超薄閘氧化層電晶體中量子效益及軟崩潰之嶄新實驗觀察

Abstract

在超大型積體電路中，閘氧化層的可靠度議題一直是十分重要的考量，雖然經過了很久的研究，但在這方面仍有許多尚未得到明確解釋的現象。近來，由於元件已達到0.18微米技術，使得氧化介電層厚度也隨之縮小到5毫米以下， 在高電場的作用下，這超薄氧化層會呈現許多未知的退化情況，因此，尋求氧化層退化機制相關的物理解釋便變得十分迫切而重要。 在本實驗中，使用2.5毫米厚度的閘氧化層，此時軟崩潰現象已可以觀察到。我們使用定電流破壞的方式，在雙向電流的衝擊下觀察氧化層的衰退與電流流向的相關性；再可由所量到的時間相關閘電壓變化圖中，經由觀察電壓擾動的變化，輕易得到軟崩潰的時間，此時，我們便可以很清楚的了解到在電子由閘極注入下所量得的軟崩潰電荷遠少於由基底注入的情況。接著，我們首次提出一個關於量子效益的實驗觀察，研究在nMOS結構中發生在邊緣區域的碰撞游離。經由實驗結果，可以很清楚的量得在源極、汲極、和閘極間生成的電子電洞對，並反應出兩點結果：(i)缺陷引致漏電流形式下的注入電子有非彈性碰撞的特性；以及(ii)在軟崩潰情況下的注入電子比缺陷引致漏電流形式下的注入電子有更多的能量損失或能障降低。

The gate oxide reliability issue is one of the most critical concerns regarding ULSI (ultra-large scaled integration) technology development. Despite long-term studies, there are many unclarified phenomena. Recently, ULSI devices in the state-of-the-art technology require the SiO2 gate dielectrics layer less than 5 nm thick. Under high field stressing, such ultrathin oxide presents a lot of anomalous degraded performances. Thus, the desire for a physical understanding of the oxide degradation is quite urgent and critical. In our experiments, the soft breakdown (SBD) can be experimentally observed on the oxide of 2.5 nm thickness. Constant current stress (CCS) conditions are adopted in studying the effect of both stress polarities to cause the oxide degradation. From the time-domain gate voltage plots, time-to-soft-breakdown (tSBD) is easily detected by the fluctuation of gate voltage. It is obviously found that charge-to-soft-breakdown (QSBD) of gate injection is lower than that of substrate injection. Secondly, we report a new experimental observation of quantum yield (QY) for the study of impact ionization occurring in the edge region of nMOSFET structure for the first time. It is experimentally proved that the generated electron-hole pairs are detected at the edge region between the source/drain extension and the gate. Measurements further reveal that (i) inelastic scattering behaviors feature the injected electrons in stress-induced leakage current (SILC) mode; and (ii) the injected electrons in soft breakdown experience more energy loss or barrier lowering than SILC.