【作者】 李俊杰；

【导师】 汤自荣；

【作者基本信息】 华中科技大学 ， 机械制造及其自动化， 2017， 博士

【摘要】 作为半导体行业中最重要的部分之一,集成电路的发展近年来已被国家推向了科技发展战略层面。为了克服摩尔定律在半导体行业高速发展中所遇到的瓶颈,三维集成电路应运而生,并对其封装工艺中的凸点密度、能量消耗、封装性能等都提出了更高的要求。然而,现阶段键合工艺中所使用的Sn或Sn基无铅焊料已无法满足更加严苛的封装需求,逐渐出现了一些问题,例如:键合过程中Sn的过度溢出、服役过程中的Sn须生长、柯肯达尔孔洞形成等。这些现象都会导致集成电路中短路、断路或者电学性能衰退等情况发生。因此,找到一种能替代Sn材料的高性能互连材料是微电子封装技术继续发展的迫切需求。因其优异的导电、导热及抗电迁移特性,Cu、Ag材料都被视为理想的互连材料。但是,其高熔点却导致Cu、Ag材料无法满足微电子封装的工艺要求,从而限制了其发展。随着纳米技术的快速发展,科研工作者们在晶体材料的低温烧结研究上有了大量突破。因此,利用纳米技术降低Cu、Ag材料的烧结温度,并实现Cu-Cu键合是一种可行的研究思路。由于Ag是一种资源短缺型材料,不适合可持续发展需求,本文研究主要针对使用Cu基纳米焊料的Cu-Cu键合展开。具体研究内容包括:一、基于纳米晶体材料的尺度效应,提出使用Cu纳米颗粒制备Cu纳米焊料,并基于Cu纳米颗粒的烧结特性降低Cu-Cu键合温度的方法。本文首先合成出平均尺寸为100 nm左右的Cu纳米颗粒,并制备成Cu纳米焊料,在250oC-400oC进行了烧结及键合研究。其中,最低的烧结电阻率在400oC下实现,仅为12.9μΩ·cm;最高的Cu-Cu键合剪切强度在400oC键合温度,40 MPa键合压力下实现,达到35.68 MPa。文中还对Cu纳米焊料的烧结机理及Cu-Cu键合的键合机理进行了阐述,为基于纳米焊料的低温键合研究提供了理论依据。使用100 nm左右的Cu纳米颗粒制备的纳米焊料,有效地将Cu-Cu键合温度降低至400oC以内。虽不能完全满足微电子封装的工艺需求,但证实了基于Cu纳米焊料烧结实现键合的可行性,为后续研究奠定了基础。二、结合尺度效应及烧结机理,实现了降低Cu纳米颗粒尺寸以达到降低Cu-Cu键合温度的目的。本文改进工艺方法,实现60 nm左右Cu纳米颗粒的合成,并制备出纳米焊料。基于此纳米颗粒,Cu纳米焊料在300oC下实现高性能烧结,薄膜电阻率低至12.0μΩ·cm。高质量Cu-Cu键合界面也在300oC下实现,键合压力降低至1.08MPa,并且剪切强度高达31.88 MPa。随后,在150oC下进行了200小时恒温老化测试,键合界面与力学特性并未发生明显变化,充分了Cu-Cu键合的高完整度。文中还使用此纳米焊料进行了晶圆级Cu-Cu键合探索,同样实现了高质量的键合界面与键合强度,并保证了晶片的完整性。研究结果表明,本文合成的60 nm Cu纳米颗粒在微电子封装中有更好的应用前景。三、利用同等尺寸下Ag材料更优异的烧结特性,本文将70 nm左右的Ag纳米颗粒混入60 nm Cu纳米颗粒中,进行了Cu基混合纳米焊料研究。在250oC下,Cu-Ag原子比为2:1的Cu2-Ag混合纳米焊料展现出优异的低温烧结特性。其烧结后的薄膜电阻率可低至19.9μΩ·cm,接近于纯Ag纳米焊料烧结后的12.0μΩ·cm。文中还结合TEM分析,观察了Cu-Ag之间的扩散形成,对Cu-Ag混合纳米焊料的烧结机理进行了分析,解释了Ag纳米颗粒在提升烧结特性中起到的关键性作用。同样在250oC下,使用Cu2-Ag混合纳米焊料实现的键合呈现出与用纯Ag纳米焊料键合类似的紧实键合界面,并且剪切强度可高达25.41 MPa,满足使用需求。因此,Cu2-Ag混合纳米焊料可提升烧结及键合性能,并保证了比纯Ag纳米材料更好经济性。四、为进一步探索基于纯Cu纳米焊料的低温Cu-Cu键合方法,本文制备出了一种表面均匀附着5 nm极细Cu纳米颗粒的Cu纳米团聚体,并基于此Cu纳米团聚体进行了烧结与键合研究。研究表明,优异的烧结特性及键合性能可在250oC以内实现。其中,烧结后的Cu纳米焊料薄膜电阻率可低至4.37μΩ·cm,仅为Cu块体的2.5倍;Cu-Cu键合的剪切强度也可达满足使用需求的25.36 MPa。此项研究在使用纯Cu纳米焊料进行Cu-Cu键合方面实现了大幅度进步,具有重要的研究意义。本文以尺度效应为理论主线,对使用Cu基纳米焊料进行的Cu-Cu键合进行了一系列研究,研究思路及研究成果对微电子封装的发展有一定的指导意义。更多还原

