Influence of reactive sputtering process parameters on the structure and properties of TiO 2 thin films

摘要

Recently, titanium dioxide (TiO2) thin films have attracted significant attention and became a major area of research since the discovery of its photocatalytic effect on water. TiO2 is characterized by high chemical stability, mechanical hardness and optical transmittance as well as by a high refractive index. Therefore it is used in a variety of applications including solar energy conversion, optical coatings and protective layers. TiO2 thin films can crystallize in two crystalline structures, anatase and rutile. Anatase is metastable at room temperature while rutile is the thermodynamically stable phase. Each phase is characterized by its specific physical properties and related applications. The rutile phase for example is known for its comparatively high mass density (4.23 g/cm3) and high refractive index of up to 2.75 at 589 nm. Hence, it is highly suitable for applications like antireflective coatings. The anatase phase in turn is characterized by a very pronounced photocatalytic activity in combination with hydrophobicity. Consequently, it is applied to fabricate self-cleaning, antifogging glass and antibacterial surfaces. It is also used for water and air purification. In this work, an atomistic understanding of the growth of TiO2 thin films under the influence of various sputtering process parameters has been developed. It has been demonstrated that tailoring the structure of the reactively sputtered TiO2 thin films is possible by controlling the sputtering process parameters. Different sputtering techniques like dcMS, IBAS and HiPIMS have been utilized to fabricate TiO2 thin films. These films exhibit two crystalline structures, namely anatase and rutile. Sample preparation has been performed at different conditions, varying e.g. energetic bombardment, oxygen partial pressure and film thickness. It has been found that the formation of each phase is governed by specific parameters. For instance, energetic bombardment promotes the growth of the rutile structure. On the other hand, the growth of the anatase phase profits from the absence or very weak ion bombardment. Additionally, the anatase phase was often found for growth at high oxygen partial pressure or for thick films, whereas a rutile structure was formed otherwise. Additional substrate heating was also found to support the formation of the anatase phase. It has been demonstrated that energetic bombardment plays a dominant role in the structure formation. It has been proven that the bombardment of the growing film with highly energetic negative oxygen ions inherent in the sputtering process promotes the growth of the rutile structure. This has been observed by an investigation of the sample profile utilizing new and aged targets, since the distribution of oxygen ion bombardment along the substrate depends on the age of the target. Further support was found from investigating films grown in a HiPIMS process, where the negative oxygen ions with high energies are the dominant species governing structure formation. Furthermore, pure rutile films have also been grown under additional ion bombardment in an ion-assisted DC sputtering process. These results also show that the ion bombardment selectively hindered the formation of the anatase phase. The investigation of the structure under the influence of O+ ion and Xe+ ion bombardment has indicated that the nature of the bombarding species does not play a role in structure formation. Reducing the intensity of the energetic oxygen ion bombardment from the sputter target has enabled the formation of pure anatase structure. It has also been shown that the ion bombardment has a strong influence on the surface topography. Two surface features can be clearly distinguished that were proposed to represent rutile and anatase grains. It has also been demonstrated that purely rutile films grown in the HiPIMS process are thermally stable. The impact of highly energetic oxygen ions to the growing film has led to the formation of compressive stress which is dependent on the various process parameters. The films show an inhomogeneous distribution of the rutile and anatase phases upon increasing the film thickness. Rocking curve scans at small incidence angle have shown that the rutile phase grows at the substrate-film interface. With increasing thickness, the anatase phase overgrows the rutile phase. First evidence for this has been found from a simulation of the rocking curve scans for different film structures. TEM measurements finally confirmed the postulated growth mode. The measurements show that the growth of the rutile phase is usually observed at the substrate interface. Few anatase grains nucleate at the interface and overgrow the rutile grains in a conical manner.