Optical Waveguide Devices And Their Anti-reflection Coatings In Silicon-on-Insulator

SOI (Silicon-on-insulator) consists of a thin silicon layer on top of an oxide cladding layer carried on a bare silicon wafer. With its silicon core (n=3.45) and its oxide cladding (n=1.45), it has a high vertical refractive index contrast. Also, both silicon and the oxide are transparent at the telecom wavelength of 1.3um and 1.5um. Due to its special structure, SOI material system, which has very good optical and electronic properties, provides a common platform for VLSI (Very Large Scale Integration) and PLC (Planar Lightwave Circuit). Fabrication process of SOI optical waveguide is very compatible with standard CMOS fabrication processes. Passive and active optoelectronic devices can be fabricated on the SOI wafer, and MEMS devices also can be integrated on the SOI wafer. Monolithic integration of optoelectronic devices on the SOI wafer is the main trend for future optoelectronic industry. Our work in this dissertation is focused on the optical waveguide devices and their anti-reflection coatings in SOI materials.For SOI-based optical waveguide devices, the reflectance and transmittance of uncoated waveguiding silicon layer is almost constant about 31% and 55%, respectively. At the air/waveguide or waveguide/coupling fiber interfaces Fresnel reflection occurs. Fresnel reflection loss of the two waveguide endfaces was calculated to be 3.22dB assuming a normally incident beam. A SOI-based waveguide device needs a high-quality anti-reflection coating on both faces of the device to minimize the Fresnel reflection. To find proper anti-reflection coatings, various methods have been explored to deposit high-quality anti-reflection coatings, including SiN_xO_y:H films deposited by plasma enhanced chemical vapor deposition (PEVCVD), silicon oxynitride films prepared by ion beam assisted deposition (IBAD), and HfO₂ films fabricated by electron beam evaporation. The optical properties and components of the films were characterized by spectroscopic ellipsometry, X-ray photoelectron spectroscopy and Perkin-Elmer Lambda900 spectrophotometer, etc.. The optical experiment results suggested that all the films were very attractive single layer anti-reflection coatings for the SOI-based optoelectronic devices. And for a coated double-side polished silicon wafer, Fresnel losses at the telecom wavelength of 1550nm have been reduced to 0.022dB by depositing HfO2 film (185nm) as single layer anti-reflection coating. For practical fabrication process, it is very difficult to deposit SiNxOy:H films onto the SOI rib waveguide endfaces due to the size of SOI waveguide devices. Silicon oxynitride films can be deposited onto the SOI rib waveguide endfaces after CMP (chemical Mechanical Polishing), but silicon oxynitride films cannot deposited onto the endfaces of the integrated SOI waveguide devices using IBAD. A clamp fixed the SOI rib waveguide, and the waveguide endfaces were perpendicular to the evaporation direction so that HfO2 could be easily deposited onto the integrated SOI waveguide endfaces using electron beam evaporation.Based on the single-mode waveguide theory, SOI rib waveguides were fabricated by inductive coupled plasma reactive ion etching with vertical sidewall. To achieve the integration of self-alignment connection between single mode fiber and rib waveguide in silicon-on-insulator (SOI) wafer, a three-mask lithography process was used. Uniformity V-grooves and U-grooves were etched by wet etching and dry etching, respectively. The experiment results indicated that the three-mask lithography ICPRIE process is easy, cost-effective and acceptable in a mass production environment. And HfO2 films can be deposited onto the endfaces of the integrated waveguide devices through U-groove.Based on these experiments, the monolithic integration of Y-branch and T-branch devices were fabricated in the SOI wafer , respectively. We measured the fiber-waveguide-fiber insertion losses as the ratio between the output and input powers using Agilent 8164A lightwave measurement system. For the symmetric 1><2 Y-branch with branch angle 28 of 0.8°, the fiber-waveguide-fiber loss was measured to be 4.4dB at X=l.55um. And the results at ?i=l .55 \im are 5.0±0.5 dB and 5.2±0.5 dB, respectively, in the two output waveguides of the 1 x2 single-mode T-branch. The split ratio is nearly 52:48.Endface roughness, surface roughness and sidewall roughness result in increasing scattering losses for waveguides. According to scalar scattering theory, Tien's theory and Marcuse's theory, scattering loss induced by the rms (root-mean-square) roughness wasstudied systematically. And the scattering loss is proportional to the square of the sidewall rms roughness.A series of atomic force microscope measurements were carried out to demonstrate the rms roughness of SOI rib waveguide etched by ICPRIE method. To smooth the sidewall surface and corner mirror surface, various methods have been explored. In order not to change the waveguide configuration, low-temperature ultra-high vacuum annealing, hydrogen annealing and mixed ICPRIE were used to reduce the rms roughness of the rough surfaces. After such treatments, the ripples of the surfaces disappeared, and the rms roughness could be reduced to approximately 9nm. With slight shape changed, oxidation and wet etching is a great way to reduce roughness. The SOI rib waveguide devices went through a dry oxidation. After the oxidation step, the SiO2 layers with the thickness of lOOnm were removed by by 40wt.% KOH solutions at 70°C. The ripples of the surfaces disappeared, and the rms roughness could be lowed down to approximately 0.5nm. This is to our knowledge the smallest reported rms roughness for a high-index-difference system such as as SOI rib waveguide.