标题: | 电迁移效应对铜金属连线之危害 Electromigration-Induced Failures in Cu-Based Metallization |
作者: | 秦玉龙 Yu-Lung Chin 邱碧秀 Bi-Shiou Chiou 电子研究所 |
关键字: | 铜;电迁移;应力迁移;Cu;electromigration;stress migration |
公开日期: | 2001 |
摘要: | 本论文中,将针对钽金属扩散阻障层(第4章)、低介电常数介电层聚亚醯铵PI2610(第5章)以及Cu/Al双层结构合金(第6章)对铜金属连线电迁移阻抗特性之影响作一深入之研究;第一个主题系研究钽金属扩散阻障层对铜金属连线之微细构造、电迁移以及热应力致电迁移特性之影响,第二个主题系研究及比较传统介电材料及低介电常数材料间之热传导特性、微细构造、电迁移以及热应力致电迁移特性之影响,第三个主题主要研究Cu/Al双层结构合金对铜金属连线之表面粗糙度、电阻、微细构造以及电迁移特性之影响。研究结果分述如下。 在钽金属扩散阻障层的研究方面,由于钽具有与铜不互熔之材料特性,且最重要的是钽金属扩散阻障层有效强化铜之微细构造-其中包括铜之Cu(111)织构以及其晶粒大小,这主要是因为铜之Cu(111)晶面与钽之β-Ta(200)晶面产生异质叠晶现象。在钽金属扩散阻障层对铜金属连线之电迁移以及热应力致电迁移特性之影响方面,Ta/Cu/Ta多层结构金属导线在225ºC时之电迁移抵抗力大约为Cu单层结构金属导线的二倍长;在历经500次热循环后,Ta/Cu/Ta多层结构金属导线之电迁移抵抗力并未改变;而且,采用Ta/Cu/Ta多层结构金属导线的电迁移抵抗力增加为Cu单层结构金属导线的三倍长;Ta/Cu/Ta多层结构金属导线与Cu单层结构金属导线的电迁移活化能分别为0.77 eV与0.65 eV;在历经500次热循环后,Ta/Cu/Ta多层结构金属导线之电迁移生命周期预测为100年,约比Cu单层结构金属导线电迁移生命周期长十倍,这主要是因为钽金属扩散阻障层有效强化铜之织构及其晶粒,在本研究中铜之微细构造对铜金属连线之电迁移以及热应力致电迁移特性扮演极重要的角色,另外,相对于铜与介电层之介面,铜与钽金属扩散阻障层之介面有效减低电迁移效应,且钽金属扩散阻障层有提供电流分流的作用。其中Ta/Cu/Ta多层结构金属导线之电迁移抵抗力低于Vaidya and Sinha的预测主要归因于铜与钽金属扩散阻障层之介面仍是电迁移之主要扩散路径。 在低介电常数介电层聚亚醯铵PI2610的研究方面,为探讨采用低介电常数介电层之可行性,我们分别比较了聚亚醯铵PI2610 与 PETEOS SiO2两种介电层的热特性及其电迁移抵抗力,实验结果显示在PI2610基材上之铜金属导线的热阻抗是在PETEOS SiO2基材上的三倍,由于焦耳热效应作用,当在PI2610基材上之铜金属导线历经电流密度1.4´107 A/cm2后75秒即因焦耳热效应所产生之高温使铜金属导线呈现开路现象,其铜金属导线开路主要是因为铜金属导线上之温度已超过聚亚醯铵PI2610的热分解温度。在低介电常数介电层聚亚醯铵PI2610对铜金属导线之电迁移以及热应力致电迁移特性之影响方面,在研究中我们发现介电层的高热阻抗特性是造成电迁移破坏过程中金属导线电阻值有较高之变化率的成因之一,铜金属导线在PI2610基材与PETEOS SiO2基材上的电迁移活化能分别为0.87 eV与0.65 eV;在历经500次热循环后,在PI2610基材上之铜金属导线的电迁移生命周期预测为79年,远优于在PETEOS SiO2基材上之铜金属导线(8年)。 在Cu/Al双层结构合金的研究方面,实验结果显示铝金属基材上有效强化铜之微细构造与提高铜膜之表面平整度之效应,这主要是因为铝与铜具有相同之立方晶体结构(面心立方),以及铝与铜其(111)面最靠近原子的间距只有10.7%,因此有效强化铜膜之I(111)/I(200)织构比;同时,当我们采用较厚之铝金属基材能更有效强化铜膜之Cu (111)优选方向。在Cu/Al双层结构合金的电阻值方面,其电阻值随毎增加1nm厚的铝金属基材大约增加0.2 mW•cm,这要比直接使用Al(Cu)合金来的低。在铝金属基材对铜金属连线之电迁移之影响方面,铝金属基材有效提升铜金属连线之电迁移抵抗力,在225ºC与电流密度1.2´107 A/cm2的测试条件下,采用50nm厚的铝金属基材的铜金属连线之电迁移抵抗力大约为Cu单层结构金属导线的十倍长。 This thesis studies the relationship between electromigration resistances of Cu-based metallization with the diffusion barrier layer of tantalum (Chapter 4), the low dielectric constant polyimide of PI2610 (Chapter 5), and the alloy of Cu/Al (Chapter 6), respectively. The first topic focuses on the effects of a Ta barrier layer on the microstructure, electromigration and thermal-stress-enhanced electromigration of Cu interconnects. The second topic is the effects of low dielectric constant underlayer on the thermal characteristics, microstructure, electromigration and thermal-stress-enhanced electromigration of Cu interconnects. The final topic studies the effects of a Cu/Al alloy on the surface morphology, electrical resistance, microstructure, and electromigration resistance of copper interconnects. The results are described as follows. The immiscible Ta barrier layer enhances the microstructures of Cu films, both the (111) texture and the median grain size of the annealed Cu films increases, due to the heteroepitaxial relationship between the hexagonal close-packed atomic array in the Cu(111) plane and the pseudohexagonal configuration of β-Ta(200). At 225ºC, the electromigration median time-to-failure (MTF) of Ta/Cu/Ta multilayer interconnects is about two times longer than that of Cu monolayer interconnects. After 500 thermal cycles, the MTF of Ta/Cu/Ta multilayer interconnects does not change and is about three times longer than that of Cu monolayer interconnects. The activation