Process And Mechanism Of Nanosecond Laser Surface Modification Assisted Brazing Si₃N₄ Ceramic And Cu

In the fields of new energy vehicles,rail transit and smart grid,etc.,ceramic base copper（Cu）clad laminates,as current carrier,support body,insulator and thermal conductivity channel in the power devices,play an essential role in ensuring the reliability of power modules in devices.Silicon nitride（Si₃N₄）ceramic is expected to be used as the next-generation substrate materials for high-power electronic devices to meet the advanced packaging requirements of power modules for next-generation wide band-gap（WBG）semiconductor devices as a result of its excellent mechanical properties,high fracture toughness,low dielectric loss and superior thermal conductivity.Therefore,the high-quality bonding of Si₃N₄ ceramic and Cu is of great significance in promoting the development of power modules for future WBG semiconductor devices.Conventional Si₃N₄/Cu joining method for fabrication package substrate in power devices has issues of violent interfacial reaction,uncontrollable joint microstructure and reaction products with high brittleness.In this work,laser surface modification（LSM）is utilized to control the surface structure and composition of Si₃N₄ ceramic,hence achieving the direct bonding Si₃N₄ ceramic to Cu and further effective control of Si₃N₄/Cu interfacial compounds and microstructure.An infrared nanosecond pulsed laser was used to modify the surface of Si ₃N₄ceramic to investigate the interaction mechanism between the laser and Si ₃N₄ceramic and the effects of LSM on the microstructure and chemical composition of the Si₃N₄ ceramic surface.The optical-thermal interaction of laser-irradiated Si₃N₄ceramic led to the local ceramic surface temperature increment during LSM,which induced decomposition and evaporation of the Si₃N₄ ceramic surface.When the laser power was 28W,the Si₃N₄ was decomposed to liquid Si and nitrogen.When the laser power was increased to 42W,the liquid Si was vaporized into gas Si.Meanwhile the sintering aid of Mg O,Si₃N₄ and Si O₂ had participated in the formation of a eutectic liquid phase during LSM and remained in the Mg-Si-O-N intergranular glassy phase in the Si precipitates.The laser energy and number of pulsed absorbed per unit area of Si₃N₄ ceramic surface increased with increasing laser power or decreasing scan speed.Severe evaporation of the liquid and the sputtering of the molten materials at higher temperatures led to an increase in the content of Si precipitations and the surface roughness of the S i₃N₄ ceramic.The effects of LSM on the joining of Si₃N₄ ceramic and Cu was investigated based on the above mechanism of laser-induced thermal decomposition of Si₃N₄.The reaction and formation of the interface were promoted by the Si precipitates produced by the LSM.The interfacial microstructure of the Si₃N₄/Cu joint was Mg-Si-O-N glass,Cu_0.875Si_0.125,Si₃N₄ particles and Cu₃Si-Si eutectic structure.Besides,the groove structure of the Si₃N₄ ceramic surface fabricated by the decomposition and evaporation during LSM was perfectly replicated,finally forming a wave-like interface.The wave-like interfacial structure could provide an effective micro-mechanical interlocking to inhibit the propagation of cracks,and the strength and reliability of the Si₃N₄/Cu joint were significantly improved.The processes of LSM and brazing were optimized based on the LSM assisted Si₃N₄/Cu joining technique,combining with microstructure and mechanical property test.When the laser powers were 14W and 28W or the joining temperature was800°C,the debonding and micro-cracks in the joins were observed as the results of the inadequate interfacial reactions.As the increase of laser power and the bonding temperature,the quantity of defects in the joint were reduced and the flawless interface was obtained.The optimum value for the maximum joint shear strength of18.3MPa was obtained at laser power of 56W,scan speed of 100mm/s,bonding temperature of 950°C and holding time of 10 min.The shear load of the joint was88.6%of the fracture load of the base metal Cu.The failure mode of the joint was combination failure.The excellent stability of the microstructure of the joint exhibited excellent thermal-impact-resistance performance that the low defect density（less than 0.6%）was obtained after thermal cycling testing.Thermaldynamic calculation was used to analyze the chemical potential and Gibbs free energy in Si₃N₄/Cu system.Correspondingly,the rection and formation of interface were related to the change in chemical compositions of Si₃N₄ ceramic surface.The formation mechanism of interfacial reaction products during the joint formation was described.During brazing,the solid diffusion and reaction between the Cu and Si precipitates on the Si₃N₄ surface produced by LSM first occurred,and Cu₃Si and Cu₁₅Si₄ were formed at the interface.The local point bonding was formed in the peak of the groove structure.When the bonding temperature increased to802°C,the eutectic liquid was formed by the eutectic reaction between Cu ₃Si and Si.Then eutectic liquid filled the gaps and groove of the Si₃N₄ ceramic surface through capillary action.The eutectic liquid also promoted the dissolution of the base materials and the Cu-Si reaction products.The Mg-Si-O-N glass remained in solid as the result of the glass transition temperature（>1000°C）being above the bonding temperature.And the Mg-Si-O-N glass did not participate in the interfacial reaction,resulting in the formation of amorphous interface layer between the Si ₃N₄ ceramic and Cu,thus bonding the ceramic to the metal.