【Abstract】 As one of the most important part in semiconductor industry,the development of integrated circuits in recent years has been regarded as a technology development strategy in our country.In order to overcome the bottleneck encountered by Moore’s Law in the rapid development of the semiconductor industry,the 3D-IC(three-dimensional integrated circuit)technology was emerged as a promising solution and put forward higher requirements for the bump density,energy consumption and packaging performance in the packaging process.However,the Sn or Sn-based lead-free solders used in the bonding process at this stage has been unable to meet the more stringent packaging requirements,and gradually appeared some problems,such as the overflow of Sn during the bonding process,the Sn whisker growth during the server process and the Kirkendall voids formation.These phenomena will lead the short circuit,open circuit,and the degradation of electrical performance in integrated circuits.Therefore,it is an urgent need to find a high-performance interconnection material that can replace Sn material for the development of microelectronics packaging technology.Because of the excellent electrical conductivity,thermal conductivity and resistance to electromigration,Cu and Ag are always considered as ideal interconnect materials.However,the high melting point of Cu and Ag limits their application in microelectronic packaging,because they can not meet the process requirements.With the rapid development of nanotechnology,researchers had achieved a lot of breakthroughs in the research field of low-temperature sintering of crystalline materials.Therefore,reducing the the Cu-Cu bonding temperature by using the Cu and Ag materials with nanotechnology is a viable research idea.Since Ag is a resource shortage material,it is not suitable for sustainable development.So,this study mainly focuses on Cu-Cu bonding with Cu-based nanosolders.The specific research contents are as follows.Firstly,based on the size effect of nanocrystalline materials,we proposed a method of reducing the Cu-Cu bonding temperature by sinterable Cu nanosolders,which were prepared by Cu nanoparticles.In this paper,Cu nanoparticles with an average size of about 100 nm were synthesized and Cu nanosolders were also prepared.The sintering and bonding experiments were carried out at 250 oC to 400 oC.The lowest electrical resistivity of 12.9 μΩ·cm was achieved at 400 oC,while the highest shear strength of 31.88 MPa was reached after bonding at the temperature of 400 oC and the pressure of 40 MPa.The sintering mechanism of Cu nanosolders and the bonding mechanism of Cu-Cu bonding process have also been proposed in this paper.These mechanisms provide a theoretical basis for the research of low temperature bonding.By using the Cu nanosolders,the Cu-Cu bonding temperature was effectively reduced to 400 oC.Although it can not fully meet the microelectronic packaging requirements,but it confirms the feasibility of Cu-Cu bonding by using sinterable Cu nanosolders,which laid the foundation for the following study.Secondly,based on the size effect and the sintering mechanism,a method of further reducing the Cu-Cu bonding temperature by smaller Cu nanoparticles were proposed.By improving the synthesis process,Cu nanoparticles were obtained with an average size of 60 nm.By using these nanoparticles,the high sintering performance of Cu nanosolders was achieved at 300 oC,and the resistivity was as low as 12.0 μΩ·cm.A high-quality Cu-Cu bonding interface was also achieved at 300 oC,with a much lower bonding pressure of 1.08 MPa.The shear of Cu-Cu joint can reach a high value of 31.88 MPa.Then,the aging test had been taken for the bonded samples by heating at 150 oC for 200 h.After testing,the bonding interface and the mechanical properties of bonded samples did not show obvious changes,which demonstrate the compactness of the bonding joint.We have also achieved the high-quality wafer level Cu-Cu bonding by using this type of Cu nanosolders.After bonding,the wafer substrates showed high degree of integrity,which indicate the Cu nanoparticles of 60 nm have a better prospect in microelectronic packaging.Thirdly,according to the fact that Ag nanoparticles have better sintering performance than Cu nanoparticles with similar size,we proposed a method of preparing the Cu base Cu-Ag composite nanosolders by adding Ag nanoparticles of 70 nm into Cu nanoparticles.At the temperature of 250 oC,the Cu2-Ag composite nanosolders(the Cu-Ag atomic ratio of 2:1)exhibited high sintering properties of low temperature.The resistivity of sintered Cu2-Ag film had reached 19.9 μΩ·cm,almost close to the resistivity of 12.0 μΩ·cm,which was brought by pure Ag nanosolders.The diffusion layer formation of Cu-Ag interface was observed by TEM,the sintering mechanism of Cu-Ag composite nanosolders was analyzed,and the key effect of Ag nanoparticles in enhancing the sintering performance was explained.With the bonding temperature of 250 oC,a compact bonding interface with a hige shear strength of 25.41 MPa was achieved by Cu2-Ag nanosolders,which was similar as that by using Ag nanosolders.Therefore,by using the Cu2-Ag nanosolders,the sintering and bonding performance were enhanced,and the cost-efficiency was guaranteed.Fourthly,to further research the low temperature Cu-Cu bonding method,we have proposed a novel type of Cu nanoaggregates with large amounts of ultra-small Cu nanoparticles on the surface.Based on these nanoaggregates,we have done the sintering and bonding researches.Excellent sintering and bonding properties can be achieved within 250 oC.By using Cu nanoaggregates,the lowest resistivity can reach 4.37 μΩ·cm,which is only 2.5 times than that of Cu bulk.The shear strength of the Cu-Cu joint can also achieve 25.36 MPa.This study has made great progress in the Cu-Cu bonding by using pure Cu nanosolders,which has a momentous research significance.In this work,a series of studies on the Cu-Cu bonding by using Cu-based nanosolders were carried out with the size effect as the theoretical main line.The research ideas and research results have the guiding significance for the development of microelectronic packaging.更多还原