Paper TF-ThP20
Low Hydrogen Silicon Nitride Films Deposited by Plasma Enhanced Chemical Vapor Deposition

Thursday, November 13, 2014, 6:00 pm, Room Hall D

Due to exceptional material properties, such as refractive index and others, the use of silicon nitride (SiN) as an optical waveguide has become common. SiN films are used in many applications, from gate dielectrics to encapsulation layers. However, the use of SiN as a waveguide is greatly affected by hydrogen incorporation, particularly at telecommunication wavelengths of ~1550 nm.

Hydrogen incorporation for plasma enhanced chemical vapor deposition (PECVD) of SiN_x is a due to the Si-H species from the use of SiH₄ as well as the standard precursor NH₃ for the N component for SiN_x. The resulting hydrogen bonds, both Si-H species and N-H species, result in absorption and thus loss in the telecommunication spectrum. In particular, the N-H bond has high absorption around 1550 nm.

Previous studies have investigated the use of N₂ as a replacement precursor for NH₃ for low-hydrogen PECVD SiN_x.^1,2 However, these studies have only investigated a few process parameters and their effect on the optical characteristics of the film. In our investigation, a detailed study was performed on a production PECVD system to look at the effect of SiH₄ flow, NH₃ flow, N₂ flow, ratios of all precursors, chamber pressure, and RF bias. The relative hydrogen content was measured for all process parameters through Fourier transform infrared spectroscopy (FTIR). Additionally, we measured the refractive index (from 375 nm to 1675 nm), deposition rate, uniformity, and stress of the SiN_x films. We are able to not only tune the relative hydrogen content of the film, but the uniformity, stress and refractive index independently, in order to fabricate SiN_x optical waveguides for photonic applications.

It was observed that although substituting N₂ in place of NH₃ for the source of N in SiN_x resulted in lower relative hydrogen content, it greatly affected the refractive index and stress of the film resulting in a film that is not ideal for photonic integrated circuits. In order to balance these film properties, we were able to exploit other process parameters, such as chamber pressure and RF bias, in order to tune the film. We also measured the insertion loss of SiN_x optical waveguides with select SiN_x films in order to further understand the role of different film properties.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

¹S. C. Mao, et al., Opt. Express 16, 20809-20816 (2008).

²F. Karouta, et al., J. Phys. D: Appl. Phys. 45, 445301 (2012).