前瞻電阻式轉態記憶體元件之製作與特性研究

Abstract

近年來，非揮發性記憶體元件帶動了攜帶性產品的發展，如手機、筆記型電腦、mp3隨身聽、隨身碟、數位相機等。傳統非揮發性記憶體的操作原理是利用複晶矽浮停閘(floating-gate)做為載子儲存單元，其操作模式是透過儲存的電荷自反轉層之通道以穿隧效應躍遷過穿遂氧化層並儲存至複晶矽浮動閘極內。當移除所施加於元件的閘極偏壓後，由於所儲存電荷無足夠的電場或動能來穿過或躍遷穿遂氧化層，該電荷便侷限於浮動閘極中，並造成電晶體之起始電壓的改變。藉此，我們透過記憶體元件起始電壓的變化來判讀其狀態。然而，隨著元件微縮技術的需求，使得傳統的浮停閘極記憶體結構也面臨了元件可靠度上的挑戰，在元件進入奈米尺度後，為了維持閘極控制能力則必須等比例微縮下的穿隧氧化層，使其厚度不再具有絕對的阻障效果，導致保存在浮停閘內的儲存電荷容易再穿隧回通道。另外，在經過長時間而高速的操作後，薄化的穿隧氧化層亦容易產生成漏電路徑，因而導致記憶體元件的失效。另外，在橫向的微縮上，當其中單一元件寫抹或讀取時，可能連帶影響到鄰近的元件的狀態。有關新世代非揮發性記憶體的發展呈現百家爭鳴的情形，其中，電阻式非揮發性記憶元件具有低功率消耗、高密度、高操作速度、高耐久性、微縮能力高及非破壞性資料讀取等優點，使其成為新世代非揮發性記憶元件的熱門人選。 在本論文中，我們首先尋找各式的電極材料應用在MnO2電阻式記憶體上，由於在實驗中發現，不同的電極材料對於電阻式記憶體的轉態特性：是否成功轉態、轉態窗口的大小、多次操作轉態的穩定性或資料保存的耐久性...等等有著明顯的差異。在多次實驗後，我們找出最佳的組合可以使電阻式記憶體的特性達到最佳化，並且我們使用二次離子質譜儀（SIMS）對不同電極材料記憶體元件做縱深分析，我們發現在不同電極材料和RRAM材料的介面中，氧離子的分佈極為不同，這影響了電阻式記憶體在電阻轉態的行為，這項發現讓我們得知電極材料和RRAM材料之介面對於轉態行為扮演了重要的角色，並且我們找到了最佳的電極材料組合。 我們利用上述的最佳電極材料組合應用在DyMn2O5電阻式記憶體材料上，我們發現在第一次操作（Forming process)其電流-電壓特性曲線會出現一個負微分電阻，以及在之後的轉態操作中，也發現存在一個負微分電阻的現象，此現象在之前的研究中並無發現，並且我們將元件操作在適當電壓下，我們是第一次發現此元件可以在另外兩個電阻態之間作切換，一般大部分對於電阻式記憶體的研究中分為兩派，一派是支持電阻絲理論(filament type)而另一派則是支持介面理論(interface type)，然而我們認為此兩種理論並不單一存在而是可以共存在同一元件中，如同我們DyMn2O5電阻式記憶體材料上，因此我們對於DyMn2O5電阻式記憶體材料做尺寸效應的實驗，我們發現我們元件中filament type以及interface type的轉態操作是共存在同一元件中。 另一方面，我們使用目前常做為電阻式記憶體的材料HfO2，對其做多組態的操作（multi-level)並用變溫量測平台萃取出不同組態的溫度係數，實驗結果發現，隨著限流(compliance current)的增加，電流和溫度的關係是從半導體態轉換到金屬態傳導，由此可知，在電阻式記憶體從OFF-state轉換到ON-state的過程中，電阻絲是分段形成，電子經由跳躍傳導(Hopping)，而後隨著限流的增加，電阻絲連成一條導通路徑，當提高更大的限流，電阻絲是慢慢的變粗。 此外，我們嘗試在電阻性記憶體中埋入金屬奈米點，期望能夠穩定電阻式記憶體的導通路徑以達到穩定操作的目的，實驗結果發現。電阻是記憶體的電流不管在開啟（ON-state)或是關閉狀態（OFF-state)會因為金屬奈米點的嵌入而穩定下來。此外由關閉狀態至開啟狀態的導通電壓（Set Voltage)以及相反的關閉電壓（Reset Voltage)變化的範圍也會縮小，經由ISE-TCAD模擬計算發現，在奈米點的上下兩側會出現很強的尖端電場，由於這上下的尖端電場，可以使氧離子更容易在奈米點上下做移動，因此導通路徑會被奈米點限制在同一路徑上以達到穩定操作的目的。

