Studies On Aluminum Nitride Ceramics And Surface Metallization

Aluminum Nitride is an ideal substrate and packaging material applied in high power devices, circuits and modules due to high thermal conductivity. In this paper, AlN ceramic and thick film substrates with high thermal conductivity were prepared by adding Y₂O₃, CaO and Y₂O₃-CaO sintering agents, and influences of composition, tape casting process, pressureless sintering on microstructure and physical properties were investigated. Surface metallization of the covalent-bonded AlN ceramics was studied by using two pastes, i.e.CaO-B₂O₃-SiO₂-BaO glass-bonded and TiBB₂ reaction-bonded Ag pastes. Phase compositions, hot processing procedures and microstructures of AlN ceramics with and without metallization were characterized by XRD, DTA-TG,SEM. Thermal conductivity, sheet resistance and adhesive strength were measured by laser flash method, four probe technique and tensile test. The obtained AlN ceramic substrates possess excellent thermal conductivity and reliable adhesion between metal and itself.Investigation into four AlN powders revealed that the AlN powder synthesized by carbothermal reduction method had fine particle size, narrow size distribution and granular morphology. Processing features were superior to that of other AlN powders. The final microstructure was uniform and thermal conductivity achieved ₂48W/m.K.Tape casting of AlN substrates adopted a non-aqueous solvent system with PVB binder. Surface smooth, dense and uniform green sheets of AlN could be achieved under carefully controlling rheology, viscosity and other processing parameters.In the study of sintering aids, CaO displayed the best improvement to sinter AlN ceramics, and Y₂O₃-added AlN ceramics had the highest value of thermal conductivity. The complex addition of Y₂O₃-CaO combined the above both characteristics. Sintering temperature and holding time played important role on density and thermal conductivity. Proper increase of sintering temperature and holding time could improve the density and thermal conductivity of AlN ceramics.In the study for CaO-B₂O₃-SiO₂-BaO glass, adhesive strength between AlN substrates and conducting layer was the highest and reached 11.74MPa when glass content was 10% under constant solid content of metal paste. With increasing of glass content, sheet resistance became large. The sheet resistance was above 46 m?/â–¡as glass content of 15%. The wetting behavior of CaO-B₂O₃-SiO₂-BaO glass to AlN substrates revealed that glass tended to penetrate into surface layer of AlN and diffuse into AlN lattice at 850℃.In the study of reaction-bonded thick film, when the content of Ag,TiB2,Co3O4 and organic carrier in metal paste was 72.8%, 1.5%, 0.7%, and 25%, respectively, and sintering condition was from room temperature to 550℃in air, then to 850℃holding for 15 minutes in 95% N2, the resultant metal layer was dense and smooth with adhesive strength of 12.7MPa and sheet resistance of 5.2 m?/â–¡.