鈦酸鍶高介電常數介電質於MIM電容之應用

Abstract

有鑒於隨機動態存取記憶體對電容密度的需求越來越高，較低介電係數的高介電係數材料已不敷未來使用，所以本論文以具有極高的介電係數(大於100)的材料鈦酸鍶(SrTiO3, STO)作為介電層的金屬/介電層/金屬(MIM)電容為研究的對象。 本論文比較低溫、功率和高溫、高功率沈積條件的STO薄膜特性差異，發現在氬氣與氧氣流量比為18:12，沉積溫度400oC，功率150W時，剛沉積未退火的STO薄膜，介電係數約240，經過500□C/30分鐘的氧氣爐管退火，介電係數可以升高到360。分析產生高介電係數的原因，STO產生高介電係數的原因仍然不明。 考慮電容密度和漏電流密度，發現有效的沈積後的退火溫度約為500oC。超過600□C會因為結晶過飽和，導致晶粒變小，電容值會下降。800□C退火再結晶的邊界充滿缺陷，會使漏電大幅增加，且對於電容密度的幫助有限。結晶程度越高，漏電流會沿著晶粒邊界缺陷通過跟著增加。比較氮氣和氧氣退火，發現經過氧氣退火的試片可以有明顯的漏電流改善。經過氮氣退火的試片對於漏電流沒有改善反而增加，而且會有能障降低，形成在低電場時有強烈的Shcottky emission機制出現。 STO濺鍍時的氬氣/氮氣流量比也會影響STO的特性。氧氣流量的增加可以降低漏電流，但是過多的氧會使得介電常數降低。在漏電流和介電係數取其平衡，較適當氬氣/氧氣比例的比為18:12。 本論文發現介電係數、漏電流密度、電容電壓係數、介電損耗之間有特定的關係，特別是電容電壓係數增加，則介電損耗降低，其間的機制還不是非常清楚，值得深入探討。

With the device down-scaling, the dynamic random access memory (DRAM) requires higher and higher capacitance density. Those high-dielectric constant (high-κ) materials with middleκvalue cannot meet the requirement for the future DRAM application. In this thesis, metal-insulator-metal (MIM) capacitors with high-κdielectric SrTiO3 which has dielectric constant higher than 100 are investigated. Characteristics of STO films deposited at 350□C/80W and 400□C/150W are compared. The STO film deposited at substrate temperature of 400oC, Ar/O2 ratio of 18:12, and RF power of 150W showsκvalue higher than 240. After post-deposition annealing at 500□C for 30 min in O2 ambient, theκvalue as high as 360 is demonstrated. With the help of various material analysis, the reason for such a high κ value is not clear. Post-deposition annealing (PDA) will change the properties of the STO films. The highest capacitance could be obtained after PDA at 500oC due to crystallization of the STO film. Above 600oC, grain size becomes small because of the super saturation of grain growth. The STO film is re-crystallized at 800□C, however, it will produce high density of defects at grain boundaries when the grain grows up. The leakage current will increase with the grain re-growth. Annealing in O2 ambient for sufficient thermal budget can reduce leakage current. The films annealed in atmosphere nitrogen have lower barrier height between STO and Pt electrode, which results in strong Shcottky emission current at low electric field. The Ar/O2 gas flow ratio also affects the STO properties. The leakage current of STO will be reduced with the increase of oxygen flow rate. But the excess oxygen will reduce the dielectric constant. We find a balance between leakage current and dielectric constant that the Ar/O2 =18:12 is the proper deposition condition for STO. The dielectric constant, leakage current, capacitance-voltage coefficient, and dielectric loss are correlated. Especially, as the capacitance-voltage coefficient increases, the dielectric loss decreases. The mechanism is not clear and is worthy to investigate.