先进封装纳米孪晶铜电镀液配方开发及材料力学性能优化

  随着微电子器件小型化发展，半导体工艺制程逐渐逼近摩尔定律的极限，封 装技术成为进一步优化器件性能的解决方案和发展方向，包括晶圆级封装（Wafer Level Packaging, WLP），倒装芯片（Flip Chip, FC）等在内的先进封装形式不断涌 现。金属铜凭借导电性好、抗电迁移能力强、价格低廉等优势，是再布线 （Re-Distribution Layer, RDL），凸点（Bump）及硅通孔（Through Silicon Via, TSV） 等晶圆级封装结构内重要的互连材料。电镀铜互连制造工艺利用电镀技术沉积速 度快、工艺参数简便、工艺兼容性良好等优势，是实现金属铜在互连结构内沉积 生长的主要方式。添加剂或其组合可以使电极表面电流分布更均匀，镀液的深镀 能力和均镀能力得到提升，是决定电镀铜微观组织形貌的核心。而铜互连结构进 一步向高密度、细线路的方向迈进，对电镀图形外形的平整度、均一性，以及材 料的力学和电学性能也提出了更高的要求。纳米孪晶铜基于高密度（111）晶面定 向分布的生长孪晶，使电镀铜兼具高强度、高韧性及高导电性等特性，因此作为 铜互连材料具有广阔的应用前景。而如何在电镀液配方中，通过添加剂这一简便 手段，实现电镀纳米孪晶铜组织形貌的调控和材料性能的提升，一直是本领域重 点关注的课题。 本文第一部分围绕无机添加剂开展纳米孪晶铜电镀液配方开发研究。首先， 在基于孪晶促进剂的纳米孪晶铜电镀液中加入镍离子，开展复配电镀研究。通过 对镀液和镀层详细的电化学和显微组织表征发现，加入 0.5ml/L 以上浓度的镍离子 能够消除纳米孪晶铜镀层中无孪晶的等轴细晶过渡层，从而制备出完全纳米孪晶 铜镀层，实现超细线路 RDL 内高比例纳米孪晶铜的生长。其生长机理在于，镍离 子通过形成氯化物占据铜表面形核位点，导致铜离子形核数量减少，并与孪晶促 进剂形成更稳定络合物，抑制其脱附，从而促进柱状晶纳米孪晶铜的生成和长大。 然后，研究氯和溴卤素离子对纳米孪晶铜电镀液的影响，重点分析卤素离子的类 型和浓度对孪晶生长和形态的调控机制。研究结果发现，氯离子与溴离子相比， 能够在较为宽泛的浓度和电流密度窗口内生成柱状晶的孪晶组织；而溴离子能够 与铜形成更稳定的化合物吸附层，从而显著增强电极极化、抑制晶粒生长，最终 获得基于等轴晶的孪晶组织。 本文第二部分围绕有机添加剂开展纳米孪晶铜电镀液配方开发研究。主要基 于纳米孪晶铜直流电镀工艺和孪晶促进剂，分别使用整平剂十六烷基三甲基溴化铵（CTAB）及加速剂聚二硫二丙烷磺酸钠（SPS）开展复配电镀实验。分析不同 添加浓度下镀液添加剂的作用效果和交互影响，对镀层组织的晶粒形态、尺寸、 取向及孪晶界的密度进行定性和定量的表征，并考察显微组织对宏观薄膜力学性 能的影响。CTAB 复配电镀实验结果表明，在优选浓度 10ml/L 下，纳米孪晶铜电 镀填充 RDL 图形的平整性和均匀性均有显著提升（计算数值下降了 30%-60%）。 SPS 及其类似物 3-(苯并噻唑基-2-巯基)-丙基磺酸盐（ZPS）和 O-乙基-S-(3-磺丙基)- 二硫代碳酸酯 (OPX)的复配电镀结果显示，加速剂与孪晶促进剂的电化学拮抗作 用导致去孪晶现象，且去孪晶效果与加速剂的浓度和加速能力正相关。少量强加 速剂 SPS 和 OPX 使得柱状晶纳米孪晶组织转变为等轴晶细晶组织，后者的晶粒进 而因自退火而长大，而弱加速剂 ZPS 则几乎不引起柱状晶纳米孪晶组织的改变。 本研究工作面向先进封装电子互连应用场景，开发了高性能纳米孪晶铜直流 电镀工艺及镀液配方。重点探索添加剂作用下纳米孪晶铜微观组织形貌的调控机 制，针对纳米孪晶铜过渡层厚、表面粗糙等问题从添加剂角度提供了解决方案， 并且丰富了不同晶粒形态纳米孪晶铜的制备方法，从而优化电镀纳米孪晶铜材料 显微组织，为其产业化应用拓展了添加剂选型方面的设计思路和实践基础。

