銅污染對閘極絕緣層的效應及影響

Abstract

金屬污染效應所導致的氧化層可靠度問題已在深次微米元件技術領域中受到越來越多的重視。本篇論文將針對銅污染對閘極絕緣層產生的效應進行研究。 我們以”後段”污染的方式在室溫下進行元件與污染溶液的直接接觸，排除因溫度高低差異而造成的過飽和效應，其後以四百度的溫度進行退火，以仿製程中來自完成第一層金屬後之化學機械研磨產生的銅污染源所經過的熱處理。為了評估銅污染在實際製程中的影響性，我們首先研究0.25微米所使用閘極氧化層厚度在污染後會產生的特性變化。量測發現，銅污染過的元件會有漏電流驟增及氧化層缺陷增多…等的趨勢，這可從其後30 Å 氧化層厚度的實驗裡進一步獲得證實，並且發現銅污染在先進的0.18微米以下製程裡會導致更嚴重的元件特性衰退，包括:更低的崩潰電場以及更容易被偏壓等加強的漏電流效應 (stress induced leakage current)。這樣的結果在以往的文獻記載中並未發現，是以，我們提出一個模型不僅在量測數據上達到了誤差6.7%以內的精確模擬，也發現銅在介面存在將降低電子所需突破的位障高度，如同把實際氧化層厚度變薄，進而加強穿透電流等的效應。此外，我們也審視了高介電性的oxynitride，認為氮含量的增加確實可有效消弭銅的穿透與影響。應用此方法的優點，不僅在於它承襲了製程的真實性，最重要的是它避免了一些化學性能引起的機制，使分析免於複雜化。

This dissertation addresses the issues related to copper (Cu) contamination effects especially induced by the process of Cu interconnects in CMOS devices. First of all, we have studied the effect of copper contamination after the front-end MOS capacitor fabrication with 50 Å oxide thickness on gate oxide integrity. The significant effect of Cu contamination on the pre-tunneling current, in combination with insensitive dependence of the Fowler-Nordheim (F-N) tunneling current, oxide charge density, and breakdown field on the Cu concentration, suggests that the current leakage mechanism may be due to neutral traps generated by Cu inside oxide. In addition to pre-tunneling oxide leakage, a small amount of Cu contamination increases the interface trap density that may degrade the device performance. Next, much higher leakage current, lower breakdown effective field, poorer charge-to-breakdown, and worse stress-induced leakage current are observed in ultra-thin 30 Å oxide even at low Cu contamination of 10ppb. The strong degradation of ultra-thin gate oxide integrity can be explained by the tunneling barrier lowering and increased interface trap tunneling due to Cu presence in oxide and oxide-Si interface. Finally, we also studied the Cu contamination in oxynitride. In comparing with thermal SiO2 at physical thickness of 30-50 Å range, the oxynitride shows much improved Cu contamination resistance. Furthermore, the Cu contamination resistance increases with increasing nitrogen content. The mechanism of improved gate dielectric resistance to Cu is due to the strong diffusion barrier property of oxynitride as observed by SIMS.