Research On Low Temperature Cu-Cu Bonding Technology Of Nano Cu Paste And Its Applications

Low temperature Cu-Cu bonding is the key technology to realize the electrical interconnection,mechanical support,and heat dissipation of electronic devices and three-dimensional integrated circuits.Electronic packaging is developing towards the integration,miniaturization,multifunction,and high-power density,which puts forward new challenges for low temperature bonding,so it is urgent to develop a novel low temperature Cu-Cu bonding technology.Nano Cu paste has important application values in low temperature Cu-Cu bonding because of its small size effect,low cost,good electromigration resistance,excellent conductivity,and high temperature resistance.However,the spontaneous oxidation of Cu nanoparticles（NPs）reduces its bonding performances,and the high-quality Cu-Cu bonding is achieved with the help of high temperature,high pressure,and reducing atmosphere,which seriously restricts the application and development of nano Cu paste in the microelectronic fields.In order to reduce the bonding temperature and eliminate the oxidation of Cu NPs,this dissertation started from the preparation and surface-modification of Cu NPs and studied on the bonding performances of nano Cu paste,and the nano Cu paste was applied on the interconnection packaging of high-power LEDs.The main research contents and innovations are described as follows:1)In order to reduce the bonding temperature,the carboxylic acid（formic acid and oxalic acid）surface-modification was proposed to remove the surface oxides of Cu NPs,and the carboxylic acid modified nano Cu paste was prepared for low temperature Cu-Cu bonding.Cu NPs with the average particle size of 65 nm were prepared by aqueous phase reduction.Carboxylic acid converts surface oxides into copper carboxylate salts,which can be decomposed at low temperature,thus reducing the sintering resistance of Cu NPs.After the oxalic acid surface-modification,the high strength Cu-Cu joints were realized at 250°C,and the electrical and mechanical properties of the sintered nano Cu layer were enhanced.2)Aiming to reduce the bonding requirements and eliminate the surface oxides,a method of preparing Cu NPs was proposed by reducing Cu²⁺-IPA complex,and a self-reducible nano Cu paste was developed for low temperature bonding.Cu NPs with the particle size of 6.5 nm have good dispersivity.Attributed to the reducibility of IPA stabilizer,the low boiling point（<200°C）of organic compositions,and the small size effect of ultra-small Cu NPs,Thanks to the reduction of IPA stabilizer,the low boiling point of organic composition（<200°C）and the small size effect of ultrafillet copper nanoparticles,the high-quality Cu-Cu joints were achieved under low temperature and low pressure conditions（250°C/2 MPa）,and the metallurgical interconnections appear at the bonding interfaces.3)To solve the problems of high cost of nano metal paste and high porosity of bonding layer,a micron-nano Cu composite paste was proposed to replace the nano Cu paste as the bonding material for low temperature bonding.The composite paste not only maintains low temperature interfacial bonding properties originated from the high diffusion coefficient and small size effect of nanoparticles,but also reduces the cost and interfacial porosity by introducing the micro particles.After optimizing the ratio of nano to micron particles,the Cu-Cu joints bonded at 250°C display the high shear strength of 45.6 MPa,and the die-attach layers yield the low resistivity of 5.44μΩ·cm and low porosity of 2.67%.Furthermore,the Cu-Cu joints still shows high strength and good thermal stability.4)Due to the low bonding temperature and high thermal conductivity,the nano Cu paste was successfully applied on the interconnection packaging of high-power LEDs for enhancing their heat dissipation performances.The experimental results were verified by heat transfer theory analysis and COMSOL thermal simulations.The effects of bonding temperature on the thermal and optical properties of LEDs were investigated and compared with the traditional die-attach materials.The results show that the sintered nano Cu layer possesses high thermal conductivity of 142.37 W/（m.K）after bonding at 250°C,and the packaged LEDs display low thermal resistance of 6.58 K/W and low junction temperature change of 3.22°C.Compared with the traditional die-attach materials,the LED samples packaged by the nano Cu paste show better heat dispersion and optical performances.