Transparent silicon nitride films prepared by surface wave plasma chemical vapor deposition under low temperature conditions

• Fabrication of transparent SiNx films using surface-wave-plasma deposition.
• Si-rich-SiNx films contain suboxides that may decrease both transparency and stability.
• Using high NH3/SiH4 gas ratio results in transparent and stable Si3N4.

Highly transparent silicon nitride films with a low absorption coefficient of only 200 cm- 1 or lower were prepared under high NH3/SiH4 source gas ratio conditions at 80 °C or lower temperature using surface wave plasma chemical vapor deposition (SWP-CVD). Rutherford backscattering measurements indicated that a silicon nitride structure Si3Nx (x > 5) with excess nitrogen could be prepared with the SWP-CVD method under high NH3/SiH4 source gas ratio conditions. X-ray photon spectroscopy (XPS) analysis provided verification that the excess nitrogen combined with the oxygen contained in the SiNx film during low-temperature film formation and that the atomic ratio of Si and N was almost stoichiometric, i.e., Si3N4.XPS study also revealed that the Si3N< 4 structure contained suboxides, which presence may reduce the transparency of the films. In contrast, suboxides were not observed in the Si3N4 structure obtained under high NH3/SiH4 source gas ratio conditions. Fourier-transform infrared spectroscopy study confirmed that the SiNx film becomes more stable when the SiNx structure approaches the stoichiometric ratio. Achieving a near-transparent Si3N4 structure requires a sufficient amount of N atoms in the periphery of the Si atom, i.e., an adequate amount of NH3 is necessary in the presence of the SiH4.