| Compared with low-temperature co-fired ceramic?LTCC?and printed circuit board?PCB?embedded integrated capacitors,microwave dielectric thin film integrated capacitors based on passive integration technology are more suitable for the miniaturization of electronic equipments due to their higher capacitance density,more excellent microwave performance and compatible fabrication techniques with the Si-based semiconductor process.In general,the film-forming temperature of low-temperature-sintering microwave dielectric materials is lower than that of high-temperature-sintering materials;therefore,low-temperature-sintering microwave dielectric materials always keep advantages over high-temperature-sintering materials in fabrication of thin film integrated capacitors.The objective of the present work is to obtain low sintering/annealing temperature dielectric ceramics and films with near zero resonant-frequency-temperature-coefficient??f?.Aimed at the compounds in Bi2O3-MoO3 binary system,the studies on the microstructure,phase composition and formation mechanism of low-temperature-sintering microwave dielectric ceramics,and the films prepared by thermal oxidation method were conducted systematically.The main results were outlined as follows:Bi2MoO6,Bi2Mo2O9 and Bi2Mo3O12 compounds were synthesized by conventional solid-state reaction route with Bi2O3 and MoO3 as the starting materials.X-ray diffraction?XRD?analysis demonstrated that Bi2Mo3O12 would appear as intermediate phase during the synthesis process of Bi2MoO6 and Bi2Mo2O9 as the temperature was below 540 °C and 610 °C,respectively;but no intermediate phase was detected during the synthesis process of Bi2Mo3O12.The optimum sintering temperature was 800860 °C for Bi2MoO6,640655 °C for Bi2Mo2O9,and 635650 °C for Bi2Mo3O12.The microwave dielectric properties were ?r=26.527.8,Qf=1350013600 GHz,?f=-95-98×10-6/°C for Bi2MoO6;er=34.836.8,Qf=1120011500 GHz,?f=5045×10-6/°C for Bi2Mo2O9;?r=19.220.3,Qf=2640026800 GHz,?f=-155-135×10-6/°C for Bi2Mo3O12.Obviously,the ?f values of these compounds still can't meet the requirement for fabrication of microwave capacitors.To make ?f ?0,doping elemental oxides with opposite ?f value is one of the most common ways.For Bi2MoO6 ceramics sintered at 860 °C and 880 °C,the ?f values were 2.3×10-6/°C and-1.6×10-6/°C by the addition of 20 mol % Sm2O3,respectively.For Bi2Mo2O9 ceramics sintered at 640 °C and 660 °C,the ?f values were adjusted to 2.2×10-6/°C and 1.7×10-6/°C by doping 8 mol % MgO,respectively.For 0.22 Bi2Mo3O12-0.78 TiO2 ceramics sintered at 690 °C and 710 °C,the ?f values were 1.9×10-6/°C and 1.4×10-6/°C,respectively.Unfortunately,although the ?f values were close to zero,some new problems,such as the sintering temperature increasing and the sintering density decreasing,were aroused.To improve the strategy,Bi2MoO6 and Bi2Mo3O12 with negative ?f values and Bi2Mo2O9 with positive ?f value were mixed and sintered,respectively.Interestingly,the results indicated that Bi2MoO6 and Bi2Mo3O12 did not react with Bi2Mo2O9.The change of microwave dielectric properties follows Lichtenecher's mixing rule.In Bi2MoO6-Bi2Mo2O9 mixing system,as the content of Bi2MoO6 was 30% in weigh percent,the ceramics sintered at 640 °C was dense,the microwave dielectric properties were ?r=30.4,Qf=10777 GHz and ?f=1.6×10-6/°C.In Bi2Mo3O12-Bi2Mo2O9 mixing system,as the content of Bi2Mo3O12 was 15% in molar percent,the ceramics sintered at 640 °C was dense,the microwave dielectric properties were ?r=31.7,Qf=12200 GHz and ?f=2.1×10-6/°C.Clearly,the new strategy is superior to the old one,and is more suitable for the fabrication of dielectric thin film capacitors.The studies on the fabrication of thin-film capacitors were carried out.Bi/Mo multilayer thin films were deposited by direct current magnetron sputtering at first,and then were annealed at O2 atmosphere to prepare bismuth molybdate thin films.As the Bi/Mo molar ratio was 2:3,it was found that the phase composition transformed gradually from Bi2O3 and MoO3 to Bi2MoO6 and Bi2Mo3O12 when the annealing temperature increased from 420 to 480 °C,and eventually Bi2Mo3O12 and Bi2MoO6 coexisted at 450 °C and 480 °C.With the annealing temperature increasing,the capacitance density of the thin film capacitor increased,the leakage current density?Id?at 6 V decreased,the capacitance temperature coefficient??c?was about +310×10-6/°C.As the Bi/Mo molar ratio was 2:1,dense dielectric thin films were obtained at the annealing temperature of 400600 °C.The phase composition at 400 °C was Bi3.64Mo0.36O6.55,Bi2Mo2O9 and Bi2O3,no Bi2MoO6 was detected.The volume fraction of Bi2MoO6 increased rapidly at the annealing temperature of 400500 °C.As the temperature was up to 500600 °C,the volume fraction of Bi2MoO6 reached a stable value of 90%.The er increased slightly with the annealing temperature increasing.At 1 kHz,the er of thin films annealed at 500 °C reached the maximum value of about 37.5,and tan? was 1.06%;at 12 V?electric field strength of about 18.2 kV/mm?,Id=1.46×10-7 A/mm2;at the applied voltage range-12 +12 V,the capacitance kept unchanged.Based on the low-temperature sintering characteristics of Bi2MoO6,Bi2Mo2O9 and Bi2Mo3O12,bismuth molybdate thin films were prepared successfully by annealing of Bi/Mo multilayer thin films at low temperature under O2 atmosphere.The thermal oxidation method was expanded from the preparation of single metal oxide thin-film to binary metal oxide thin-film,which offers some enlightenment to the preparation of multi-element metal oxide thin-films.The dielectric thin-film preparation by thermal oxidation method is a promising approch due to its simple technology and high efficiency.The bismuth molybdate dielectric thin-films prepared by thermal oxidation method at low temperature are expected to be applied to the fabrication of thin film integrated capacitors. |
PDF Full Text Request