原子層沉積技術成長二氧化鈦薄膜之應力分析及應用

Abstract

本篇論文是探討原子層沉積法成長之二氧化鈦薄膜的特性並應用於電阻式記憶體元件當中。為了將成長之薄膜應用在電阻式記憶體元件中，實驗中使用了包含穿透式電子顯微鏡、X光光電子能譜等技術來進行薄膜性質的分析。結果中顯示藉由原子層沉積法成長參數的調控可以達到薄膜組成成分、晶體結構以及缺陷數目的改變。此外，研究發現薄膜在結晶態時會發生破裂的情形，因此研究中利用穿透式電子顯微鏡技術，進行殘留應力之計算。根據實驗結果可推斷此應力的成因為降溫時所產生之熱應力，且當薄膜成長厚度達到臨界厚度後，會於降溫過程中藉由結晶的方式以消除內部所累積之熱應變能。根據薄膜形貌與應力分析結果，在元件的應用上，我們採用非晶相薄膜以減少薄膜中的缺陷，並減少薄膜厚度，以避免應變能的累積而造成薄膜破裂發生，維持元件的性能。最後，根據以上的實驗結果我們成功地將原子層沉積法成長之二氧化鈦薄膜應用於電阻式記憶體當中，並將Pt/TiO2/Pt和FTO/TiO2/ITO兩種結構之元件進行電性量測。結果顯示由於電阻轉換反應的差異使得利用惰性金屬薄膜作為電極的元件穩定度較差；而使用導電金屬氧化物(FTO、ITO)薄膜作為電極之元件穩定度及操作特性有明顯地改善。

In this study, we investigate the properties of TiO2 thin films grown by atomic layer deposition method (ALD) and apply them to the resistive random access memory devices (ReRAM). For the application in the ReRAM devices, various characterization methods, such as transmission electron microscopy (TEM), X-ray photoelectron microscopy (XPS), are used to analyze the properties of TiO¬2 thin films. Based on the experimental results, it shows that TiO2 films with different properties, such as the crystal structure, the chemical composition and the concentration of oxygen vacancies, can be grown by tuning the growth parameters. Besides, because some fractures in the polycrystalline TiO2 films are observed in the cross-sectional TEM images, it proves the existence of the residual stress. In this study, TEM techniques are used to measure the residual stress in thin films. According to the experimental results, we infer the residual stress is thermal stress. During the growth, amorphous TiO2 films are deposited on the substrates, and crystallize to release the thermal strain energy at the critical thickness in the cooling process. In the ReRAM devices, amorphous TiO2 thin films are used to reduce the concentration of the defects; at the same time, the thickness of thin films is reduced to avoid the high accumulated strain energy and the fracture problems. Based on these experimental results, we successfully apply ALD-grown TiO2 films to the ReRAM devices and investigate the I-V characteristics of the two different device structures, Pt/TiO2/Pt and FTO/TiO2/ITO. It shows that the stability of the devices with the Pt/TiO2/Pt structure is not good enough because its resistive switching mechanism. On the other hand, the stability and the operating characteristics are improved significantly using the conducting metal oxides, such as the FTO and ITO thin films, as electrodes. Key words: Titanium oxide, Atomic layer deposition, Transmission electron microscopy, Strain analysis, Resistive random access memory.