應用於射頻/類比電路的三氧化二鑭金屬-絕緣層-金屬電容之特性研究

Abstract

在本篇論文中，我們使用高介電常數的三氧化二鑭絕緣層，來製作應用於高頻或類比電路的低溫金屬-絕緣層-金屬(金氧金)電容。然後深入探究其操作原理，和實際應用面上所產生的問題，包含漏電流和其傳導機制、類比特性和其訊號失真機制、受電應力行為和劣化過程，以及介電絕緣層崩潰和可靠度特性探討等。 首先，就三氧化二鑭絕緣層電流的傳導機制而言，它不像傳統二氧化矽絕緣層般，有這麼明確的載子傳輸行為。因為此三氧化二鑭介電質屬於低溫製備，擁有許多的缺陷密度和介面態在能隙中，致使其傳導機制較複雜且需要被修正。第二，此三氧化二鑭金氧金電容的電容密度與外加電壓、溫度和頻率之間的關係亦在此論文中詳加探討。有鑑於過往研究對描述這些關係之背後成因的論點分歧，我們從高介電常數絕緣層最根本的介電行為：極化與鬆弛的角度來切入。於是歸結出，電容的電壓係數主要受空間電荷的極化與鬆弛影響，而電容的溫度係數則最主要受控於電偶極的極化和鬆弛行為。而此電容在定電壓應力測試中，因為電應力會造成缺陷的產生，進而降低空間電荷的遷移率，導致其電容的電壓係數降低。反之，缺陷引發的電偶極極化效應則促使電容的溫度係數增加。由此可進一步證實，這兩種機制主宰了高介電常數金氧金電容之類比特性的精確度。 接下來，我們討論此三氧化二鑭金氧金電容之電應力行為及其時間依存性可靠度的分析。由實驗結果可歸納得，三氧化二鑭金氧金電容在定電壓應力下的劣化機制是缺陷產生與電荷補捉，這可借由量測低電場中的應力誘生漏電流及偵測出應力貢獻的電容密度變化來分別確定。我們也觀察到三氧化二鑭金氧金電容在定電壓應力下的二階段時間依存性崩潰現象，這歸因於金屬與介電質間的界面層先崩潰並導致劇烈的電荷補捉與釋放效應，爾後介電質本體也隨之崩潰。這層不可避免的界面層是因高介電常數介電薄膜直接沉積在金屬表面上所形成，由於它原本就具有較高的缺陷密度而易於崩潰，於是當介電層微縮時因它的不可微縮性就變成一個可靠度的問題。是故，界面層較高的初始缺陷密度不僅影響金氧金電容的漏電流和類比特性，在元件失效上亦佔有舉足輕重的地位。 最後，本實驗所製作的低溫十奈米三氧化二鑭金氧金電容，其具有低漏電流（在外加電壓□1 V時為9.4 nA/cm2），很高的崩潰電場 (在25 □C時大於7 MV/cm)，低的電容電壓係數(頻率在100 kHz時為671 ppm/V2)，足夠高之電容密度(11.4 fF/□m2)，以及高度穩定性和良好的可靠度等眾多優良特性。因此，鑭系高介電常數絕緣層金氧金電容是超大型積體電路技術中後段的射頻和類比電路裡最具潛力的被動元件。

In this study, the low-temperature metal-insulator-metal (MIM) capacitor with the high dielectric constant (high-k) lanthanum oxide (La2O3) film deposited by electron beam (e-beam) evaporation was fabricated and characterized for radio frequency (RF) and analog applications. The operational principles and the implementation issues of the high-k La2O3 MIM capacitor are discussed, including leakage current and conduction mechanisms, analog properties and distortion mechanisms, stress behaviors and degradation processes, as well as dielectric breakdown and reliability characteristics. To begin with, we evaluate the conduction mechanisms for high-k La2O3 MIM capacitors. Unlike the conventional SiO2 MIM capacitor where the quantum-mechanical tunneling is pronounced, the trap-related mechanisms are important for high-k MIM capacitors with low temperature fabrication, due to the high trap and interface state density in the high-k dielectric. Secondly, the effects of voltage, temperature, and frequency on the capacitance of high-k La2O3 MIM capacitors are investigated in detail on the basis of fundamental high-k dielectric behaviors: polarization and relaxation. The space charge polarization and relaxation are principally responsible for the positive voltage coefficient of capacitance (VCC). However, the dipolar polarization and relaxation dominate the positive temperature coefficient of capacitance (TCC). Interplay of these two effects on analog characteristics is crucial for developing the precise MIM capacitors with high-k dielectrics. The changes in VCC and TCC caused by the constant voltage stress (CVS) also verify the above inferences. VCC decreases since the space charge mobility reduced by stress induced traps, but TCC increases because the quantity of trap induced dipoles grows during stress. Furthermore, the stress behaviors and the reliability issues of high-k La2O3 MIM capacitors under various CVS conditions are also studied. The wear-out mechanisms of La2O3 MIM capacitors during electrical stress are trap generation and charge trapping. This could be identified by measuring the stress induced leakage current (SILC) at low field and by detecting the capacitance variation under electrical stress, respectively. Moreover, the very distinct two-step time-dependent dielectric breakdown caused by CVS testing could be observed. It is ascribed to firstly the interfacial layer (IL) breakdown leading to the severe charge trapping/detrapping, followed by the breakdown of the bulk high-k layer. Therefore, the high intrinsic defect density in the IL not only affects the leakage current and analog characteristics of MIM capacitors, but also plays an important role on the device failure rate. In summary, a highly stable and reliable 10-nm La2O3 MIM capacitor with low leakage current (9.4 nA/cm2 at □1 V), high breakdown strength (□ 7 MV/cm at 25 □C), small VCC (671 ppm/V2 at 100 kHz), low thermal budget (□ 400 □C), and sufficient high capacitance density (11.4 fF/□m2) has been successfully demonstrated. The results highlight the promise of the La-based high-k MIM capacitors as the next-generation passive component in RF/analog circuits.