高溫鋁製程乾蝕刻底切最佳化之研究

Abstract

為了達到降低生產成本的目的，本論文以六吋晶圓金屬層之鎢塞製程的設計法則(design rule)下，將金屬層之鎢塞製程改為高溫鋁塞的製程。由於鋁塞製程中需要大開口方式的傾斜導槽式接觸窗，也就是接觸窗上方開口較大，由於材料已經改變，選擇比也不同，因此在金屬蝕刻製程中，如果金屬側壁保護不足，容易造成鋁塞接觸窗底切(undercut)的現象，本論文研究金屬乾蝕刻接觸窗底切的最佳化，找出適用的金屬蝕刻程式參數條件。 本文主要研究金屬線下方的接觸窗底切之改善，在金屬蝕刻製程中，藉由蝕刻程式架構分析、關鍵因子分析、及調整金屬蝕刻程式的參數(如射頻電源/ 偏壓射頻電源 /三氯化硼(BCl3) /氯(Cl2)/氮氣(N2)/氬氣(Ar))，再利用實驗計畫手法得到各因子的最佳條件及最適當的程式來改善底切。 鋁蝕刻主要的蝕刻氣體為氯氣(Cl2)，蝕刻行為為化學性，形同等向性蝕刻，如果對金屬側壁的保護不足，容易造成金屬側壁的侵蝕或底切。在過蝕刻步驟將氯(Cl2)與三氯化硼(BCl3)比例由一比一，增加到一比三，降低金屬鋁(Al)與介電層(IMD)的選擇比，可以增加側壁保護改善底切。同時增加射頻電源(RF Top)與偏壓射頻電源(RF Bot)或加大偏壓射頻電源(RF Bot) 也可獲得底切的改善且較顯著。 射頻電源(RF Top)與偏壓射頻電源(RF Bot)比例及氯(Cl2)與三氯化硼(BCl3)比例，如果比例同時都偏低，極容易造成底切。底切與射頻電源(Soure RF)電漿功率及氯氣(Cl2)成正比，與偏壓射頻電源(Bias RF)電漿功率及三氯化硼(BCl3)成反比。透過電漿解離的氯離子越多，等向性的蝕刻機率越高，底切的機率相對的高。射頻電源(Soure RF)電漿功率可以增加非等向性蝕刻，三氯化硼(BCl3)除了幫助非等向性的蝕刻，也提供側壁的保護降低底切的機率。改善底切可以朝增加偏壓射頻電源(Bias RF)電漿功率及三氯化硼(BCl3)，降低射頻電源(Soure RF)電漿功率及氯氣(Cl2)的方向著手，等同增加物理性的轟擊(physical bombardment)，減少化學性的反應((chemical reactivity)對底切有改善的作用。本實驗也證明，在設計法則不變的情況下，將鎢塞製程改鋁塞製程是可行的。

To reduce the fabrication cost, we replaced the conventional tungsten plug process based on the design rule for 6-inch wafers with the aluminum plug process. The replacement of the plug material significantly changes the process characteristics of patterning of the plug and the first metal interconnect layer, such as the etch rate, etch selectivity and etch profile. Undercuts are easily formed during the dry etch of the Al metal layer because a part of the Al plug is subject to etch erosion as a result of the larger top area of the sloping Al contact structure compared with the tungsten plug process. In this study, we studied the cause leading to the Al undercut. We first analyzed key factors resulting in the Al undercut during the etch process, followed by an experimental design for optimizing the etch process. The key etch parameters include source RF power, bias RF power, and the flow rate of etchants, such as BCl3 ,Cl2 ,N2 and Ar. Using a model developed for optimizing process conditions and verified by experiments, we have decided the best etch process condition for the Al interconnect using the Al plug structure. To alleviate the undercut failure, it is important to effectively passivate the sidewall of the metal line structure during the etch process by minimizing chemical erosion, which is an isotropic process, and increasing physical sputtering, which is an anisotropic process. From the study, effective sidewall passivation can be achieved by optimizing the plasma condition and the concentration of reactant gases; these include increasing the BCl3 flow rate, increasing the bottom power, decreasing the Cl2 flow rate and the RF source power.