Structural characteristics and optical properties of plasma assisted reactive magnetron sputtered dielectric thin films for planar waveguiding applications

Thin films of aluminum oxide (Al₂O₃), tantalum pentoxide (Ta₂O₅), titanium oxide (TiO₂), yttrium oxide (Y₂O₃) and zirconium oxide (ZrO₂) were deposited by plasma assisted reactive dual magnetron sputtering to determine their suitability as a host for a rare earth doped planar waveguide upconversion laser. The effect of deposition parameters such as cathode, plasma power and oxygen gas flows were studied and the operational working points were determined. Both power and lambda control were used to optimize the optical quality of each material. By using lambda control feedback system, the magnetron power fluctuates to sustain a fixed oxygen flow in the target area reducing the compound layer growth on the material and maintaining a healthy deposition rate. The optical properties, structure and crystalline phase of each film were found to be dependent on the process parameters. X-ray diffraction (XRD) analysis revealed that the thin films varied from amorphous to highly crystalline depending on the deposition conditions. X-ray photoelectron spectroscopy (XPS) was utilized for surface compositional analysis revealing that films had varying stoichiometric ratios which are controlled for each material by the deposition parameters chosen. The waveguide loss for the thin film layers was investigated and Ta₂O₅ was shown to have a slab waveguide loss of ~ 1 dB/cm at both visible and infra-red wavelengths making it ideal for planar waveguide and laser applications. TiO₂, Y₂O₃ and ZrO₂ were found to deposit in a highly crystalline phase. Waveguiding in the TiO₂ layers was not possible at 633 nm or in the infrared region. The Y₂O₃ samples gave low loss (2–4 dB/cm) at the 1.3 and 1.5 μm wavelengths but no waveguiding at 633 nm or 833 nm was possible. Atomic force microscopy showed rough surface topography for TiO₂, Y₂O₃ and ZrO₂ akin to their crystalline growth with the SEM images confirming the regular crystalline columnar structure for the case of Y₂O₃ and ZrO₂.