一維奈米結構之電子傳輸現象觀察研究

Abstract

近年來，由於電子尺寸微縮，一維奈米材料因具有良好的可擴展性，被廣泛地研究。一維奈米材料在物理及化學性質之表現上，與塊材不盡相同，因此，許多研究深入探討其在電子元件中之電性行為以及其動態反應。在本研究中，利用一維奈米線作為研究材料，應用於以下研究主題中，非揮發性電阻式記憶體，非揮發性相變化記憶體，銅導線中電致遷移之行為研究。 在第一部分，我們利用臨場觀察電阻式記憶體其阻態轉變時材料之變化，將有助於機制之釐清。電阻式記憶體由於具有非揮發性、速度快、讀寫次數高、記憶容量大且成本低等優點被視為下個世代極有可能取代Flash之記憶體，但其阻態轉變機制尚眾說紛紜。本研究中，因奈米線尺寸較薄膜小，較易限制燈絲尺寸，以便觀察燈絲路徑產生之位置，故我們選擇氧化鋅奈米線作為觀測材料。電極部分，則利用電子束微影製程及電子槍真空蒸鍍系統製作金電極。再以臨場穿透式電子顯微鏡觀察氧化鋅奈米線在高阻態及低阻態之影像，藉由穿透式電子顯微鏡以及X 光微區分析得知在低阻態時，氧原子往陽極移動，使氧化鋅奈米線以鋅原子與氧空缺為主要元素，以形成燈絲路徑。本研究提供直接證據，證明阻態轉變是由於氧原子遷移至陽極部分生成燈絲所產生。 第二部分則針對相變化記憶體中電性表現以及晶相間的關係作深入地探討。相變化記憶體亦被視為下一個取代Flash的非揮發性記憶體。相變化材料在加熱前後，晶相轉換造成高低組態改變，而具有記憶效果。本研究中，利用臨場超高真空穿透式電子顯微鏡，觀察臨場施加電壓導致電阻轉換時，材料原子級形貌及結構之變化。由不同的電性量測，證實硒化銦奈米線相變化記憶體具備高電阻比值以及良好耐久性等性質。元件導通利用電壓掃描完成，元件重置由於需要快速升溫之過程，則利用脈衝量測模式來完成。在重置過程中，發現不同的脈衝寬度，影響重置時相的變化。較大的脈衝寬度，重置過程行為由退火取代淬火，因此硒化銦奈米線由單晶轉變為多晶而非非晶。脈衝振幅的不同，則影響單晶轉變為多晶之區域大小及晶粒大小。脈衝振幅大，導致溫度提高，使得多晶晶粒較大。本研究經由瞭解電性及量測模式對硒化銦奈米線相變化所造成的影響，並探討不同量測模式造成不同重置結果，以推論相變化過程可能產生的問題。 最後，我們利用臨場觀察銅奈米線電致遷移以及熱致遷移之行為。銅導線在半導體中占有相當重要的位置，在元件中，高電流密度可能導致之電遷移效應將會影響元件之可靠度。本研究中，利用臨場超高真空穿透式電子顯微鏡，觀察臨場施加電壓時，銅原子的移動現象。當電流密度達到一定強度時(106 A/cm2)，電致遷移之行為開始發生，在電流密度持續上升後，元件造成崩潰，另外，亦發現在電致遷移產生的同時，試片其餘地方會有銅顆粒的產生。由研究結果中，可得知電致遷移之方向，取決於電子靜電以及電子風之力的總和。銅導線中，電子風相較於電子靜電力之影響較甚，因此，電致遷移的方向為陰極往陽極移動。此外，經由熱致遷移效應，可以發現大顆粒將小顆粒的銅溶解並成長之過程。本研究經由了解及探討電以及熱致遷移現象，期許能改善銅導線導致元件失效之問題。

For the last decades, the one dimentional nanowires are widely studied due to its potential as a functional components in nanodevices. While its physical properties are different from the bulk materais, the relationship between electric properties and dynamic behaviors are important and interesting for the IC device application. For continuance of Moore's Law, 1-D device was devoloped to scale down the size of IC device. 1-D nanowire device not only has high scalability, but also has the restricted area, which let dynamic processes could be observed and studied easily. The thesis utilize 1-D nanowires in the following topics, resistive random-access memory (ReRAM), phase change random access memory (PCRAM) and electromigtration of Cu nanowires. The electric properties and dynamic behaviors of 1-D nanowires device will be investigated. In the first part, the switching mechanism of ReRAM has been studied. ReRAM has been of wide interest for its potential to replace flash memory in the next-generation nonvolatile memory roadmap. In this study, we have fabricated the Au/ZnO-nanowire/Au nanomemory device by electron beam lithography and, subsequently, utilized in-situ transmission electron microscopy (TEM) to observe the atomic structure evolution from the initial state to the low resistance state (LRS) in the ZnO nanowire. The element mapping of LRS showing that the nanowire was zinc dominant indicating that the oxygen vacancies were introduced after resistance switching. The results provided direct evidence, suggesting that the resistance change resulted from oxygen migration. In the second part, the relationship between switching behavior and dynamic evolution of PCRAM has been investigated. PCRAM has been extensively investigated for its potential applications in nonvolatile memory of next-generation. In this study, Indium (III) Selenide (In2Se3) was selected due to its high resistivity ratio and lower programming current. Au/In2Se3-nanowire/Au phase change memory devices were fabricated and measured systematically in the in-situ TEM to perform RESET/SET process under pulsed- and dc voltage swept-mode, respectively. During the switching, we observed the dynamic evolution of the phase transformation process. The switching behavior resulted from crystalline / amorphous change and revealed that a long pulse width would induce the amorphous or polycrystalline state by different pulse amplitudes, supporting the improvement on the writing speed, retention and endurance of PCRAM. Finally, Cu nanowires connected to Au electrodes were fabricated and observed by in-situ TEM to investigate the electro- and thermo- migration processes that are induced by DC current sweeps. Electromigration in Cu has been widely investigated as the root cause of typical breakdown failure in Cu interconnects. In this study, we observed the dynamic changes of different mass transport mechanisms. A current density on the order of 106 A/cm2 and a temperature of approximately 400°C were sufficient to induce electromigration and thermomigration, respectively. Observations of the migration processes activated by increasing temperature indicated that the migration direction of Cu atoms depends on the net force from the electric field and electron wind. This work is expected to support future design efforts to improve the robustness of Cu interconnects.