銅膜化學氣相沉積之技術開發與銅膜整合參氟二氧化矽在積體電路上的應用

Abstract

本論文研究及開發銅膜化學氣相沉積技術，以及整合低介電常數物質(參氟二氧化矽)與銅膜之可靠度研究。銅膜化學氣相沉積為一項正在開發中的技術，現有之技術資料十分有限，亦無商用設備可供購置，因此，本研究首先針對銅膜化學氣相沉積所須之各種功能和需求的考量，自行設計、組裝、建立一垂直式低壓化學氣相銅膜沉積系統，並且隨著周邊零組件的發展更新其系統，使其能更精確、再現性更高的沉積出適用於深次微米積體電路連線使用的銅膜。 第一階段以氣泡式(bubbler)銅源輸送系統配以冷壁式反應腔體來沉積銅膜，一方面探討化學氣相沉積銅膜之基本性質，如沉積速率、活化能、銅膜電阻係數、雜質含量、及其微結構等，另一方面試圖瞭解Cu-CVD在不同載氣(carrier gas)下之銅膜性質並探討其化學反應機制。本研究發現Cu-CVD使用氫氣作為載氣時比起以氮氣或氬氣時能夠沉積出較低電阻係數、較高沉積速率、及較為緻密的銅膜。氫氣不僅扮演還原劑的角色將原本無法繼續反應的附產物轉變為銅膜(在沒有還原劑的狀況下，兩個銅源分子僅能產生一個銅原子，而當還原劑存在時則將另一個原本反應成銅化合物的附產物還原成銅原子)，以增加沉積速率外，也扮演了清道夫的工作將殘餘的附產物反應成較易揮發之氣體以利排除。另一方面本研究也發現當銅膜的雜質含量較低及結構較緻密時其電阻係數較低，而此銅膜性質乃由反應條件所控制。當反應條件在溫度180 ℃、壓力300 mTorr、以流速100 sccm的氫氣作為載氣的狀況下得以沉積出低雜質含量及結構較緻密的銅膜，亦即具有較低電阻係數的銅膜性質。 事實上氣泡式(bubbler)輸送系統是當初也是現今許多液態前驅物質所常使用的方式，但是對Cu-CVD而言由於銅源蒸氣壓很低致使銅膜沉積速率非常低(約100 Å/min)，對實際應用而言非常不符合經濟效益，所以當周邊零組件有新突破時我們也更新了我們的輸送系統，改以液體直接輸入(DLI)方式輸送銅源，這樣不僅能夠更精確的輸送銅源，其再現性及操控性都非常優異，尤其銅源是以室溫型態放置大大的降低了前反應(Premature)的機率。同樣的其更新系統的基本性質被探討，另外在階梯覆蓋型態上(Step coverage)本研究採一特殊Cantilever結構來探討其反應機制，其結果顯示re-emission的反應機制扮演了非常重要的角色，能使銅膜的階梯覆蓋性好。本研究完成了尺寸0.2 μm及aspect ratio為3的填洞，其Cu-CVD的反應條件為溫度200 ℃、壓力800 mTorr、以流速150 sccm的氦氣作為載氣、銅源流速為0.48 ml/min的狀況下以超過800 Å/min的沉積速率鍍出2.0 μΩ-cm電阻係數的銅膜。 本研究針對整合低介電常數物質(參氟二氧化矽)與銅膜之可靠度研究做了探討。就參氟二氧化矽之性質而言其介電常數隨著氟含量的升高而降低，但其抗水性卻增加，事實上參氟二氧化矽吸了水之後不僅造成介電常數的升高亦使得元件的可靠度產生了問題。本研究提出了以一氧化二氮電漿(N2O Plasma)後置處理來改變參氟二氧化矽的表面性質，藉以改善吸水問題。在整合的議題上本研究採Cu/SiOF/Si MIS的結構來探討，其中銅膜的製作採物理氣相沉積(PVD, i.e., sputtering deposition)及化學氣相沉積(chemical vapor deposition)兩種方式。對於物理氣相沉積所製作的銅電極而言此MIS元件劣化發生於溫度400 ℃以上，若加入25 nm厚的擴散阻礙層TaN可將元件的穩定溫度提升至500 ℃。一氧化二氮電漿(N2O Plasma)的後置處理被發現能在銅電極與參氟二氧化矽界面形成一有效擴散阻礙層，將元件的穩定溫度提升至500 ℃。另一方面對於以化學氣相沉積來製作的銅電極而言，在銅膜沉積的同時形成了界面物質，此界面物質乃由化學氣相沉積的附產物與參氟二氧化矽表面反應而成，此界面物質在本研究中被發現為一有效的擴散阻礙層，可以有效阻止銅擴散進入參氟二氧化矽，將此MIS元件的穩定溫度提升至500 ℃。

This thesis consists of three major topics dealing with equipment and process issues of Cu metallization and reliability and compatibility issues regarding integration of Cu with SiOF films. For the Cu CVD study, a low pressure chemical vapor deposition (LPCVD) system with a vertical reactor was built and remodeled for accurate and reproducible deposition of Cu films. We considered the Cu CVD’s specific purpose, the handling capability, the compatibility with other processing equipment, and the financial limitation in this home-made Cu CVD system. The apparatus used in the first stage of this thesis study was a home made cold wall reactor LPCVD system using a bubbler scheme for precursor delivery. Various basic properties of Cu CVD were investigated including activation energy, deposition rate, film resistivity, impurity content, microstructure, as well as their dependence on the deposition pressure, substrate temperature, and the kind of carrier gases. We found that the Cu-CVD using H2 as a carrier gas resulted in Cu films of lower resistiviity, denser microstructure and faster deposition rates than using Ar or N2 as the carrier gas. Hydrogen, not only acts as a reduction agent to improve the deposition rate, but also reacts with the residual precursor fragments. On the other hand, we also found that