淺溝渠隔離氧化層對三維閘極電晶體的可靠性影響

Abstract

近年由於元件尺寸的微縮，許多問題接踵而至，像是短通道效應、隨機摻雜擾動效應、以及一些元件的漏電問題等。使用先進的元件，這些問題也在這些年慢慢克服。其中三閘極的場效電晶體，在越過奈米之後，已成為未來世代的主流。由於其有三個閘極，所以對通道的控制力較佳，可以有效地改善短通道效應。加上其通道使用極低的摻雜濃度，所以使得隨機摻雜擾動效應也變得微小。儘管如此，這種元件仍然存在著一些潛在的問題，像是：通道側壁表面的不均勻性，金屬功函數的變異，以及電流引發的自體加熱反應等，這些因素都需要被嚴謹的考慮以做為未來元件設計的參考目標。 由於通道寬度越來越窄，使得原本在三閘極場效電晶體兩旁的溝渠隔離氧化層(STI)靠通道越來越近，極有可能對通道造成影響。在本篇論文中，我們提出了一個方法來定性研究淺溝渠隔離氧化層上的陷阱電荷分佈以及其對元件可靠性的影響。這個方法利用元件在長時間操作下，例如：用加溫產生熱載子的操作方式(NBTI)，觀察其臨界電壓標準差的變化，再經由我們提出的三維閘極電晶體空乏區模型的轉換就可以得到這些電荷在通道底下的分佈。 另一方面，我們也運用此方法來量測放射線所引發的STI氧化層陷阱電荷。X光對STI氧化層的影響不會受到元件微縮的影響，因為STI氧化層是無法微縮到像閘極氧化層，所以X光仍然是對其有影響的。先前的研究都集中於核反應的模型探討以及放射線本身條件對元件的影響，我們將首先把NBTI可靠度測試以X光照射做結合。三閘極的場效電晶體經由X光照射之後再去做NBTI的可靠度分析。經由實驗，照射過X光的電晶體確實會產生大量STI陷阱電荷，並且影響元件的可靠性，元件的操作電流會隨著靠近通道的STI陷阱電荷的增加而衰減，臨界電壓也會隨著增加，且我們也利用實驗排除對閘極氧化層電荷的影響。這結果對於未來元件的設計特別是太空科技的應用，提供重要參考。

For the continuing scaled down of the device’s size, many problems will be encountered such as short channel effect, random dopant fluctuation, and leakage currents. Several of these problems have been handled well in advanced CMOS technology. Among these, the trigate structure has been considered to be the candidate for the future generations. Trigate MOSFET has better gate controllability to solve the short channel effect. It also has immunity from the random dopant fluctuation because of the lightly doped channel. However, trigate devices also suffer from another problems such as side wall surface roughness, metal work function variation, and self-heating effect etc. The trigate device needs to be thoroughly studied for the future design and applications.

Since the fin width is getting thinner, the STI (shallow trench isolation) are closer to the channel region, and it might have some impacts on that. In this thesis, we propose a new method to characterize the STI traps under the fin channel. Under the long term NBTI stress, the standard deviation of threshold voltage will change and by using the trigate depletion model we proposed, we can easily find the distribution of these traps under the channel region.

The second part, we apply the technique to examine the radiation induced random STI traps. It is known that the X-ray will have an influence on STI traps. As the size of MOSFET shrinks down, the gate oxide is becoming so thin. However the STI oxide cannot become as thin as the gate oxide, so there are still some problems when exposing under the radiations. In the previous studies that were mainly focusing on the nuclear models or the effects of different kinds of radiations. We first combined the reliability test with the X-ray. After being exposed to the radiations, the trigate devices were then taken to the NBTI testing. The pre X-ray irradiated procedure affects the reliability a lot due to the massively increased STI traps. The on current of the device will be degraded and threshold voltage will increase due to the increased STI traps near the channel region. This result is helpful to the design of the future devices especially in the space technology.