化學機械研磨阻障層鉭與其電化學特性的研究

Abstract

超大型積體電路(ULSI)發展下，化學機械研磨(Chemical Mechanical Polishing, CMP)已成為提昇銅製程(Cu Process)效能的關鍵技術。雖然研磨銅的研究與應用目前已漸趨成熟完善，然而受限於銅製程上常必須藉由阻障層的使用以防止銅擴散入介電層中，因此快速磨銅後會面臨異質材料間的研磨選擇率控制等問題，又成為銅製程之另一研發重點。本研究選擇研磨目前被認為具潛力的阻障層材料-鉭系(Ta) ，探討不同研磨液組成對Ta CMP效能的影響。利用直流極化曲線與開環電位等電化學量測，探討了包括表面鈍化、腐蝕等反應以及其與機械作用間的關係，再利用交流阻抗技術分析Ta在不同研磨液組成中的反應機制，並架構其等效電路。原子力學顯微鏡(AFM)與X光光電子分光儀(XPS)則被採用作為研磨前後表面平坦度和組成成分的分析。實驗結果顯示，Ta在不同氧化劑系統表面皆自然產生鈍化層阻止進一步的溶除，在單純氧化劑研磨液中，化學溶除速率過小，不利阻障層的移除。選擇添加劑如氨水、草酸、甘胺酸和甘醇酸等皆可藉由與Ta及其氧化物的錯合作用，加速阻障層的溶除速率。實驗結果亦顯示，在過氧化氫研磨液系統中添加醋酸和磷酸，醋酸和磷酸會以類似吸附作用附著於待磨表面改變其表面狀態，有效延遲Ta表面鈍化成緻密Ta2O5的時間，使研磨表面有較長時間處在較容易腐蝕和機械移除的狀態，而研磨狀態下，腐蝕電流密度都有大幅增加的趨勢，且磨後電位降也變大。交流阻抗分析的結果證實醋酸和磷酸的添加改變了鉭和研磨液界面的反應機制，磨後的反應阻抗的下降代表增進了表面鈍化膜移除的效率，AFM的實驗結果亦顯示分別在醋酸和磷酸添加的研磨液系統，其膜後表面粗糙度可下降至16.21 和13.81nm。此外，硬度較低的研磨粒子如SiO2，可藉由其與Ta的反應性，得到較佳的研磨效能。

In ULSI development, chemical mechanical polishing (CMP) has become the key technique that can help achieve global planarity and enhance the efficiency of the Cu metallization process. The Cu CMP process is constrained by the deposition of barriers between Cu and the dielectric layer, which is carried out to prevent Cu diffusion into dielectrics. Thus far, the process has problems related to polishing rate selectivity and compatibility between different materials, which then becomes another significant issue of the process. Tantalum (Ta) has been considered the barrier material with the greatest potential. In this study, the effects of slurry compositions on Ta CMP were investigated. By performing electrochemical measurements such as polarization curves, open-circuit potentials and impedance spectroscopy, the slurry compositions and electrochemical characteristics during CMP were discussed; further, surface morphological analysis after CMP was carried out by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The experimental results showed that Ta is difficult to polish mechanically because of the formation of the hard oxide, Ta2O5, on its surface in various oxidant slurry systems. Ta dissolution rate could be enhanced by adding additives such as ammonia, oxalic acid, glycine and glycolic acid due to the chelating effects. The experimental results also indicated that CH3COOH and H3PO4 could be adsorbed on the surface that is to be polished in order to modify the surface status; in particular, the time taken for the Ta surface to be passivated into dense Ta2O5 would be effectively increased so that the surface could remain for a longer time in a status where it could be easily corroded and easily removed. The corrosion current density and the potential drop both increased when CH3COOH or H3PO4 weas added to slurries. The impedance study also confirmed that the addition of CH3COOH and H3PO4 changed the reaction mechanism between Ta and the slurries. The decrease in the reaction impedances during CMP was indicative of the enhanced removal efficiency. In addition, lower surface roughness after CMP could be achieved; in this study, surface roughness of 16.21 and 13.81 nm were achieved when CH3COOH and H3PO4 were added to slurries, respectively. Finally, SiO2 abrasives although with low hardness could still achieve good removal performance due to the interaction between surface functional group and Ta.