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

Thin films of titanium dioxide (TiO2) are important high refractive index coatings in optical multilayer stacks. Especially interesting is the rutile crystalline phase of TiO2 films because it's refractive index in the range of 2.6 ... 2.7 is the highest available refractive index of materials being transparent in the visible and near infrared wavelength region. Rutile TiO2 films in optical interference coatings therefore in principle allow achieving the desired optical function 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 nm thickness before going over to an anatase or rutile crystalline structure. The amorphous start layer typically has a refractive index in the range of 2.4 ... 2.5. This significant difference to the index of the rutile film leads to a typical gradient in the refractive index of TiO2 single layers and makes application of TiO2 films in precision optics practically difficult. The paper explores the possibilities of obtaining rutile TiO2 films right from the beginning of film growth by significantly increasing energetic ion bombardment of the growing film and adatom mobility. The influence of a variety of process parameters such as substrate temperature, deposition pressure, magnetic field strength and RF bias is investigated. The main focus of investigations was the comparison of a standard pulse process, a pulsed process 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 ring magnetron (DRM). Film characterization was carried out by XRD as well as by spectroscopic ellipsometry and photometry. Total optical losses were characterized by cavity-ring-down spectroscopy (CRD) and absorption was measured by laser induced deflection. Results show that in a rather narrow range of process parameters the desired crystalline growth of TiO2 with the rutile phase throughout the film can be obtained. This parameter set includes both the pulsed process with significantly increased pulse current and the genuine HiPIMS mode, but in all cases requires a combination with substrate heating, RF substrate bias and adapted process pressure. A significant reduction of crystallite size and scattering losses was achieved by adding a small amount of silicon dioxide (SiO2) to the film. (C) 2015 Elsevier B.V. All rights reserved.