利用原子層沉積系統成長氧化鋁閘極介電層於砷化鎵基板之研究

Abstract

砷化鎵擁有高電子遷移率、較大的能帶隙、功率耗損低及較大的崩潰電場等優點，因此用它取代矽基板做電晶體等應用具有可行性。 就砷化鎵晶圓的濕式化學清洗而言，我們希望能夠達到抑制原生氧化層的效果。從XPS分析圖發現，當鹽酸濃度的比例下降，對於抑制原生氧化層的能力愈佳。然而，低於鹽酸濃度10%，對於砷的氧化物的去除能力，是呈現負成長。除此之外，如果在鹽酸中加入雙氧水，基板更容易被氧化，產生更厚的原生氧化層，因此降低了清洗的能力。同時，我們也發現硫化銨溶液對於原生氧化層的抑制也具有效用。 我們利用原子層沉積系統成長氧化鋁作為閘極介電層。就漏電因素而言，我們選擇基板加熱至300oC，然後再沉積氧化鋁，除此之外，亦使用交界鈍化層(IPLs)試圖使沉積在基板上的薄膜能夠有較佳的效果，其中，疊上一層薄薄的矽及硫化銨鈍化是兩種最常見的交界鈍化層。並且發現，這兩種方法確實對於抑制原生氧化層有很大的功效，而且有助於沉積高品質的氧化鋁薄膜，尤其是將清洗完的砷化鎵基板浸泡在硫化銨溶液中。我們亦研究出，使用硫化銨溶液約2%並浸泡至其中30秒為硫鈍化的最佳效果。 在長完氧化鋁之後，將晶圓做沉積後快速熱退火(PDA)並試圖讓薄膜較佳化。在做完600oC快速熱退火之後，C-V曲線的確變得更陡峭，而遲滯現象也降低了，但是卻造成漏電流變大，尤其是在氮氣環境下做快速熱退火。這是因為在氮氣環境下比起在氧氣環境下產生了較多的砷與砷的鍵結(As layer)於介電層裡，而這種鍵結是在氧化鋁薄膜中形成漏電流的最主要的因素。 最後討論電容結構的可靠度問題。發現，隨著應力的增加，做硫化銨鈍化的樣品，更能承受應力使得漏電流不至於增加太快。當我們考慮漏電流、遲滯現象以及接面載子補捉密度(Dit)，則認為有做硫鈍化以及在氧氣環境下做快速熱退火，是最佳的選擇。

GaAs offers the advantages of high electron mobility, rich band gap engineering, low power consumption and high breakdown fields and thus is expected to outperform Si in the specific metal-oxide-semiconductor (MOS) applications. With wet-chemical cleaning of GaAs wafers, it is necessary to effectively suppress the formation of native oxide before dielectric deposition. From XPS spectra, we found that HCl solution could better eliminate native oxide with decreasing concentration, but with a limit of around 10%. Less than 10%, the situation became worse. Moreover, as we used the mixture solution of HCl and H2O2, the substrates would be oxidized and cleaning effect decreased. We also found that (NH4)2S passivation was effective in suppression of native oxide formation. In this thesis, we grew Al2O3 by atomic layer deposition system (ALD). For low leakage current, the growth temperature was set at 300oC. Prior to dielectric deposition, we employed additional interfacial passivation layers (IPLs) in order to deposit excellent Al2O3 film. Si capping and (NH4)2S passivation were the two most commonly used IPLs techniques. We found that these two methods were effective in eliminating native oxide and in turn very helpful in achieving high-quality Al2O3 film deposition. In particular, we found that dipping in 2% (NH4)2S solution for 30s was the optimized treatment condition. After depositing Al2O3, we conducted post deposition annealing (PDA) to further improve the deposited film. We observed that the C-V curves of the samples became sharper and their hysteresis decreased significantly with PDA at 600oC concomitant with larger leakage current, especially in N2 atmosphere. We believe it is due to the fact that N2 annealing would cause more As layer incorporated into the dielectric than O2, which is likely to form the leakage current path. Finally, the reliability of MOS capacitors has been characterized and discussed. It was observed that (NH4)2S treatment was effective in preventing rapid Jg increase during stressing. Considering not only Jg, hysteresis but also Dit, O2 PDA with sulfide treatment was the best treatment condition.