紫外光輔助退火對電漿輔助化學氣相沉積碳氮化矽薄膜在結構和光學性質上之影響

Abstract

碳氮化矽薄膜在目前半導體已被廣泛研究應用在CMOS後段製程中作為低介電蝕刻阻擋層和抗銅擴散層的材料，由於良好的抗氧化和防止銅擴散能力，隨著14奈米甚至更低奈米的製程需求，碳氮化矽薄膜(k < 5.0)在機械性質上的問題也就更加重要，已有研究顯示紫外光退火後處理對於碳矽氧薄膜在機械強度上能有所提升(從3至5GPa)，但目前很少有研究關於紫外光退火後處理對於碳氮化矽薄膜的影響和機制，所以本研究藉由電漿輔助化學氣相沉積法，並使用前驅物1,3,5,7-tetravinyltetra methylcyclotetrasilazane (TVSZ)和N-methyl-aza-2,2, 4-trimethyl-silacyclopentane (MTSCP) 在不同沉積溫度製備碳氮化矽薄膜。接著將薄膜經過紫外光輔助退火處理，探測薄膜處理後在化學鍵結、密度、機械強度、光學性質的變化，藉此探討處理後薄膜在結構和機制的變化，並用兩種組成結構差異很大的碳氮化矽薄膜TVSZ和MTSCP做比較。而紫外光後處理所引發的薄膜應力問題本研究也將藉由彎柄儀測量並探討。 在經過紫外光輔助退火處理後，結構部分在低溫沉積薄膜(100°C)中大量且脆弱的官能基(如N-H、CHx、Si-H)會被打斷，並形成更多的矽氮(Si-N)鍵結，而對高溫沉積薄膜(300°C)原本就少量脆弱的官能基則會被幾乎移除，形成更堅固交聯的矽氮(Si-N)鍵結，對於機械強度上，經過紫外光輔助退火處理後，低溫沉積薄膜的彈性係數可以從29.2顯著提升到67.1GPa(提升130%)，高溫沉積薄膜也仍然有一定幅度的提升從113至133GPa(提升18%)。薄膜應力的部分則變得更少壓應力，低溫沉積碳氮化矽薄膜(100°C)的應力由-83 MPa降至-42 MPa，而高溫沉積矽碳氮薄膜(300°C)則由-195 MPa減少至-165 MPa，此應力結果是由於矽氫鍵(Si-H)的移除和矽氮(Si-N)鍵結的交聯使得薄膜收縮所造成之影響。在光學穿透度部藉由大量移除化學鍵結在官能基(CHx)與更加提升Si-N主結構的影響下，在經過紫外光輔助退火處理後，可以對穿透率有明顯的影響。總體而言，紫外光輔助退火處理對碳氮化矽薄膜在機械性質上和光學穿透度都能有很好的提升，此種後處理方法將會是在調整碳氮化矽薄膜性質上的很好選擇。

Silicon carbonitride films (SiCxNy) have been extensively used as a low-k etch-stop/Cu diffusion barrier materials in CMOS backend interconnections due to their excellent resistance to oxidation and Cu diffusion. In order to meet the mechanical requirements for 14 nm node and below, the mechanical strength of silicon carbonitride films (k < 5.0) requires further improvement. UV-assisted annealing of low-k SiCOH films has been well studied and used to enhance the modulus from 3.5 to 5.9 GPa due to increased crosslinking. However, UV-assisted thermal annealing of SiCxNy films and its mechanism are still lacking. In this thesis, a single precursor 1,3,5,7-tetravinyltetra methylcyclotetrasilazane (TVSZ) and N-methyl-aza-2,2, 4-trimethyl-silacyclopentane (MTSCP) were used to deposit silicon carbonitride films by plasma-enhanced-chemical-vapor-deposition (PECVD) at various deposition temperatures (100-300oC). The changes such as chemical bonding, density, mechanical strength, thermal stress, and optical property in SiCxNy film after UV-assisted annealing were investigated. In addition, the impact of N/Si and C/Si ratios in TVSZ (more Si-N like) and MTSCP (more Si-C like) precursor on the as-deposited and UV-treated SiCxNy films was examined and compared. Compared to SiCxNy films prepared by other precursors, TVSZ-deposited SiCxNy films possess higher film density and better mechanical strength due to its Si-N rich structure. After UV annealing of SiCxNy deposited at lower temperature (100oC), large amount of terminal groups like N-H, CHx, and Si-H bonding were removed and Si-N bonding was reformed into a cross-linked structure, leading to a significant increase of Young’s modulus from 29.2 to 67.1 GPa, i.e. 130% increase. For as-deposited SiCxNy films deposited at higher temperature (300oC), the trace amount of Si-CH3 terminal groups was completely broken up and more cross-linked Si-N was formed upon UV annealing. Its modulus increased from 113 to 133 GPa (18%). According to bending beam measurement, the stress in the 100 oC-deposited SiCxNy films became less compressive from -83 MPa to -42 MPa and in the 300 oC-deposited SiCxNy films changed from -195 MPa to -165 MPa. This result may be owing to the removal of Si-H combined with enhanced Si-N crosslinking. The transmittance of PECVD SiCxNy films prepared by using TVSZ and MTSCP precursors were investigated. Based on the changes of density and chemical structure in the terminal groups and the main structure about Si-C or Si-N bonding, UV curing can adjust transmittance by removing hydrocarbon and enhancing Si-N main structure. Overall, UV-assisted thermal annealing is a promising post-treatment for enhancing the mechanical and optical properties of SiCxNy films.