Effects of Forming Gas Annealing on the Recovery of Radiation Damaged Power MOSFETs
|摘要:||本論文主要探討金氧半(MOS)功率電晶體的輻射損傷，以及在原始氣體(forming gas)中的退火處理對輻射損傷的修復效應。電晶體被鈷60照射後，臨界電壓向負電壓方向飄移，而且次臨限電流對閘極電壓的變動率減少；產生這些現象的主因，乃是因為鈷60照射導致氧化層內產生正電荷，並且在矽與二氧化矽的界面形成界面陷阱之故。特別是遭受大劑量照射的電晶體，輻射造成的氧化層陷阱密度遠大於輻射產生的界面陷阱密度。因此，輻射產生的氧化層正電荷是造成電晶體臨界電壓偏移的主因。鈷60照射也造成閘極氧化層漏電流 (gate leakage current) 的增加。但鈷60照射對於汲極與源極間的耐壓(BVds)，至200 kGy輻射劑量為止，並未發現明顯影響。
在含有5%氫氣的原始氣體中作400℃退火處理，可以有效修復輻射所造成的元件損傷。由於原始氣體中的氫氣作用可以修復輻射所造成的氧化層內部的斷鍵。電晶體在原始氣體中退火處理後，受損傷的閘極氧化層得以復原 (Ig-Vg特性得到復原) ，而且輻射產生的氧化層正電荷大量的減少。不過，原始氣體中的退火處理都無法去除輻射所產生的界面陷阱。由於輻射產生的氧化層正電荷是造成電晶體臨界電壓飄移的主因。所以400℃的原始氣體退火處理，對於金氧半功率電晶體輻射損傷的修復還是非常有效。本論文也用純氮氣作退火處理，發現也有修復元件輻射損傷的功能，但效果較差。|
This thesis study investigates the radiation effect of power MOSFETs as well as the forming gas annealing effect on the recovery of the radiation damaged devices. It is found that Co-60 irradiation resulted in threshold voltage (Vth) shift toward the negative voltage direction and the increase of sub-threshold swing, which are due to, respectively, the generation of radiation induced positive oxide charge and interface trapping states. However, the radiation induced oxide trap density is much higher than the radiation induced interface trap density, particularly for the devices irradiated to a higher dose. Thus the trapped oxide charges play a dominant role in determining the radiation induced threshold voltage shift. The Co-60 irradiation also resulted in increase of gate oxide leakage current. However, the drain to source breakdown (BVds) did not exhibit any obvious degradation by Co-60 irradiation up to a radiation dose of 200 kGy. The radiation damaged power DMOSFETs can be efficiently recovered by forming gas annealing at 400℃. Presumably, the radiation induced broken bonds in the damaged gate oxide were healed by hydrogen reaction during the forming gas annealing, resulting in the recovery of gate oxide quality (recovery of Ig-Vg characteristic) as well as substantial decrease in radiation induced oxide charges. However, it is found that the forming gas annealing was not able to anneal out the radiation induced interface traps. Since the threshold voltage shift arisen from Co-60 irradiation is mainly due to radiation induced positive oxide charge, the threshold voltage of the power MOSFETs can be efficiently recovered by the forming gas annealing at 400℃. Thermal annealing in pure N2 ambient also exhibited restorative ability for the radiation damaged devices, but to a much less extent.