Development of dielectric materials for low temperature co-fired ceramics (LTCC) application
Creators: name: Wang, Xiaozhou
Publisher: name: The Australian National University
Year: 2021

description: Low temperature co-fired ceramics (LTCC) technology allows constructing three-dimensional structure of electronic components and facilitates the assembly of devices made of different types of materials and presented with different functions. The development of high performance LTCC materials is a big challenging in this field due to the limitation of the processing conditions and the selection of potential candidate materials. The controllable stability and repeatability are also critical issues. This thesis is to challenge these difficulties by conducting systematic research on the chemical composition, phase formation, structure, electrical and thermal properties of potential candidate materials to develop a series of LTCC materials with optimal performance. In this thesis, the novel sol-gel process is introduced to prepare the amorphous phase, rather than using traditional molten glass, for better chemical composition control. Two amorphous materials, Amorphous phase-1 (AP-1) and Amorphous phase-2 (AP-2), are synthesized in the temperature range of 600oC and 700oC. The final chemical composition of AP-1 is 79.51% SiO2-0.81% Na2O-2.41% K2O-2.50% CaO-14.77% B2O3, and that of AP-2 is 80.78% SiO2-0.27% Na2O-0.87% K2O-3.45% CaO-5.25% BaO-9.38% B2O3 (mole ratio). The processing condition is optimized to reduce residual carbon and avoid crystallisation out of the amorphous phase. The controllability of low-melting chemical components in amorphous phases will reduce the sintering temperature of LTCC materials in a manageable manner. The sol-gel derived amorphous phases are used to develop the LTCC composites with dielectric constant at different values and optimized thermal expansion property. As results, the hexagonal Al2O3 is mixed with 50.19wt% AP-1 and 46.65wt% AP-2, respectively, to form the composites which show dielectric constant of 7-8 and the coefficient of thermal expansion (CTE) of ~6ppm/oC when sintered between 850oC and 900oC for 6h. The composite made up of 60wt% monoclinic ZrO2 and 40wt% AP-1 presents dielectric constant of ~11 and CTE of ~6ppm/oC when sintered at 850oC for 0.5h. The composite with the composition of 42.86wt% AP-1+52.38wt% fused SiO2+4.76wt% Al2O3 exhibits dielectric constant of ~5 and CTE of 3.26ppm/oC when sintered at 850oC for 6h. This series of LTCC materials with adjustable dielectric constant and appropriate thermal expansion property can provide a flexibility in electronic circuit designs. Besides the sol-gel derived amorphous phases, we also introduced a cubic KBSi2O6 phase with simpler composition as the precursor to develop the LTCC materials for the first time. Different from the amorphous phase forming vacuous liquid to densify the composites, this silicate exhibits good tolerance for ionic substitutions (such as Al substituting B), and thus lead to intimate contact of phases in the sintered sample to achieve well densified microstructure. As the result, the composition of developed sample is Al2O3: KBSi2O6: CaO=1.07: 1: 0.4 (mole ratio). It shows dielectric constant of ~8 and CTE of 11.04~12.46ppm/oC when sintered between 850oC and 900oC. Overall, utilizing the sol-gel derived amorphous phases and unique crystalline KBSi2O6 phase as the precursors enables controllable design of LTCC materials. As the result, through systematic investigation presented in this thesis, we have developed five LTCC materials with tuneable dielectric constant in the range of 4-12. These materials show dielectric loss below 0.003 and resistivity above 10^12 ohm centimeter. The thermal expansion property is also adjusted to be in the acceptable range to match with the silicon, alumina substrates or printed circuit board. All designed LTCC materials can be co-fired with silver electrode at the temperature range of 850-900oC without any internal diffusion. Also industrial trial has been conducted by industrial collaboration partner which confirmed the validation of these LTCC materials.