準分子雷射退火之低溫高介電常數鈦酸鍶鋇薄膜BaxSr1-xTiO3於動態隨機存取記憶體電容器之研究

Abstract

隨著積體電路密度的增加與元件尺寸的縮小，動態隨機存取記憶體的電容面積與操作電壓亦隨之減小，但為了維持一定的訊號/雜訊比而不產生錯誤的判讀，其所儲存的電荷必須保持一臨界值，因此，我們必須在不增加儲存單元投影面積的前提下有效地提升電容器的電容值。 根據基本電容公式，有三種提升電容器電容值的方法：增加電極板有效面積、縮減介電層厚度以及以高介電常數材料取代傳統之介電層。但為增加電極板有效面積，隨著電容需求日增，製程亦愈形困難；而介電層厚度縮減至某一程度，則會造成穿遂效應，形成極大的漏電流。因此，使用高介電常數材料必將是未來之趨勢。 鈦酸鍶鋇是近年來備受注意的材料，其提供了下列優點：高介電常數、低漏電流、長使用週期、優良的化性、熱穩定性及低損耗因子等。縱使鈦酸鍶鋇具有上述諸優點，但對於達到高介電常數值，則需高結晶溫度(T>450°C)。當熱預算增加，則隨之衍生多問題，如前段製程MOS元件離子佈植原子之再分佈、底層薄膜間原子(Si，及底電極金屬原子...etc.)之內擴散，還有鈦酸鍶鋇薄膜與底電極介面之反應物生成。如何在低溫環境下達到結晶佳、高介電常數值，將是未來高密度DRAM極微波整合IC之製程整合最重要之課題。 本論文研究及利用248nm準分子雷射退火，成功的將一射頻磁控濺鍍系統於極低溫(T<150°C)下成長之鈦酸鍶鋇非晶薄膜熔融-再結晶，而達到異常高之介電常數值(er >1000)特性。並針對不同準分子雷射退火條件下之薄膜，研究其薄膜電性與物性。且應用準分子雷射對於鈦酸鍶鋇薄膜退火之物理機制亦有深入探討。

