Fracture origins in LiNbO 3 wafers due to postprocessing micro-repolarization
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Fracture origins in LiNbO3 wafers due to postprocessing micro-repolarization Hirotoshi Nagata and Junichiro Ichikawa Optoelectronics Research Division, New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan
Mitsuru Sakima Optoelectronics Division, New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan
Kaori Shima and Eungi Min Haga Advanced Materials Research Division, New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan (Received 23 November 1998; accepted 5 April 1999)
In the process of developing electro-optic devices from ferroelectric z-cut LiNbO3 wafers, a repolarization throughout the wafer thickness occurs due to a localization of electric charges on the wafer. The repolarization not only generates microdomains causing light to scatter but also large defects in the crystal that become the origin of wafer fracture. The size of such defects is comparable to the wafer thickness (0.5 mm), and an anomaly in the chemical and crystalline structures can be found in them. X-ray diffractometry and x-ray photoelectron spectroscopy confirm that a chemical reduction in the defective region occurs.
Since the worldwide expansion of optical fiber communication systems, the production of optical external modulators and other electro-optic waveguide devices made of ferroelectric LiNbO3 crystalline wafers has proliferated.1 High-speed optical phase modulators and optical polarization scramblers, in particular, are unique devices made from LiNbO3. Z-cut LiNbO3 wafers (i.e., crystallographic c-axis normal) are frequently used in the fabrication of such LiNbO3 devices because they have the highest electro-optic constants and lowest dielectric constants.2 The device fabrication process generally consists of a photolithograph of a waveguide pattern, a waveguide formation by Ti-indiffusion, a SiO2 buffer layer deposition by the sputtering or evaporation technique, an oxygen-atmosphere annealing for the buffer layer, and an electro-plating of patterned electrodes.3 Sometimes, a plasma etching of the surface is carried out to improve radio frequency performance.4 But there is a disadvantage to using z-cut LiNbO3 wafers; that is, at times, a postprocessing repolarization occurs during the fabrication process.5 When observed through an optical microscope, this phenomenon appears as microdomains. Such repolarization can be attributed to extrinsic electrical charges on the wafer surface induced by the plasma process and the intrinsic charges caused by a pyro-effect of the crystal. With respect to the optical performance of these devices, the prevalence of microdomains causes light to scatter, increasing the optical propagation loss of the waveguides. In addition to such problems, postprocesJ. Mater. Res., Vol. 15, No. 1, Jan 2000
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