超薄氧化層崩潰特性之研究

Abstract

閘極氧化層的可靠度問題一直是超大型積體電路技術中的一關鍵課題，隨著氧化層厚度不斷的縮小（< 5.5 nm），許多不同於以往的氧化層劣化現象陸續被發現，其中以軟性崩潰（Soft Breakdown，SBD）最為特別，因此本論文主要在探討超薄氧化層的崩潰特性。 軟性崩潰（SBD）會造成閘極漏電流明顯的增加，但是其增加的幅度卻比傳統熱崩潰（Hard Breakdown，HBD）現象小的許多，此外SBD還有許多不同的特性，像是RTS（random telegraph signal）、電流特性符合power law、對元件操作特性不會造成明顯的影響等，這些特性在近幾年的文獻中都曾被提出來，因此本論文的一大主題就是驗證這些在文獻中提到的現象。 除了SBD之外，另一種不同於SBD的崩潰現象亦在此研究中被發現，命名為新軟性崩潰（New Soft Breakdown，NSB）。NSB像SBD一樣會造成閘極漏電流的增加，但在本質上卻與SBD是不同的，例如它不遵守power law、且閘極漏電流比SBD的大好幾個等級等。此外，一個合理的模型在論文中亦被提出來解釋NSB現象，即NSB可以被看成有殘餘氧化層存在的熱崩潰（HBD）現象。因為有殘餘氧化層的存在，所以閘極漏電流的增加會被限制在某個範圍之下，不致使元件特性完全喪失。NSB的發生與外加的測試條件或是元件的散熱能力非常有關係，而且發生機率會隨著厚度的變薄而增加。當氧化層厚度薄到某個程度之下（<3.0 nm），NSB跟SBD一樣不會對元件特性造成明顯的影響，而且在某個厚度（<20 nm）之後，元件直接穿透電流（Direct tunneling current）會比SBD的電流還來的大，所以SBD現象會不易被觀察到，因此NSB會成為主要的崩潰模式。

The reliability of oxide films is an important issue for ultra large scale integrated (ULSI) IC's, especially as oxide is getting thinner and thinner. It is known for several years that thinner oxides can have anomalous failure mode. However, despite the large research attention in recent years, the detailed mechanisms of the so-called soft breakdown (SBD) in ultra thin (<5.5 nm) oxide remain unsettled. The main purpose of our study is to focus on the breakdown characteristics of ultra thin oxides. Soft breakdown will cause an obvious increase of gate leakage current, but not as large as that in conventional hard breakdown (HBD) with thermal runaway. Besides, SBD depicts distinctively different characteristics such as random telegraph signal (RTS) noise, post-breakdown I-V characteristics that follows power law, no significant shifts in device operation, et al. These properties were proposed only very recently in literature. So one part of this study is devoted to verify these characteristics on oxides grown in our laboratory. Besides SBD, another similar but distinctively different breakdown mode is identified in this study, and is called new soft breakdown (NSB). NSB will cause even larger gate leakage current, compared to SBD, but smaller than HBD. NSB is caused by physical mechanism different from that of SBD. As NSB not only depicts much more larger leakage current, but also disobeys the power law, a characteristic signature of SBD. A plausible model is also proposed in this thesis to explain NSB. NSB can be considered as HBD but with the presence of some residual oxide. Because of the presence of residual oxide, gate current is confined to certain level and therefore does not seriously affect the device operation. The occurrence probability of NSB is very sensitive to the stress condition and the heat release ability of the device. Moreover, the probability will increase with decreasing oxide thickness. NSB, similar to SBD, does not cause no significant shifts in device operation when oxide thickness is less than 3.0 nm. NSB will become dominant breakdown mode as oxide is thinner than 2.0 nm, as the large direct tunneling current will screen the SBD current. Abstract (English)…………………………………………………………………....II Acknowledge…………………………………………………………….………….IV Contents……………………………………………………………………………...V Table Captions……………………………………………………………………...VII Figure Captions…………………………………………………………………....VIII Chapter 1 Introduction…………………………………………………………….1 1.1 Background……………………………………………………………...…..1 1.2 Oxide Hard Breakdown and Soft Breakdown……………………………....2 1.3 Organization of This Thesis…………………………………………………4 Chapter 2 Experimental Description………………………………………………6 2.1 Fabrication Process for Deep Sub-micron nMOSFET with Varied Thickness of Ultra Thin Gate Dielectric…………...………………………..6 2.2 Details of Measurement Techniques………………………………………...9 Chapter 3 The Characteristics of Soft Breakdown for Thin Gate Oxide…………....12 3.1 The Polarity Dependence of Oxide Breakdown……………………...……12 3.2 Soft Breakdown Characteristics…………………………………………....14 3.3 Model for Soft Breakdown and Power-Law I-V Relationship…………….19 3.4 The Correlation between Soft and Hard Breakdown……………………....20 3.5 Summary……………………………………...……………………………21 Chapter 4 A New Breakdown Event and Its Evolution………………………….22 4.1 A New Breakdown Mode Different from Soft and Hard Breakdown……..22 4.2 A Proposed Breakdown Model to Explain New Soft Breakdown………....25 4.3 Supporting Evidence of New Breakdown Model…………………...……..27 4.4 Summary………………………………………………………………...…30 Chapter 5 The Influence of Soft and New Soft Breakdown on nMOSFET Device Characteristics…………………………………………….….32 5.1 Experimental……………………………………………………………….32 5.2 Influence of Soft and New Breakdown on nMOSFET Device Characteristics……………………………………………………………...33 5.3 The Location of Breakdown Spot in Oxide………………………………..34 5.4 Summary………………………………………………………….………..35 Chapter 6 Conclusions…………………. ……………………………………….37 6.1 Conclusions……………………………………………………………...…37 6.2 Future work………………………………………………………...………38 Reference………………………………………………………………...………….39