低壓化學氣相沉積鎢膜在極大型積體電路上之運用

Abstract

本論文研究低壓化學氣相沉積鎢膜技術在極大型積體電路上之運用。內 容涵蓋了選澤性鎢沉積（selective CVD-W）在深次微米矽接觸（contact ）與鋁層金屬通道（via）的成長機制，沉積參數例如︰壓力、溫度、矽 烷╱六氟化鎢（SiH4╱WF6）氣體流量與流量比等等在沉積速率、鎢膜阻 值、晶相以及選擇率等特性上的影響。同時鎢沉積前的基板預處理（ pretreatment）技術也在研究之列，並且探討沉積所得鎢╱矽與鎢╱鋁接 觸元件的一些重要特性。對於活性接觸區的矽基板，傳統式鎢沉積之預處 理包括氫氟酸（HF）、緩衝氫氟酸（BHF）溶液的濕式蝕刻無法消除源於 活性離子蝕刻（RIE）步驟中所造成的破壞層與污染層。根據實驗觀察， 破壞層導致大量的矽耗損（silicon consumption）、粗糙的鎢╱矽界面 、高元件漏電流。另一方面，污染層則是造成低沉積率、橫向侵蝕（ lateral encroachment）、選擇率損失與高鎢膜阻值現象的主要因素。因 此，在論文中將介紹一種新穎的四氟化碳╱氧氣（CF4/O2）混合電漿配合 氫氟酸蒸氣的清潔技術，可以有效消弭上述問題，進而獲得高沉積率、低 鎢膜電阻值、低選擇率損失、平坦的鎢╱矽界面以及低鎢╱矽接觸元件漏 電流等特性。對於鋁基板特別是深次微米鋁層通道的預處理是十分困難的 。傳統的六氟化硫（SF6）或三氯化硼（BCl3）電漿乾式蝕刻易於將鋁層 通道中的鋁與氧化鋁濺射到絕緣層上，間接造成鎢沉積時選擇性的損失。 除此之外，由六氟化鎢（WF6）與鋁層直接反應所產生之非揮發性的三氟 化鋁（AlF3），是導致低沉積率與高通道阻值（via resistance）的主要 因素。因此我們發展出一種氣相預處理技術，將鋁基板曝露於氫氯酸蒸氣 （HCl vapor）環境中約六十秒，將可以有效清除氧化鋁層。另一方面， 預處理後鋁層表面的含氯披覆層亦有抑制三氟化鋁產生的功能。實驗結果 顯示，經過氫氯酸蒸氣清潔之後，可以得到高沉積率、緻密而平坦的鎢膜 、低選擇率損失以及低通道阻值。同時，本論文亦述及覆蓋性化學氣相鎢 （blanket CVD-W）沉積技術之研究。一種新穎之化學氣相沉積非晶鎢膜 （CVD a-W）將取代傳統式覆蓋性鎢沉積製程中用以作為黏著層（ adhesion layer）的氮化鈦（TiN）。經由控制矽烷╱六氟化鎢（SiH4╱ WF6）流量比、沉積壓力與溫度等參數，可獲得一高沉積率、均勻且步階 覆蓋性（step coverage）極佳的非晶鎢膜。此一技術不僅可簡化製程， 也可以解決氮化鈦黏著層造成的高接觸阻值、對氫氣╱六氟化鎢（H2╱ WF6）系統有過於冗長的初始階段、步階覆蓋不良等問題。實驗發現利用 兩段式的沉積方法：第一階段為短時間的非晶鎢沉積步驟作為黏著層。控 以12.5/5每分鐘立方公分的矽烷╱六氟化鎢流量，使其達到2.5的高流量 比，其它參數分別為300℃以及100 mTorr。如此可得400 nm/min沉積率， 具有覆蓋性、非晶相（amorphous phase）、160((-cm的沉積鎢。第二階 段再以典型之選擇性鎢沉積技術來沉積厚鎢膜。典型之選擇性沉積鎢步驟 乃採用低矽烷╱六氟化鎢流量比（小於1），其阻值極低（約13((-cm）且 具複晶相性質。實驗證明，此二階段沉積法將可獲得優良的覆蓋性鎢膜。 再者，為探討此種非晶鎢膜在金屬化技術中的運用，我們檢視了非晶鎢膜 應用在擴散障礙層（diffusion barrier）的效果。對於二種金屬化系統 （鋁、銅）擴散障礙層測試實驗發現，非晶鎢膜的屏障能力明顯地比複晶 鎢膜好。此外，在同一腔體內不破真空的情況下經過氮化處理的非晶鎢膜 ，可以更進一步改善其熱穩定度。首先，針對Al/W/n+p接面二極體的研究 分析，鎢對鋁的屏障能力乃決定於鎢消耗於形成WAl12的程度。氮化處理 將會鎢膜表面形成一約五百埃的氮化鎢層（WNx），降低了鎢膜對鋁層之 反應活性。因此增加非晶鎢膜對鋁的屏障能力。Al/WNx/W/n+p接面二極體 在經過30分鐘575℃退火處理之後，金屬層阻值並無升高趨勢。同時Ｘ光 繞射儀也未探測到W(Si,Al)2，WAl12等化合物的產生。說明鋁矽層間原子 擴散現象並未發生。再經電性量測，大多數Al/WNx/W/n+p接面二極體的逆 偏壓漏電流仍小於10-7 A/cm2。綜合上述數據，可證實由非晶鎢膜氮化處 理所得的氮化鎢，具備了高熱穩定度（thermal stability）特性，為一 良好的擴散障礙層材料。其次，在Cu/W/p+n接面二極體熱穩定度的實驗中 發現，非晶鎢膜缺少可以提供銅原子快速擴散路徑的晶粒邊界（grain boundaries）結構。以典型沉積鎢膜（具複晶性質）作為擴散障礙層的接 面二極體，在經30分鐘675℃退火處理，材料分析發現大量的銅原子穿透 鎢層與矽基板反應形成Cu3Si化物，造成約10-3 A/cm2左右的漏電流。相 反地，採用非晶鎢膜的接面二極體則擁有700℃的熱穩定度。然而，對於 超過700℃以上之溫度，非晶鎢會因結晶化（crystallization）現象而喪 失其擴散障礙層的作用。因此，我們發現利用氮化處理可以引入氮原子填 塞在鎢膜的晶粒間，以達杜絕銅原子的擴散，更進一步強化擴散障礙層的 效果。這種現象使得Cu/WNx/W/p+n接面二極體在歷經30分鐘725℃的退火 處理之後仍保有良好的電性(漏電流小於10-6 A/cm2)。另外，氮化處理亦 具有抑制非晶鎢結晶化的功能。實驗顯示即使在750℃的退火溫度，Ｘ光 繞射圖譜也無鎢晶相的信號。 