Effect of pulse mode in reactive magnetron sputtering on structure and properties of titanium dioxide films

The state of the art to obtain crystalline TiO2 layers by vacuum deposition methods is the use of elevated substrate temperature during deposition or annealing after deposition above 250 °C. Therefore these processes are limited to temperature resistant substrates like glass. On the other hand there is a call for photocatalytic and hydrophilic layers on temperature sensitive materials. An innovative reactive pulse magnetron sputtering (PMS) system allowing to change the pulse mode (unipolar pulse, bipolar pulse or pulse packet) has been used for the sputter deposition of transparent TiO2 films. Investigations were carried out to determine the influence of the pulse mode on the substrate peak temperature during deposition for different substrate materials, on the film structure and on different film properties (hardness, Young's modulus, refractive index and surface roughness). The experimental results show that it is possible to achieve crystalline TiO2 films by pulse sputtering without heating the substrate prior to or annealing the substrate after the coating procedure. Bipolar powering as well as pulse packet powering tend to affect the structure of the layers towards rutile. On thermal sensitive polycarbonate substrates we deposited 500 nm thick amorphous TiO2 films by unipolar pulse sputtering. To deposit crystalline TiO2 layers on these substrates is still a challenge.