湿法化学制备的超薄和极薄二氧化硅／硅膜的光学性能研究

Study on optical properties of ultra-thin and very-thin silicon oxide/silicon structures prepared by chemical wet methods

二氧化硅薄膜至今依然属人们广泛研究的材料,这是因为当这种材料制备为高质量的超薄、极薄的氧化物时,可实际应用于不同方面,如超大规模集成电路(VLSI)的栅氧化层以及液晶显示屏(LCD)的生产。本文考察了厚度为3nm和5nm的极薄二氧化硅层的结构性质,这些薄层是通过适度掺杂n-型硅(100)晶片而形成。在形成氧化层之前用标准RCA方法清洁,并随后在氮气氛围中退火,部分样品在HCN溶液中钝化。本研究中用傅立叶变换红外光谱(FT-IR)技术获取了复合结构中不同类型的化学键信息。对SiO2钝化试样和非钝化试样中的Si-O-Si的不对称伸缩振动分别用纵光(LO)和横光(TO)模式进行了鉴别,发现TO模式位置(约1 107cm-1)和振幅与试样的厚度无关。另一方面,LO模式的位置从约1 230cm-1(厚度约为1.5nm)改变为1 244cm-1左右(厚度约为4.5nm)。根据红外光谱峰的偏移,认为超薄和极薄SiOx复合结构并不均匀。对红外光谱获得的结果进行了反褶积处理并获取相关信息。用次级离子质谱分析法(SIMS,Secondary Ion Mass Spectrometry)考察了试样的原子组成,发现NH键的数量也与技术条件相关。基于记录的试样X-反射率数据的理论处理结果,用原始方法确定了材料的结构性质、层密度、表面粗糙度以及相应界面,并将所得结果与原子力显微镜所获得的结果进行了对比和讨论。借助于深能阶瞬态光谱学中的电荷变形,证实HCN溶液对二氧化硅/硅界面密度的强钝化影响。钝化后,发现新形成的界面深处缺陷阱其密度可以忽略不计,这是因为其形成原因与钝化过程中在界面处引入的NH原子对存在相关。

SiO2 thin films still belong to extensively studied material because if they are prepared in the form of high quality ultrathin and very thin oxides, they can be used in different actual applications, e.g. as gate oxides in the formation of VLSI as well as in the LCD production. We analyzed properties of very-thin SiO2 layer structure with thickness of 3 nm and 5 nm formed on moderately doped n-type Si （100） wafers, which were cleaned before formation of oxide layers using the standard RCA method and consequently they were annealed in nitrogen and parts of samples were passivated in aqueous HCN solutions. FTIR spectroscopy was used for obtaining information on different types of bonds in the structure. In passivated and non-passivated samples were identified the longitudinal optical （LO） and transverse optical （TO） modes, respectively, of the Si-O-Si asymmetric stretching vibration for SiO2. It was found that TO mode position （-1 107 cm 1） and amplitude are independent on thickness of the sample. On the other hand, LO mode position varied from -1 230 cm 1 （thickness -1.5 nm） to -1 244 cm-1 （thickness -4. 5 nm）. From FTIR peak shifts we were able to suggest inhomogeneity in ultra-thin and very thin SiOx based structures. Detailed deconvolution of FTIR spectra was performed to obtain relevant information. Atomic composition of the samples was investigated by SIMS method. Surprisingly, there were identified also NH bonds of different amounts in dependence on technological conditions. Structural properties, densities of layers, roughnesses of surfaces and corresponding interfaces were determined, also, by original method based on theoretical treatment of data recorded of samples by X-ray reflectivity. The results are compared and discussed with ones obtained by AFM. By charge version of deep level transient spectroscopy we have confirmed strong passivation influence of HCN solutions on the SiO2/Si interface density of states. After passivation procedure we have registered, on negligible density level, also newly formed interface deep defect traps. Their origin we relate with just formed NH atomic couples introduced to the interface region by passivation procedure.