紫外光輔助退火後處理對碳氮化矽薄膜的影響

Abstract

隨著VLSI元件尺寸不斷的縮小，為了降低隨著尺寸縮小而不斷上升的後段封裝中的RC延遲，具備低介電常數的內層介電層(ILD)與蝕刻終止層/擴散阻障層(ES/DB)是不可或缺的，然而最大的問題是在於低介電常數與因為低介電常數而造成的極差機械性質間的取捨；當介電常數大幅度降低的同時所帶來的較差的機械性質，可能導致元件在化學機械研磨(CMP)與覆晶封裝(CPI)等製程中遭到破壞， 為了解決這個問題，紫外光輔助退火已經被廣泛的應用在以碳氧化矽(SiCO)製作的內層介電層中，達到在降低介電常數的同時又可以提升機械性質；然而，目前仍然缺乏有關於紫外光輔助退火是否也能如預期般對蝕刻終止層/擴散阻障層(ES/DB)造成影響的研究。 本研究注重於探討紫外光輔助退火對於應用在銅連接製程中的碳氮化矽材料在結構以及性質上的影響，特別是對於用N-methyl-aza-2,2,4-trimethylsilacyclopentane (MTSCP) 高含碳單一前驅物並用射頻電漿輔助化學氣象沉積法在80 – 200 ℃基板溫度與1 Torr工作壓力下製備的碳氮化矽薄膜；研究中比較了在不同溫度沉積的碳氮化矽薄膜經過兩種後處理：(1) 單純退火：在400 ℃下持續5 分鐘 (2)紫外光輔助退火：在400 ℃下並同時用80 mW/cm2 強度的紫外光照射持續5 分鐘。 在本研究中發現，紫外光輔助退火可以移除有機相、打斷高極性的氮氫鍵結(NH)並且提升矽氮(SiN)主體結構，因此，紫外光輔助退火可以有效提升碳氮化矽薄膜的性質。本研究得到的最佳碳氮化矽薄膜是以MTSCP單一前驅物在150℃基板溫度、1Torr工作壓力以及0.27 W/cm2的電漿密度之下所製備，經過紫外光輔助退火之後，介電常數降低了0.22從4.05降低到3.83，同時沒有彈性模數的降低維持在大約20 GPa，根據這個結果，紫外光輔助退火是個對應用在後段連結封裝中的碳氮化矽材料很有效的後處理製程。

As the feature size of VLSI devices scaling down, a lower dielectric constant both in the interlayer dielectric (ILD) and etch-stop/diffusion barrier (ES/DB) layer is necessitated to slow down the increasing trend of RC propagation delay in the backend interconnect. However the critical problem is the dilemma between dielectric constant and mechanical properties of such dielectrics. When the dielectric constant is much reduced, its mechanical properties may be too weak to survive processes such as chemical-mechanical polish and packaging due to chip-package interaction. To solve this problem, UV-assisted thermal annealing has been used to post-treat carbon-doped silicon (SiCOH) ILD for reduced k-value but enhanced mechanical strength. Yet, there is a lack in the understanding if UV-assisted thermal anneal can favorably affect the dielectric constant and mechanical strength on the low-k ES/DB materials. This work investigates the effect of UV-assisted thermal annealing on the structure and properties of silicon carbonitride (SiCxNy) films for application in copper interconnects. In particular, SiCxNy films are deposited by plasma-enhanced chemical vapor deposition at substrate temperature 80 – 200 ℃ and 1 Torr using a single precursor, N-methyl-aza-2,2,4-trimethylsilacyclopentane (MTSCP). Comparative studies on post-treatment of SiCxNy films deposited at various temperatures are carried out using (1) thermal annealing at 400 ℃ for 5 minutes and (2) UV-assisted thermal annealing at 400 ℃ with 80 mW/cm2 UV light exposure for 5 minutes. In this study, UV-assisted thermal annealing is found to remove the organic phase, break up the high-polarizability NH bonding, and enhance SiN backbone structure. Thus, UV-assisted thermal anneal can enhance the properties of SiCxNy film compared to conventional thermal annealing under same curing time, 5 min. The optimal result of low-k SiCxNy film using MTSCP precursor was obtained by deposition at 150 ℃,1 Torr, and under a plasma power density of 0.27 W/cm2. Under these conditions, the dielectric constant of low-k SiCxNy film drops 0.22 from 4.05 to 3.83, while the Young’s modulus maintains the same at about 20 GPa. As a result, UV-assisted thermal annealing can be a powerful post-treatment technique to enhance silicon carbonitride films for backend interconnect applications.