Study On LTCC Microwave Dielectric Materials And Application Of Low Permittivity And Low Loss

With the rapid development of 5G communication,Internet of things,wearable devices,vehicle-mounted radar and medical electronic devices,the miniaturization,integration and multifunction of electronic devices are required.Low temperature co-firing ceramics?LTCC?technology is an important approach to realize system packaging because of its high integration and independent design.For LTCC substrate materials,it is required to low dielectric constant??_r?,high quality factor?Q×f?,near-zero resonance frequency temperature coefficient??_f?and low sintering temperature??950oC?.In order toachieve low temperature sintering and maintain good microwave dielectric properties,the CaMgSi₂O₆ and Mg₂SiO₄ microwave dielectric ceramics,which have a low dielectric constant?⁷.5 and 6.8?,were investigated.In order to satisfy the application of LTCC substrate,low-temperature sintering,high Q×f value,as well as near-zero?_f value were investigated by optimizing the sintering aid and doping ions.The relationship between the microstructure and the macroscopic dielectric properties of the material was revealed through XRD refinement,Raman spectrum,XPS analysis methods,as well as the first principle?DFT?analysis.The details are as follows:?1?The influence mechanism of sintering aid on the microstructure and microwave properties of dielectric ceramics was revealed.Low temperature sintering and low loss of CaMgSi₂O₆ and Mg₂SiO₄ microwave dielectric ceramics were realized.The CaMgSi₂O₆ and Mg₂SiO₄ ceramics could be sintered at low temperature sintering?900°C?through the LiF addition,and the excellent microwave dielectric properties of CaMgSi₂O₆ and Mg₂SiO₄ were also achieved.Compared to pure CaMgSi₂O₆ samples with sintering at 1250oC,CaMgSi₂O₆–2 wt%LiF ceramic was sintered at low temperature?900oC?,and it has excellent microwave dielectric properties:?_r=7.45,Q×f=64800 GHz and?_f=–34 ppm/°C.High Q×f value is mainly caused by high relative density and Li⁺ions as the acceptor dopant.The?_f value and MgO₆ octahedral distortion are positive correlation.Adding of 8 wt%LiF can realize that the sintering temperature of Mg₂SiO₄ ceramics reduced from 1400oC to 900oC,also has excellent microwave dielectric properties:?_r=6.1,Q×f=109719 GHz and?_f=–61 ppm/°C.The?_f value is also positive correlation with the MgO₆ octahedral distortion.Compared with the sintering temperature and Q×f value of CaMgSi₂O₆–x LiF and Mg₂SiO₄–xLiF ceramics,we found that adding LiF is an effective approach to achieve both the low sintering temperature and keeping good microwave dielectric properties.This method can be widely used in low temperature sintering of microwave dielectric ceramics and is an important technical approach to realize the LTCC application.?2?Combined with the ion occupation and structural distortion,the correlation between ion substitution and the dielectric properties of materials was revealed,which effectively guided the design of material formulation,and realized the controllable adjustment from the material microstructure to the dielectric properties,as well as obtained LTCC microwave dielectric ceramics with high Q×f value.CuO was also served as sintering aids and ionic substitution source in the nominal compositions of CaMg_1-xCu_xSi₂O₆.CuO did not form solid solution with CaMgSi₂O₆,and rich–Cu₂O eutectic phase located at the grain boundary.Moderate CuO can promote the densification of the ceramic.Excess CuO can lead to a phase transition?from C2/c to P2₁/c?in Mg-deficient CaMgSi₂O₆.Enhanced microwave dielectric properties were achieved in the x=0.04 ceramics sintered at 1250°C with?_r=7.41,Q×f=160 100 GHz?two times better than the previously reported values?,and?_f=-42ppm/°C.The high Q×f value is mainly caused by the densification and order structure.In Mg_2-xCu_xSiO₄,Cu²⁺and Mg₂SiO₄ form a solid solution,and Cu²⁺occupies Mg?1?position.The?_r values of the ceramics increase with the increase of Cu²⁺content and sintering temperature due to the effect of ion polarization and relative density.The Q×f values increase firstly and then decrease with the increase of Cu²⁺content and sintering temperature.The Q×f value is mainly determined by crystallinity and relative density.The?_f value changes nonlinearly with the increase of Cu²⁺content,whereas first increases and then decreases with the increase of sintering temperature.At the same sintering temperature,the?_f value of the ceramic was mainly affected by Mg?2?O₆octahedra distortion.The excellent microwave dielectric properties were achieved with Mg_1.92Cu_0.08SiO₄ ceramic sintered at 1250oC:?_r=6.35,Q×f=188500 GHz and?_f=–2.0 ppm/°C.By replacing Mg²⁺in Mg₂SiO₄ with Cu²⁺ion,the microwave dielectric property is regulated from the microstructure,providing important guidance for material formulation design in the future studies.?3?By optimizing the compound scheme of positive and negative?_f values material,obtained with near-zero?_f value and high Q×f composite LTCC material system,realized the material match co-firing with Ag electrode,meet the needs of LTCC technology application.The?_f value of CaMgSi₂O₆–2 wt%LiF and Mg₂SiO₄–8 wt%LiF ceramic is further adjusted by CaTiO₃ ceramic.Near-zero?_f value can be obtained by adding an moderate CaTiO₃,but it could deteriorate the Q×f value.Comparisons with CaTiO₃,the Mg₂SiO₄–8 wt%LiF–45 wt%Li₂TiO₃ ceramic has the excellent microwave dielectric properties:?_r=10.7,Q×f=237400 GHz and?_f=–3.0 ppm/°C.This material has low sintering temperature,near-zero?_f and high Q×f value,wich is one of the candidate materials of millimeter wave device.The CaMgSi₂O₆ and Mg₂SiO₄ have good chemical compatibility with the Ag electrode at low temperature sintering.In addition,LTCC substrate of Mg₂SiO₄–8 wt%LiF–9 wt%CaTiO₃ has the good co-firing matching with Ferro SP0803 silver paste,which meets LTCC application.