E-Thesis 101 views
Deposition and Characterisation of Silicon Oxynitride Dielectric Films for Waveguide Applications / PARISA RAHBARI
Swansea University Author: PARISA RAHBARI
Abstract
Within the last decade, chemical vapor deposition (CVD)-grown silicon oxynitride (SiOxNy) thin films have become increasingly important for applications in low cost, compact, integrated optical devices. The major advantage of SiOxNy is the tunability of its refractive index over a wide range (n = 1....
| Published: |
Swansea
2021
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| Institution: | Swansea University |
| Degree level: | Master of Research |
| Degree name: | MSc by Research |
| Supervisor: | Elwin, Matt |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa59929 |
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| Abstract: |
Within the last decade, chemical vapor deposition (CVD)-grown silicon oxynitride (SiOxNy) thin films have become increasingly important for applications in low cost, compact, integrated optical devices. The major advantage of SiOxNy is the tunability of its refractive index over a wide range (n = 1.45– 2), resulting in a large degree of freedom for integrated optics design. The plasma-enhanced chemical vapor deposition (PECVD) technique is attractive because of the possibility of working with relatively high deposition rates and at low deposition temperatures (300 – 400 0C), so that it is compatible with well-established microelectronic processing . Furthermore, the CVD technique permits control of the structural, mechanical and optical properties of the deposited films by adjusting the deposition parameters of the CVD process. Silicon oxynitride films (SiOxNy) grown using PECVD have been studied for many applications in the microelectronics and optoelectronics industry, i.e. as passivation coatings, thin gate dielectrics, as well as membranes and optical waveguides for micromechanical systems (MEMS). Controlling the gas flow of the precursor reactant gases permits deposition of SiOxNy films with different silicon, oxygen and nitrogen concentration ratios. The tuneable film composition allows properties such as refractive index, mechanical stress , dielectric constant and optical gap to be varied in a controlled manner. SiOxNy is a highly appropriate material for the combination of waveguides with silicon micromachining technology, since the ability to tune the refractive index is very important for the development of optical applications. These applications involve the integration of planar optical waveguides with micromechanical components on silicon substrates for applications including integrated sensors and optical communication devices. The SiOxNy films also provide advantages in terms of low mechanical stress, high deposition rates and high resistance to chemical corrosion, a very important characteristic for MEMS applications. Knowledge of the bond structure, elemental composition and stoichiometry of different silicon oxynitride (SiON) layers is useful in order to understand the optical behaviour and properties of the fabricated material thin films, since these physical and chemical properties strongly influence the material quality, and subsequently the material’s optical loss. A high concentration of Si-H and Si-Si bonds is well known to be responsible for the formation of undesired grains, pores and columnar microstructures that can cause scattering and absorption losses. Thus, reduction of these bonds is desirable in order to improve material quality. NH3 has been the precursor gas of choice (as opposed to N2) when deposition SiOxNy films, because the energies of N–H bonds are almost three times smaller than those of N ≡ N bonds (391 kJ/mol versus 941 kJ/mol), and therefore require less energy to break. In practice, this allows lower energy or lower temperature deposition processes. However, the use of NH3 gas in the deposition process has been reported to lead to unwanted N–H bonds in the deposited films.[1] The use of N2O - bond energies N-O (201 KJ/mol); N=O (607 KJ/mol) - instead of NH3 during the deposition process can reduce the overall amount of hydrogen in the deposition processes gas chemistry and hence reduces the overall presence of N−H and Si-H bonds, which act as absorption centres. The use of N2O reduces the overall Hydrogen content of the deposited films, even though some hydrogen is present, pertaining to the use of SiH4 as the silicon precursor. In this work, films with different nitrogen, silicon and oxygen composition ratios were obtained in PECVD deposited SiOxNy films, by varying the N2O, and SiH4 gas flows while maintaining constant all other deposition conditions. Spectroscopic ellipsometry was employed to obtain the refractive index and the thickness of the SiOxNy films at a variety of points across the surface of a 100 mm wafer. These measurements were performed at in order to determine the standard deviation of the refractive index and the thickness uniformity across the wafer. The RI and film thickness have been monitored at three different wavelengths (636 nm, 1310 nm and 1550 nm). Extensive characterisation has been performed on these films, including Energy Dispersive X-ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS), and Fourier Transform Infra-Red (FTIR) spectroscopy. Optical losses were measured using a prism coupler (Metricon), in order to assess the optical performance of the SiOxNy thin films. The application of these films is in optical waveguides for data communications. Reducing the hydrogen content of the films minimises optical losses through reduced absorption and reduced scattering. Deposition of high refractive index SiOxNy films also allows a high index contrast (the difference in refractive index between the core and cladding layers in an optical waveguide device). A high index contrast permits a higher radius of curvature of a waveguide bend – allowing a more tightly packed curved waveguide structure to be fabricated – thus reducing the waveguide footprint and consequently reducing the ultimate device cost. |
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| Keywords: |
Dielectric Films |
| College: |
Faculty of Science and Engineering |