Synthesis And Properties Of Barium Strontium Titanate Thin Films On Copper Foils

With the development of the miniaturization and muti-function of electronic products, Barium strontium titanate (BST) thin film is a key material for the development of embedded capacitors which perform the charge-storing function but do not require space on the surface of the Printed-Circuit-Board (PCB). Most of this work however, has utilized deposition of films on refractory or noble metal substrates. As the competition of the electronics market become stronger, substrate requires inexpensive materials and a flexible material that will facilitate lamination into printed wiring boards. Copper foils are the optimal choice because of its excellent properties. However, acquiring a useful BST thin film on the flexible copper foils is a great challenge, due to the ease of copper oxidization, cracking, poor crystallinity, and poor adhesion of the films to the flexible copper foil. In 2006, Laughlin et al had reported that BST thin film was prepared by sputtering. Up to now, however, there is rarely more report about thin film prepared on copper foil.In this paper, BST thin films are prepared on copper foil by sol-gel, followed by annealing in almost inert atmosphere. Barium acetate, strontium acetate and titanium butoxide are used as starting materials. PEG was directly added to the BST sol precursor to solve the cracking of BST thin film. The electric properties of BST thin films are improved by adding La₂O₃ buffer layer between BST thin film and substrate, or adding dopant.(1) The properties of BST thin films which are prepared from BST precursor modified by 20wt%, 30wt %, 40wt %, 50wt %, 60wt %, 70wt %PEG200 are analyzed by DTA-Tg, XRD and the electrical measurement system. The results indicate that the properties of the BST thin films made from the BST solution modified by 40wt % and 50wt % PEG200 are the optimal among these thin films. Their dielectric constants are 421 and 291 respectively, their dielectric losses are 0.11 and 0.108 respectively at 1 MHz. Additionally, the leakage current density is also much lower, which is about 2000uA/cm² for the BST thin film made from the BST solution modified by 40wt % at 116.7 kV/cm (7V). Therefore, the optimal amount additive for PEG200 is around 40%.(2) It is the first time to use La₂O₃ as the buffer layer between BST thin film and substrate. XRD and FESEM indicate that La₂O₃ buffer layer is helpful for the crystallization of BST thin film. The dielectric properties of Ba0.7Sr_0.3TiO₃ thin film is not obviously improved, but the dielectric properties of Ba_0.5Sr_0.5TiO₃ thin film is obviously improved by La₂O₃ buffer layer. For example, the dielectric constant of Ba_0.5Sr_0.5TiO₃ thin film without La₂O₃ buffer layer annealed at 750℃at 1 MHz is 141, whereas, Ba_0.5Sr_0.5TiO₃ thin film with La₂O₃ buffer layer is 248. The reason is possibly that La³⁺ enter into the lattice of BST, which make the Curie temperature (Tc) move to the lower temperature for Ba0.7Sr_0.3TiO₃, whereas to the higher temperature for Ba_0.5Sr_0.5TiO₃, which is proved by the hysteresis loop. XPS show that Ti³⁺ is present in BST thin film because of a lot of oxygen vacancies in the thin film. La₂O₃ buffer layer decrease the concentration of oxygen vacancies in the thin film (which can be seen from the intensity of Ti³⁺peaks between the two BST thin films), which is helpful for the decrease of the leakage current density of BST thin film.(3) It is the first time to study the A-site Mn-doping effect on the microstructure and electrical properties of BST thin film. XRD supports that Mn occupies the A site of perovskite ABO3 structure in BST thin film. And Mn doping to BST thin film obviously suppress the secondary phase of barium titanate to form. The Ba_0.7-xSr_0.3Mn_xTiO₃ (x=0.025) thin film have the higher dielectric constant and lower dielectric loss which are 1213 and 0.06 respectively at 1MHz. Additionally Mn doping to BST thin film move the Tc, which make Ba_0.7-xSr_0.3Mn_xTiO₃ exhibit the paraelectric behavior at room temperature, and make Ba_0.5-xSr_0.5Mn_xTiO₃ exhibit the ferroelectric behavior at room temperature. This is confirmed by the hysteresis loop. The leakage current density of Mn-doped BST thin film is greatly lower than that of BST thin film, from 2.5×10⁴ uA/cm² for undoped BST thin film to 100uA/cm² for Ba_0.675Sr_0.3Mn_0.025TiO₃ thin film.(4) XPS indicates that Mn ions in the thin film exist as Mn²⁺, with a small amount of Mn³⁺. Mn ions decrease the concentration of oxygen vacancies (V..) in the thin film through Mn³⁺ which attracts the electrons, which is confirmed by Ti³⁺ peak hardly appearing in Mn-doped BST thin film. The optimal amount of Mn doping in Ba_0.7-xSr_0.3Mn_xTiO₃ is x=0.025.(5) Zr doping to BST occupies the B site of perovskite ABO3 structure in BST thin film. XRD indicates that the diffraction peak of Ba0.7Sr_0.3(Ti_1-xZr_x)O₃ shift towards smaller 2θvalue with increasing Zr content, and that Zr doping to BST obviously suppress the secondary phase of barium titanate to form and reduce the compressive strain of the thin film from1.05 for x=0 to 0.403 for x=0.2. FESEM shows that the grain size of the thin film decreases with increasing the Zr content. HRTEM indicates that 90°and 180°domain coexist in the thin film.(6) Zr doping to BST thin film greatly decrease the dielectric loss of the thin film over the applied field, from 0.364 for x=0 to 0.09 for x=0.1 at 333 kV/cm and 10 kHz. In addition, the electricity breakdown voltage of BSZT thin film increases with increasing Zr content. The optimal amount of Zr in Ba0.7Sr_0.3(Ti_1-xZr_x)O₃ thin film is x=0.1.