先進元件中原子尺寸缺陷之研究

Abstract

隨著電子元件尺寸縮小的未來趨勢，將會面對更多的單電子效應，尤其以隨機擾動的電子訊號 (RTS) 更為之重要。研究小尺寸元件中，經由單一一個氧化層缺陷所造成的雜訊行為，可以提供更不一樣的當元件操作過程中氧化層退化的訊息。因此，一個單一缺陷是否為中性或是帶電性，可經由電流的擾動行為來觀察判斷。低頻雜訊 (又稱1/f noise) 可以視為一個在頻率域把所有不同的RTS訊號的貢獻都加起來的電子訊號，低頻雜訊可以當成研究半導體和絕緣層之間的介面性質的有效方法。 在本篇論文中最主要的目的是，更深入的研究探討雜訊與擾動在超薄氧化層元件、擁有高介電常數之絕緣層 (high-k) 的元件以及受製程應力 (process strained) 之元件。根據在不同元件上的研究，本篇論文的架構如下所描述。 首先，第一章是針對雜訊與擾動的介紹。接下來，在超薄氧化層元件中 RTS 現象的研究將在第二章中會有所描述，庫倫能量 (Coulomb energy) 可以視為對一個奈米尺寸的缺陷充電過程中所不可忽略的要素，在本論文，我們首次發表實驗上在1.7奈米厚度的氧化層元件所做的研究分析，發現更深入氧化層的缺陷將會面對到更高的庫倫能量，另一個成果是，利用 multiphonon 理論成功的解釋電子被缺陷抓取以及釋放的能量問題。相對應的能量配置座標圖也建立在本論文中。在第三章中，我們將探討經由RTS振幅大小所淬取出庫倫散射在二維電子氣中所造成的影響，基於對一個單一缺陷所造成的RTS現象的相關實驗，發現到更深的氧化層缺陷將會造成更小的庫倫散射現象。但是當在強反轉 (strong inversion) 範圍，庫倫散射對在表面的缺陷而言，會造成更強烈的變化現象，一個更強烈的遮蔽效應反映在庫倫電位是造成這這現象的主因，庫倫散射和缺陷的位置的關係將是未來奈米元件中更需要重視的問題。 接下來在第四章中，我們將研究有關高介電的絕緣層元件，打開-關閉切換的行為或是兩個級別的隨機擾動雜訊都是在低電壓下量測N型通道超薄閘介電層( 1 奈米的氧化層 + 1 奈米的氮化矽 )金氧半電晶體的邊緣直接穿透電流。起因是由於製程所造成的缺陷可以說是局部的閘極堆疊變薄(或是等效於具傳導性的漏電流蘇)。在這個非固有的狀況中，電流中電子被陷阱抓住或是電子被陷阱釋放出來的理論可以適當的解釋我們所量到的數據，尤其隨機擾動雜訊振幅的大小比例高達百分之十八。電流對電壓的特性曲線很直覺的關聯著某些個缺陷點，展現和氧化層變薄的事實十分的一致。RTS可以用來觀察電流流通過一個奈米線狀的缺陷點的有效工具，一個敏感的監督製程的角色就像是展示出缺陷發生的機率及位置所在。 之後在第五章中，低頻雜訊拿來使用在監控受不同的製程應力程度下的氧化層介面品質，在低頻雜訊在承受製程應力的金氧半電晶體中的量測中，發現靠近表面的缺陷密度隨著通道寬度而變化。這個發現可以解釋為在矽和氧化矽的介面間，因為晶格長度不匹配所造成的 Pb 中心可視為靠近表面的缺陷的主要來源。在低頻雜訊的實驗中，對於通道寬度的縮減，相對應於應力的提高，也降低晶格長度不匹配的程度。最後，我們把所有所做過的研究結論放在第六章。

The continuous shrinking in the feature dimensions of metal-oxide -semiconductor field-effect transistors (MOSFETs) brings into prominence the single electron effects, among which the most important is the Random Telegraph Signals (RTS). Studies on noise from individual oxide traps in small structures can supply new information of device operation as well as degradation phenomena. Thus, individual traps can be observed in their neutral or charged state and, as a consequence, the current fluctuates between two discrete levels. The low-frequency noise (so-called 1/f noise) can be considered as the superposition of several random telegraph signals (RTS) in frequency domain. The 1/f noise can be used as a potential tool for studying the interface between the semiconductor and insulator. The main purpose of this dissertation is to deeply investigate the fluctuations and noise in ultrathin oxide devices, high-k devices and process strained devices. Based on the study of different devices, the organization of this dissertation is described below. First, an introduction to the RTS and noise is described in Chapter 1. The study of the RTS phenomenon in ultrathin oxide devices is demonstrated in Chapter 2 of the dissertation. Coulomb energy is essential to the charging of a nanometer-scale trap in the oxide of a metal-oxide-semiconductor (MOS) system. In this dissertation, we present for the first time experimental evidence from a 1.7-nm oxide: substantial enhancements in Coulomb energy due to the existence of a deeper trap in the oxide. Other corroborating evidence is achieved on a multiphonon theory, which can adequately elucidate the measured capture and emission kinetics. The corresponding configuration coordinate diagrams are established. Then, Chapter 3 presents the study on Coulomb scattering in a two-dimensional electron gas (2DEG) system through the relative amplitude of RTS. Experiments on an individual nanoscale trap in the oxide responsible for random telegraph signals lead to remarkable results. In this work, we demonstrated a study for relationship between the capture time, emission time, and the relative amplitude. Initially, the deeper trap in oxide corresponds to weaker Coulomb scattering in a 2DEG. However, as the 2DEG enters into the strong inversion regime, the amount of Coulomb scattering with an interface trap drops with a faster rate than the deep trap. A stronger screening potential confinement is shown to be the physical origin of this effect. The near-distance effect is expected to remain a challenging issue in the area of nanoscale devices. Second, the study of the high-k devices is described in Chapter 4. On-off switching behaviors or two-level RTS are measured in the low voltage direct tunneling currents in ultrathin gate stack (10 Å oxide + 10 Å nitride) tunneling diode. The plausible origin is the process-induced defects in terms of localized gate stack thinning (or equivalently the conductive filament). In such extrinsic case, the current trapping-detrapping theories can adequately elucidate the data, particularly the RTS magnitude as large as 18%. The current-voltage (I-V) characteristic associated with a certain defective spot is assessed straightforwardly, showing remarkable compatibility with existing oxide thinning case. The current tunneling through the wire-like weakened spot can be probed by RTS. The role as a sensitive process monitor is demonstrated in terms of the occurrence probability of the defects as well as their locations. Third, the 1/f noise used to monitor the quality of oxide interface with different tensile stress is presented in Chapter 5. Low-frequency noise measurement in process tensile-strained n-channel metal-oxide-semiconductor field-effect transistors yields the density of the interface states, exhibiting a decreasing trend while decreasing the channel width. This finding corroborates the group of Pb centers caused by the lattice mismatch at (100) Si/SiO2 interface as the origin of the underlying interface states. The present noise experiment therefore points to the enhancement of the tensile strain in the presence of channel narrowing, which in turn reduces the lattice mismatch. Finally, we summarize the conclusion of our works in Chapter 6.