Development of Ultra-smooth Si Surface by the Control of SiO_2/Si Interface Formation on an Atomic-Scale

为了形成原子扁平的氧化硅薄膜，使用原子扁平的硅衬底和逐层氧化工艺是至关重要的。首先，在1000ﾟC的干氧中形成200 nm厚的热氧化膜；其次，用缓冲氢氟酸去除氧化膜。第三，在硅衬底上用1100ﾟC的SiHCl_3气相反应外延生长硅膜，然后在氢气中冷却，直到衬底温度降到400ﾟC以下。第四，在稀氢氟酸中去除氮气中生长的自然氧化物，得到氢化的Si(III)-1*1和Si(100)-2*1。在40%的NH_4F溶液中，通过对上述第二步得到的硅衬底进行处理，还制备了氢封端的Si(III)-1*1表面。为了形成均匀的氧化膜，在300ﾟC的1Torr干氧中生长了厚度为0.6 nm的预氧化物，而不破坏Si-H键。在1Torr干氧气氛中，在600ﾟC以上的温度下生长了热氧化物，在Si(Ⅲ)和Si(100)表面形成的热氧化膜上的凸起高度分别小于两个原子台阶的高度0.314 nm和单原子台阶的高度0.135 nm。特别是在Si(100)表面上形成了非常平坦的氧化膜。从表面微观粗糙度与SiO_2/Si界面结构的相关性来看，SiO_2/Si(100)界面非常平坦，有望成为未来最先进的MOSFET的最终界面。这些结果是通过使用非接触式原子力显微镜对氧化膜表面形貌的原子尺度观察和利用X射线光电子能谱测量界面结构而获得的。

In order to form atomically flat silicon oxide films, it is essentially important to use atomically flat silicon substrates and the layr-by-layr oxidation process. First, 200-nm-thick thermal oxide films were formed in dry oxygen at 1000ﾟC.Second, this oxide films were removed by buffered hydrofluoric acid. Third, on silicon substrates thus obtained silicon films were epitaxially grown by the gaseous reaction of SiHCL_3 at 1100ﾟC followed by the cooling in hydrogen gas until the substrate temperature decreases to be smaller than 400ﾟC.the silicon substrates thus obtained were maintained in nitrogen gas. Fourth, hydrogen-teminated Si(III)-1*1 and Si(100)-2*1 were obtained by removing native oxides, which were grown in nitrogen gas during relativelely long storage time, in dilute hydrofluoric acid. Hydrogen-terminated Si(III)-1*1 surface was also prepared by treating silicon substrates, which were obtained after the second procedure described above, in 40% NH_4F solution. In order to form uniform oxide film, the 0.6-nm-thick preoxides were grown in 1 Torr dry oxygen at 300ﾟC without breaking Si-H bonds. Through this preoxide thermal oxides were grown in 1 Torr dry oxygen at more than 600ﾟC.The height of protrusions on thermal oxide films thus formed on Si(III) and Si(100) surfaces are smaller than the height of two atomic steps, that is, 0.314 nm and that of single atomic step, that is 0,135 nm, respectively. Especially, the extremely flat oxide films were formed on Si(100). From the correlation between surface microroughness and SiO_2/Si interface structures the SiO_2/Si(100) interface is expected to be extremely flat and can be the ultimate interface which will be used in the most advanced MOSFET in the future. These results were obtained from the atomic-scale observation of surface morphologies of oxide films using noncontact-mode atomic force microscope and the measurement of interface structures using X-ray photoelectron spectroscopy.