快速加熱低壓化學氣相沈積氧化鉭薄膜電容的底電極材料分析

Abstract

本篇論文探討應用於動態隨機存取記憶體（DRAM）之氧化鉭薄膜的製程與性質。主要利用快速加熱製程的化學氣相沉積法（RT-LPCVD）來成長薄膜，配合不同的後續退火程序，包括在氧氣氛及一氧化二氮氣氛中以不同的溫度做快速退火處理，並採用不同的底電極材料，以期改善薄膜的電性與探討其差異原因。 在快速退火處理的研究中，我們發現當溫度上升時薄膜有更好的結晶性，介電常數也較高，最高可達66左右，但是漏電流卻並非一直隨著退火溫度的上升而降低，而是會有一最佳化的溫度，原因是因為當薄膜的厚度很薄時，晶粒邊界提供了漏電流路徑並主導了漏電流機制而造成此一效應，此外介面在高溫處理後的反應在超薄厚度的氧化鉭薄膜中影響也逐漸重要。在快速高溫退火的氣氛處理探討上，於一氧化二氮中作快速加熱退火的薄膜電性比在氧氣氛中有更好的結果，原因在於一氧化二氮較能有效率的分解出活化氧原子，並能有效填補結構內的氧缺陷及大幅降低薄膜內碳、氫雜質的含量，故有助於漏電流之降低；此外，相對於在氧氣氛中熱處理，一氧化二氮處理之薄膜介電常數值也較高。並在底電極比較整體特性方面，我們發現銥(Ir)會是最佳的底電極材料，而鉑(Pt)的表現也不錯。 最後，我們對於漏電流機制作一分析和探討，可瞭解在中電場區時漏電流是以Schottky和Poole-Frenkel導通機制的混合。而在變溫量測方面，我們發現當量測溫度上升時漏電流也隨之上升。當處在高溫時會發生『軟性崩潰』的現象，一般相信是由介電崩潰與Poole-Frenkel導通機制所造成。而以熱穩定度來看，釕(Ru)的整體表現是比較好的。

Tantalum oxide (Ta2O5) thin films have been demonstrated to be highly promising storage dielectrics in DRAM applications. Rapid thermal low-pressure chemical vapor deposition (RT-LPCVD) method was employed to grow the thin film in this study. The objectives was to study the effect of bottom electrode materials and the effect of O2 and N2O annealing at various temperatures on the electrical and dielectric properties of Ta2O5 thin films. The experiment shows that the Ta2O5 films have better crystallinity and dielectric constant (about 66) at higher annealing temperature, and the leakage current doesn't decrease when the annealing temperature increases but has an optimal condition. The reason is that the grain boundary provide the leakage path and the grain boundary limited conduction dominates the leakage when the Ta2O5 film is very thin and the grain size is relatively large compared with the film thickness. Besides, the effect of reaction of the interface will become more important after high temperature annealing process. The RTA in N2O can effectively improve the film quality and electrical properties of Ta2O5 CVD films. The reason is that it reduces the carbonhydrogen contamination more effectively than O2 gas does, and releases the oxygen atoms to fill the oxygen vacancies more effectively. From a viewpoint of the whole property, the Ir is the optimal electrode material and Pt is also good. Finally, we investigate the conduction mechanism of the leakage current and find that the leakage current follows the conduction mechanism of the mixture of Schottky emission and Poole-Frenkel emission at the middle electric field. In the different temperature measuring, we find the leakage current increases with the temperature. The phenomenon called "Soft Breakdown" occurs at high measuring temperature and it was believed this is due to the dielectric breakdown and Poole-Frenkel conduction mechanism. The performance of Ru is better than the other materials in the viewpoint of thermal stability. ABSTRACT (CHINESE)…………………………………………………………………..i ABSTRACT (ENGLISH)………………………………………………………………….ii ACKNOWLEDGEMENTS…………………………………...………………………….iii CONTENT………..……………………………………………………….……………….iv FIGURE CAPTIONS…………………………………………………...………………...vi TABLE LISTS……………………………………………………………………………..viii CHAPTER 1 INTRODUCTION………………………………………………………….1 1-1 General Background………………………………………………………...1 1-2 Motivation…………………………………………………………………..3 1-3 Thesis Organization…………………………………………………………6 CHAPTER 2 EXPERIMENTAL DETAILS……………………………………………..8 2-1 Process Flow………………………………………………………………...8 2-2 Equipment……………………………………………………….…………..9 2-2.1 RT-LPCVD System…………………………………….……….…….9 2-2.2 Metal-Organic Precursor……………………………….……………..11 2-2.3 Sputtering System……………………………………………………12 2-3 Preparation Procedures…………………………………………………….13 2-4 Measurement………………………………………………………………15 2-4.1 Physical Characterization……………………………………………15 2-4.1.1 Ellipsometer………………………………………………….15 2-4.1.2 X-Ray Diffraction Analysis (XRD)………………………….16 2-4.1.3 Atomic Force Microscope (AFM)……………………………16 2-4.2 Electrical Characterization…………………………………………..16 2-4.2.1 I-V Measurement…………………………………………….16 2-4.2.2 C-V Measurement……………………………………………17 CHAPTER 3 RESULTS AND DISCUSSION…………………………………………..18 3-1 As-Deposited Ta2O5 Thin Films……………………………………………..18 3-1.1 Physical Characterizations…………………………………………...18 3-1.2 Electrical Characterizations………………………………………….20 3-2 Effect of Rapid Thermal Annealing………………………………………..25 3-2.1 Physical Characterizations…………………………………………...26 3-2.2 Electrical Characterizations………………………………………….35 3-3 Conduction Mechanisms………………………………………………..….56 3-3.1 Theory………………………………………………………..………56 3-3.2 Leakage Current and Conduction Mechanisms………………………58 3-3.3 Different Temperature Measurement………………………………...67 CHAPTER 4 CONCLUSIONS AND FUTURE WORK..………………………………76 4-1 Conclusion…………………………………………………………………76 4-2 Future Work………………………………………………………………..77 REFERENCES……………………………………………………………………………78