不同浮動閘極材料N通道快閃記憶體資料保存特性研究

Abstract

近幾年來，快閃式記憶體(Flash Memory)已廣泛地成為非揮發性記憶體的主流產品。就一個具有較佳資料保存特性的先進快閃式記憶體元件設計來說，多晶矽層間介電質(Interpoly Oxide-Nitride-Oxide Dielectric)的厚度設計是主要的考量重點。因為主要的資料流失來源是氧化氮層(Nitride)中的電荷移動與電荷從多晶矽層間介電質漏出，在過去，快閃式記憶體元件在Oxide-Nitride-Oxide(ONO) Dielectric的設計考量多著重在薄化Nitride層與厚化頂氧化層(Top oxide)及底氧化層(Bottom oxide)的厚度，這樣所得到的Interpoly ONO Dielectric具有較小的漏電流和較佳的資料保存能力。 本研究論文旨在提出一個新的設計方向，即是以浮動閘極工程來改善元件特性。其中，吾人首次採用P型浮動閘極N通道的快閃記憶體來進行資料保存特性的研究，從浮動閘極雜質濃度及雜質型態，對浮動閘極材料的改變，進而影響快閃式記憶元件特性做一深入探討。根據實驗結果，吾人發現：(1)快閃式記憶體其基本的電流特性與浮動閘極的摻雜種類(Doping Type)無關。(2)與傳統的N型浮動閘極快閃記憶體相比較，P型浮動閘極的快閃式記憶體會有較佳的資料保存特性，主要是因為P型浮動閘極在寫入操作時形成較高的Tunnel Oxide電場和較低的Interpoly ONO dielectric電場，所以P型浮動閘極之快閃記憶體在寫入操作時會有較少的電子數目會注入到ONO介電層中，換言之，P型浮動閘極之快閃記憶體具有較佳的本質資料流失(Intrinsic Charge Loss)特性。此外，在本論文中，吾人進一步對多次寫入(抹除)後產生的影響與保存電荷能力的關係，做一完整的比較，其中N型浮動閘極的快閃記憶體有較嚴重的資料保存特性劣化現象，這可由擾動現象的結果加以解釋之。最後，由本研究論文可知，N通道P型浮動閘極之快閃記憶體的元件結構比傳統元件結構具有更多的優點，更適用於未來高可靠性(Reliability)快閃記憶體的應用上。

Recently, the flash memory has become the main stream of nonvolatile semiconductor memory products. For the design of advanced flash memories with better data retention characteristics, the ONO interpoly dielectric thickness is a major concern. The main sources of charge loss are charge movement in the nitride and charge leakage out of the ONO film. In the past, the design of ONO film of flash memories is mainly focused on thinning the nitride and thickening the top and bottom oxides. The resultant interpoly ONO film shows much smaller current leakage and superior charge retention capability.In this thesis, we proposed a new design viewpoint by floating-gate engineering. First, a new n-channel flash memory cell is developed for the first time with the p-type floating-gate material. From the experimental results, several new issues have been addressed: (1) The current-voltage characteristics of flash memories is independent of the floating-gate doping. (2) The p-type floating-gate flash cell has much better data retention characteristic than the conventional n-type floating-gate one, because the p-type floating-gate flash cell has smaller electric field across the ONO dielectric during programming. As a consequence, fewer electrons will be injected into the ONO佁ielectric in p-type flash cell. Namely, the p-type floating-gate flash cell has better intrinsic charge loss behavior. In addition, we compare the charge loss behavior after program/erase cycles, where we see that data retention characteristics become worse after program/erase cycles. This can be explained by observing the disturb characteristics. Finally, the n-channel flash memory cell with p-type floating-gate structure is more advantageous for high reliability cell design and in particular for future flash memory applications. English Abstract iii Acknowledgements v Contents vi Figure Captions viii Table Captions xi Chapter 1 Introduction 1 Chapter 2 Device Fabrication and Experimental Setup 3 2.1 Introduction 3 2.2 Device Fabrication 3 2.3 Equipment Setup 5 2.4 Basics of Program and Erase Operations 10 2.4.1 Channel-Hot-Electron Injection (CHE) Program 10 2.4.2 Channel Fowler-Nordheim Tunneling (CFN) Erase 13 Chapter 3 Basic I-V Characteristics and Comparison of Electric Field for Different Floating-gate Types 17 3.1 Introduction 17 3.2 Capacitance Model of Flash Memories 17 3.3 The Effect of Electric Field for Different Floating-gate Type 19 3.4 Basic Current-Voltage Characteristics for Different Floating-gate Types 22 3.5 Summary 26 Chapter 4 Intrinsic Charge Loss Characteristics in N-Channel Flash Memories with Different Floating-gate Materials 29 4.1 Introduction 29 4.2 Effect of Floating-gate Doping Types on Flash Cell Charge Loss Behavior 29 4.3 Effect of Floating-gate Doping Concentration on Flash Cell Charge Loss Behavior Performance33 4.4 Temperature Effect on Charge Loss Behavior 33 4.5 Charge Loss Behavior Affected by Programmed Charges 37 4.6 Different Program Scheme Effect on Charge Loss Behavior 40 4.7 Summary 40 Chapter 5 Data Retention Characteristics in N-Channel Flash Memories with Different Floating-gate Materials after Program/Erase Operations 42 5.1 Introduction 42 5.2 Charge Loss Properties in Different Floating-gate After P/E Cycling 42 5.2.1 Program/Erase Cycling Effect on Charge Loss Behavior 43 5.2.2 Program/Erase Cycling Effect on Disturb Behavior 43 5.3 Charge Loss Mechanisms in N-Channel Flash with Different Floating-gate Material 51 5.4 Summary 54 Chapter 6 Conclusion 57 References 58