With the miniaturization of microelectronic devices, the semiconductor manufacturing process is approaching the limit of Moore's Law, while integrated circuits packaging has become the technical solution and development direction for further optimization of device performances. Novel advanced packaging techniques, for example, wafer-level packaging (WLP), flip chip (FC), etc. spring up in the industry. Copper is the important interconnect material used in wafer-level packaging including redistribution layer (RDL), bump and through silicon via (TSV), by virtue of good electroconductivity, better electromigration resistance, and low cost. The copper interconnect electroplating allows rapid deposition rate, simple control parameters, and facile process compatibility, which is the mainstream to realize bottom-up growth of copper within interconnect structures. Moreover, additive or its combinations, on one hand, can modulate current distribution on cathode surface for improved covering and leveling power, and on the other hand, is the core element in determining the microstructure of electroplated copper. As copper interconnect moves towards even higher density and finer pitch, higher demands for surface flatness and pattern uniformity of electroplated copper, along with its mechanical and electrical properties, have been put forward. Nanotwinned copper, based on high-density (111)-oriented growth twins, is an advanced material with superior strength, ductility and electroconductivity, showing broad application prospects. Great research efforts have been put in realizing microstructure manipulation and performance optimization simply through plating bath additives. The first research part of this article revolves around inorganic additives in formulation development for nanotwinned copper electroplating. First, nickel cations were co-added with twin growth prompter, and a detailed electrochemical and microstructure characterization to baths and plating films were carried out. It is found that addition of over 0.5ml/L nickel ions can eliminate the equiaxial grained transition layer between cathode and nanotwinned columnar grains to obtain a complete nanotwinned microstructure, which enables deposition of high-proportioned nanotwins in the ultra-fine line RDL. The mechanism lies in that nickel cations adsorb at cathode in the form of chloride and occupy copper nucleation sites, leading to a decrease in the number of copper nuclei. The nickel chloride further complexes with twin growth promotor and prevents its dynamic desorption, thereby facilitating formation of nanotwinned columnar grains. And then, addition of chloride and bromide in plating baths were studied to reveal the grain morphology evolution of nanotwinned copper by control of the type and concentration of halides. The results show that chloride, compared to bromide, allows the growth of columnar grained nanotwinned copper in a wide concentration and current density range. While bromide is able to form a more stable compound adsorption layer with copper, which significantly enhances cathodic polarization and inhibits copper growth, resulting in equiaxial grained nanotwinned copper eventually. The second research part of this article focuses on direct-current based organic additive formulars in nanotwinned copper electroplating. Twin growth promoter were co-formulated with leveler cetyltrimethylammonium bromide (CTAB) and accelerator sodium polydithiodipropane sulfonate (SPS) in the plating baths, respectively. The additive effects and interactions under different concentrations in the plating baths are analyzed, the microstructure characters such as morphology, size, orientation and twin density are qualitatively and quantitatively characterized, and lastly effects of microstructure on the mechanical properties of thin copper film are investigated. The results with CTAB indicate that, at a preferred concentration of 10 ml/L, the flatness and uniformity of nanotwinned copper RDL are significantly improved (numerically reduced by 30%-60%). Besides, the results with SPS and its analogues 3-(benzothiazol-2-ylthio)-1-propanesulfonate (ZPS) and O-ethyl-S- (3-sulfopropyl)-dithiocarbonate (OPX) show that the accelerators are strongly antagonistic to twin growth promotor, resulting in a detwinning phenomenon in electroplated copper, and this effect is positively corelated to the concentration and depolarizing ability of accelerators. A small content of strong accelerators like SPS and OPX will cause nanotwinned columnar grains to evolve to detwinned equiaxial fine grains which further self-annealed into coarser grains, while weak accelerator ZPS hardly changes microstructures of nanotwinned columnar grains.
Abstract This study develops direct-current nanotwinned copper electroplating techniques for high-end copper interconnect scenarios in advanced electronic packaging. Particularly, the microstructure and morphology manipulated by plating additives are explored. Aiming at optimizing the microstructure of nanotwinned copper materials, our study provides feasible solutions to address practical problems about thick transition layer, rough surface topography, etc. in electroplating nanotwinned copper, meanwhile enriches the preparation methods of nanotwinned copper with different grain morphologies, which thereby expands the scope of additive selection and lays a pioneer basis for industrial application of nanotwinned copper.

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