In conventional memory devices, poly-silicon is used as the “floating-gate” to store charge. However, the conventional floating-gate non-volatile memory device has faced the challenge of reliability due to the requirement of down-scaling device. The scaled tunneling oxide is difficult to prevent the stored charge in the floating-gate from tunneling back into the Si-substrate. To improve the retention time of conventional floating-gate memories, several novel memory devices have been proposed. The resistive switching random access memories (RRAMs) have advantages of low power consumption, high-density integration, and high speed operation as one of the next-generation nonvolatile memory candidates. In the thesis, the influence of top electrode material on the resistive switching properties of MnO2-based memory film using Pt as a bottom electrode was investigated. In comparison with Pt/ MnO2/Pt and Al/ MnO2/Pt devices, the Ti/ MnO2/Pt device exhibits resistive switching current–voltage (I–V ) curve, which can be traced and reproduced more than 105 times only showing a little decrease of resistance ratio between high and low resistance states. Furthermore, transmission electron microscopy analyses are used to confirm the crystalline structure of MnO2 on Pt bottom electrode. Secondary ion mass spectrometry reveals a change of oxygen distribution in MnO2 thin film due to material characteristic of variant top electrodes. We suggest that the interface between MnO2 and electrodes play an important role on the resistive switching behaviors. Then, we use the above mentioned optimize electrode composition on the DyMn2O resistive switching memory device. A negative differential resistance (NDR) phenomenon was first found at the forming process and resistive switching I-V curve. By applying approach voltage, dual resistive switching characteristics can coexist in on memory device. The typical switching effect could be attributed to the formation and rupture of the conducting filament in DyMn2O5 films. The parasitic switching behavior can be observed in the specific operation condition. Dual bipolar resistance switching behaviors of filament-type and interface-type can coexist in the devices by appropriate voltage operation. In addition, the relationship between filament-type and interface-type switching behaviors were studied In addition, HfO2 which is a popular resistive switching material is investigated for multi-level non-volatile memory applications. We develop a gradual soft breakdown technology combine with the different temperature measurement system to extract the temperature coefficient of the different resistance state. The results indicate as the compliance current increases, the transportation mechanism switches from hopping (semiconductor-like) to ohmic conduction (metal-like). Due to hopping barrier decreases, the ρ become lower before the critical compliance current of 300uA. When the compliance current becomes larger than 300uA, the electrons go through a metal-like conducting path. As the compliance current become larger, the corresponding filament area A become larger to make more electron can transport though the metal filament. The formation of filament of HfO2 RRAM should be divided into two steps, the purification and the reproduction. Besides, nonvolatile memory characteristics of Ti/Al2O¬3/Pt embedded with Ni nanocrystals is investigated in order to improve the behavior of RRAM. The cross-section TEM and XPS results shows that the formation of Ni nanocrystals after the 500℃ annealing. The device characteristics of RRAM can be stabilized by embedding nanocrystals. It is due to the conductive path can be confined through the nanocrystals instead of forming randomly in the device which is without nanocrystals. Therefore, the structure with embedded nanocrystals can be applied in next generation nonvolatile memory application.