基于化学约束刻蚀剂层技术的铜表面加工研究
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摘要
铜具有低的电阻率和高的电子迁移阻力,在半导体产业中作为为集成电路的互连材料而得到广泛的应用。为了解决随集成电路小型化而出现的RC延迟、层间串扰等问题,low-k材料成为互连线的主要层间介质。化学机械抛光(Chemical Mechanical Polishing, CMP)是目前进行铜互连层平坦化加工的主流加工工艺,该技术整合了抛光液的化学作用和抛光垫的机械作用,是实现高精度平坦化的有效手段。但是,low-k材料的低硬度和多孔性特征使low-k/Cu结构的机械强度很低。抛光液中的磨粒,抛光压力、摩擦力等会引起铜互连线和low-k材料之间的界面变形甚至剥离破坏。
为了实现铜的无应力高精度加工,本文提出了基于化学约束刻蚀剂层技术(Confined Etchant Layer Technique, CELT)的铜加工方法。作为距离敏感性技术,CELT通过化学刻蚀的方式实现微/纳米尺度的材料选择性去除,并且没有机械损伤和热作用。将CELT技术用于铜的加工需要开发铜的化学约束刻蚀工作液。本文选择Fe(bpy)3(ClO4)2和Ru(bpy)3Cl2作为工作液的前驱体,采用循环伏安法分别研究了乙二酸、三正丙胺和氨基乙酸等三种不同约束剂的约束效果。根据分析结果,乙二酸的作用最显著,同时,约束刻蚀工作液的pH值和乙二酸浓度也进行了优化。
为了验证工作液的效果,分别对优化后的铁基/钌基约束刻蚀工作液(约束剂为乙二酸)进行了系统的刻蚀实验。刻蚀试验平台由三维运动平台、电化学工作站和数据处理中心等三部分组成,并且设计了工作电极的卡具和电解池。实验结果表明,对于两种刻蚀工作液,乙二酸的加入明显限制了刻蚀剂沿电极表面的径向扩散。与不含乙二酸的工作液相比,含有乙二酸的工作液可以得到边界更加清晰的刻蚀坑。除了传统的铂丝点状电极外,本文还设计了新型的带状铂丝电极,该电极的应用突破了传统电极的单点加工,实现了具有一定宽度区域的连续加工,为铜的大面积加工奠定了基础。
最后,为了将本文开发的铜约束刻蚀工作液用于铜的大面积刻蚀,本文提出了大面积电极静态刻蚀和小面积电极动态刻蚀两种策略。对于前一种策略,本文研究了大面积工具电极和工件之间的距离控制；基于挤压膜的理论和试验验证表明,对于大面积的电极和工件,两者之间液膜会在开始的几分钟内迅速进入相对稳定状态,产生的超薄液膜会极大地阻碍间距的进一步减小。对于动态刻蚀,铜表面的动态刻蚀试验表明,电极和工件之间的运动会引发刻蚀体系内组分传质方式的改变,从而对刻蚀结果造成一定影响。
Copper has been widely employed as integrated circuits'interconnect material in the semiconductor industry due to its low electric resistance and high electro-migration resistance. In order to solve the problems like RC delay and interlayer crosstalk, the low-k materials have been widely used as the inter-level dielectric for interconnects in the manufacturing of integrated circuits. Chemical Mechanical Polishing (CMP) is the traditional method for planarization of copper interconnects. As an effective method for high precision planarization, it integrates the chemical effects from the polishing slurry and the mechanical effects from the polishing pad. However, because of low-k materials' low hardness and high porosity, the low-k/Cu structure has a low mechanical strength. The abrasives in the polishing slurry, the polishing pressure and related polishing friction will deform the structure of low-k/Cu and even cause the separation between the copper wire and the low-k material.
In order to achieve stress-free and high precision machining of copper, a new method was proposed based on Confined Etchant Layer Technique (CELT). As a distance sensitive method, it can realize micro/nano precision material removal by chemical etching, which has no mechanical damage and heat effect. The core issue of this method is the development of the working solution for copper. In this paper, Fe(bpy)3(ClO4)2and Ru(bpy)3Cl2are selected as the precursors of the working solution. Cyclic voltammetry method was used to investigate the effects of Oxalic acid, Tri amphetamine and Aminoacetic acid. According to the results, Oxalic acid has a better effect. Moreover, the pH of the working solution and the concentration of oxalic acid were optimized.
