Fe-Ni新型UBM材料的电镀工艺开发及CSP封装可靠性研究

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	Fe-Ni新型UBM材料的电镀工艺开发及CSP封装可靠性研究
	张昊
学位类型	硕士
导师	刘志权
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	Fe-ni合金 Ubm 电镀 Csp封装可靠性 Fe-ni Alloy Electroplating Ubm Csp Packaging Reliability
摘要	"随着无铅化进程的不断推进，无铅焊料相对较高的回流温度造成传统凸点下金属层（under bump metallization, UBM）如Cu或Ni等材料的过快消耗；另外，随着封装密度的不断提升，焊点尺寸越来越小。因此，现有的高密度封装工艺面临新的界面反应以及界面可靠性问题。据报道，相对于传统的UBM材料，Fe-Ni合金UBM与典型无铅焊料发生反应时，具有较慢的界面反应速率。Fe-Ni合金UBM与SnAgCu焊料反应焊点的剪切强度，也稍强于Cu UBM与SnAgCu焊料形成的焊点。此外，如果钎焊前的处理工艺得当，Fe-Ni UBM的润湿性能也可以得到保证——Fe-50%wt Ni UBM与SnAgCu焊料之间的润湿角已与Cu UBM与SnAgCu焊料之间的润湿角相当。本文进行了Fe-Ni 新型UBM材料的电镀工艺开发及其产业化探索。利用定制的晶圆电镀系统，在8寸Cu种子层晶圆及UBM图形晶圆上实现了不同组分Fe-Ni合金UBM镀层的制备。研究了各电镀参数对镀层成分的影响规律，对镀层在不同工艺条件下的生长速度、镀层的晶体结构演化以及镀层表面质量进行了测量及表征。结果表明，镀层成分主要受到电流密度、温度、镀液中Ni2+、Fe2+离子浓度比的影响。镀层晶体结构随着Fe含量的增加经历从γ固溶体向α固溶体的转变。在1~3A/dm2电流密度、50℃、镀液中FeSO4·7H2O含量35~45g/l条件下，镀层生长速度为0.16~0.21μm•min-1•(A/dm2)-1。镀层表面质量达到超镜面水平，表面高度差小于200nm。焊点可靠性方面，针对产业芯片级封装（CSP）样品，考察了Fe-Ni/Sn-3.5Ag-0.7Cu焊点互连结构在150℃、180℃以及200℃下的热老化可靠性，以及640周次热循环条件下的界面破坏情况。结果表明，在150℃条件下，1μm的Fe-75Ni镀层在时效9天后化合物层迅速增厚，且增厚主要来自FeSn2外层的CuNiSn化合物，而1μm厚的Fe-50Ni及Fe-30Ni镀层可承受长达27天时效而Fe-Ni层消耗量尚不及二分之一，同时，界面化合物FeSn2生长速度随着镀层Fe含量的增加而降低。200℃条件下，Fe-75Ni、Fe-50Ni、Fe-30Ni合金镀层均在4天内达到其极限生长厚度，且极限生长厚度随镀层Fe含量的增加而上升。经过640周次热循环后，Fe-75Ni/SnAgCu互连体界面出现大量沿Ti/Si界面产生的横向裂纹，破坏率20%~40%；Fe-30Ni/SnAgCu互连体界面在Si基体下方出现少量弧形基体裂纹，Ti/Si界面破坏较少；Fe-50Ni/SnAgCu互连体具有最佳的热循环可靠性，两种形式的裂纹发生率均在10%以下。"
其他摘要	Accompanying the fast development of lead-free soldering in microelectronic industry, traditional UBM materials such as copper and electroless nickel are consumed quite quickly due to the higher reflow temperature of lead-free solders than that of SnPb solders. On the other hand, solder joints are getting more smaller due to the increased package density. Therefore, advanced high density packaging process like chip scale packaging (CSP), is facing new challenges of interfacial reaction and reliability issues. As it had been reported before, Fe-Ni alloy exhibits a much slower interfacial reaction rate than that on traditional UBMs using typical Pb-free solders, and the shear strength of SnAgCu solder joint could be slightly improved by replacing the Cu/Ni UBM with the Fe-Ni UBM. Moreover, with adequate pre-treatment during soldering, the wettability of Fe-Ni UBM is also quite acceptable. Thus Fe-Ni alloy is a good candidate as novel UBM materials, which has not been industrialized yet. In this study, the electroplating process of Fe-Ni films was systematically explored for industrialization purpose, and the thermal reliability of Fe-Ni/SnAgCu solder joints was investigated. Firstly, using customized wafer electroplating system, Fe-Ni UBM films with different compositions had been successfully prepared on 8-inch Cu seed layer wafer and 8-inch UBM patterned wafer. The influence of electroplating parameters on the composition of Fe-Ni UBM films, the growth speed of deposit under various process conditions, the evolution of deposit’s crystal structure, and the surface quality of deposited films, had been clarified through series experiments. It was found that the composition of deposited films was mainly determined by the current density, the electroplating temperature, and the concentration ratio of Ni2+/Fe2+ ions. With the increasing of Fe concentration in the deposited films, the crystal structure of deposit changed from γ solid solution to α type. The growth speed of deposited films was in the range of 0.16~0.21μm•min-1•(A/dm2)-1 under the electroplating conditions of current density at 1~3A/dm2, temperature at 50℃ and concentration of FeSO4·7H2O at 35~45g/l. The surface quality of deposited films had reached ultra-flat standard, the largest difference in surface height was less than 200 nm. Secondly, the interfacial reliability of Fe-Ni/SnAgCu solder joints was tested by high temperature storage, temperature cycling, and ball shear tests. The thermal stability of Fe-Ni/SnAgCu joints was investigated under 150, 180, and 200°C. Storage at 150°C, Fe-75Ni films (1μm thick) exhibited an obvious IMC growth after 9 days’ aging, which was mainly caused by the formation of CuNiSn compound outside the FeSn2 layer. While Fe-50Ni and Fe-30Ni films could endure 27days’ aging, and the consumption of Fe-Ni layer was less than half of the original thickness (1μm thick). Moreover, the growth speed of FeSn2 layer slowed down with the increasing of Fe concentration in the deposited films. When aged at 200°C, Fe-75Ni, Fe-50Ni and Fe-30Ni film would reach the growth limit of FeSn2 within 4 days, and the limited thickness of FeSn2 layer increased with the increasing of Fe composition in the deposit. The interfacial fracture conditions after 640 temperature cycles had been evaluated. A large amount of cracks could be observed at the Ti/Si interface of Fe-75Ni/SnAgCu solder joints with a destructive rate of about 20~40%. Fe-30Ni solder joint exhibited a small amount of arcuate cracks under Ti/Si interface, while less cracks could be discovered on Ti/Si interface. Fe-50Ni solder joint had relatively better thermal cycle reliability than other two compositions, and the destructive rate of both cracks was less than 10%. The shear strength for Fe-75Ni, Fe-50Ni, and Fe-30Ni solder joints were 4.344, 5.504, and 5.508 mg/mm2 respectively, which are all satisfied with the requirement of industrialization (＞3.5 mg/μm2).
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64541
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	张昊. Fe-Ni新型UBM材料的电镀工艺开发及CSP封装可靠性研究[D]. 北京. 中国科学院金属研究所,2012.