Voltage-controlled Oscillator Using Bst Thin Film Varactor Tube Micromachining Study

Perovskite barium strontium titanate (BaxSr1-xTiO3) thin films are potential candidates for application in frequency-agile microwave electronic components due to their large field-dependent dielectric constant. Barium strontium titanate thin film varactor integrating with microwave circuit can effectively reduce the size of circuit, and be applied in the voltage controlled oscillators. For barium strontium titanate thin films using for voltage controlled oscillators with high Q and phase noise, low dielectric loss as well as moderate tunability is essentially required.In this thesis, the dielectric properties of barium strontium titanate thin films using for voltage controlled oscillator have been optimized, and micro-fabrication of barium strontium titanate thin film varactor have been studied. The major conclusions are as follow:1. Low loss Ba_0.5Sr_0.5TiO₃ (BST) thin films have been achieved by optimizing sputting process. As is well known, Mn2+ ions doping was usually thought to be an effective way to compensate oxygen vacancy defects in BST thin films thus to reduce the dielectric loss. By analyzing the energy state of particles during sputtering process, substrate temperature, sputtering power and atmosphere were optimized for superior electrical properties. We have produced the Mn2+-doped BST thin films with a tunability of 50% and dielectric loss of less than 1% on Pt/Ti-coated sapphire substrates.2. In order to further reduce the dielectric loss of BST thin film, a SiN buffer layer was prepared on BST thin film in this paper. Because silicon nitride has a low dielectric loss and could act as a good diffusion barrier, we suggested that the heterolayered structure of BST/SiN thin films with SiN buffer would decrease the overall dielectric loss. It was found that the dielectric loss decreases with the increase of SiN thickness. The BST /SiN bilayered thin films at thickness ratio dSiN/dBST of 0.2 give low dielectric loss with 0.49% and the largest FoM of 50.1, at which the tunability and dielectric loss exhibit an optimum balance. In this paper, the thickness effect of SiN buffer layer on dielectric properties was discussed using a series connection model of multilayered thin film capacitors. 3. The electrical characteristics of BST thin films have been improving using oxygen-plasma treatment technique. The plasma treatment can reduce contaminations and effectively passivate the oxygen vacancy of films. It was found that the capacitance and dielectric loss of the BST films was reduced below 200W. The dielectric loss of BST thin films reduced form 2.4% to 1.1% by 200W and short-duration (5 min) process. When the process power is higher than 200W, the dielectric loss of BST thin films increased because of rough surface morphology.4. The BST thin films varactor used in voltage-controlled oscillator were fabricated using micro-fabricated techniques in this paper. The effective size, capacitance value of the varactor fabricated are 80²00μm2, 0.1pF¹.5pF respectively, and the Q value, tenability of device are 156, 25.6% respectively at the frequency of 50 kHz.