Improved optical loss characteristics of PECVD silicon oxynitride films using low frequency plasma
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Improved optical loss characteristics of PECVD silicon oxynitride films using low frequency plasma S. Naskara,b, C.A. Bowera, S.D. Wolterb, B.R. Stonera,b, and J.T. Glassb a CMET1, RTI-International, Research Triangle Park, NC 27709 b Department of Electrical Engineering, Duke University, Durham NC 27709 ABSTRACT With the continued growth of photonics, silicon oxynitride (SiOxNy) is becoming a popular material for optoelectronic applications owing to its large tunable refractive index. However, with the increase in refractive index, these films tend to show poor optical transmission characteristics. In this research we have investigated the influence of growth conditions on the loss characteristics of PECVD SiOxNy films. The films are grown at 350 °C substrate temperature and 1 Torr pressure with silane (SiH4) and nitrous oxide (N2O) precursor gases. The precursor flow rate and power input to the system are varied as the two primary parameters. It is observed that films grown at 100 kHz plasma frequency proved to be more transmissive than films grown at 13.56 MHz plasma frequency. Elastic recoil detection analysis results showed the hydrogen content is less in the low frequency films than the high frequency films, which is believed to be the reason for the low loss behavior. The details of these analysis results are discussed below.
INTRODUCTION Silicon oxynitride has been under investigation over the past decade as a possible candidate for photonics applications. The tremendous advantage of this material is its large tunable refractive index that can theoretically range between 1.46 and 2.0 coupled with its compatibility with existing silicon processing technology. Thus, it is a popular choice for integrated photonics applications. Several researchers1-5 have demonstrated the deposition of this material using PECVD and have shown a wide range of index of refraction for this material. However, with the increase in the refractive index, this material shows poor loss characteristics as the films tend to become highly absorbing. Previous researchers have optimized low loss SiOxNy films below n = 1.50 for photonic application beyond which the loss increases rapidly. Since high index contrast waveguide devices are becoming rapidly popular, this material so far can face severe competition owing to its poor transmission properties in the high index range. In the present research, the parameter conditions have been varied and films with much lower loss compared to the conventional films at higher index values have been deposited. By using 100 kHz plasma frequency instead of the more popular 13.56 MHz, silicon oxynitride films with greater stoichiometric consistency are achieved. Direct comparison of the loss data has shown improved transmission for the lower frequency films. In order to understand the compositional changes and their impact on loss behavior, ERD analysis and XPS analysis were carried out. The details of this study are discussed below. 1
Formerly part of MCNC-RDI, Research Triangle Park, NC 27709
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