In order to verify the effect of the optimized working solution, systematic etching experiments were carried out for both the [Fe(bpy)3]2+and the [Ru(bpy)3]2+optimized working solution with oxalic acid as the scavenger. The experiment table for copper etching has three parts, a3-D motion platform, an electrochemical working center and a data processing center. Also, the fixture of the working electrode and the electrolytic cell were designed according to related requirements. The experiments showed that, the scavenger, oxalic acid, can greatly hinder the radial diffusion of the etchant for both the [Fe(bpy)3]2+and the [Ru(bpy)3]2+working solutions. Compared with the etching results from working solutions without scavenger, the working solution with scavenger can result in clearer etched pits and improved etching resolution. Moreover, besides the traditional single point cylinder electrode, a novel band electrode was designed. With the new electrode, a2-D band area can be machined, which laid foundation of large area machining of copper.
At last, in order to realize large area material removing of copper using the developed working solutions, two different strategies were presented, that is, static etching with a large working electrode and dynamic etching with a small electrode. For the first method, the strategy for the control of the distance between the working electrode and the workpiece was studied. Based on the squeeze film theory and related experiments, the distance will fall into a relative constant stage after first few minutes'rapid falling, and the the ultra-thin liquid film will deter the decrease of the distance between the large area electrode and the workpiece. For the dynamic etching of copper, dynamic etching experiments showed that the motion between the electrode and the workpiece will change the style of mass transfer, which has an effect on the etching result.
为了实现铜的无应力高精度加工,本文提出了基于化学约束刻蚀剂层技术(Confined Etchant Layer Technique, CELT)的铜加工方法。作为距离敏感性技术,CELT通过化学刻蚀的方式实现微/纳米尺度的材料选择性去除,并且没有机械损伤和热作用。将CELT技术用于铜的加工需要开发铜的化学约束刻蚀工作液。本文选择Fe(bpy)3(ClO4)2和Ru(bpy)3Cl2作为工作液的前驱体,采用循环伏安法分别研究了乙二酸、三正丙胺和氨基乙酸等三种不同约束剂的约束效果。根据分析结果,乙二酸的作用最显著,同时,约束刻蚀工作液的pH值和乙二酸浓度也进行了优化。
为了验证工作液的效果,分别对优化后的铁基/钌基约束刻蚀工作液(约束剂为乙二酸)进行了系统的刻蚀实验。刻蚀试验平台由三维运动平台、电化学工作站和数据处理中心等三部分组成,并且设计了工作电极的卡具和电解池。实验结果表明,对于两种刻蚀工作液,乙二酸的加入明显限制了刻蚀剂沿电极表面的径向扩散。与不含乙二酸的工作液相比,含有乙二酸的工作液可以得到边界更加清晰的刻蚀坑。除了传统的铂丝点状电极外,本文还设计了新型的带状铂丝电极,该电极的应用突破了传统电极的单点加工,实现了具有一定宽度区域的连续加工,为铜的大面积加工奠定了基础。
最后,为了将本文开发的铜约束刻蚀工作液用于铜的大面积刻蚀,本文提出了大面积电极静态刻蚀和小面积电极动态刻蚀两种策略。对于前一种策略,本文研究了大面积工具电极和工件之间的距离控制；基于挤压膜的理论和试验验证表明,对于大面积的电极和工件,两者之间液膜会在开始的几分钟内迅速进入相对稳定状态,产生的超薄液膜会极大地阻碍间距的进一步减小。对于动态刻蚀,铜表面的动态刻蚀试验表明,电极和工件之间的运动会引发刻蚀体系内组分传质方式的改变,从而对刻蚀结果造成一定影响。
Copper has been widely employed as integrated circuits'interconnect material in the semiconductor industry due to its low electric resistance and high electro-migration resistance. In order to solve the problems like RC delay and interlayer crosstalk, the low-k materials have been widely used as the inter-level dielectric for interconnects in the manufacturing of integrated circuits. Chemical Mechanical Polishing (CMP) is the traditional method for planarization of copper interconnects. As an effective method for high precision planarization, it integrates the chemical effects from the polishing slurry and the mechanical effects from the polishing pad. However, because of low-k materials' low hardness and high porosity, the low-k/Cu structure has a low mechanical strength. The abrasives in the polishing slurry, the polishing pressure and related polishing friction will deform the structure of low-k/Cu and even cause the separation between the copper wire and the low-k material.
