鉿-鉬二元合金之線性功函數調變在奈米元件之應用

Abstract

當金氧半場效電晶體的閘極長度微縮到100 nm以下，閘極氧化層厚度同時變得極薄時，多晶矽空乏效應及高閘極電阻等問題將日益嚴重，而使用金屬做為閘極則可避免這些問題。本次實驗是利用同時濺鍍的方式沉積鉿-鉬二元合金當金屬閘極，藉由改變對鉿及鉬的濺鍍功率，改變鉿-鉬二元合金的組成比例變化，以達到功函數的調變。 在實驗中我們分別選用鉿及鉬來取代傳統的多晶矽閘極，但是鉿的熱穩定性很差，並不算是好的閘極材料;相對的，鉬的熱穩定則十分優異。藉由鉬較佳的熱穩定性，我們期望沉積出具有較佳熱穩定性之鉿-鉬合金以取代鉿電極。75% 鉿-鉬二元合金被驗證具有較鉿為佳的熱穩定性，且其功函數(~ 4.15 eV)更接近N型多晶矽閘極，故可被用來取代鉿做為閘極之用。此外，在實驗中發現鉿-鉬二元合金的功函數有線性調變的特性，比起非線性調變而言，對於調變效率和製程變異能夠取得折衷。在奈米元件應用方面，因為所需的功函數值在N型場效電晶體介於4.4~ 4.6電子伏特，而在P型場效電晶體則是4.8 ~ 5.0電子伏特。因為所需的功函數值並不是固定的，因此功函數的調變更顯得重要。利用鉿-鉬二元合金的功函數線性調變之特性，免去通道掺雜的步驟，使得奈米元件能有較低且對稱的臨限電壓以符合未來低功率操作的目標。

With the downscaling of CMOS gate length to 100 nm regime as well as the drastic thinning of gate oxide thickness, poly depletion effect and high gate resistance encountered in poly-silicon gates will be more pronounced. Metal gates can eliminate poly depletion effect and have lower gate resistance. In our experiment, HfxMo(1-x) films were deposited by co-sputtering to be observed as gate electrodes. The sputtering power of each target was varied to modulated the composition of alloy and then the metal work function. In this work, we select Hf and Mo to replace n+ and p+ poly-silicon, respectively, but the thermal stability of Hf is too poor to be served as gate electrode. Because Mo has excellent thermal stability, the use of HfxMo(1-x) alloy is expected to provide better thermal stability as well as to replace Hf gate electrode. Hf0.75Mo0.25 alloy provide suitable work function value (~ 4.15 eV) and better thermal stability (up to 400 ℃) so that the combination of Hf0.75Mo0.25 / Mo may be a new candidate for dual work function metal gate bulk CMOS technology in gate-last process. Furthermore, the linear work function modulation using binary alloy HfxMo(1-x) was achieved. Compared with non-linear Φm modulation, linear modulation behavior is a compromise between modulation efficiency and immunity to process variation. For nanometer devices, the required work function to be applied to low voltage application are 4.4 ~ 4.6 eV and 4.8 ~ 5.0 eV for N- and P-MOSFET, respectively. Linear work function modulation technique can be used to obtain appropriate work function values for nanometer devices threshold voltage control without implanting additional channel doping.