low-resistivity Cu films can be obtained by reducing impurity concentrations in the films and by forming films with a compact microstructure. At a deposition temperature of 180 ℃ using 100 sccm H2 as the carrier gas, Cu films with a low impurity content can be deposited at low deposition pressure of 300 mTorr, resulting in deposited Cu film with denser microstructures. Thus, the resistivity of Cu films is dependent on the deposition conditions. Since the “bubbler” precursor delivery scheme resulted in very low deposition rates (〜 100 Å/min), which is not adequate for practical applications, we remodeled our Cu CVD apparatus by introducing a direct liquid injection (DLI) precursor delivery system to provide highly accurate, reproducible, and controllable flow of precursor to the reaction chamber. This remodeled system was used for the study of Cu CVD in the second stage. Profiles of Cu films deposited on a specialized cantilever structure were studied with respect to the key processing parameters. The kinetics and the via filling capability of the Cu CVD were investigated by Cu film deposition on this cantilever structure. We found that re-emission of the rate-limiting precursor species plays an important role in enhancing the conformality of Cu films deposited on the cantilever structure. Process parameters of enhancing such mechanisms (multiple adsorption/re-emission events) were investigated. It was found that complete Cu filling of vias with a size of 0.2 μm and an aspect ratio of 3 can be achieved by the Cu-CVD at 200 ℃ and 800 mTorr with the precursor flow rate of 0.48 ml/min and the He carrier gas flow rate of 150 sccm; with such deposition conditions, the deposition rate of Cu films exceeded 800 Å/min and the as-deposited film resistivity was below 2.0 μΩ-cm. The thermal stability of fluorine-doped silicon oxide (SiOF) low-k dielectric as well as the integration issues of SiOF and Cu metallization is also investigated in this thesis. It is found that the dielectric constant of SiOF films decreased with increasing the fluorine concentration in the films; however, higher concentration of fluorine in the films also resulted in lower resistance to moisture uptake. The moisture absorption led to increase in dielectric constant and caused device reliability problems. N2O plasma treatment resulted in SiOF films of increased moisture resistance, presumably due to modification of the SiOF film, especially in the surface region. In the study of integration issues of SiOF and Cu metallization, the Cu electrodes of the Cu/SiOF/Si MIS capacitors were deposited by physical vapor deposition (PVD, i.e., sputtering deposition) as well as chemical vapor deposition (CVD) methods. For the sputter deposited Cu electrode, the Cu gated MIS capacitors were able too remain intact after a 30 min annealing at temperature up to 400 ℃. With a 25 nm thick reactively sputter deposited TaN barrier layer sandwitched between Cu and SiOF, the thermal stability temperature of the MIS capacitors were raised to 500 ℃. Post deposition in-situ N2O plasma