 × 700 nm² down to 4 dB/cm in the frequency band from 185 to 196 THz.

1. Ji X., Roberts S., Corato-Zanarella M., Lipson M. Methods to Achieve Ultra-High Quality Factor Silicon Nitride Resonators. APL Photonics. 2021, vol. 6, iss. 7, p. 071101. doi: 10.1063/5.0057881

2. Liu J., Huang G., Wang R. N., He J., Raja A. S., Liu T., Engelsen N. J., Kippenberg T. J. High-Yield, Wafer-Scale Fabrication of Ultralow-Loss, DispersionEngineered Silicon Nitride Photonic Circuits. Nature Communications. 2021, vol. 12, iss. 1, p. 2236. doi: 10.1038/s41467-021-21973-z

3. Jin W., Yang Q. F., Chang L., Shen B., Wang H., Leal M. A., Wu L., Gao M., Feshali A., Paniccia M., Vahala K. J., Bowers J. E. Hertz-Linewidth Semiconductor Lasers Using CMOS-Ready Ultra-High-Q Microresonators. Nature Photonics. 2021, vol. 15, iss. 5, pp. 346–353. doi: 10.1038/s41566-021-00761-7

4. El Dirani H., Youssef L., Petit-Etienne C., Kerdiles S., Grosse P., Monat C., Pargon E., Sciancalepore C. Ultralow-Loss Tightly Confining Si3N4 Waveguides and High-Q Microresonators. Optics Express. 2019, vol. 27, iss. 21, pp. 30726–30740. doi: 10.1364/OE.27.030726

5. Ooi K. J. A., Ng D. K. T., Wang T., Chee A. K. L., Ng S. K., Wang Q., Ang L. K., Agarwal A. M., Kimerling L. C., Tan D. T. H. Pushing the Limits of CMOS Optical Parametric Amplifiers with USRN:Si7N3 Above the Two-Photon Absorption Edge. Nature Communications. 2017, vol. 8, iss. 1, p. 13878. doi: 10.1038/ncomms13878

6. Brasch V., Chen Q. F., Schiller S., Kippenberg T. J. Radiation Hardness of High-Q Silicon Nitride Microresonators for Space Compatible Integrated Optics. Optics Express. 2014, vol. 22, iss. 25, pp. 30786– 30794. doi: 10.1364/OE.22.030786

7. Marpaung D., Yao J., Capmany J. Integrated Microwave Photonics. Nature Photonics. 2019, vol. 13, iss. 2, pp. 80–90. doi: 10.1038/s41566-018-0310-5

8. Blumenthal D. J., Heideman R., Geuzebroek D., Leinse A., Roeloffzen C. Silicon Nitride in Silicon Photonics. Proc. of the IEEE. 2018, vol. 106, iss. 12, pp. 2209– 2231. doi: 10.1109/JPROC.2018.2861576

9. Kippenberg T. J., Gaeta A. L., Lipson M., Gorodetsky M. L. Dissipative Kerr Solitons in Optical Microresonators. Science. 2018, vol. 361, iss. 6402, p. eaan8083. doi: 10.1126/science.aan808

10. Capmany J., Novak D. Microwave Photonics Combines Two Worlds. Nature Photonics. 2007, vol. 1, iss. 6, pp. 319–330. doi: 10.1038/nphoton.2007.89

11. Marpaung D., Yao J., Capmany J. Integrated Microwave Photonics. Nature Photonics. 2019, vol. 13, iss. 2, pp. 80–90. doi: 10.1038/s41566-018-0310-5

12. Pfeiffer M. H. P., Herkommer C., Liu J., Morais T., Zervas M., Geiselmann M., Kippenberg T. J. Photonic Damascene Process for Low-Loss, High-Confinement Silicon Nitride Waveguides. IEEE J. of Selected Topics in Quantum Electronics. 2018, vol. 24, iss. 4, pp. 1–11. doi: 10.1109/JSTQE.2018.2808258

13. Wang L., Xie W., Van Thourhout D., Zhang Y., Yu H., Wang S. Nonlinear Silicon Nitride Waveguides Based on a PECVD Deposition Platform. Optics Express. 2018, vol. 26, iss. 8, pp. 9645–9654. doi: 10.1364/OE.26.009645

14. Ay F., Aydinli A. Comparative Investigation of Hydrogen Bonding in Silicon Based PECVD Grown Dielectrics for Optical Waveguides. Optical Materials. 2004, vol. 26, iss. 1, pp. 33–46. doi: 10.1016/j.optmat.2003.12.004

15. Shaw M. J., Guo J., Vawter G. A., Habermehl S., Sullivan C. T. Fabrication Techniques for Low-Loss Silicon Nitride Waveguides. Micromachining Technology for Micro-Optics and Nano-Optics III. 2005, vol. 2720, pp. 109–118. doi: 10.1117/12.588828

16. Vasilev V. Yu. Silicon Nitride Thin Film Deposition for Microelectronics and Microsystems Technologies. Part 8. Hydrogen Influence on Basic Film Properties. Nanoi mikrosistemnaya tekhnika. 2019, vol. 21, no. 6, pp. 352–367. doi: 10.17587/nmst.21.352-367 (In Russ.)

17. Ershov A. A., Eremeev A. I., Nikitin A. A., Ustinov A. B. Extraction of the Optical Properties of Waveguides Through the Characterization of Silicon‐ On‐Insulator Integrated Circuits. Microwave and Optical Technology Letters. 2023, vol. 65, iss. 8, pp. 2451–2455. doi: 10.1002/mop.33675

18. Bogaerts W., De Heyn P., Van Vaerenbergh T., De Vos K., Kumar Selvaraja S., Claes T., Dumon P., Bienstman P., Van Thourhout D., Baets R. Silicon Microring Resonators. Laser & Photonics Reviews. 2012, vol. 6, iss. 1, pp. 47–73. doi: 10.1002/lpor.201100017

19. Nikitin A. A., Vitko V. V., Cherkasskii M. A., Ustinov A. B., Kalinikos B. A. Nonlinear Frequency Response of the Multi-Resonant Ring Cavities. Results in Physics. 2020, vol. 18, p. 103279. doi: 10.1016/j.rinp.2020.103279