energy Ea of electromigration of Ta/Cu/Ta multilayer interconnects is 0.77 eV, which is higher than that of Cu monolayer interconnects (0.65 eV). A lifetime of 100 years is predicted for Ta/Cu/Ta multilayer interconnects with and without 500 thermal cycle stresses. Since the Ta/Cu/Ta specimen has enhanced crystallographic texture and larger grain size, it has both higher electromigration endurance and thermal stress resistance than Cu. In this work, the microstructure of thin film conductors is of vital importance to influence electromigration behavior, where crystallographic texture, grain size, and grain size distribution impact the reliability of Cu-based metallizations. Moreover, compare with the Cu/liner surfaces, the capping layers of Ta suppress the interface migration and act as a shunt layer. However, the measured MTF is shorter than that predicted by an equation proposed by Vaidya and Sinha. The shorter MTF and smaller Ea values for the Ta/Cu/Ta multilayer compared to the predicted ones are due to the weak Ta/Cu interface. To evaluate the feasibility of the low dielectric constant polyimide for the intermetal dielectric applications, the thermal characteristics and electromigration resistance of two dielectrics, PI2610 and PETEOS SiO2, are investigated. It is shown that the thermal impedance of metal lines on polyimide is about three times of those on PETEOS SiO2. The open-circuit failure occurs after 75 sec stressing at a current density of 1.4´107 A/cm2 for specimens with polyimide underlayer due to the high joule heating effect which causes the extreme high temperature rise at interconnects. Hence, decomposition of polyimide occurred which further accelerated the breakdown of interconnects. The thermal impedance is one of the major reasons which cause the electrical resistance change during electromigration test. The activation energies for electromigration of Cu are 0.87 eV for SiO2/Cu/PI2610 and 0.65 eV for SiO2/Cu/ SiO2. After 500 thermal cycles, both the median time to failure and activation energy Ea decrease for EM, and lognormal standard deviation increases. At 225ºC, the median time to failure of SiO2/Cu/PI2610 is about two times longer than that of SiO2/Cu/ SiO2. A lifetime of 79 years is predicted for SiO2/Cu/PI2610 specimens with 500 thermal cycles stresses. The effects of Al underlayer with respect to texture control and electromigration resistance of Cu films are investigated in this study. The Al underlayer enhances the (111) texture of Cu films and the surface smoothness of the annealed Cu film. The same cubic crystal structure (face-centered cubic) and small difference of the nearest interatomic distance of the (111) plane between Cu and Al result in the increase of the peak ratio I(111)/I(200) of Cu films. The thicker the Al film, the more preferred the Cu (111) orientation. The resistivity of the Cu/Al bilayer film, which increases at about 0.2 mW•cm per 1-nm-thick Al underlayer, is smaller than the reported value of single phase Cu(Al) solution. The electromigration resistance of Cu interconnects is also improved when the Al underlayer is present. The median time to failure of Cu interconnects with a 5-nm-thick Al underlayer is longer by one order of magnitude than that without an Al underlayer at 240°C and 12 MA/cm2. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#NT900428081 http://hdl.handle.net/11536/68773 |
显示于类别: | Thesis |