Research On Process And Mechanism Of Dissimilar Transient Liquid Phase Bonding Of Al Uminum Nitride And Copper

Third-generation semiconductors such as silicon carbide(SiC)and gallium nitride(GaN)are widely used in high-temperature power devices due to their advantages of high band gap,large chemical bond energy and stable high-temperature crystal structure.Aluminum nitride(AlN)has been considered as a potential high temperature packaging materials due to its excellent insulation property and thermal conductivity as well as its coefficient of thermal expansion(CTE)matching with the third-generation semiconductor.In practical application,it is usually required to cover the surface with copper(Cu)before connecting with the power device.The higher service temperature poses a challenge to the interconnection technology of AlN and Cu.In this research,the indirect brazing method combined with the advantages of ultrasonic effect and transition liquid diffusion(TLP)bonding technology was utilized to propose a high temperature resistant bonding method of AlN and Cu.Initially,a layer of active filler metal was coated on the surface of AlN with the aid of ultrasound.Subsequently,the AlN coated with active filler metal was connected with Cu by TLP bonding,forming the high melting AlN/Cu IMCs joint.The microstructure evolution and interfacial bonding mechanism of AlN/Cu joints were discussed.In this study,a layer of Sn-Al-Cu active filler metal with a thickness about 20?m was coated on the surface of AlN under the atmospheric environment at 350? with the aid of ultrasound.The Sn-Al-Cu active filler metal combines well with AlN but no obvious transition layer or reaction products were observed at the interface of AlN/active filler metal at SEM scale.Transmission electron microscope(TEM)analysis on the interface of AlN/filler metal shows that a mixed transition layer of amorphous alumina(Al₂O₃)and nanocrystalline SnO with a thickness of 20nm was formed.The formation of transition layer can be attributed that Al atoms and Sn atoms in the filler metal react with O atoms which dissolved in the filler metal under the impact of ultrasonic cavitation resulting in the deposition of amorphous Al₂O₃ and nanocrystalline SnO at the interface between AlN and the filler metal,forming a transition layer by the composition of amorphous Al₂O₃and nanocrystalline SnO.Subsequently,the AlN coated by filler metal was connected with Cu at 300? by TLP bonding.With the extension of isothermal time of TLP process,the content of Cu-Sn intermetallic compounds in the weld increased gradually.The Cu₆Sn₅in the weld showed a scallop shape while the Cu₃Sn showed a plane shape.Additionally,due to the addition of Al in the filler metal,the solubility of Cu in Sn is reduced to some content which made the Cu particles precipitated in the filler metal and grew into Cu₆Sn₅which is distributed in the welding seam.Furthermore,the content of Al in filler metal is rare so that no obvious Al-Cu phase was observed.When the isothermal time reached 60 min,the weld seam was mainly composed of large block Cu₆Sn₅grains.When the isothermal time went to 240 min,Cu₆Sn₅ was entirely consumed and transformed into Cu₃Sn with uniform grain size through sloid diffusion.TEM analysis of AlN/Cu₆Sn₅ and AlN/Cu₃Sn interfaces showed that the transition layers with a thickness of about 200nm was observed at both AlN/Cu₆Sn₅ and AlN/Cu₃Sn interfaces.Compared with the AlN/active filler metal interface,the thickness of the transition layer increased and partial crystallization occured.Some?-Al₂O₃ grains appeaed in the amorphous Al₂O₃ layer,and some SnO₂ nanocrystals also appeared in the transition layer.With the extension of isothermal time,the mechanical properties of the AlN/Cu joints decreased.The fracture of the AlN/Cu joints composed of all Cu₃Sn always occured at the interface of AlN/Cu₃Sn and the shear strength of the joint is about 16.7Mpa.In order to investigate the reason for the decrease of mechanical properties of all intermetallic compound joints,the bonding properties of AlN/amorphous Al₂O₃/?-Sn and AlN/?-Al₂O₃/Cu₃Sn interface were studied by first-principles calculation.Through the establishment of AlN/amorphous Al₂O₃/?-Sn and AlN/?-Al₂O₃/Cu₃Sn interface model,the bonding strength of different interfaces was evaluated by the separation work.The separation energy of AlN/amorphous Al₂O₃,amorphous Al₂O₃/?-Sn,AlN/?-Al₂O₃,?-Al₂O₃/Cu₃Sn interface is 0.66J/m²,1.00 J/m²,0.69J/m² and 0.16J/m²,respectively.For the AlN/amorphous Al₂O₃/?-Sn interface,the disordered Al and O atoms in the amorphous Al₂O₃ interface have higher electron state energy distribution and are more likely to participate in interface bonding which is conducive to interface bonding.In the AlN/?-Al₂O₃/Cu₃Sn interface,the Cu-O and Sn-O bonds between Cu₃Sn and?-Al₂O₃ are weak and the AlN/Cu₃Sn interface becomes the weak region in the joint which weaken the mechanical properties of the all Cu₃Sn AlN/Cu joint.