紫外光輔助退火對電漿輔助化學氣相沉積低介電碳氮化矽薄膜之影響

Abstract

低介電碳氮化矽薄膜(k = 5.0–6.0)在目前的半導體製程應用中，當作CMOS後段製程中低介電蝕刻阻擋層的材料。 在14奈米或更低的製程中，碳氮化矽薄膜(k < 5.0)有良好的機械性質是非常重要的。在此研究中，選擇使用高碳矽比(C:N:Si=7:1:1)的單一前驅物N-methyl-aza-2,2,4-trimethyl silacyclopentane (MTSCP)利用電漿輔助化學氣相沉積法在不同沉積溫度製備碳氮化矽薄膜。研究中利用紫外光輔助退火將碳氮化矽薄膜在400度進行處理。探討薄膜在處理後結構、介電性質、機械性質上的改變。經紫外光輔助退火處理後，發現碳氮化矽薄膜在組成和結構上都有明顯的改變。藉由傅立葉轉換紅外光譜儀，可以觀察到對應矽氮(Si-N)的波鋒強度提高且有紅位移現象，代表生成更多緻密的矽氮(Si-N) 交聯結構。此外紫外光輔助退火處理也被觀察可以很有效地移除極性官能基，例如氮氫(N-H)和矽氫(Si-H)。由於大量的極性官能基移除和碳氫鏈的減少，對於在低溫沉積的碳氮化矽薄膜經過紫外光輔助退火處理後可將介電常數從3.6降至3.2。此外，更多的較強的矽氮(Si-N)鍵結交聯使得楊氏模數在後處理後從 6.0GPa 增加至7.4GPa。在300度沉積的碳氮化矽薄膜經過紫外光照射後，矽氮(Si-N)鍵結交聯使介電常數從4.2至4.4。楊氏模數也從50GPa提升至58GPa。另外，紫外光後處理對薄膜的應力也有明顯的影響，藉由彎柄儀測量薄膜曲綠半徑的變化，得到在紫外光退火處理後低溫沉積之碳氮化矽薄膜應力由+10 MPa 升至+45 MPa，對高溫沉積矽碳氮薄膜應力由-120MPa減少至-58 MPa，此應力結果是由於矽氫(Si-H)的移除再加上矽氮(Si-N)交聯使得薄膜收縮所造成之影響。 本研究結果了解由MTSCP單一前驅物製備之碳氮化矽薄膜經過紫外光輔助退火之後，能有效增加薄膜機械性質並持有低的介電常數(3.2~4.4)，並同時具有良好的薄膜殘留應力。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。

Low-k silicon carbonitride films (SiCxNy) (k = 5.0–6.0) have been introduced as low-k etch-stop/dielectric barrier materials in CMOS backend interconnections. For 14 nm node and below, SiCxNy films (k < 5.0) with good mechanical strength are highly desirable. In this study, a single precursor, N-methyl-aza-2,2,4-trimethyl silacyclopentane (MTSCP) with high C/Si ratio (C:N:Si=7:1:1) was used to fabricate SiCxNy films at various substrate temperatures by PECVD. This thesis examines the effect of UV-assisted annealing at 400oC on the dielectric constant, structural change, and mechanical properties of SiCxNy films, compared to those by conventional thermal annealing. UV light is found to significantly alter the composition and structure of silicon carbonitride films. Upon UV irradiation onto SiCxNy film at 400oC, its Si-N peak intensity increased and stretching band blue-shifted according to Fourier transform infrared spectroscopy, indicating a denser and more cross-linked Si-N structure. In addition, UV radiation is effective in removing polar groups such as N-H and Si-H. For SiCxNy film as-deposited at low temperature, such as 100oC, UV-assisted thermal annealing reduced the k-value from 3.6 to 3.2, presumably due to the removal of polar groups, -NH and SiH and a decrease in density by breaking up hydrocarbon in carbon chain. Still, the enhanced crosslinking led to 23% improvement of mechanical strength from 6.0GPa to 7.4GPa. UV-assisted annealing of 300oC-deposited SiCxNy film showed an increase of Young’s modulus from 50GPa to 58GPa, while the dielectric constant increased slightly from 4.2 to 4.4 due to slight increase of film density by enhanced Si-N crosslinking. The film stress is also strongly affected by UV radiation as characterized by a wafer curvature method, i.e. bending beam. After UV-assisted annealing, the stress of 100oC-deposited SiCxNy film changed from +10 MPa (tensile) to +45 MPa. In contrast, SiCxNy films deposited at 300oC showed a compressive stress of -120 MPa, then increased to -58 MPa upon UV annealing. This can be attributed to the removal of Si-H combined with enhanced Si-N crosslinking by UV-assisted annealing. The applicability of UV-assisted thermal annealing as a post-treatment technique to improve silicon carbonitride films is also addressed.