In order to achieve stress-free and high precision machining of copper, a new method was proposed based on Confined Etchant Layer Technique (CELT). As a distance sensitive method, it can realize micro/nano precision material removal by chemical etching, which has no mechanical damage and heat effect. The core issue of this method is the development of the working solution for copper. In this paper, Fe(bpy)3(ClO4)2and Ru(bpy)3Cl2are selected as the precursors of the working solution. Cyclic voltammetry method was used to investigate the effects of Oxalic acid, Tri amphetamine and Aminoacetic acid. According to the results, Oxalic acid has a better effect. Moreover, the pH of the working solution and the concentration of oxalic acid were optimized.
In order to verify the effect of the optimized working solution, systematic etching experiments were carried out for both the [Fe(bpy)3]2+and the [Ru(bpy)3]2+optimized working solution with oxalic acid as the scavenger. The experiment table for copper etching has three parts, a3-D motion platform, an electrochemical working center and a data processing center. Also, the fixture of the working electrode and the electrolytic cell were designed according to related requirements. The experiments showed that, the scavenger, oxalic acid, can greatly hinder the radial diffusion of the etchant for both the [Fe(bpy)3]2+and the [Ru(bpy)3]2+working solutions. Compared with the etching results from working solutions without scavenger, the working solution with scavenger can result in clearer etched pits and improved etching resolution. Moreover, besides the traditional single point cylinder electrode, a novel band electrode was designed. With the new electrode, a2-D band area can be machined, which laid foundation of large area machining of copper.
At last, in order to realize large area material removing of copper using the developed working solutions, two different strategies were presented, that is, static etching with a large working electrode and dynamic etching with a small electrode. For the first method, the strategy for the control of the distance between the working electrode and the workpiece was studied. Based on the squeeze film theory and related experiments, the distance will fall into a relative constant stage after first few minutes'rapid falling, and the the ultra-thin liquid film will deter the decrease of the distance between the large area electrode and the workpiece. For the dynamic etching of copper, dynamic etching experiments showed that the motion between the electrode and the workpiece will change the style of mass transfer, which has an effect on the etching result.
引文
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[3]谢鑫.互连线对集成电路设计的影响[J].科技资讯,2009(27)：99.
[4]张文杰,易万兵,吴瑾.铝互连线的电迁移问题及超深亚微米技术下的挑战[J].物理学报,2006(10)：5424-5434.
[5]赵冬月.以Ti-Al薄膜为阻挡层的硅基铁电电容器的集成[D].保定：河北大学,2011.
[6]钟波.考虑温度的纳米级互连线延迟和功耗研究[D].西安：西安电子科技大学,2010.
[7]何旭曙.ULSI多层金属布线中的热效应和可靠性研究[D].广州：华南师范大学,2007.
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[15]Kondo S, Sakuma N,Homma Y, et al. Abrasive-free polishing for copper damascene interconnection[J].J Electrochem Soc,2000,147(10):3907-3913.
[16]张伟,路新春,刘宇宏等.氨基乙酸-H_2O_2体系抛光液中铜的化学机械抛光研究[J].摩擦学学报,2008,(4)：366-371.
[17]张伟,路新春,刘宇宏等.缓蚀剂在铜化学机械抛光过程中的作用研究[J].摩擦学学报,2007,(5)：401-405.
[18]Okiharu K, Toshiyuki E. Ultra flat and ultra smooth Cu surfaces produced by abrasive free chemical mechanical planarizationpolishing using vacuum ultraviolet light [J]. Precision Engineering,2011,35:669-676.
[19]Lappin D, Mohammadi A, Takahata K. An experimental study of electrochemical polishing for micro-electro-discharge-machined stainless-steel stents [J]. Journal of Materials Science:Materials in Medicine,2012,23(2):349.
[20]许旺,张楷亮,杨保和.新型铜互连方法--电化学机械抛光技术研究进展[J].半导体技术,2009(6)：521-524.