This thesis studies properties of the low pressure chemical vapor deposited tungsten film (CVD-W) for ultra-large-scale- integrated (ULSI) circuit applications. The deposition mechanisms of selective CVD-W filling on submicron contact holes and aluminum vias, the substrate pretreatment technology prior to tungsten deposition and the interface characteristics of W/Si and W/Al devices are investigated.For the silicon substrate, the conventional pretreatments include HF or buffer HF wet etching cannot completely remove damage and contamination introduced by the reactive ion etching (RIE) process, which will cause a grave degradation in tungsten film properties. A CF4/O2 plasma etching followed with an O2 plasma ashing step are developed and that exhibits the capability of efficient surface cleaning. After this post-RIE process, the excellent characteristics of tungsten films such as high deposition rate, low film resistivity, low selectivity loss (i.e., 0.25 pcs/cm2), elimination of encroachment and creep-up, smooth W/Si interface and very low leakage current of W/p-Si Schottky contact are obtained.For the aluminum substrate, pretreatment of the submicron aluminum via patterned substrate is vital to successful selective CVD-W. A traditional pretreatment uses in-situ SF6 or BCl6 plasma etching to remove the native metal oxide prior to conducting the CVD-W. During plasma etching, however, the outsputtered aluminum atom and aluminum oxide can be redeposited on the sidewall and adjacent region of vias, where tungsten nucleation is induced, resulting in creep-up and selectivity loss. Moreover, nonvolatile AlF3 compound comes from the direct reaction between WF6 and aluminum substrate can dramatically degrade the CVD-W/Al contact properties. In this study, a new pretreatment by using a 60 s hydrochloric acid vapor exposure on aluminum underlayer before selective deposition of tungsten has been proved able to remove aluminum oxide as well as to suppress the aluminum fluorides at CVD-W/Al. As the results, a high deposition rate, dense and smooth tungsten film with low resistivity, and low F impurity concentration formed at CVD-W/Al interface that results in low via resistance are obtained by using HCl vapor phase precleaning.Simultaneously, specifics of the blanket CVD-W technology are also established in this thesis. A novel chemical vapor deposited amorphous tungsten (CVD a-W) film deposited by an in-situ SiH4-WF6 gas phase reaction used as an adhesion layer to replace TiN in blanket CVD-W process. By controlling SiH4/WF6 flow ratio, deposition pressure and temperature, a conformal CVD a-W film with high deposition rate and superior step coverage is obtained. This CVD a-W film not only reduce process complexity, but it also can solve inherent problems of the sputtered TiN layer including long initiation time characteristics of H2/WF6 chemistry, high contact resistance to Si substrate and poor step coverage. As a result, a new two-step deposition of tungsten is developed as follows: the first step involves a short deposition of CVD a-W film with 2.5 SiH4/WF6 flow ratio as the adhesion layer. The second step includes typical CVD-W process (i.e., SiH4/WF6 flow ratio < 1) to grow thick tungsten film. It is verified that this two-step deposition technique has been proved to provide an excellent deposition for blanket CVD-W process. Furthermore, we have examined effects of the CVD a-W film as diffusion barrier for ULSI metallization. As the experimental results of two metallization systems (Al and Cu), the barrier characteristics of CVD a-W film is more superior than those of polycrystalline tungsten. In addition, an in-situ nitridation of CVD a-W film by N2 plasma has shown to further improve the thermal stability of CVD a-W film.Firstly, for Al/W/n+p junction diodes, the barrier capability of tungsten film is dependent on the consumption of tungsten through reaction of Al with tungsten at elevated temperature to form WAl12. It is observed that the in-situ nitridation will form a 50 nm-thick WNx layer on CVD a-W film which can reduce the reactivity of tungsten while contact to aluminum. The interdiffusion of aluminum and silicon substrate is thus suppressed and the Al/WNx/W/n+p diodes maintain excellent interfacial integrity and thermal stability. Moreover, the design of a diffusion barrier to Cu metallization poses different issues from those presented with Al. Copper is found to penetrate through a barrier layer without reacting with the barrier while the aluminum is highly reactive and usually induces failure by consuming barrier to form aluminides. The absence of grain boundaries in CVD a-W film efficiently eliminate the rapid diffusion path for Cu migration. The experiments give no evidence of interdiffusion and structural change for Cu/CVD a-W/Si samples up to 675C for 30 min annealing in N2. At higher temperature (700C), Cu penetration results in the formation of (Cu3Si precipitates at the CVD a-W/Si interface. It is due to the crystallization of CVD a-W film above 650C, rendering grain boundary structure and hence fast pathways for Cu diffusion. The Cu/CVD a-W/p+n diodes thus exhibited large increase in reverse leakage current. The effectiveness of nitrided barrier is attributed to blockade of grain boundaries and interstitial sites in tungsten film by N atoms. This slows down Cu diffusion significantly. Physical and chemical analyses indicate that Cu/WNx/W/Si multilayer maintained the integrity of interface while the annealing was carried out at 750C. Moreover, the reverse leakage current densities of Cu/WNx/W/p+n junction diodes retain at 10-6 A/cm2 after 30 min 725C annealing. Furthermore, the addition of nitrogen to tungsten film also plays an important role in the phase transformation of barrier material during annealing. The experimental results indicate that the crystallization of CVD a- W film was suppressed in Cu/WNx/W/Si sample even after annealing at 750C.