高介電係數鈦酸鍶鋇薄膜BaxSr1-xTiO3於動態隨機存取記憶體電容器之研究

Abstract

隨著積體電路密度的增加與元件尺寸的縮小，動態隨機存取記憶體的電容面積與操作電壓亦隨之減小，但為了維持一定的訊號/雜訊比而不產生錯誤的判讀，其所儲存的電荷必須保持一臨界值，因此，我們必須在不增加儲存單元投影面積的前提下有效地提升電容器的電容值。 根據基本電容公式，有三種提升電容器電容值的方法：增加電極板有效面積、縮減介電層厚度以及以高介電常數材料取代傳統之介電層。但為增加電極板有效面積，隨著電容需求日增，製程亦愈形困難；而介電層厚度縮減至某一程度，則會造成穿遂效應，形成極大的漏電流。因此，使用高介電常數材料必將是未來之趨勢。 鈦酸鍶鋇是近年來備受注意的材料，其提供了下列優點：高介電常數、低漏電流、長使用週期、優良的化性、熱穩定性及低損耗因子等；而射頻磁控濺鍍法則為一普遍且成熟之鍍膜技術。本論文研究即利用一多靶式射頻磁控濺鍍系統，成功地在低基板溫度(T=300~350℃)沈積具高介電常數(er > 350)，及低漏電(~ 10-8A/cm2)的鈦酸鍶鋇薄膜，並針對不同之薄膜沈積條件，研究其薄膜電性與物性，並加以分析了解其物理機制；此外，快速退火方式亦被證實能有效提昇阻障層抗氧化能力及對內擴散的阻擋能力。最後，我們研究不同電漿方式處理對鈦酸鍶鋇薄膜的影響，結果顯示適當的氧電漿處理能夠改善負偏壓漏電流特性，若配合適當沈積氣體壓力所沈積之薄膜，預期能獲得最佳之結果。

With increasing the density of devices on ULSI, the minimum feature size is scaled down into deep-submicron regime. For DRAM capacitors, the cell areas and applied voltage are also rapidly shrinking, which lead to lower storage capacitance while the minimum value of capacitance should be maintained in order to achieve a reasonable signal-to-noise (S/N) ratio. According to the calculating equation for flat-type capacitor, there are three ways to increase the value of capacitance, including the decrease of dielectric thickness, the enlargement of the effective surface area of storage nodes, and the utilization of high dielectric constant materials. However, large tunneling current limits the shrinking of dielectric thickness and more complicated processes are needed with enlarging the effective surface area of storage nodes. Therefore, the utilization of high dielectric constant materials is the trend for Giga-bit scale DRAMs in the future. Barium Strontium Titanate (BaxSr1-xTiO3) is the most attractive material due to its high dielectric constant, low leakage current, TDDB over 10 years, good chemical and thermal stability, and low dissipation factor. And RF magnetron sputtering is a prevalent and mature technique for thin film deposition. In this thesis, a high dielectric constant (er > 350) and low leakage current (~10-8A/cm2) BST thin film was successfully deposited at low substrate temperature (T=300~350℃) by utilizing a RF magnetron co-sputter system. The physical and electrical properties of BST films deposited with different deposition parameters were also investigated in order to realize the associated mechanisms. Moreover, Rapid Thermal Annealing (RTA) was proved to enhance the resistance of barrier layers against inter-diffusion and being oxidized during BST deposition. Finally, BST films were treated with different plasma, and the results showed improved leakage current at negative bias. The optimum leakage characteristic at negative bias is expected in BST thin films deposited at proper gas pressure with O2 plasma post-treatments.