Interface Reactions in LiNbO 3 Based Optoelectronics Devices
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Interface Reactions in LiNbO3 Based Optoelectronics Devices Hirotoshi Nagata, Yasuyuki Miyama, Kaoru Higuma, Yoshihiro Hashimoto, Futoshi Yamamoto, Yuuji Yamane, and Miki Yatsuki Optoelectronics Research Division, New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-9601, Japan
ABSTRACT We present secondary ion mass spectrometry (SIMS) study results on interfaces of LiNbO3 based optoelectronic devices, which have been performed in order to examine the cause of device failures. The devices are widely used in current high-speed optical fiber communication systems, and such investigation from a materials-viewpoint is important to improve the device quality. Especially, the device long-term stability is strongly affected by alkali-contaminants diffused into the SiO2 buffer layer of device, and here we confirmed that an adoption of common Si3N4 passivation is effective in preventing the process-induced contamination without any influence to device performance.
INTRODUCTION Since LiNbO3 based optical waveguide modulators are widely used in global fiber communication systems, their long-term reliability has been carefully investigated from the view point of stability of device performance, such as dc drift phenomena. The latest Telcordia GR-468-CORE standard comments on reliability and quality requirements for LiNbO3 modulators in addition to conventional laser devices [1]. However, to our knowledge, reports on problems in device quality due to the LiNbO3 modulator fabrication processes are limited, although a demand for LiNbO3 modulators is rapidly increasing. For instance, the magnitude of the dc drift in modulator optical output is largely enhanced by alkali-contaminants injected into a SiO2 buffer layer covering the LiNbO3 substrate [2]. Because the dc drift is a main cause of device wear-out failures, the drift must be suppressed [3]. We found previously that the alkali-contamination was caused by wet-processes for exposed SiO2 layer, such as photolithography, wet-etching, etc. In this report, applicability of a Si3N4 passivation layer, common material in Si device processes, to LiNbO3 modulators is shown. The Si3N4 layer works not only as the passivation layer but also as the glue layer for Au/Ti electrodes formed on modulator surface.
STRUCTURE OF LiNbO3 MODULATORS Figure 1 shows a schematic cross-section of the LiNbO3 optical intensity modulators, mainly consisting of an LiNbO3 substrate with the buried optical waveguides, the SiO2 buffer layer covering the LiNbO3 surface, and the thick gold electrodes. The optical waveguides were formed by a thermal diffusion of metallic Ti lines at approx. 1000 oC. After the waveguide formation, the SiO2 layer was deposited by a vacuum evaporation method and annealed at 600 oC in an oxygen atmosphere. On the SiO2 surface, the Au/Ti binary film was AA3.2.1
deposited by a sequential vacuum evaporation of Ti and Au, as a glue layer for the thick Au electrodes prepared by an electro-plating method. The role of the Ti l
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