超薄氧化鉿及氧化鋯鉿介電層之可靠度研究

Abstract

在本論文中，以二氧化鉿(HfO2)及二氧化鋯鉿(HfZrO)為閘極介電層的金氧半場效電晶體之可靠度為我們研究的主要內容。介電層中的缺陷主導了以鉿為基本材料之高介電係數介電層在電壓溫度不穩定性測試之下的劣化情形和特性。一開始我們探討了同以二氧化鉿為閘極介電層、厚度卻有所不同的金氧半場效電晶體之可靠度表現，發現在正電壓溫度不穩定性(PBTI)的操作之下，較薄的二氧化鉿介電層的確具有較少的氧化層缺陷，以及較低載子被捕捉在缺陷之內的機率，因而能有效降低臨界電壓的飄動。然而，在負電壓溫度不穩定性(NBTI)的操作下，卻是以介電層所受的電場大小主導氧化層內捕捉的載子數量，以及劣化的情形。和PBTI相反，此時卻是較厚的二氧化鉿介電層具有較好的可靠度表現。另一方面，我們也比較了同樣厚度之下，二氧化鉿及二氧化鋯鉿介電層之可靠度。 由於鋯元素的參雜，能有效降低氧化層主體中的缺陷數量，因此二氧化鋯鉿之於二氧化鉿，在PBTI操作下的確具有較穩定的臨界電壓。然而，二氧化鋯鉿的共價帶補償卻是較二氧化鉿為低，因此在NBTI測試之下，會有較大量的載子穿隧進閘極介電層被捕捉，進而導致臨界電壓飄動幅度較大。 此外，我們也探討了在直流和交流電壓溫度不穩定性測試之下，載子trapping/ detrapping的機制。在常溫時，交流電壓具有的off-time會使被捕捉在氧化層中較淺缺陷裡的載子能穿隧回通道，而被捕捉在較深缺陷中的載子，則因能量不夠而留在原處。除此之外，當脈衝電壓重新上升至on時，必須先將較淺處的缺陷填滿，進而載子才會移動至較深的缺陷，形成臨界電壓漂移。這樣重新填滿的過程，使得氧化層中被捕捉的載子總數量較直流電壓測試下來的少，有效改善了臨界電壓的不穩定性。然而將操作溫度升至 100oC 時，當脈衝電壓降至off-time，被捕捉在較深氧化層缺陷中的載子因具有較大的熱能而能在介電層中稍微的往回移動，在空間中分布於較靠近通道的位置，增加氧化層缺陷捕捉的載子對於臨界電壓的影響，使交流電壓溫度不穩定性更為嚴重。

In this dissertation, the reliability issues for HfO2 and HfZrO MOSFETs with metal gate electrode has been successfully demonstrated. The oxide trap during BTI stress will dominate the characteristics for Hf-based gate dielectrics. For the same material HfO2, the reduction of Vth shift under PBTI stress due to the less trap states and possibility to be trapped for 2nm thickness HfO2, compared to 2.5nm. On the contrary, the electric field dominates the charge trapping and NBTI degradation so that enhancement observed in thicker HfO2. On the other hand, as compared to HfO2 dielectric, HfZrO apparently improves the reliability under PBTI stress because of the reduced bulk oxide traps. However, HfZrO gate dielectric would induce more degraded Vth shift under NBTI stress with respect to the lower valence band offset, which promotes more charges tunneling and being trapped. In addition, the trapping and detrapping mechanism under DC and AC stress was developed. At room temperature, charges in the shallower oxide trap detrap at off-time of the pulse under AC stress, while the charges in the deeper trap would be hold there. Besides, when the pulses rise again, the shallower traps would be refilled first and the charges move into the deeper traps sequentially. As the reason mentioned above, total occupied oxide trap and Vth shift is actually decreased under AC operation. Nevertheless, when elevating the temperature, the charges in deeper traps would move backward and approach the channel at off-time of the pulse. The space redistribution nearer the surface will increase the impact of oxide trap on Vth shift. Consequently, the severer degradation is induced under AC stress at 100oC.