With increasing the density of devices on ULSI, the minimum feature size is scaled down into deep-submicron regime. For DRAM capacitors, the cell areas and applied voltage are also rapidly shrinking, which lead to lower storage capacitance while the minimum value of capacitance should be maintained in order to achieve a reasonable signal-to-noise (S/N) ratio. According to the calculating equation for flat-type capacitor, there are three ways to increase the value of capacitance, including the decrease of dielectric thickness, the enlargement of the effective surface area of storage nodes, and the utilization of high dielectric constant materials. However, large tunneling current limits the shrinking of dielectric thickness and more complicated processes are needed with enlarging the effective surface area of storage nodes. Therefore, the utilization of high dielectric constant materials is the trend for Giga-bit scale DRAMs in the future. Barium Strontium Titanate (BST, BaxSr1-xTiO3) is the most attractive material due to its high dielectric constant, low leakage current, TDDB over 10 years, good chemical and thermal stability, and low dissipation factor. Although BST has such advantages above, but a relative high temperature (> 450 oC) is needed to crystallize BST films to obtain high er. When the process thermal budget increases, many problems can occur; for example, redistribution of implanted atoms in MOS devices fabricated in front end, inter-diffusion of under-layer films like Si and bottom electrode metal, reaction between the thin BST films and metal film used as the bottom electrode metal. How to implement high crystallized, high □r thin BST films at low temperature will be the most important theme for the process integration of future high density DRAM and microwave monolithic IC. In this thesis, 248 nm excimer laser annealing was undertaken, and the amorphous-like BST film deposition at low substrate temperature ( 150°C ) was successfully melted and recrystallized. Very high dielectric constant (er > 350) was achieved. Moreover, the physical and electrical properties of BST films annealed with different parameters were also investigated. Finally, the detailed physical mechanisms of excimer laser annealing on BST thin films were offered. Abstract (in English)----------------------------------------iii Acknowledgment (in Chinese)------------------------------------v Contents------------------------------------------------------vi Table Lists---------------------------------------------------ix Figure Captions------------------------------------------------x Chapter 1 Introduction---------------------------------------------------1 1-1 Motivation------------------------------------------------1 1-2 Thesis Outline--------------------------------------------2 Chapter 2 Theoretical Background-----------------------------------------4 2-1 Basic Characterization of Perovskite Materials-----------4 2-2 Deposition Methods of Perovskite Materials---------------7 2-3 Electrical Characterization of Perovskite Materials------8 2-3.1 Dielectric Constant----------------------------------8 2-3.2 Leakage Current--------------------------------------9 2-3.3 Tangent Loss----------------------------------------10 2-3.4 Degradation-----------------------------------------12 2-4 Integration Issues--------------------------------------13 2-4.1 Dielectric------------------------------------------13 2-4.2 Barrier Layers--------------------------------------13 2-4.3 Electrode Materials---------------------------------14 2-5 Annealing Mechanisms of ELA-----------------------------15 2-5.1 Introduction of ELA---------------------------------15 2-5.2 Energy Deposition and Heat Flow---------------------16 2-5.3 Absorption Mechanisms of Intense Laser Radiation in Semiconductors--------------------------------------17 2-5.4 Grain Gowth Mechanisms of ELA-----------------------17 2-5.5 Process Prameter Effects of ELA Poly-Si Films-------18 Chapter3 Experimental Procedures---------------------------------------20 3-1 RF Magnetron Co-sputter System--------------------------20 3-2 KrF Excimer Laser Annealing (ELA) system----------------21 3-3 Experimental Procedures---------------------------------21 3-4 Physical Characterization Techniques----------------22 3-4.1 X-Ray Diffraction Analysis (XRD)--------------------22 3-4.2 Ellipsometry----------------------------------------23 3-4.3 Optical Properties-------------------------------23 3-4.4 Scanning Electron Microscopy (SEM)------------------23 3-4.5 Auger Electron Spectroscopy (AES)-------------------24 3-5 Electrical Measurements 3-5.1 Current-Voltage (I-V) Measurements------------------24 3-5.2 Capacitance-Voltage (C-V) Measurements--------------24 3-5.3 Impedance-Frequency Measurements--------------------25 Chapter 4 Physical and Electrical Characterization of BST Thin Film ELA on Bottom Electrode Ir----------------------------------------26 4-1 Introduction--------------------------------------------26 4-2 Experimental Procedures---------------------------------27 4-3 Results and Discussions---------------------------------28 4-3.1 The Result of Extinction and reflectivity analysis--28 4-3.2 The Result of Crystallinity Analysis----------------29 4-3.3 The Result of Capacitance-Voltage (C-V) Analysis----29 4-3.4 The Result of Current-Voltage (I-V) Analysis--------30 4-3.5 The Cnduction Mechanisms----------------------------31 4-4 Summary-------------------------------------------------32 Chapter 5 The Detail Analysis of BST Thin Films ELA with Pt/TiN/Ti/Si Substrate ----------------------------------------------------33 5-1 Introduction--------------------------------------------33 5-2 Experimental Procedures---------------------------------34 5-3 Results and Discussions---------------------------------35 5-3.1 The Result of Extinction and reflectivity analysis--35 5-3.2 The Result of Crystallinity Analysis----------------35 5-3.3 The Result of Depth Profile Analysis----------------36 5-3.4 The Result of Capacitance-Voltage (C-V) Analysis----36 5-3.5 The Result of Current-Voltage (I-V) Analysis--------37 5-3.6 The Cnduction Mechanisms----------------------------38 5-4 Summary-------------------------------------------------39 Chapter 6 Conclusions---------------------------------------------------41 Reference