Research On Properties And Mechanism Of Substrate Materials With Low Thermal Expansion Coefficient

With developing technology, micro-electronic devices are developing in the direction of high integration and miniaturization, which puts forward new requirements for the substrate materials. After years of research, some foreign research institutions have implemented the LTCC substrate material business, but in this field in C hina are still in the stage of catching up. Therefore, the preparation of the materials that could meet the requirements of LTCC substrate, has become an extremely important topic. Li2O-Al2O3-SiO2(LAS) and CaO-Al2O3-SiO2(CAS) glass-ceramics which have with low coefficient of thermal expansion, low dielectric constant could be seen as candidate used in LTCC substrate materials. So LAS and CAS glass-ceramics were prepared by the melt-quenching method and solid-state reaction method, respectively. The effect of glass composition and the sintering procedure on the crystal precipitation, microstructure, sintering and properties are studied.1. Li2O-Al2O3-SiO2 glass-ceramic1) The addition of MgO can reduce the crystallization te mperature of the glass ceramic, it is conducive to the densification sintering and promotes the precipitation of the crystalline phase of CaMgSi2O6. With the addition of MgO, the bending strength of glass-ceramic increases from 75 MPa to 155 MPa. By adjusting the doping amount of MgO and the sintering procedure, the type and content of crystal phase can be adjusted, and the coefficient of thermal expansion can be increased from 0.5 to 4.3 * K-1·10-6.2) The glass transition temperature and crystallization temperature of the glass-ceramic decrease with the increasing of ZnO. Excess ZnO addition will promote the precipitation of the crystalline phase of ZnAl2O4. A moderate addition of ZnO will increase the bulk density of the material, and is beneficial to densification sintering of glass-ceramic. The best sintering temperature is 800 oC, and the best sintering time is 30 min. Because the big radius of Zn2+, it will hinder the movement of the alkali metal ion, which could reduce the dielectric loss of the mater ial. The properties of crystal phase will affect the bending strength of glass-ceramic, the formation of ZnAl2O4, which has a large elastic modulus, will increase the bending strength of glass-ceramic. Moreover, the ZnAl2O4 phase has a big coefficient of thermal expansion, so the increase of ZnO content will increase the coefficient of thermal expansion of glass-ceramic.3) Because the CaO could depolymerize the network of glass, the increase of CaO content will increase the crystallization tendency of the material, and it is benefit to the precipitation of CaMgSi2O6 phase. The addition of CaO could lower the viscosity of the glass and increase the liquid content which is conducive to densification sintering. Due to the difference of the dielectric constant between the CaMgSi2O6 phase and the main crystal phase of the glass-ceramic, the addition of CaO has a little effects on the dielectric constant of the material, and the dielectric constant of glass-ceramic mainly effect by the microstate. The increase of CaMgSi2O6 content also lead to the increase of material coefficient of thermal expansion.4) By doping oxide and the adjusting of sintering procedure, the thermal expansion coefficient of LAS glass-ceramic could accomplish serialization. The sample sintered at 800 oC for 30 min has the best properties: high bending strength(155 MPa), low dielectric constant(7.2), low dielectric loss(2*10-3) and low coefficient of thermal expansion(3.2*10-6·K-1) matching that of silicon. The prepared LAS glass-ceramic could meet the requirements of LTCC substrate materials.2. LAS glass-ceramic+Al2O3 ceramic compositeThe bending strength of glass-ceramic which has a low coefficient of thermal expansion will increase as the introducing of the crystal phase with a high coefficient of thermal expansion. For the objective of increasing the bending strength, we prepared the LAS glass-ceramic+Al2O3 ceramic composite. The liquid phase deriving from the glass is benefit to the densification sintering, so the reduction of glass leads to the increase of crystalline temperature and the increase of sintering temperature. With the increasing Al2O3 content, the Al2O3 phase with a high dielectric constant and high coefficient of thermal expansion precipitates in the glass-ceramic, which will leads to the increase of the bending strength, dielectric constant and coefficient of thermal expansion of the glass-ceramic. The sample sintered at 800 oC for 30 min has the best properties: high bending strength(173 MPa), low dielectric constant(8), low dielectric loss(2.4*10-3) and low coefficient of thermal expansion(2.9*10-6·K-1) matching that of silicon. The prepared composite could meet the requirements of LTCC substrate materials.3. CAS ceramic1) The increase of Al2O3/SiO2 ratio will lead to the reconnection of the broken glass network, which will make it difficult for crystallization in ceramic. It also lower the viscosity of glass and reduce the shrinkage rate of ceramic. The change of the glass composition and sintering procedure result in the variation of the type and content of crystal phase in ceramic, which will affect the properties of the material.2) The addition of ZnO can weaken the network structure, which can reduce the glass transition temperature and the crystallization temperature, and lead to the increase of the precipitation of quartz and anorthite. With the increasing ZnO content, the crystal phase with high coefficient of thermal expansion leads to the increase of the thermal expansion coefficient of ceramic.3) By changing the glass composition and sintering system, we can adjust the type and content of the crystalline phase, which will affects the performance of ceramic. The sample sintered at 900 oC for 2 h has the best properties: high bending strength(145 MPa), low dielectric constant(5.5), low dielectric loss(3.4*10-3) and low coefficient of thermal expansion(5.3*10-6·K-1). The prepared ceramic could meet the requirements of LTCC substrate materials.4. The calculation of coefficient of thermal expansion and the study of crystal ization kinetics1) According to the analysis of crystal form from XRD, we get the content of the crystal phase and glass phase. This is helpful for the calculation of thermal expansion coefficient of glass-ceramic. The calculated values are close to the measured value, which means that we can get desired coefficient of thermal expansion through controlling the type and content of crystalline phase.2) The crystallization activation energy of LAS glass-ceramic increases from 143.1 to 219.5 kJ/mol with the doping of ZnO. The Avrami constant n is in the range of 3.1 to 3.5, which means that the crystallization mechanism for LAS glass-ceramic is bulk crystallization. The crystallization activation energy of CAS glass-ceramic increases from 339 to 275 kJ/mol with the doping of ZnO. The Avrami constant n is in the range of 1.41 to 1.82, which means that CAS glass-ceramic is surface crystallization mechanism.