**3.4 Silicon nitride (Si3N4)**

Most applications of Si3N4 have been used in terms of the improvement of properties such as surface modulation for orthopedic applications [25] and biomedical applications [26]. Also, Si3N4 owns good optical properties. The Si3N4 was fabricated as photonic circuits to spectroscopic sensing [27]. The Si3N4 was used for nonlinear signal processing applications [28]. Furthermore, Si3N4 was microfabricated as the waveguides and grating couplers for new nanophotonic approach of light delivery for optogenetic applications [29].
