【作者】 余春；

【导师】 陆皓；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 近年来,高端电子产品(计算机、手机等)的高性能化和微型化发展趋势对集成电路的设计和封装提出了越来越高的要求。凸点互连,如Flip Chip (FC),Ball Grid Array (BGA),Chip Scale Package (CSP)等是实现密集型、多I/O端口集成电路的主要封装工艺之一。凸点互连工艺使用的连接材料为钎料,其成分逐渐从Sn-Pb合金向无铅Sn基合金过渡。传统的Sn-Pb合金钎料在电流载荷作用下会发生电迁移失效(EM),对集成电路的可靠性造成了极大的威胁。因此,合金钎料电迁移问题成为过去10年封装技术领域研究的热点问题之一。无铅钎料出现后,电迁移问题仍然引起了广泛的关注。特别是随着互连结构尺寸持续减小,工作电流不断增加的发展趋势,无铅钎料接头的电迁移失效现象不容忽视。根据凸点互连结构的特点,本文设计了Cu/钎料/Cu(钎料为Sn-37Pb,Sn-0.5Cu,Sn-3.5Ag和Sn-3.0Ag-0.5Cu)三明治接头,在1×103 A/cm2/60℃条件下进行了EM加速实验。电迁移过程采用数字万用表进行监测。同时,采用扫描电镜(SEM)分析了EM阶段接头反应界面的微观组织形貌及演变,界面相成分采用电子探针(EPMA)和能谱仪(EDS)分析。同时,引入有限元方法分析了焊点中的电流密度分布和元素浓度分布,通过改变凸点互连的几何结构和材料属性,定性的研究了影响电流塞积的主要因素。最后,采用第一性原理方法,从电子结构、扩散激活能等方面分析了合金元素对Sn基钎料EM的影响。主要结论如下:1.在相同载荷和结构条件下,Sn-37Pb钎料凸点结构的EM寿命明显不如SnTM(TM=Cu或/和Ag)。187小时后,前者的阴极Cu6Sn5/钎料界面出现了尺寸超过10μm的空洞,空洞的尖端位于Cu6Sn5/钎料界面;在阳极则出现Pb的富集层。经过320小时,阴极已经完全熔化,形成断路。而后者在700小时后,电阻仍然没有显著的变化。2.以上四种钎料接头界面化合物的生长明显具有极性特征,即阳极化合物的生长速度比阴极快。阳极界面化合物的生长主要受Cu原子浓度的控制,生长动力学方程为满足抛物线关系。阴极界面化合物的生长则受电流密度控制,当电流密度较小时,化合物的生长仍然满足抛物线方程;而电流密度较大时,化合物溶解占主导,甚至会引起空洞形核和长大。3.电迁移过程中,Cu3Sn/Cu界面Kirkendall空洞的形核速度增加,但同样具有极性效应,阳极界面Kirkendall空洞的浓度比阴极高。发展到一定阶段,空洞吞并、长大,形成裂纹。由于Kirkendall空洞的尺寸不如阴极Cu6Sn5/Cu界面的EM空洞明显,因此,对Sn-37Pb钎料接头的可靠性影响不明显。但是,Kirkendall空洞对SnTM钎料接头的可靠性威胁应引起足够重视。4.影响凸点结构电流密度分布的因素主要为两方面,即几何结构和材料属性。通过有限元分析发现,宽的引线和厚的UBM都有利于减少钎料凸点中的电流塞积;同样,采用电阻率较小的Cu导线,降低UBM的电阻率以及适当增加钎料的电阻率,都能缓解凸点中的电流塞积。部分计算结果得到试验验证。5.通过对Sn31M(M=Pb、Bi、Cu或Ag)超晶胞电子结构的第一性原理计算发现,Cu和Ag对于稳定最邻近Sn原子的效果比Pb和Bi原子明显。前者获得电子,而后者会向邻近的Sn原子转移电子。因此,可以预测,在电场条件下,Ag和Cu能够散射掉电子流对Sn原子的作用,从而提高电迁移抗性。6.根据扩散能的计算结果可知,以上四种合金元素都能够增加邻近Sn原子的扩散能障,并且Cu和Ag溶质原子的效果更佳。同时,在同一体系中,溶质原子的扩散能障都比对应的最近邻主原子低,因此,溶质原子将成为主要的扩散元素。虽然Pb和Bi的计算结果与结论5矛盾,但由于试验观察的现象是在Sn-Pb共晶钎料中,Sn和Pb都有可能成为主要的扩散元素,决定于环境温度。因此,计算结果在一定程度上与试验现象相吻合。7. SnCu、SnAg体系的扩散激活能高于纯Sn体系,而SnPb、SnBi体系的扩散激活能与纯Sn体系相当。该结果可用于解释为什么Cu和Ag能改善Sn基钎料的电迁移抗性。更多还原

【Abstract】 The trend of electronic products (like notebook, cell phone) to high performace and miniaturization requires many tough improvements for integrated circuit. Bump-to-trace interconnect technologies, such as Flip Chip, Ball Grid Array, and Chip Scale Package, are the most important methods to achive the integrated circuit with denser characteristic and much more I/O ports. Alloy solder is the joining consumable used in bump-to-trace interconnecting process, and nowadays, the components of solder transfers from Sn-Pb alloys to lead-free Sn-based alloys. Traditional Sn-Pb eutectic alloy solders is sensitive to electromigration (EM) under current stressing, and therefore, EM failure has great threat to the realiability of integrated circuit. In fact, EM problem of alloy solders has been the hot issue for the past 10 years. The EM problem has still attracted a lot of attention even after development of lead free solders. Especially, since the dimensions of interconnects decrease continuously and current increases, the EM phenomenon can not be ignorated.In this paper, a Cu/solder/Cu sandwitch structure was designed, where, solder is Sn-37Pb, Sn-0.5Cu, Sn-3.5Ag and Sn-3.0Ag-0.5Cu eutectic or near eutectic alloys, respectively. EM accerated tests were conducted under 1×103 A/cm2 + 60℃. The EM process was monitored by using a resistance detector. After the tests were done, we analyzed the interfacial microstructures with scanning electronic microscope (SEM), and the products at the interface were indentified with EPMA. Electron wind is the main driving force to induce EM, so we simulated the current density distribution and concentration distribution of the solder bump, as