以高含碳前驅物利用電漿輔助化學氣相沉積法製備低介電碳氮化矽薄膜

Abstract

本研究著重於探討銅連接製程中碳氮化矽薄膜作為低介電蝕刻終止層之沉積、結構及性質。因為半導體元件尺寸的縮小，需要降低等效介電常數來減少訊號傳遞的時間延遲。然而蝕刻終止層因為其較大的介電常數而對等效介電常數造成提升。所以開發具有低介電常數與優良性質的新穎蝕刻終止層是重要且迫切的。為了更加了解不同化學結構之前驅物對於低介電碳氮化矽薄膜性質的影響，兩個不同結構的高含碳前驅物(碳含量73 %)—1,3-divinyl-1,1,3,3-tetramethyldisilazane (DVTMDS) 與bis(dimethyl amino) diethylsilane (BDMADES) 被選擇來利用射頻電漿輔助化學氣相沉積法在25-400 ℃的基板溫度與90-1500 mTorr的沉積壓力製備碳氮化矽薄膜。這兩個前驅物在碳氫結構上有很大的差異，例如DVTMDS含有兩個乙烯基、四個甲基且碳矽比為4；而BDMADES則含有兩個二甲胺基、兩個乙基且碳矽比為8。本研究主要著重於碳氮化矽薄膜的化學結構、元素比例、機械性質、蝕刻選擇率、介電常數與漏電流之性質比較。 在DVTMDS分子中之乙烯基結構在電漿聚合中傾向於形成具有較高熱穩定性的Si-(CH2)2-Si亞甲基橋交聯結構，使得相較於BDMADES (k=3.5-5.2)，以DVTMDS (k=3.2-4.5) 為前驅物在較高基板溫度下沉積的碳氮化矽薄膜仍保有較多的碳氫結構，這讓碳氮化矽薄膜有較低的介電常數且有較佳的熱穩定性。另一方面，在BDMADES分子中的N-CH3 和Si-CH2CH3結構容易在電漿中裂解，使的在電漿沉積的過程中損失大量的碳元素。再者，吾人發現在較高的沉積壓力下高極性N-H鍵結在碳氮化矽薄膜中的含量會降低，並使碳氮化矽薄膜有較低的介電常數和漏電流密度。 本研究中，具有最佳性質的碳氮化矽薄膜是使用DVTMDS為前驅物，在基板溫度為350 ℃，壓力為1 Torr，電漿功率為0.15 W/cm3狀態下沉積。其介電常數為3.44，漏電流密度在1 MV/cm之下為1.2×10-9 A/cm2，彈性係數約為32 GPa，以及具有不錯的蝕刻選擇率，約為4.0。此研究所開發出的碳氮化矽薄膜可作為下一世代之低介電蝕刻終止層的材料選擇之一。

This thesis work examined the properties of silicon carbonitride (SiCxNy) films for application as the low-k etch-stop layer (ESL) in copper interconnects. Due to the downscaling of feature size, the lowering of effective dielectric constant (keff) is needed to reduce RC delay. However, the ESL has a significant impact on the keff because of higher dielectric constant. As a result, development of novel ESL with low dielectric constant and good properties is very important. To investigate the effect of different chemical structures in the single precursors on the properties of low-k SiCxNy thin films, two different single-precursors with high carbon content: 1,3-divinyl-1,1,3,3-tetramethyldisilazane (DVTMDS) and bis(dimethylamino)diethyl- silane (BDMADES) were chosen to prepare SiCxNy films by using radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) at 25-400 ℃. In this study, emphasize had been laid on the chemical structures, atomic compositions, mechanical properties, etch selectivity, dielectric constant and leakage current of the SiCxNy films. Upon plasma polymerization, vinyl groups of DVTMDS could gain the formation of thermally stable Si-(CH2)n-Si methylene bridge crosslinking, leading to higher content of hydrocarbon (CHx) could be preserved than BDMADES during plasma process and even at high substrate temperature. This resulted in a lower dielectric constant and higher thermal stability. On the other hand, the N-CH3 and Si-CH2CH3 in BDMADES exhibited the unstable fragments in the plasma polymerization process, causing large loss of carbon. Furthermore, we found that with higher deposition pressures the N-H bonds that exhibited high polarity decreased, leading to a lower dielectric constant and leakage current density. Among these SiCxNy films, low-k SiCxNy films with k value of 3.44, leakage current density of 1.2×10-9 A/cm2 at 1 MV/cm (breakdown strength >3 MV/cm), mechanical strength of around 32 GPa and good etch selectivity (deposited at 400℃, etch selectivity=4.0) were prepared at TS of 350 ℃, 1 Torr and power density of 0.15 W/cm3 using DVTMDS as single-precursor, and it could be the candidate materials of low-k ESL meeting the requirements of device integration in the next generation .