treatment was found to result in decrease of dielectric constant as well as barrier effectiveness against Cu diffusion for the SiOF film. With the N2O plasma treated SiOF, the Cu/SiOF/Si capacitors were able to remain stable up to 500 ℃ For the Cu electrode chemically vapor deposited on SiOF, the by-products of Cu CVD reacted with SiOF, leading to the formation of an interfacial layer at the Cu/SiOF interface. It turned out that the interfacial layer acted as an effective barrier and prevented Cu from permeating into the SiOF films; thus, the CVD-Cu/SiOF/Si MIS capacitors were able to remain stable at temperature up to 500 ℃. Abstract (Chinese) i Abstract (English) v Acknowledgement (Chinese) ix Contents x Table Captions xiii Figure Captions xiv Chapter 1 Introduction 1 1.1 General Background 1 1.2 Motivation for Copper Chemical Vapor Deposition 2 1.3 Motivation for Low Dielectric Barrier Study for Cu Metallization 3 1.4 Thesis Organization 4 References 6 Chapter 2 System Configuration of Cu CVD 10 2.1 Introduction 10 2.2 Functional Consideration of Precursor 11 2.3 Cu CVD System with Bubbler Delivery Scheme 13 2.4 Direct Liquid Injection (DLI) Delivery System 15 2.5 Conclusions 18 References 19 Chapter 3 Copper Chemical Vapor Deposition from CuI (hexafluoro-acetylacetonate) trimethylvinylsilane 29 3.1 Introduction 29 3.2 Experimental Procedure 30 3.3 Deposition Mechanism 31 3.4 Characterization of Cu Films 33 3.4.1 The Properties of CVD Copper Films 33 3.4.2 Copper Deposition at Different Temperatures 34 3.4.3 Copper Deposition at Different Pressure 36 3.5 Effects of Different Carrier Gases 37 3.6 Conclusions 39 References 40 Chapter 4 Process Optimization for Conformality of Copper Chemical Vapor Deposition 68 4.1 Introduction 68 4.2 Experimental Precedure 69 4.2.1 Deposition of Cu Films 69 4.2.2 Cantilever Structure 70 4.2.3 Characterization of Cu Films 70 4.3 Fundamental Mechanism of Conformal Deposition 70 4.4 Temperature Dependence of Deposition Rate 72 4.5 Copper Deposition at Different Pressure 74 4.6 Reactant Concentration Dependence of Deposition Rate 74 4.7 Cu Profiles in Cantilever Structure 75 4.8 Conclusions 76 References 78 Chapter 5 Fluorine-doped Silicon Oxide (SiOF) and its Integration with Cu Metallization 93 5.1 Introduction 93 5.2 Experimental Procedure 94 5.2.1 Sample Preparation 94 5.2.2 Electrical Measurement 96 5.3 Characteristics of SiOF 97 5.3.1 Dielectric Constant 97 5.3.2 Water Absorption 98 5.3.3 Effect of N2O Plasma Treatment 99 5.4 Thermal Stability of TaN/Cu/SiOF/Si Capacitors 100 5.5 Barrier Effectiveness of TaN Layer 101 5.6 Effect of N2O Plasma Treatment 102 5.7 Cu Electrode Deposited by CVD Method 103 5.7.1 Thermal Stability of TaN/Cu/SiOF/Si Capacitors 103 5.7.2 Effects of TaN Barrier Layer 105 5.7.3 Effect of N2O Plasma Treatment on SiOF 106 5.8 Comparative Study Between the Sputter Cu Electrode and the CVD Cu Electrode MIS Capacitors 106 5.9 Conclusions 107 References 109 Chapter 6 Conclusions and Future Study 6.1 Conclusions 142 6.2 Future Works 145 Publication List 147 Vita 148