前瞻非揮發性電阻式記憶體元件之製作與特性研究

Abstract

隨著數位行動生活的到來，非揮發性記憶體在可攜式電子產品，如：手機、數位相機跟筆記型電腦扮演著重要的角色。以傳統浮閘(Floating gate)記憶體為基本元件之非揮發性固態半導體記憶體是現今非揮發性記憶體的主流，但是它有著許多缺點，包含：高的操作電壓、低的操作速度與較差的耐久力，且隨著尺寸微縮的趨勢下，面臨了難以解決之難題，即儲存在懸浮閘極中之電荷，因穿遂氧化層過薄而隨時間漸漸流失，造成資料流失，如此瓶頸，加快了下世代非揮發性記憶體之研究腳步。其一類預期能取代傳統浮閘記憶體的非揮發性元件者為電阻式非揮發性記憶體(RRAM)，主要是由於其製程簡單且與動態隨機存取記憶體(DRAM)製程相似，可以被整合到半導體的後段製程。電阻式記憶體擁有高速、非揮發性與低電壓操作的特性等優點。故本論文所研究之主題，將針對電阻式性記憶體這類型之非揮發性記憶體原件觀念為主，而提出一以鐵原素為製程基礎之一系列具簡易製程方式的記憶體原件之備製，並探討其元件之原理與特性，以及更進一步地提供改良其元件特性之製備方式：對此鐵原素電阻式性記憶體而言，我們提供了一種利用氧化鐵電極的表面之方式，來獲得一層具備阻值轉態特性的薄氧化鐵薄膜，因為阻值轉態特性的存在主要與非完全化學當量組成的過渡金屬氧化物的存在相關，是故藉由氧化鐵電極表面所獲得之薄氧化鐵薄膜的最大優勢為其本身所具備之不完全化學組成之氧化層成分，而能利用以提供較佳的電阻式記憶體特性。另一方面，藉由鉑(Pt)元素於鐵電極的參雜而製成的鉑化鐵(FePt alloy)合金，而能間接控制鐵電極的氧化與擴散，以利於進一步地探討其特性與原理。就其基本轉態原理之推測上，我們藉由不同極性之高壓施加而於電極上所產生之不同特性之泡泡現象(Bubble Effect)，以及元件電阻值對不同元件電極面積之關係分析，來更進一步來間接地佐證此氧化鐵之阻值轉態現象的機制。甚者，我們亦於此兩組含鐵電極之元件，研究於不同的後續熱退火製程處理之特性，發現藉由退火處理能有效地降低形成電壓、操作電壓以及其電性參數之變異性，而能獲得更佳的電阻式記憶體特性，其原因也藉由X光光電子能譜儀(XPS)，穿透式電子顯微鏡(TEM)，X光粉晶繞射儀(XRD)分析來討論；我們更藉由做不同退火處理樣本之電性結果的統計分析，來釐清其更深入的機制。此外，我們亦發現藉由設定量測時之電流限流限制條件與電壓掃描限制條件之設定，而能任意調整此氧化鐵薄膜其所具有的阻值狀態，能達到具多重阻值位階之特性，以利於成為高儲存資料密度之運用；同樣地，此不同電流限流與電壓掃描限制條件下之電性結果統計分析，亦能對這尚未完全明瞭的電阻轉態機制，進一步地來輔助此元件轉態機制之確認。最後對全文作一總結，並對未來可行的研究工作做一建議。

With the arrival of Digital Age, nonvolatile memory (NVM) plays an important role in portable electronic products, such as the mobile phone, digital camera, and notebook computer. Floating gate composed nonvolatile memories have been widely applied in electronic devices in recent years, but it has many drawbacks, including high operation voltage, low operation speed, and poor endurance. Moreover, as the device dimensions are continuously scaled down, the floating gate composed memory faces the challenge of thin tunneling oxide that causes an unsatisfactory retention time. Consequently, resistive random access memory (RRAM) is one promising candidate to substitute for conventional floating gate memory. As for RRAM, the digital data can be stored in two memory states with high and low resistivities, ON-state and OFF-state, respectively. The two memory states can be easily switched by voltage biases or pulses, which enhance the possibility of the application in circuit level. Therefore, the topic of this thesis discusses this advanced nonvolatile memory devices by fabricating and characterizing iron-based RRAM devices and, furthermore, proposes the methods to improve the characteristics of the proposed devices for NVM applications. The proposed iron-based RRAM device was fabricated by a structure of TiN/SiO2/FeOx/Fe, where the FeOx is a thin transition layer at the SiO2/Fe interface and produced spontaneously during the plasma-enhanced tetraethyl orthosilicate oxide deposition process due to the ease of oxidation of iron atoms. The basic idea of the proposed structure is that the resistive switching effect is associated with the existence of nonstoichiometric materials, so the thin FeOx transition layer produced by partially oxidizing the iron electrode surface exhibits a richer nonstoichiometry property because of containing the compositions varied from Fe-rich FeOx (close to Fe electrode) to oxygen-rich FeOx (close to SiO2 layer). Moreover, in order to clarify the detailed mechanism of resistive switching effects, a method of adding platinum (Pt) into Fe electrode, which affects oxidation and diffusion characteristics of Fe layer, was also proposed by a TiN/SiO2/FeOx/FePt structure. In addition, observation of bubble effects occurred at the top electrode after biasing highly opposite polarity voltage stresses as well as the electrode area dependence of resistance values also provides another indirect method to clarify the mechanism of resistive switching effects. Moreover, the influence of thermal annealing treatments on the FeOx resistance switching behaviors was also researched because of the thermal sensitivity of the iron oxide layer. The distinct reduction of memory switching parameters in forming voltage, set/reset voltages, and even their dispersions was obtained after annealing. The cause was also discussed by XPS, TEM, and XRD analyses. Additionally, statistical electrical results, including set/reset current, set/reset voltage and set/reset power, also help for understanding the mechanism of resistive switching effects. Furthrmore, multiple resistance states were easily observed to obtain in our proposed FeOx-contained structure by justifying the sweeping voltage during the reset process region and the compliance current during the set process region, which allow more bits to be stored per cell for application. Moreover, extraction of statistical results, such as set/reset voltage, set/reset current and set/reset power, further provides more details to clarify the mechanism of the FeOx-contained resistive switching behaviors.In the final part of this dissertation, the conclusions and the suggested future works are presented.