高密度三维集成低温Cu-Sn固态扩散键合技术基础研究

3D integration has been acknowledged as one of the candidates to extend Moore’s Law with the advantages of smaller form factor, lower power consumption, higher bandwidth, and heterogeneous integration. Because of low cost and high process compatibility, metal microbump-to-microbump bonding of Cu-Sn binary system is a popular technology to realize vertical interconnection between wafers or chips in 3D integration. However, the bonding temperature is increased to above the melting point of Sn (232℃) in traditional Cu-Sn solid-liquid-interdiffusion bonding. Sn overflow usually occurs and leads to risk of electrical short between adjacent fine-pitch bumps, which is unfavorable for future’s 3D integration with higher density. In this project, the fine-pitch (＜20μm) Cu and Sn bumps with low roughness and good uniformity are firstly designed, optimized and fabricated. A novel pretreatment on bumps is proposed to clean and activate the surface. At low temperature (＜200℃), wafer-level Cu-Sn bonding is performed with solid-state-diffusionin technology in a short time (＜30min). Bonding performances, including interface, strength, resistance, and electromigration, are studied using modern analysis methods of materials. Meanwhile, Cu-Sn solid-state-diffusion theory is established, and electromigration failure mechanism is also explained. Therefore, the research of this project will further promote the application and development of 3D integration technology.

三维集成由于具有小尺寸、低功耗、高带宽和异质集成等特点，已成为延续“摩尔定律”的重要手段。而基于成本和兼容性的考虑，Cu-Sn二元体系的金属微凸点键合是实现三维集成片间垂直互连的主流技术之一。目前常规的Cu-Sn固液互扩散键合的致命缺点在于其工艺温度远高于Sn材料的熔点(232℃)，键合过程中极易发生Sn的“外溢”并导致相邻较窄节距微凸点的短路，这将不利于未来高密度三维集成的实现。本项目拟设计、优化和制作粗糙度低、一致性好的窄节距(＜20μm)Cu、Sn微凸点；通过新型表面预处理方法，获得超洁净、高活性的凸点表面；在低温(＜200℃)条件下,利用固态扩散技术，短时间内(＜30min)完成晶圆级的Cu-Sn键合；依靠材料现代分析方法研究键合的界面、力学、电学和电迁移特性，并建立Cu-Sn固态扩散理论和阐明电迁移失效机理。因此，本项目的研究将进一步促进三维集成技术的应用和发展。

基于成本和兼容性的考虑，Cu-Sn二元体系的金属微凸点键合是实现三维集成片间垂直互连的主流技术之一。本项目设计、优化和制作了厚度为1.5~1.6μm的Cu凸点及厚度为1.6~1.7μm的Sn凸点，在Cu表面溅射沉积了10nm的Cr防护层，使Cu表面的均方根粗糙度由最初的10nm降至8.3nm。随后在Ar中混入5% H2，在200W功率、200sccm气流量的条件下，对沉积Cr的Cu表面等离子体预处理60s，接触角已从未处理的30°左右降至10°以下；微凸点经过200℃、38.4MPa、15min的键合，退火后形成了稳定的Cu-Cu3Sn-Cu结构，平均键合强度达到了27.0MPa。Cu/Sn/Cu三明治结构在键合、退火过程中，伴随着Sn层的减小、分割及耗尽，相应的Cu6Sn5 层的生长、合并及转化，最终将形成稳定的三层结构—Cu/Cu3Sn/Cu；温度为 200°C 的Cu–Sn键合的IMC生长机制仍主要受晶格扩散控制，且存在 Sn 时，Cu6Sn5的生长削弱了Cu3Sn的生长；低温Cu-Sn键合的抗EM能力达到1.0*10^4A/cm^2以上，经过EM测试的菊花链未发生严重的电学性能退化。因此，本项目的研究将进一步促进三维集成技术的应用和发展。