Development of high-k embedded capacitors on printed wiring board using sol-gel and foil-transfer processes

Fabrication of high-kappa embedded capacitors on printed wiring board (PWB) is limited due to the inherent low-temperature process required for the organic packaging. Current dielectrics for embedded capacitors are mosty organic with insufficient dielectric constant. The limitations of the organic and the ceramic routes for fabricating high-k PWB-compatible capacitors can be addressed using a foil transfer process. This is achieved by implementing a modified sol-gel process to grow ultra thin films on a carrier foil that can be laminated onto the printed wiring board. To avoid temperature limitations of organic build-up processes, all high-temperature processing steps required by the oxide dielectric are performed before the embedding process. The goal of this work is to develop a low-cost solution-based process that can substitute for costly MOCVD and RF sputtering processes. Sol-gel was selected because of its inherent ability to provide precise stoichiometry control, and confortnal, continuous, large-area deposition via cost-effective spin or dip-coating. This research focuses on the effect of heat treatment conditions (i.e., heating method and gas atmosphere) on the dielectric properties of barium titanate (BaTiO3) and strontium titanate (SrTiO3) thin films. A novel heat treatment method, i.e., Rapid Thermal Processing (RTP), was investigated. The advantages of this method are improved properties because of the tendency to form more textured films, faster processing times, less metal oxidation, and minimized film substrate interfacial reactions. In our study, base-metal nickel (Ni) and titanium (Ti) foils with 12 micron thickness have been used as carrier substrate to minimize cost. High-kappa BaTiO3 and SrTiO3 capacitive layers that are compatible with low-temperature-processed polymer packages were synthesized using a sol-gel process with barium/strontium 2-ethylhexonate and titanium isopropoxide as precursors. RTP lowers the process time to 3 minutes for the development of a well-crystallized titanate film as opposed to the few hours of processing time required for conventional heat treatment. The oxidation was correspondingly reduced under these conditions as determined by X-ray diffraction (XRD) and Raman spectroscopy studies. Capacitance densities ranging from similar to45-700 nF/cm(2) have been achieved by varying the film thicknesses from similar to250 to 900 nm and by varying the heat treatment conditions. By following the RTP with a 1 hr heat treatment in nitrogen (N-2) atmosphere, the dielectric loss was reduced to 0.005. These films were subsequently integrated onto organic boards using conventional lamination and lithography methods, followed by low-cost wet etching. The dielectric constants of the films from 100 MHz to 8 GHz were measured using a Coplanar Waveguide (CPW) transmission lines with multi-line calibration technique.