Research Of Copper Bonding Wire Performance And Bonding Performance

Copper bonding wire is popular performance in microelectronic packaging due to excellentproperties and low cost. But copper wire has been limited in advanced packaging because ofcorrosion （oxidation） and immature bonding technology. It is a effective method to improvecopper bonding wire properties through doping some other elements （micro-alloy）. But onlylittle people do some research about doping little other elements to high purity copper andimproved oxidation properties.In this study, copper oxidation kinetics and oxidation mechanism were investigated byoxidation weighing experiment, and calculating the oxidation weight gain rate, drawingweightgain rate vs. time curve. The factors, such as structures, purity, grain size and micro alloyelement, to copper oxidation were investigated by analyzing atom energy of copper surfaceand oxidation film morphology by EDS and SEM. and recrystallization temperature/heataffected zone length and its mechanism were investigated by testing the mechanical andelectrical properties, grain size and organizational structure of high purity copper bondingwire and micro-alloy bonding wire at different temperatures. Based on optimized copperperformance,the factors of micro alloy to high purity copper wire were researched byanalyzing the bonding quality and bonding strength of high purity copper wire andmicro-alloy copper wire. Bonding quality was investigated by analyzing the different copperproperties and different bonding parameters. The ball bonding Cu-Al interface and strengthafter HTS and PCT were investigated by EDS and ball shear strength test and stitch pull test,and researched the reliability and failure mechanism of copper bonding wire. The mainconclusions as follows:1.Oxidation mechanism of copper, different energy atoms of copper surface andmorphology of the oxide film were analyzed, and draw a conclusion: the free electrons in thecopper is gradually shifted tooxygen atom in the microscopic process of copper oxidation,and reduce non-crystal compound with copper and oxygen, then thenon-crystal organizationbecome large due to further be oxidized. Then the oxygen atom gains an electron of thecopper atoms format porous Cu₂O particles and attached on a copper substrate.When thesubstrate is completely covered by the Cu₂O film, Cu₂O continues to be oxidized into denseCuO, rather than the oxygen atoms gain enough electronic and generate crystals of Cu₂O orCuO crystals.2. Copper oxidation has a significant orientation. In the non-close-packed （100） planes,the interfacial energy is high, and the interfacial atoms stacking is relatively loose, and thoseare rough in atomic scale. And the copper oxidation film is continuous growth, the oxidationrate is higher than the close-packed （111） crystal surface. The adhesion and dense of oxidationfilm influence the oxidation rate of pure copper, the oxidation rate is slow when the oxide filmis more dense and matrix adhesion. Oxidation film of single crystal copper （111） crystalsurface is dense and good adhesion,and dense oxidation film and adhesion of polycrystallinecopper oxide film is followed. The density and adhesion of single crystal copper （100） crystalsurface is poor than single crystal copper （111） crystal surface and polycrystalline copper, andthe oxidation rate is larger than polycrystalline copper. The oxidation film of high puritysingle crystal copper （100） crystal surface is fall off easily, and oxidation film of low puritycopper （100） crystal surface is combination mechanical pinning with Cu substrate, and adhesion is poor.The area of copper surface （111） crystal surface is increased after grainrefinement, and crystal surface atom with smaller energy is increased, then the oxidation ratedecreases.3. When high purity copper is doped micro Sn, the affinity of Sn atoms to oxygen atomsare stronger than copper atoms to oxygen atoms, and SnO and CuO are non-solution, soCu-Sn alloy produced protect oxidation layer between oxidation and copper base and betweenCu₂O and Cu₂O, then the anti-oxidation property of high purity copper is improved.4. Chemical interaction and elastic interaction between Sn atoms and Cu atoms cause thelattice distortion of high purity copper, and micro Sn causes the total grain boundary energy tobe reduced, then the chance of recrystallization nucleation in high purity copper are reduced,and influence the nucleation action of copper recrystallization, and grain boundary migrationin recrystallization process are prevented. And recrystallization nucleation rate and graingrowth are reduced, so the recrystallization is prevented, and recrystallization temperature isimproved, the length of HAZ is reduced, and the high purity copper wire HAZ length is150um, then the micro alloy copper wire HAZ is120um. The fusing current and fusing timeof micro alloy copper wire fit I3.49exp.10³+0.61, the confidence interval0<t<10, and fusing current and loop length fit I0.06675t³+0.7861l²-3.25.2562,the confidence interval0<l<5.5. Trace Sn in high purity copper wire is more favorable to increase the dislocationdensity in the bonding process, the dislocation density increased is the main factors offormatting the bonding interface and effective connection, so bonding strength of micro alloycopper wire is excellent than high purity copper wire.The number of metal grain is larger inthe same volume due to dope micro Sn, and the grain orientation is different, so increasesynergy when deform, reduce inhomogeneity of deforming, so that the same deformation aredistributed to more grain when deforming, and get a uniform deformation, then the first bondis uniformed. The mechanical properties of micro-alloy copper wire influences the FAB HAZlength, and hardening state of copper wire, with a high dislocation density,dislocation canreduce the recrystallization temperature, so the HAZ length of hardening wire increased by8.7%.6. Ultrasonic power is the key to bonding process, excessive ultrasonic power cause thebond region deformed and deduced obvious cracks, and can cause serious stress concentrationat the near bonding region, resulting micro cracks in the device during performance.Excessive ultrasound power will destroy the bonded area that has been formed, and result indecreased bond strength, then form a insufficientconnection. But small ultrasonic power cannot provide sufficient energy to form a bonding strength, and form a non-strength connectionor bonding drop. In bonding processing, improper bonding force lead to crater defects. Inbonding processing, the location of max force that copper ball passed to aluminum pad locatearound the capillary edge, and form a reliability connection. In the middle of aluminum pad,the force is small, and copper ball does not enough plastic deformation, and form a weakconnection.7. Bonding interface do not find any voids and cracks after copper ball bonding HTS, andforms some IMC, such as CuAl and Cu9Al4, the bonding interface strength is increased, andfailure mode is aluminum stripping, and the same to no-HTS. Bonding interface find somevoids after PCT, and bonding strength decreased, IMC Cu9Al4disappear. To Cu/Al interface,Cu active phase is lower than Al. In humid environment, the surrounding environment presents weak acid because of halogen atoms, and forms an electrochemical corrosionconditions, and Cu content is prodid in IMC Cu₉Al₄, and its electrochemical potential is lowerthan Cu, and do not form a protect layer easily, and corrosion easily, the reaction isCu₉Al₄+12Br^-=4AlBr₃+9Cu+12e^-, and reduce the device reliability.This study will be a new understanding to copper oxidation mechanism, the factors ofoxidation, the factors of micro-alloying element Sn to the performance of bonding copperwire and copper wire bonding reliability. And this will improve the copper bonding wireperformance, speed up the application about copper bonding wire in the microelectronicsindustry, and has important theoretical significance.