well as the factors influencing current crowding with finite element method (FEM). Lastly, the effects of alloy elements on the EM process of Sn based solder were analyzed by employing the first principles method. Main conclusions are as follows,1. EM lifetime of Sn-37Pb solder joint is shorter than that of SnTM (TM=Cu or/and Ag) solder joint under same stressings and structure. After 187 h, there forms void with diameter of about 10μm in the former solder joint, and 320 h later, short circuit arises due to complete dissolution of Cu at cathode side. While, the resistances change little after 700 h for the latter solder joints.2. It is found that the current stressing has a polarity effect on the growth of intermetallic component (IMC) layer, i.e., growth rate of IMC layer at anode side is faster than that at cathode side. Growth of IMC at anode side is controlled by concentration of Cu element migrated into anode interface, the growth dynamic equation is w , obeys with parabola rule. While, growth of IMC at cathode side is governed by current density,growth also obeys parabola rule as the maximum current density in the solder bump is small relatively. However, as the current density increases, IMC at cathode side would dissolve, even lead nucleation of micro-voids.3. Nucleation rate of Kirkendall voids at Cu3Sn/Cu interface is accerated under current stressing relative to that under thermal aging. Formation of Kirkendall voids is also of polarity characteristic, density of Kirkendall voids at anode side is higher than that at cathode side. Microvoids coalesce into larger voids or cracks. Since the dimension of Kirkendall voids is much less than than formed at Cu6Sn5/Sn-37Pb interface at cathode side, failure caused by Kirkendall voids at Sn-37Pb solder joints can be neglect. However, for SnTM solder joints, Kirkendall voids should be attracted enough attention.4. Geometry structures and materials’properties are the main factor influencing the current density distribution in the solder bump according to finite element analysis. It is found that wider trace, thicker UBM, low resistivity trace and UBM, and solder with higher resistivity are beneficial in decreasing current crowding in solder bump.5. From first principles calculations, we found that the effects of Cu and Ag on the stability of the nearest neighbor (NN) Sn are more significant than those of Pb and Bi. The former alloying elements obtain charges from Sn, while the latter transfer charges. It can be predicted that the effects of electron flux on Sn atom can be scattered by Cu and Ag, the EM resistance of Sn is therefore improved.6. According to the calculated results of diffusion activation energy, it is found that Pb, Bi, Cu and Ag alloying elements are all increasing the barrier energy of NN Sn, and the effect from Ag and Cu is more significant. In the same system, the barrier energy for alloying element is lower than that of the NN Sn. Therefore, alloying elements would be the diffusing species. It seems that this conclusion is inconsistent with conclusion 5, however, experimental results all show that both Sn and Pb can be the dominated diffuser in Sn-Pb eutectic alloy solder, as determined by the environment conditions. Therefore, our results are reasonable.7. Both the diffusion energies of SnCu and SnAg sytems are higher than, while those of SnPb and SnBi systems are equal to that of pure Sn system. This result is another reason why Cu and Ag can improve the EM resistance of Sn based solder.更多还原