【作者】 岳武；

【导师】 张新平；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2014， 博士

【摘要】 现代电子产品向微型化、多功能化和高可靠性方向的快速发展对高密度封装的需求越来越急迫，为此必须不断减小互连焊点尺寸和互连间距。随着焊点尺度的减小，焊点中承载的密度越来越高的电流驱使微焊点中金属原子发生定向扩散迁移，即出现电迁移现象。电迁移可导致焊点微观组织中出现偏聚和粗化、空洞和裂纹、小丘和晶须以及焊点界面金属间化合物（Intermetal compound，IMC）层发生极性或异常极性生长等，上述变化可恶化焊点的力学性能、降低焊点的可靠性，甚至直接导致焊点断裂而引发电连接断路等；因此，研究微焊点中的电迁移行为具有重要意义。目前虽有不少关于电迁移问题的研究，但关于微焊点自身的结构和微观组织不均匀性对电迁移行为影响的研究还非常缺乏。针对上述问题，本论文研究首先利用自行设计的直角型焊点，通过对比研究直角型和线型Cu/Sn-58Bi/Cu焊点中的电迁移现象，阐明了焊点结构不均匀性对原子扩散迁移微观机制的影响；然后通过对比研究热时效条件下Cu/Sn/Cu、Cu/Sn-3.5Ag/Cu和Cu/Sn-3.0Ag-0.5Cu/Cu三类焊点中的电迁移行为，阐明了钎料成分差异影响电迁移扩散通量和Cu基底晶体特性影响阴极局部溶解的微观机理；还对Cu/Sn-3.0Ag-0.5Cu/Cu焊点两侧存在不同厚度IMC层时的电迁移行为进行了系统研究，揭示了不同初始厚度的界面IMC层在电迁移作用下的演变规律；最后研究了Cu/Sn-58Bi/Cu线型焊点中含有半开放式界面微气孔（空洞）时的电迁移行为，探讨了电迁移过程中微气孔对原子扩散迁移和缺口周围裂纹形成过程影响的微观机制。研究结果表明，沿电流方向具有非对称结构的Cu/Sn-58Bi/Cu直角型焊点中电流向微区电阻较小的底部尖角处聚集而形成电流拥挤，导致底部尖角附近产生严重的电迁移问题，表现为阳极侧小丘和裂纹共存、阴极侧凹陷和裂纹同时出现以及Sn/Bi两相完全分离等；随离尖角距离增大上述问题越来越弱，分析结果表明直角型焊点中不均匀分布的微区电阻是导致尖角附近出现电流拥挤并产生严重电迁移问题的根本原因；由于表面Sn原子氧化后形成的氧化膜对扩散组元Bi原子的抑制，焊点内部的物相偏聚和裂纹等缺陷远比表面严重；电迁移后Cu/Sn-58Bi/Cu焊点中阳极侧体积明显增大，其原因为Bi原子数量的增加以及Bi原子本身体积较大（为21.3cm3/mol，Sn原子体积为16.3cm3/mol）而引起阳极侧物相体积增大，电迁移程度越严重则体积增大越多。对热时效条件下Cu/Sn/Cu、Cu/Sn-3.5Ag/Cu和Cu/Sn-3.0Ag-0.5Cu/Cu三类焊点中电迁移行为的研究结果表明，电流作用下三种焊点的阳极界面IMC层生长受Cu原子的电迁移扩散通量控制、电迁移扩散系数依次降低，钎料中的Ag元素与Sn反应后生成的细小Ag3Sn颗粒分布于Sn晶界中形成的网状组织对扩散组元Cu原子的抑制作用是引起扩散系数依次降低的微观机制。阴极侧Cu基底晶粒的（020）晶面垂直或近似垂直于电流方向可引起严重的局部溶解，而（111）或（111）晶面垂直或近似垂直于电流方向时局部溶解可以得到抑制，其本质原因是（111）和（111）晶面上的原子释放率要低于（020）晶面。阴极侧较高的形核率、充足的Sn和Cu原子供应致使该侧Cu3Sn层中新生成的晶粒多为等轴晶，而阳极侧Sn原子的缺乏导致新生成的Cu3Sn多为柱状晶。对Cu/Sn-3.0Ag-0.5Cu/Cu焊点两侧存在不同厚度IMC层时电迁移行为的研究结果表明，阴极侧IMC层较厚而阳极侧IMC层较薄时焊点的抗电迁移性能优于阴极侧IMC较薄而阳极侧IMC较厚时的情况。阴极界面IMC层的初始厚度存在临界值，如果初始厚度小于临界值，电流作用下其厚度先增加后减小，而大于临界值时则不断减小；由浓度梯度引起的流入阴极侧IMC层中的化学扩散通量和由电流应力导致的从IMC层中流出的电迁移扩散通量之间的平衡关系是存在临界厚度的微观机制。Cu原子在垂直于电流方向的截面上扩散并不均匀，在阳极界面前沿局部位置聚集后与周围Sn原子反应生成Cu6Sn5相并引发体积膨胀，若新生成的Cu6Sn5相的截面积较小且离焊点表面较近则容易引起焊点表面出现明显小丘，否则表面出现轻微凸起。新生成的Cu6Sn5相还可导致阳极附近形成压应力，在压应力作用下空位沿着与电子流相反的方向向Cu/Cu3Sn界面和IMC层内扩散迁移，聚集后在界面附近和IMC层中形成空洞。对Cu/Sn-58Bi/Cu线型焊点中含有半开放式界面微气孔时的电迁移行为的研究结果表明，气孔的缺口尖角周围容易形成微裂纹且尖角两侧原子的聚集程度存在差异，分析表明尖角对Bi原子的阻滞作用是造成上述现象的主要原因。内壁光滑的气孔对原子的扩散迁移影响较小，Bi原子可以顺利通过内壁光滑的气孔周围的钎料组织；同时，由于气孔减小了焊点的有效接触面积，当气孔在阴极侧附近时电迁移后阳极富Bi层厚度有增加的趋势。更多还原

【Abstract】 The rapid development of modern electronic devices and products towardsminiaturization, multifunction and high reliability has brought about increasingly urgentdemand for high density electronic packaging, accordingly the pitch size and dimension ofsolder interconnects have been continuously scaling down. With decreasing the dimension ofsolder interconnects, the current density applied to solder interconnects gets higher andhigher, leading to the electromigration (EM) problem, i.e., the directional migration of theatoms under high-density current stressing. During EM, the microstructure of solderinterconnect will be significantly changed and a number of structural problems may occur,such as microstructure coarsening, voiding, cracking, formation of hillocks and whiskers, andthe intermetallic compound (IMC) polarity or abnormal polarity growth, which can cause agradual deterioration of the mechanical property and reliability of solder interconnect, evencan directly lead to the fracture of interconnects and the open circuit. Therefore, the study ofEM in solder interconnects is significantly