超薄五氧化二鉭閘極介電層之特性研究

Abstract

當場效電晶體的閘極介電層厚度微縮至1.5奈米厚時，將產生一些諸如電子穿遂效應等嚴重的問題。因此亟需以較厚等效厚度之高介電常數材料取代二氧化矽作為閘極介電層，其中五氧化二鉭就是極有希望的材料之一。本實驗利用直流濺鍍法沈積鉭金屬於矽基材上，接著以低溫通氧氣的爐管，長時間(50分鐘)氧化金屬鉭，得到氧化鉭薄膜。金屬薄膜的厚度有30Å以及40Å兩種，分別做750℃到950℃的快速熱退火處理。薄膜中的元素分佈由二次離子質譜儀(SIMS)測得，其結晶結構由X光繞射（XRD）圖譜判斷，而薄膜表面的粗糙度則由原子力顯微鏡（AFM）探得。不同處理條件下的薄膜電性，經由I-V、C-V量測、SILC、磁滯效應、以及頻率散射的量測得知。同時也討論量子效應的漏電流機制。沈積30Å鉭金屬的樣本，在氧化後經過750度30秒的快速熱退火處理後，有最低的漏電流以及最佳的電性。而沈積40Å的鉭金屬樣本在氧化後，經過850度30秒的熱處理有最佳的電性。這些薄膜幾乎沒有磁滯效應，但是頻率散射頗為明顯。從C-V量測萃取出等效二氧化矽厚度（EOT）大約16 Å到19Å，介電係數為18到20。這些薄膜在1伏特時的漏電流密度約為10-2（A/cm2），比相同EOT的二氧化矽薄膜約小二個數量級。

When the MOSFET gate insulator is scaled below 1.5nm, some serious problems such as direct electric tunneling will occur. Therefore, high dielectric constant material is very desirable to replace SiO2. Tantalum oxide is a promising material for future MOSFET gate oxide applications. In this study, we use DC sputter system to deposit tantalum metal on silicon surface and proceeded with furnace low temperature and long time (50 min) oxidation to prepare Ta2O5 thin film. Two kinds of thickness were prepared and several RTA conditions from 750℃ to 950℃ were performed. The element distribution, crystallinity, and morphology of the films were investigated by second ion mass spectrometer (SIMS), X-ray diffraction (XRD), and atomic force microscope (AFM), respectively. The electrical properties of the films subjected to different treatment were measured, such as C-V curve, I-V curve, stress induced leakage current (SILC), hysteresis effect and frequency dispersion. Their equivalent oxide thicknesses (EOT), flatband voltage (VFB) and dielectric constant were also extracted. Moreover, the conduction mechanism with quantum effect was discussed. The as-deposit 30Å and 40Å samples have least leakage current and good electric characteristics under 750℃ 30sec and 850℃ 30sec RTA treatment, respectively. These samples have negligible C-V hysteresis effect but suffer severe frequency dispersion. From the C-V curve, we get equivalent oxide thicknesses ranging from 16 to 19Å, dielectric constants ranging from 18 to 20, and leakage current about 10-2 (A/cm2) at 1V. The magnitude of leakage current was about 2 orders less than pure oxide with the same EOT.