Fabrication and Characterization of SOI FinFETs with Schottky Barrier Source/Drain
Fu Ju Hou
Tiao Yuan Huang
Horng Chih Lin
這新元件是以金屬矽化物代替高摻雜的半導體作為源/汲極之用，使得其在製程上較為簡單且製程溫度較低。然而傳統的蕭特基源/汲極元件由於金屬與半導體接面在汲極為高電場下易產生場發射(field emission)的漏電流機制，造成其具有較大之漏電流而大大地降低其開/闗之電流比，也因此扼殺了蕭特基電晶體的實用性。然而在這新元件中加上的汲極延伸(field-induced extensions)結構能完全地抑制此一漏電流機制，且元件導通電流亦隨著延伸區電壓加大而增大，完全地改善了傳統蕭特基源/汲極元件的缺點。
在元件的操作及特性上，以二矽化鈷作為源/汲極的元件在同一個元件上能藉著改變兩個閘極(main-gate and sub-gate)的電壓極性而具有兩種模式(n通道及 p通道)的操作能力，且兩種操作模式都展現了良好的特性。從量測中，我們在同一元件中得到兩種模式操作下的開/闗電流比都接近於109，且因Fin結構的作用， 亦獲得了接近物理極限的次起始斜率(subthreshold slope)值，即趨近於60mV/decade。白金矽化物(PtSi)對p通道而言有著較低之能障高(barrier height) ， 因此擁有較大之導通電流，但對n通道而言卻大大地降低了導通特性。另外就傳導係數(transconductance)而言，在p通道操作模式下，白金矽化物源/汲極元件亦比二矽化鈷源/汲極元件來得高。|
In this thesis, we proposed and demonstrated a novel nano-scale silicon-on-insulator (SOI) FinFET device. The new device features a metallic silicided source/drain and field-induced S/D extensions. For the device fabrication, the patterning of nano-scale Si lines using electron-beam lithography with NEB-22 or hydrogen silsesquioxane (HSQ) resist was examined firstly. Since the HSQ resist has the advantages of high contrast and less line width fluctuation up to 1nm, the sub-50nm silicon lines can be more easily achieved. Nevertheless, the required high dosage up to several hundreds µC/cm2 and the severe proximity problem make the HSQ unlikely to be used in practical applications. Therefore, NEB-22 e-beam resist, with its potentially higher commercial applicability in the future, was chosen in this work to generate sub-50nm silicon fin patterns. Concomitantly, high etch selectivity between silicon and the underlying silicon dioxide is essential to the nano-scale device fabrication, owing to the use of ultra-thin gate oxide. To overcome this issue, an advanced TCP-9400 poly-Si etcher was employed. An excellent recipe having high etching ratio (up to 200) as well as anisotropic etched profile was successfully developed in this work. Schottky barrier (SB) MOSFETs generally enjoy simpler and low-temperature processing compared to conventional MOS transistors by employing metallic silicide, in lieu of heavily-doped region, as the source/drain. However, conventional Schottky barrier (SB) MOSFETs were known to suffer from intolerantly high leakage current caused by the field emission of carriers from the drain junction. The high leakage severely degrades the on-/off-state current ratio and essentially rules out their applications to mainstream integrated circuits. In our new device, this problem was effectively solved by the formation of an electrical drain junction which was induced by the sub-gate bias, VG,sub. Our results show, for the first time, that the new device with Co-silicide source/drain exhibits superior ambipolar characteristics by simply switching the bias polarity on the main-gate and the sub-gate bias. Excellent subthreshold characteristics with high on-/off-state current ratio (close to or higher than 109) and near-ideal subthreshold slope (~ 60 mV/decade) are realized, for the first time, on a single device. Moreover, we show that the new device with Pt-silicided source/drain can further improve the p-channel drivability and transconductance, albeit compromising the capability of bi-channel operation, due to its low barrier height for holes (Φbop = 0.24 V) and a high barrier height for electrons (Φbon = 0.86 V).