important for the reliability of electronic products.Thus far, considerable studies focused on the microstructure evolution and failure mechanismof solder interconnects during EM. However, there is a severe lack of attention looking at theinfluence of inhomogeneous configuration and microstructure on EM behavior of solderinterconnects. To clarify these issues, in this thesis study, a right-angle-type solderinterconnect is designed and fabricated to investigate the EM behavior of the interconnect incomparison with the line-type solder interconnect. The focus is placed on clarifying theinfluence of the inhomogeneity of interconnects’ configuration on the mechanism of theatomic diffusion under current stressing. Then, the EM behavior in Cu/Sn/Cu,Cu/Sn-3.5Ag/Cu and Cu/Sn-3.0Ag-0.5Cu/Cu line-type solder interconnects under thermalaging condition was comparatively studied to reveal the mechanism of influence of thecomposition of solder alloys on the EM diffusion flux and the effect of the crystallinecharacteristics of Cu substrate on the cathodic dissolution. Further, the effect of the IMClayers, which existed initially at the two interfaces of a solder interconnect with unequalthicknesses, on the EM behavior of Cu/Sn-3.0Ag-0.5Cu/Cu solder interconnect was studiedsystematically to clarify the evolution of the interfacial IMC layers during EM. Finally, theEM behavior of the Cu/Sn-58Bi/Cu line-type solder interconnects containing the microscalesemi-open interfacial void was investigated and the influence of the void on the atomicdiffusion and the formation of the microcrack near the edge-notch of the void wascharaterized. The results show that the electric current easily flows into the microregion with smallerelectrical resistance, which locates at the bottom corner of the right-angle-typeCu/Sn-58Bi/Cu solder interconnect with inhomogeneous configuration, leading to the currentcrowding which further brings about the severe EM problem in terms of co-existence ofhillocks and cracks at the anode together with the concaves and cracks at the cathode and thecomplete segregation of Bi-rich and Sn-rich phases. The amount of these EM induced defectsdecreses with the distance away from the bottom corner. It has been shown that theinhomogeneous distribution of microregional resistance in the right-angle-type solderinterconnect is the key factor resulting in the current crowding effect and consequently thesevere EM problem at the bottom corner. Due to the suppression effect of superficial film oftin on the diffusion of Bi aotms, the segregation of Bi/Sn phases and cracking in the innermicrostructure of solder interconnect are much severer than that on the surface. The phasevolume near the anode of Cu/Sn-58Bi/Cu solder interconnect expands obviously after currentstressing, owing to the increasing quantity of Bi atom which has a larger atomic volume (21.3cm3/mol) than Sn atom (16.3cm3/mol). Apparently, serverer EM results in the increasingvolume expansion at the anode.The results of characterization of the EM behavior of Cu/Sn/Cu, Cu/Sn-3.5Ag/Cu andCu/Sn-3.0Ag-0.5Cu/Cu solder interconnects under thermal aging condition show that thegrowth of the interfacial IMC layer at the anode is controlled by the EM diffusion