[21]Lee S, Lee Y, Du M. The polishing mechanism of electrochemical mechanical polishing technology [J]. Journal of Materials Processing Technology,2003,140(1-3):280-286.
[22]王晓明.脉冲电化学及其复合光整加工机理和表面特性的研究[D].大连：大连理工大学,2002.
[23]蒋利民.约束刻蚀剂层技术(CELT)用于金属材料表面复杂三维微结构的加工研究[D].厦门：厦门大学,2007.
[24]马信洲.约束刻蚀剂层技术用于Ni-Ti合金、熔融石英和ZnO纳米线阵列的三维微加工的研究[D].厦门：厦门大学,2007.
[25]黄海苟.约束刻蚀剂层技术用于砷化镓三维规整细微图形的复制加工[D].厦门：厦门大学,2001.
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[2]刘静,吴振宇,汪家友等.铜互连电迁移可靠性的研究进展[J].微电子学,2007(3)：364-368.
[3]谢鑫.互连线对集成电路设计的影响[J].科技资讯,2009(27)：99.
[4]张文杰,易万兵,吴瑾.铝互连线的电迁移问题及超深亚微米技术下的挑战[J].物理学报,2006(10)：5424-5434.
[5]赵冬月.以Ti-Al薄膜为阻挡层的硅基铁电电容器的集成[D].保定：河北大学,2011.
[6]钟波.考虑温度的纳米级互连线延迟和功耗研究[D].西安：西安电子科技大学,2010.
[7]何旭曙.ULSI多层金属布线中的热效应和可靠性研究[D].广州：华南师范大学,2007.
[8]朱冬平,黄河,邝嘉.集成电路的互连线延迟分析[J].现代计算机(专业版),2008(3)：18-22.
[9]苏建修,高虹,陈锡渠等.基于磨损行为的单晶硅片化学机械抛光材料的去除特性[J].纳米技术与精密工程,2009(3)：265-269.
[10]苏建修,杜家熙,陈锡渠等.基于磨损行为的单晶硅片化学机械抛光界面接触形式研究[J].摩擦学学报,2008(2)：108-111.
[11]苏建修.IC制造中硅片化学机械抛光材料去除机理研究[D].大连：大连理工大学,2006.
[12]李佩,汪辉,周华等.铜互连CMP碟形缺陷及铜残留的研究[J].集成电路应用,2008(6)：33-36.
[13]李军,左敦稳,朱永伟等.无磨料化学机械抛光的研究进展[J].机械制造与自化,2008(6)：5-8.
[14]Matsuda T, Takahashi H, Tsumgaya M, et al. Characteristics of abrasive-free micelle slurry for copper CMP[J].J Electrochem Soc,2003,150(9):G532-G536.
[15]Kondo S, Sakuma N,Homma Y, et al. Abrasive-free polishing for copper damascene interconnection[J].J Electrochem Soc,2000,147(10):3907-3913.
[16]张伟,路新春,刘宇宏等.氨基乙酸-H_2O_2体系抛光液中铜的化学机械抛光研究[J].摩擦学学报,2008,(4)：366-371.
[17]张伟,路新春,刘宇宏等.缓蚀剂在铜化学机械抛光过程中的作用研究[J].摩擦学学报,2007,(5)：401-405.
[18]Okiharu K, Toshiyuki E. Ultra flat and ultra smooth Cu surfaces produced by abrasive free chemical mechanical planarizationpolishing using vacuum ultraviolet light [J]. Precision Engineering,2011,35:669-676.
[19]Lappin D, Mohammadi A, Takahata K. An experimental study of electrochemical polishing for micro-electro-discharge-machined stainless-steel stents [J]. Journal of Materials Science:Materials in Medicine,2012,23(2):349.
[20]许旺,张楷亮,杨保和.新型铜互连方法--电化学机械抛光技术研究进展[J].半导体技术,2009(6)：521-524.
[21]Lee S, Lee Y, Du M. The polishing mechanism of electrochemical mechanical polishing technology [J]. Journal of Materials Processing Technology,2003,140(1-3):280-286.
[22]王晓明.脉冲电化学及其复合光整加工机理和表面特性的研究[D].大连：大连理工大学,2002.
[23]蒋利民.约束刻蚀剂层技术(CELT)用于金属材料表面复杂三维微结构的加工研究[D].厦门：厦门大学,2007.
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