Deposition of rutile TiO2 films by pulsed and high power pulsed magnetron sputtering

Thin films of titaniumdioxide (TiO2) are important high refractive index coatings in opticalmultilayer stacks. Especiallyinteresting is the rutile crystalline phase of TiO2 films because it's refractive index in the range of 2.6EL2.7is the highest available refractive index ofmaterials being transparent in the visible and near infrared wavelengthregion. Rutile TiO2 films in optical interference coatings therefore in principle allow achieving the desired opticalfunction with a lower number of layers and a simpler design compared to alternative high index materials.However the crystalline growth of TiO2 layers usually starts with an amorphous start layer of typically 40 nmthickness before going over to an anatase or rutile crystalline structure. The amorphous start layer typicallyhas a refractive index in the range of 2.4EL2.5. This significant difference to the index of the rutile film leads toa typical gradient in the refractive index of TiO2 single layers andmakes application of TiO2 films in precision opticspractically difficult.The paper explores the possibilities of obtaining rutile TiO2 films right from the beginning of film growth by significantlyincreasing energetic ion bombardment of the growing film and adatommobility. The influence of a varietyof process parameters such as substrate temperature, deposition pressure, magnetic field strength and RFbias is investigated. The main focus of investigations was the comparison of a standard pulse process, a pulsedprocess with significantly increased pulse current and a pulse process in genuine high power pulsed (HiPIMS)mode.Film deposition was done by stationary sputtering using the double ringmagnetron (DRM). Film characterizationwas carried out by XRD as well as by spectroscopic ellipsometry and photometry. Total optical losseswere characterizedby cavity-ring-down spectroscopy (CRD) and absorption was measured by laser induced deflection.Results showthat in a rather narrow range of process parameters the desired crystalline growth of TiO2 with therutile phase throughout the film can be obtained. This parameter set includes both the pulsed process with significantlyincreased pulse current and the genuine HiPIMS mode, but in all cases requires a combination withsubstrate heating, RF substrate bias and adapted process pressure. A significant reduction of crystallite size andscattering losses was achieved by adding a small amount of silicon dioxide (SiO2) to the film.