超大型積體電路鋁合金薄膜應力及微結構之變化

Abstract

超大型積體電路技術朝向尺寸不斷縮小的多層金屬內連線發展，從穩定性 的觀點來看，目前普遍用於內層金屬導線的鋁在材料性質方面面臨了更嚴 格的要求；例如，鋁的熔點低、機械性質不佳及高擴散性等性質,導致電 子遷移及應力遷移的發生,造成鋁導線的破壞。關於鋁導線在製程後應力 遷移的報導始於1984年,應力的產生來自於鋁及其周圍介電材料之熱膨脹 程度不同，為了舒緩此應力而導致應力遷移的現象發生。至此,應力引發 的孔洞生成或應力遷移已成為影響ULSI內連線之可靠度的一重要因素，特 別是在次微米線寬的要求下。 在積體電路多層導線結構中，金屬線的變 形不只受到矽基地的限制,亦受到絕緣層、阻障層與保護層的影響。這些 材料遠較金屬硬,並在金屬應力狀態的控制中伴演著相當重要的角色，也 因此影響著金屬化的穩定性。除了導線的應力外,微結構亦決定導線的應 力狀態。 在本研究中,我們首先採用了兩種不同濺度溫度製成的鋁合金薄膜: 200℃ 濺度的Al-1wt.%Si-0.5wt.%Cu薄膜與500℃濺的Al-1wt.%Si-0.5wt.%Cu薄 膜。利用這兩種薄膜探討Al/Ti與Al/TiN之熱應力、微結構及界面反應對 鋁薄膜應力層之影響。Ti或TiN可強化200℃濺鍍之鋁薄膜,而在500℃濺鍍 的鋁薄膜中,此強化作用消失,這是因為高溫導致較少之晶界及缺陷而使得 塑性變形易發生。200℃濺度的Al薄膜之晶粒較500℃濺度的Al薄膜之晶粒 小,這是因為高溫有較大之成長驅動力所致。 此外,本文探討鋁合金薄膜上覆蓋SiOF及其他介電材料時在熱循環期間鋁 合金的應力與晶粒大小的變化。並討論這些變化對內層金屬之材料穩定性 的影響。結果顯示，SiOF可幫助舒解鋁合金中因熱膨脹差易所造成的拉伸 應力,但退火時釋出的F對鋁薄膜的可靠度造成嚴重之威脅。此外，當鋁薄 膜上鍍上高強度之金屬,其晶粒成長亦受限制。 Al/Ti與Al/ TiN上覆以SiOF於熱循環時之應力與晶粒大小的改變亦被探討。並比較了 不同結構之薄膜的應力鬆弛與彈、塑性行為。Al/Ti與Al/TiN 的結構明顯 地強化鋁薄膜，而當SiOF覆於此結構上時會促使鋁薄膜產生塑性變形。以 介電材料保護之鋁薄膜,其晶粒成長程度較小。以TiN做為阻障層可阻礙退 火時F原子穿透至鋁層。 The continuing advances in ultralarge scale integration (ULSI) technology necessitates the development of multilayered metal wiring interconnects with ever-shrinking feature size. Alum- inum, the most commonly used metal for interconnects so far, has to meet much more stringent material requirements from reliability point of view. For example, the low melting point, mediocre mechanical strength, and high diffusivity of Al are di- rectly responsible for the failure of Al lines by electromigration and stress migration. Stress migration in the Al lines after processing was first reported in 1984. This phenomenon was attributed to the relaxation of thermal stresses generated during chip processing, the source of which was the thermal expansion mismatch between the Al and its surrounding dielectrics. Since then, stress-induced void formation, or stress migration, has become an important reliability concern for ULSI interconnects, particularly for sub-micron line structures. In intergrated circuits, the metal lines are incorporated into a multilayered wiring structure where they are confined not only by the substrate but also by the dielectric, passivation and bar- rier layers. The confinement of the relatively soft metal lines by the surrounding rigid layers plays an important role in control- ling the stress state and hence reliability of metallization. Be- sides the stress issues in metal lines, the microstructure also has an important effect on the stress level of Al alloy films. In this study, we have two different kinds of Al alloy films: 200 ℃sputtered Al-1wt%Si-0.5wt%Cu and 500℃sputtered Al- 1wt%Si-0.5wt%Cu films The thermal stresses, microstructures and interfacial reaction of Ti- and TiN-capped Al alloy thin films and the effect of microstructure on the stress levels of Al alloy thin films are investigated first. Ti or TiN can strength the singleAl alloy films sputtered at 200℃. The strength effect disappear in the 500℃ Al alloy films capped by Ti and TiN due to the lower grain boundary and defect density which make plastic deformation arises easily. Grain size increase of 200℃ sputtered Al alloy films is smaller than that of 500℃sputtered Al alloy films because of the larger driving force for grain growth. Than variations in stress and grain size of Al alloy thin films passivated by multilayered fluorinated silicon dioxide (SiOF) and other dielectrics during repetitive thermal cycling are in- vestigated. Implications of these variations in the materials reli- ability of multilevel interconnects are discussed. The results in- dicate that the presence of SiOF helps relieve part of the tensile stress induced in Al alloy films due to thermal mismatch while the release of free fluorine-containing radicals upon annealing may pose potential threat to the reliability of Al layers. In addi- tion, grain growth of Al films is limited when a mechanically harder dielectric layer, such as nitride, is imposed on top of Al. Variations in stress and grain size of Ti- and TiN- capped Al thin films passivated by fluorinated silicon dioxide (SiOF) durng repetitive thermal cycling are also investigated in this study. The amount stress relaxation, elastic and plastic behav- iors of these thin film structures are compared. Ti and TiN cap layers strengthen the single Al film significantly while the pres- ence of SiOF induces plastic deformation of metal layers. Less amount of grain growth is associated with dielectric passivated Al film. The penetration of fluorine into Al upon annealing can be reduced by TiN barrier layer.