退火過程中介層對於奈米雙晶銅薄膜之異常晶粒成長的影響

The Effect of Interlayer on Abnormal Grain Growth of Nanotwinned Copper Thin Film during Annealing Process

陳昕遠(Chen, Hsin Yuan)

指導教授 ： 歐陽汎怡

國立清華大學/原子科學院/工程與系統科學系/碩士(2015年)

摘要

為了研究介層對奈米雙晶銅薄膜熱穩定性的影響，我們利用非平衡磁控濺鍍系統將兩種介層材料分別鍍在不同的矽基板上，接下來再將銅鍍在介層之上，而我們選用的介層材料分別為CrN和TiN。接著，我們將這兩批試片送入高溫爐之中，並分別進行退火30、60和120分鐘以了解介層對熱穩定性的影響。
藉由穿透式電子顯微鏡我們發現了在熱處理前的銅薄膜中含有非常多的雙晶，而這些雙晶都是平行基板生長的。退火之後，我們在使用CrN為介層的試片中發現了許多巨大的晶粒，而隨著退火時間的增加，這些晶粒也隨之長大。 晶粒大小從退火前的0.49μm變成退火120分鐘後的60μm。在退火過程中，這些巨大的晶粒會持續長大，甚至長到比膜厚還大。而這些晶粒經過各種儀器鑑定後，可以確定為(200)的晶粒，此外，我們同時也在這些巨大的晶粒中發現雙晶，和之前的雙晶不同，這些雙晶傾向傾斜於基板生長，並會從薄膜底部成長而且達到表面。而退火後，我們並沒有發現以TiN為介層的試片中發現微結構上有任何的改變。即便是退火兩小時後，晶粒大小和雙晶的成長方向都沒有發現到顯著的改變。我們推測造成這類結晶方向轉變得原因為表面能和應變能的競爭，若應變能為主宰之驅動力，則此時(200)晶粒較為穩定，且由於CrN經量測後發現是偏向(200)之生長方向，因此許多(200)的銅晶粒在其表面成核，造成了在退火時以CrN為介層的銅薄膜擁有較大的驅動力進行異常晶粒成長。而在性質的方面，我們發現若試片發生異常晶粒成長，其硬度、電阻、殘留應力皆會下降；若沒發生，則這些性質在退火後不會有明顯的改變。在此實驗中，以TiN為介層之銅薄膜展現出較好的熱穩定性，而以上結果也指出介層的種類的確會影響銅薄膜的熱穩定性。

關鍵字

銅 ； 雙晶 ； 薄膜 ； 異常晶粒成長

並列摘要

Highly pure nanotwinned copper thin films which would be applied in semiconductor as interconnects were fabricated by using unbalanced magnetron sputtering (UBMS) system. In order to investigate the influence of different interlayers on microstructure and thermal stability of Cu film, CrN and TiN thin film was selected and respectively deposited onto a silicon wafer as an interlayer before coating Cu thin films. Then, the samples with different interlayers were subject to isothermally anneal at 250℃ in 5×10-6 torr for 30, 60 and 120 minutes, respectively, to study the thermal stability of Cu thin films.
After deposition, the Cu thin film with high density nanotwin structure was observed in both interlayer samples, and all twin boundaries was found to be parallel to the Si substrate. However, some abnormal large grains were found in the samples with CrN interlayer and their grain size increased from 0.49μm to 60μm after annealing at 250℃ for 120 minutes. These huge grains were (200) grains and some huge grains were found to grow until penetrate the thickness of Cu thin film when annealing time increases. Furthermore, the direction of twin boundary in these huge grains was observed to tend to be perpendicular to Si substrate. On the other hand, there was no significant microstructure change in the samples with TiN interlayer after annealing for 120 minutes; the grain sizes of nano-twinned Cu thin film still remain and the boundary of nano-twin Cu is still parallel to the Si substrate. We proposed that the texture evolution from {111} to {200} of Cu film was resulted from the competition between surface/interface minimization and strain energy minimization, and strain energy dominates in Cu thin film with CrN interlayer which cause the {200} grains become stable. The CrN interlayer was found with a {200}-preferred orientation, which may cause many {200} Cu seeds nucleate onto the interlayer. During annealing process, lots of {200} Cu seeds become the driving force of abnormal grain growth, and the {200} abnormal grains appear. Regarding to the properties of nanotwinned copper thin films, the residual stresses, electrical resistivity and hardness of Cu thin films decreased for the samples with CrN interlayer due to abnormal grain growth of Cu thin film, whereas these properties of Cu thin films with TiN interlayer didn’t change a lot after 120 minutes of annealing time. The above results suggest that the thermal stability of nano-twinned Cu thin film is strongly related to the interlayer and the samples with TiN interlayer exhibits more thermal stability than CrN interlayer.

並列關鍵字

copper ； twin ； thin film ； abnormal grain growth

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