Study On The Growth, Properties And Infrared Optical Applications Of Yttrium Oxide Films Based On Reactive Sputtering Methods

With the development of the aerospace technology going into hypersonic speed stage, infrared window and dome components are suffered from extreme pneumatic heat/force enviorment, thus, the antireflection and protection of the windows is important and the material investigation is so urgent. Rare-earth oxide is an important and complicated oxide group, possessing the rich physical and chemical properties. Thus, they have attracted much attention and been widely used in scientific and enginnering fields. Rare-earch oxide group includes La group oxide, Z from 57 to 71, as well as Sc(Z=21) and Y(Z=39) oxide due to the similar external electron structures. Rare-earth oxide group presents the common properties and distinct features which is from the difference of electronic structure on 4f orbit. Due to the many advantages of yttrium oxide, such as the superior stability in high temperature, the high and stable strength, the compatibility with infrared materials et al, yttrium oxide becomes the potential film material for anti-reflection and protection of infrared windows and domes, provoking many activites o n yttrium oxide film fundamental research and infrared applications.Compared with bulk material, 2-D thin film material presents distinctive properties due to the huge differnence between the thickness direction and the others. The film properties nol only rely on the material system, but also depend on material synthesized methods and growth conditions. Reactive magnetron sputtering becomes one of the most indispensable film-synthesis methods due to the advantages, such as high deposition rate, good film quality, the easy equipment and its scalability. In this work, reactive sputtering was used to grow films. After that, we studied the influence of growth conditions on the microstructures and properties of yttrium oxide films. Finally, yttrium oxide film was studied to use the anti-reflective and protective coatings of infrared windows and domes. Therefore, the whole work was conducted as the following steps.Firstly, the strong hysteresis loop of metal yttrium target was measured when sputtering metal target in the oxygen atmosphere. The hysteresis loop can be tuned by argon pressure and pumping speed. The hysteresis loop can be totally removed under high pumping speed(500 l/s) instead of the influence of argon pressure, obtaining the three stable sputtering processes: metallic, transition and poisoned mode. The results of theoretical simiulation show that the process of oxidized layer formation on the target can be slowed down under high pumping speed, resulting in the free hysteresis loop.Secondly, using direct current magnetron sputtering(DCMS), we studied the influences of sputtering mode and different substrate areas on film strctures and properties. The results show that the deposition rate is dependent on the different sputtering modes and the material yield. Yttrium oxide film prefers to grow cubic(111) orientation at metallic mode, however, the orientation turns into cubic(421) direction under poisoned mode. The film shows the mixture of cubic and monoclinic phases in the transition mode. The film shows the main cubic(111) orientation in the center and monoclinic(40-2) orientation in the edge. It demonstrates the low oxygen pressure could cause monoclinic phase instead of plasma uniformity.Using radio frequencymagnetron sputtering(RFMS), we studied the influences of oxygen pressure and the combination of temperature and bias voltage on the film growth and structures. The hysteresis loop is less obvious than that using DC sputtering. The transition from crystalline cubic(111) to amorphous s tate can be observed when the oxygen flow rate increases. Negative bias voltage makes the films in two different growth states: regular and etching state regardless of temperature. When fixed the oxygen pressure, the film prefers to grow cubic phase at high temperature and low bias, however, it changes into the monoclinic phase at high bias and low temperature, the creation and recombination of defects can be controlled by temperature and bias. The out-of-plane O/Y/O periodical arrangement transforms into O/Y causes the transformation from cubic to monoclinic phase combined with the worse crystallinity.The films grow as columar structure at metallic mode, but this phenomenon disappears at poisoned mode. The column structure depends on the crystallinity rather than crystal phase. As for the surface roughness, it is dependent on both the sputtering mode and bias voltage. The film shows rough surface under metallic mode, however the surface swithes into flat surface under poisoned mode. In regular area, the film has large roughness, but it turns the opposite side under etching area.After that, the optical, mechanical, wettable and electrical properties of yttrium oxide films were studied. The optical properties closely correlate with the crystallinity. The film has large refractive index under metallic mode, but the role reverses under the poisoned mode. The enhanced refractive index is attributed to high temperature and bias voltage. It can be realized by high crystallinity driven by temperature and the mixed role of both the crystallinity and transformation from cubic to monoclinic phase driven by bias voltage. The packing density of films has the similar change compared with the variation of refractive index, but the porosity has the opposite trend.Films at metallic mode have superior hardness, modulus and the H/E ratio compared with these at poisoned mode. High temperature and bias voltage could enhance the mechanical properties. The excellent mechanical properties are from the high crystallinity of out-of-plane direction and fewer defects strengthen the mechanical properties.The two factors, i.e. surface chemistry and roughness, determine the surface wettability. As for the physical-vapor-deposition films, the surface chemstry is the critical factor rather than roughness. The wetting angel of the both water and glycol decreases when the surface shows oxgen-deficient state, since it is easy to bond with OH. High temperature and bias voltage could play the positive role in the enhancement of dielectric properties. Temperature has larger driven force than bias voltage since cubic phase has less defects than monoclinic phase.Finally, we focus on the anti-reflective and protective applicaions of yttrium oxide films on infrared windows. Both the theoretical an d experimental results show that the transmittance of film/Zn S/film is as high as 93%. XPS profile results demonstrate the less absorved oxygen in the interface, which can be realized by high bias voltage, could enhance the adhesive strength of film/Zn S. Y ttrium oxide film could effectively prohibit the emissivity of Zn S and sapphire especially at high temperature when coated on them.