電遷移效應對銅金屬連線之危害

Abstract

本論文中，將針對鉭金屬擴散阻障層（第4章）、低介電常數介電層聚亞醯銨PI2610（第5章）以及Cu/Al雙層結構合金（第6章）對銅金屬連線電遷移阻抗特性之影響作一深入之研究；第一個主題係研究鉭金屬擴散阻障層對銅金屬連線之微細構造、電遷移以及熱應力致電遷移特性之影響，第二個主題係研究及比較傳統介電材料及低介電常數材料間之熱傳導特性、微細構造、電遷移以及熱應力致電遷移特性之影響，第三個主題主要研究Cu/Al雙層結構合金對銅金屬連線之表面粗糙度、電阻、微細構造以及電遷移特性之影響。研究結果分述如下。 在鉭金屬擴散阻障層的研究方面，由於鉭具有與銅不互熔之材料特性，且最重要的是鉭金屬擴散阻障層有效強化銅之微細構造－其中包括銅之Cu(111)織構以及其晶粒大小，這主要是因為銅之Cu(111)晶面與鉭之β-Ta(200)晶面產生異質疊晶現象。在鉭金屬擴散阻障層對銅金屬連線之電遷移以及熱應力致電遷移特性之影響方面，Ta/Cu/Ta多層結構金屬導線在225ºC時之電遷移抵抗力大約為Cu單層結構金屬導線的二倍長；在歷經500次熱循環後，Ta/Cu/Ta多層結構金屬導線之電遷移抵抗力並未改變；而且，採用Ta/Cu/Ta多層結構金屬導線的電遷移抵抗力增加為Cu單層結構金屬導線的三倍長；Ta/Cu/Ta多層結構金屬導線與Cu單層結構金屬導線的電遷移活化能分別為0.77 eV與0.65 eV；在歷經500次熱循環後，Ta/Cu/Ta多層結構金屬導線之電遷移生命週期預測為100年，約比Cu單層結構金屬導線電遷移生命週期長十倍，這主要是因為鉭金屬擴散阻障層有效強化銅之織構及其晶粒，在本研究中銅之微細構造對銅金屬連線之電遷移以及熱應力致電遷移特性扮演極重要的角色，另外，相對於銅與介電層之介面，銅與鉭金屬擴散阻障層之介面有效減低電遷移效應，且鉭金屬擴散阻障層有提供電流分流的作用。其中Ta/Cu/Ta多層結構金屬導線之電遷移抵抗力低於Vaidya and Sinha的預測主要歸因於銅與鉭金屬擴散阻障層之介面仍是電遷移之主要擴散路徑。 在低介電常數介電層聚亞醯銨PI2610的研究方面，為探討採用低介電常數介電層之可行性，我們分別比較了聚亞醯銨PI2610 與 PETEOS SiO2兩種介電層的熱特性及其電遷移抵抗力，實驗結果顯示在PI2610基材上之銅金屬導線的熱阻抗是在PETEOS SiO2基材上的三倍，由於焦耳熱效應作用，當在PI2610基材上之銅金屬導線歷經電流密度1.4´107 A/cm2後75秒即因焦耳熱效應所產生之高溫使銅金屬導線呈現開路現象，其銅金屬導線開路主要是因為銅金屬導線上之溫度已超過聚亞醯銨PI2610的熱分解溫度。在低介電常數介電層聚亞醯銨PI2610對銅金屬導線之電遷移以及熱應力致電遷移特性之影響方面，在研究中我們發現介電層的高熱阻抗特性是造成電遷移破壞過程中金屬導線電阻值有較高之變化率的成因之一，銅金屬導線在PI2610基材與PETEOS SiO2基材上的電遷移活化能分別為0.87 eV與0.65 eV；在歷經500次熱循環後，在PI2610基材上之銅金屬導線的電遷移生命週期預測為79年，遠優於在PETEOS SiO2基材上之銅金屬導線(8年)。 在Cu/Al雙層結構合金的研究方面，實驗結果顯示鋁金屬基材上有效強化銅之微細構造與提高銅膜之表面平整度之效應，這主要是因為鋁與銅具有相同之立方晶體結構(面心立方)，以及鋁與銅其(111)面最靠近原子的間距只有10.7%，因此有效強化銅膜之I(111)/I(200)織構比；同時，當我們採用較厚之鋁金屬基材能更有效強化銅膜之Cu (111)優選方向。在Cu/Al雙層結構合金的電阻值方面，其電阻值隨毎增加1nm厚的鋁金屬基材大約增加0.2 mW•cm，這要比直接使用Al(Cu)合金來的低。在鋁金屬基材對銅金屬連線之電遷移之影響方面，鋁金屬基材有效提升銅金屬連線之電遷移抵抗力，在225ºC與電流密度1.2´107 A/cm2的測試條件下，採用50nm厚的鋁金屬基材的銅金屬連線之電遷移抵抗力大約為Cu單層結構金屬導線的十倍長。

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.