磷原子擴散對奈米片電晶體之影響

Abstract

隨著摩爾定律演進，奈米片電晶體被視為實現2奈米節點的關鍵結構。然而，n型奈米片電晶體面臨了前所未見的問題：通道有效遷移率不如模擬所預測的高。推測造成此差異的原因可能為磷原子自源極/汲極擴散至通道，奈米片電晶體製程與前幾代電晶體相比多了矽鍺犧牲層，矽鍺犧牲層的存在可能進一步促進了磷原子擴散。由此本論文將著重探討磷原子在奈米片電晶體結構中可能的擴散路徑，其中分為兩大類擴散路徑，塊材擴散與介面擴散。 對於塊材擴散，本論文分別考慮矽塊材、矽鍺塊材與氮化矽塊材三種擴散路徑。成長無摻雜/磷摻雜/無摻雜之薄膜結構，進行熱擴散製程後以二次離子質譜進行濃度分析。將所得濃度分布以所選函數進行擬合，再藉由擬合參數計算擴散率。考慮到奈米片結構對擴散的影響，覆蓋層的有無與晶向差異（(100)、(110)）皆被納入塊材擴散的實驗條件之中。結果顯示，有覆蓋層之磷原子擴散率較無覆蓋層低，兩者擴散活化能無明顯差異；而不同晶向上的擴散率未呈現固定大小關係，但(110)晶向上的磷原子擴散活化能低於(100)晶向。 對於介面擴散，本論文考慮矽/矽鍺介面與矽/氮化矽介面兩種擴散路徑。實驗設計以「間距」為變數的電阻量測。在p型矽基板上以磷原子離子佈植定義n型源極/汲極，其中源極與汲極之間的距離定義為間距，範圍為50 nm至5 μm，並且間距上具有已沉積之矽鍺或氮化矽薄膜，後進行不同溫度熱擴散製程，同時形成鎳矽合金以降低接觸電阻。實驗結果以電阻對間距的關係圖來表示。矽/矽鍺介面在間距小於2 μm時電阻將隨間距增加而以指數形式增加，之後則趨於飽和，此趨勢與穿通效應模擬一致。相較之下，矽/氮化矽介面則與模擬結果不同，電阻值的增加不呈指數形式的變化，推測原因為介面中存在大量缺陷而助長了磷原子的擴散。

As Moore’s Law continues to advance, nanosheet transistors have been used as a unit structure for the 2-nm technology node and beyond. However, n-type nanosheet transistors might face an unprecedented challenge due to the diffusion of n-type dopants, which could degrade the channel’s effective mobility. Compared with previous generations of transistors, the nanosheet process introduces SiGe sacrificial layers, which may further enhance phosphorus diffusion. This thesis investigates the possible diffusion paths of phosphorus atoms in a nanosheet transistor structure. There are two types of diffusion of phosphorus in a nanosheet: bulk and interface diffusion. For bulk diffusion, this work considers three pathways: diffusion in Si, SiGe, or SiNx. Undoped, phosphorus-doped, and undoped thin-film structures were prepared, followed by thermal diffusion processes and analysis of the phosphorus concentrations using secondary ion mass spectrometry (SIMS). The measured concentration profiles are fitted with selected functions to extract the diffusion coefficients. To account for the structural impact of nanosheet architectures, the effects of the presence of capping layers and crystal orientation differences ((100) and (110)) on the phosphorus diffusion are considered. The results suggest that phosphorus diffusivity is lower with a capping layer, while the activation energies show no significant difference. Moreover, although no consistent trend is observed in diffusivity across different crystal orientations, the activation energy of phosphorus diffusion is found to be lower for the (110) orientation than along the (100) orientation. For the interface diffusion, two pathways are considered in this study: the Si/SiGe interface and the Si/SiNx interface. A experiment is designed to characterize the resistance between two heavily n-doped regions by varying its spacing on a p-type Si substrate. Between the heavily-doped regions, SiGe or SiNₓ thin films are covered to investigate the phosphorus diffusion at various temperatures. The results are analyzed by plotting resistance as a function of spacing. For the Si/SiGe interface, the resistance increases exponentially with the spacing below 2 μm and then approaches saturation, which is attributed to the punch-through effects across two n+/p junctions. In contrast, the Si/SiNₓ interface does not follow an exponential increase with spacing, which might imply the presence of high-density defects at the interface, enhancing the phosphorus diffusion.