Characterization Of Ti In-Diffused Waveguides Using Secondary Ion Mass Spectrometry (SIMS) And Electron Microprobe Analysis (EMPA)

Abstract

Characterization of Ti In- Diffused Waveguides Using Secondary Ion MassSpectrometry (SIMS) and Electron Microprobe Analysis (EMPA)E. A. LeoneAllied - Signal Corporation, Columbia Road, Morristown, NJ 07960A. J. SignorelliAllied -Signal Corporation, Columbia Road, Morristown, NJ 07960S. SriramAllied Amphenol Products, 1925 Ohio Street, Lisle, IL 60532P. VohralikVG Scientific, The birches Industrial Estate, Imberhorne Lane,East Grinstead, Sussex RH19 lUbAbstractTi in- diffused LiNb03 crystals were examined using EMPA and SIMS with a liquid metalion probe. The switching region of the waveguide consisted of two 5 Um channels ofsputter deposited Ti with a 5 0m gap. We found that after in- diffusing the Ti at -1050 °C,the Z -cut crystal had a sharper Ti distribution profile when compared to a Y -cut crystal.This is consistent with previous studies showing that the bulk diffusion coefficient forZ -cut crystals is greater than that for Y -cut crystals. However, the maximum lateralextent of the Ti was similar in both cases. It was shown that under optimum conditions,the electron microprobe analysis volume was approximately 3 0m. This is much less thanthe measured values for the Ti FWHM and maximum lateral extents, which were on the orderof 20 and 60 pm, respectively. Using a relatively new SIMS technique employing a Galiquid metal ion gun (LMIG -SIMS), it was possible to image the surface Ti distribution andobtain SIMS line scans of the individual channels. There was excellent agreement in theshapes of the Ti distributions observed between the two techniques. Considerable overlapof the two Ti channels was evident in the switching region. The Y -cut crystal had essen-tially a single Ti profile, while two partially resolved peaks were observed tor the Z-cutcrystal.introductionThe electro -optic property of LiNb03 makes it attractive for applications in opticalswitching and modulation in single mode optical fiber communication and sensor systems'.Numerous research papers have been written in the last fifteen years outlining the perfor-mance of LiNb03 based devices2,3. Typical devices are phase modulators, amplitude modula-tors, and switches. These devices depend on the electro -optic effect in which theapplication of an external field changes the refractive index in the crystal, therebymodulating the phase of the guided beam in the waveguide. Extremely high quality singlecrystal 3 inch diameter LiNb03 crystals are available in a variety of crystal orien-tations. There are two possible crystal orientations to choose from for the manufactureof guided wave devices; Y or Z cut. The performance of devices fabricated on Z cutcrystals have shown to be superior to the Y cut crystal orientation. On the other hand,the surface quality of Z cut LiNb03 atter the high temperature diffusion exhibits amottled or orange peel type texture which can be detrimental to the light propagation inthe waveguide. Waveguides fabricated on a Y cut surface do not exhibit this problem.However, the lateral diffusion in a Y cut crystal is anomalously high which leads to poormode confinement and hence excess waveguide loss.Numerous studies in the literature can be found describing the Ti distribution ofsingle indiffused waveguides and correlate the distribution with refractive index changesin the crysta14,5,6,7. Of particular interest is the Ti distribution profile within thecoupling region where the degree of cross -talk between the two channels can affect thedevice performance. The Ti profiles are usually determined with an electron microprobehaving an x -ray analysis volume on the order of