coefficientof Cu atom, which decreases orderly in three solder interconnects. The decreasing diffusioncoefficient is due to the blocking effect of the network structure of fine Ag3Sn particlessurrounding primary β-Sn phase grains on the diffusion Cu atoms. The results of selected areadiffraction patterns (SADPs) reveal that when the （020）plane of the copper grain in thecathodic substrate adjacent to the Cu3Sn/Cu interface is vertical or nearly vertical to thecurrent direction, severer cathodic dissolution occurs while the local dissolution hardly takingplace at the （111）or （111） plane, and this is resulted from the difference of the release rateof Cu atoms along different crystallographic planes. At the cathode, high nucleation rate andsurfacient supply of Sn and Cu atoms bring to the equiaxed Cu3Sn grains while less supply ofSn atoms at the anode leads to columnar grains.The results of the investigation of the influence of unequal thcknesses of IMC layersexisting at both anode and cathode on the EM behavior in Cu/Sn-3.0Ag-0.5Cu/Cu solderinterconnects manifest that for the interconnects with a thick IMC layer at the cathode and athin IMC layer at the anode, the electromigration becomes less severer compared to those with a thick IMC layer at the anode and a thin IMC at the cathode. There is a critical value ofthe initial IMC layer thickness at the cathode, below which the IMC layer grows firstly andthen decreases during EM, while above which the IMC layer at the cathode decreasesmonotonically. This is because there is an equilibrium relationship between the inflow fluxinduced by the chemical potential gradient and the outflow flux induced by the currentstressing during EM. The migration (diffusion) of Cu atoms takes place non-uniformly in thecrosssection of the solder joint under current stressing. Cu atoms easily migrate andcongregate at the anode and react with Sn atoms there, consequently Cu6Sn5IMC forms. Thenewly formed Cu6Sn5phase can lead to the volume expansion at the anode, which in turnbrings about obvious hillocks or tiny protrusions depending on the location of the Cu6Sn5phase formed either near or beneath the surface of the interconnect. In addition, the newlyformed Cu6Sn5phase also results in the compressive stress near the anode, which drivesvacancies to migrate towards the anodic Cu/Cu3Sn interface and IMC layer along the oppositedirection of electron current, and then congregate there, and finially the voids form near theinterface and in the IMC layer.The results of the study on the influence of the microscale semi-open void pre-existing atthe interface on the EM behavior of Cu/Sn-58Bi/Cu line-type solder interconnect show thatthe sharp notch of the void can easily result in congregation of Bi atoms and further lead tothe formation of cracks. The supressing effect of the sharp tip at the void on migration of Biatoms is the key factor resulting in those EM defects. Because the smooth inner surface of thevoid has little influence on atom migration, Bi atoms can pass smoothly across the soldermatrix near the void. Meanwhile, the presence of the void near the cathode side results inthickening of the layer of Bi-riched phase at the anode, owing to the reduction of the effectivecontact area.更多还原