Study On The Relationship Between Microstructures And Properties Of Ba-hexaferrite Thin Films

With the development of microwave technology, the next generation of microwave devices, such as circulators, isolators, phase shifters and filters, require that the ferrite materials employed in them must be planar, non-reciprocal, self-biased, low loss and so on. Owing to their high saturation magnetization, large uniaxial magnetocrystalline anisotropy field, high resistivity and dielectric constant, high chemical stability and mechanical durability, M-type Ba-hexaferrite(BaFe12O19, BaM) thin films have been considered to be the most potential candidate for the next generation of magnetic microwave devices. Therefore, the preparation of BaM thin films having large uniaxial magnetocrystalline anisotropy field, high saturation magnetization(Ms), high remanence ratio(Mr/Ms) and low ferromagnetic resonance linewidth(△ H) has become a hot research topic.In this dissertation, Ba M thin films were deposited by radio frequency(RF) magnetron sputtering. The effects of the preparation process on the microstructure, magnetic properties and residual stress of the film were investigated and discussed. Then based on these, BaM thin films having large uniaxial magnetocrystalline anisotropy field, high saturation magnetization, high remanence ratio and lower ferromagnetic resonance linewidth were obtained. The main research contents and results are as follows:BaM thin films were directly deposited on Si(100) substrates by using a RF magnetron sputtering system. The process parameters for preparing BaM thin films having perpendicular c-axis orientation were investigated. Experimental results indicate that appropriate sputtering power, gas pressure and the substrate temperature can make the sputtered particles obtain enough energy, then migrate on the substrate surface and move to the correct position to form a stable structure. However, too high or too low sputtering power, gas pressure and substrate temperature leads to an increase in film internal defects, grain size unevenness generated and flatness decline, which are not beneficial to the perpendicular c-axis orientation of BaM thin films. With the oxygen partial pressure ratio increasing from 0% to 8% during deposition, the films change from a state of oxygen deficiency to oxygen excess, which then results in the different properties of BaM thin films. When optimization of process parameters is as follows: sputtering power of 140 W, sputtering pressure of 1.4 Pa, oxygen partial pressure of 1%, substrate temperature of 300 ℃, and then annealed in air at 800 ℃ for 2 h, the prepared BaM thin films have better microstructure and c-axis oriented perpendicular to the film plane.By studying the influence of different types of substrates and buffer layers on the microstructure and properties of the BaM thin films, it is found that using thermally oxidized silicon(SiO2/Si) or single crystal Al2O3(001) as substrate can effectively prevent diffusion of atoms between the substrates and BaM thin film, thereby improving the microstructure and magnetic properties of the BaM thin films. By introducing a 20-nm-thick AlN(001) or BaM layer acting as interfacial buffer layer, the number of nucleis tend to form c-axis perpendicularly oriented BaM grains increases. Thus, the growth of platelet-like grains is significantly enhanced, and the crystallographic texture and perpendicular c-axis orientation of the double layered film are effectively improved.Via Raman spectrometer and XRD, BaM thin films deposited on different substrates and buffer layers have been analyzed. The results show that by introducing a 20-nm-thick BaM layer acting as interfacial buffer layer, the E1 g symmetry are not detected in Raman spectra of the BaM/BaM(20 nm)/Al2O3(001) and BaM/BaM(20 nm)/SiO2/Si(100) films, indicating that both the films are highly c-axis oriented perpendicular to the film plane. For the not annealed and annealed BaM/BaM(20 nm)/Al2O3(001) films, XRD shows that the not annealed film is amorphous, but their Raman spectra are nearly the same, indicating that the not annealed BaM/BaM(20 nm)/Al2O3(001) film is not totally amorphous and there are microcrystallites already present in the film. These microcrystallites are ranging from 10 to 40 nm and can not be detected by XRD, but their crystal orientation determine the c-axis orientation of crystal grains generated in the subsequent annealing.In the BaM thin films with different thickness, by comparing their microstructure and magnetic properties, the residual stress are calculated and their formation mechanism are analyzed. It is found that the crystallographic, morphological and magnetic properties of BaM thin films are strongly dependent on the film thickness. For the film thinner than 150 nm, the residual stress is mainly composed of epitaxial stress. Owing to the strain effect caused by the substrate, the BaM amorphous precursor prefer the growth of the epitaxial and are crystallized into platelet-like grains having their c-axis oriented perpendicular to the film plane, which then result into the excellent perpendicular c-axis orientation of the film. With the increase of the film thickness, the number of defects, dislocations, and oxygen vacancies generated in the BaM thin film increases, the epitaxial stress gradually get released while the intrinsic stress is increased. Thus, c-axis randomly oriented grains gradually increase in the film. For the film thicker than 200 nm, the residual stress in the film is mainly manifested as intrinsic tensile stress, the number of grains with c-axis randomly oriented further increases, then the perpendicular c-axis orientation and magnetic properties of BaM thin films deteriorate.By using multilayer sputtering and ex-annealing processes, multilayer(BaM/BaM)n(n=1~7) films were prepared on Al2O3(001) substrates. XRD and FESEM analysis show that most of the grains in the multilayer films are uniform platelet-like grains having their c-axis oriented perpendicular to the film plane. The static magnetic and microwave properties measurement demonstrate that for the multilayer(BaM/BaM)n(n=1~7) films, the Ms is 320~345 kA/m, the Mr/Ms is 0.81~0.92, the △ H is 1.51~3.50 kA/m(at 55~67 GHz). Especially, when n=1, the film has the smallest △ H, which is 1.51 kA/m at 67 GHz. When n=7, the film thickness reaches about 1050 nm, it has higher saturation magnetization(Ms=335 k A/m) and remanence ratio(Mr/Ms=0.81), and the ferromagnetic resonance linewidth is 2.08 kA/m at 67 GHz.