銅應用於氧化鈦基電阻式隨機存取記憶體之研究

Abstract

電阻式隨機存取記憶體是新一代的記憶體，由於結構簡單和製程容易，有很大的潛力能替代快閃記憶體成為主要記憶體元件。電阻性記憶體在各方面的性能皆有一定水準，目前主要缺點為: 無確定統一的機制、重複性不佳、高阻態電流值不穩。本篇論文中，我們使用兩種方式去改進Cu/TiO2電阻式記憶體之特性。 首先我們提出氧化銅為電極的氧化鈦記憶體，對銅電極快速加熱氧化，使其產生氧化銅、氧化亞銅和銅成份，氧化銅會限制銅的擴散進而控制導電通道，氧化亞銅會釋放銅並建構通道。我們可以藉由調整氧化銅、氧化亞銅和銅三者的組成比例來減少同在主動層的殘留並降低高組態電流。另外電極表面崎嶇度會影響操作的重複性，導電通道會建立在幾個電場較高的固定點，總而言之，我們由適當的快速熱氧化製程得到了最佳的重複性和高低組態電流比值。此研究中最佳化後的樣品擁有三個等級的高低電流比值、1000個操作次數和10000秒的儲存時間。 其次，我們提出了利用共濺去製程的TiO2:Cu電阻式隨機存取記憶體，TiO2:Cu主動層是在氧分壓為30%時，共同通氧濺鍍鈦與銅靶所製程，鈦與銅的比例可由濺鍍瓦數調變，當氧化銅、氧化亞銅和銅三者有適當的組成比例時，TiO2:Cu電阻式隨機存取記憶體可擁有較低的操作電壓，這是由於TiO2:Cu樣品中銅所需要建構導電通道的距離比快速熱氧化的樣品還短。當鈦的濺鍍瓦數為190瓦而銅為10瓦時，TiO2:Cu樣品有最佳特性，它達到了1個等級的高低電阻比值、 1000個操作次數、10000秒的儲存時間和-5V/2V的寫入/抹除操作電壓。 此篇論文，藉由簡單的加熱氧化和共鍍，得到了良好的電阻式記憶體特性，而由此研究得知氧化銅、氧化亞銅和銅的組成比例對含有銅應用的氧化鈦基電阻式隨機存取記憶體的特性有很重要的影響。

Resistance random access memory (RRAM) has been regarded as a new generation memory. Owing to the simple structure and fabrication process, it has a great potential to replace the traditional flash memory and become the most popular memory in future. RRAMs show the well performance in many respects, however, it still has drawbacks such as no certain mechanism, short endurance, and unstable current in high resistive state (HRS). In this thesis, we proposed two methods to improve the performance of titanium dioxide (TiO2) based RRAM devices with the application of copper.

First, we demonstrated the RRAM device fabricated by the oxidized copper electrode and the titanium dioxide active layer. The copper electrode has been oxidized by a rapid thermal oxidation (RTO) system, and formed the cupric (CuO), cuprous (Cu2O) and the metallic Cu in the oxidized electrode. The formed CuO serve as the Cu ions blocker and control the conduction filaments (CFs) . On the other hand, the formed Cu2O and the metallic Cu could play the role of Cu ions source for constructing the CFs. We could control the amounts of Cu ions to reduce the residual Cu ions in the active layer and the HRS current by adjusting the ratio of three compositions (CuO, Cu2O and Cu) in the electrode. Furthermore, the electrode surface roughness would affect the repeatability of the SET/RESET process. The CFs path could be constructed at several fixed positions because the enhanced local electric field at the tips during the RTO process. Consequently, we optimized the endurance and the ratio of low resistive state current to high resistive state current via an appropriate RTO process. The optimization samples in this work achieve 3 orders of LRS/HRS current ratio, endurance of 1000 cycles, and retention of 10000 seconds.

Secondly, the RRAM devices fabricated by the co-deposited TiO2:Cu active layer and the platinum electrode have also been demonstrated in this research. The TiO2:Cu active layers were deposited by the co-sputtering titanium and copper targets with 30% oxygen partial pressure of the argon-oxygen mixed sputtering gases. The ratio of the compositions Cu to Ti in the active layer could be adjusted by the variable sputtering powers. Thus, the TiO2:Cu samples might exhibit lower SET/RESET voltage than the RTO Cu/TiO2 samples by the appropriate proportion of mixed CuO, Cu2O and Cu in the TiO2 active layer. It is because the Cu drift lengths for constructing the CFs is shorter than the RTO Cu/TiO2 samples. TiO2:Cu sample has the optimization condition when the co-sputtering power is 190W of Ti and 10W of Cu. It achieved 1 orders of LRS/HRS current ratio, endurance of 1000 cycles, and retention of 10000 seconds, with the operation SET/RESET voltages of -5V/2V. In this thesis, the simple-processed RTO Cu/TiO2 and co-deposited TiO2:Cu samples could achieve the good performances of RRAMs. Furthermore, the ratio of CuO, Cu2O and Cu compositions played the important role in the characteristics of the TiO2-based RRAM with copper according to ours results and discussion.