Layr-by-layr oxidation mechanism of hydrogen-terminated silicon surfaces and their interface structures

结果表明，在pH=3.8的BHF溶液或0.1%HF+1%H_2O_2溶液中进行湿化学处理，可以显著降低Si（100）晶圆的表面微粗糙度，在FT-IR-ATR光谱中，SiH_2出现明显的拉伸振动峰，SiH和SiH_3出现微弱的振动峰。在BHF溶液中抑制OH离子对Si的各向异性刻蚀，或先用H_2O_2氧化，再用HF氧化刻蚀，选择性去除表面上的化学活性位点，被认为是产生平坦Si（100）表面的有效方法。考虑到与粗糙的Si（100）表面相比，平坦的Si（100）表面的天然氧化速率在氧化的早期阶段被显著抑制，自然氧化很可能是从表面的微切面和台阶边缘等化学反应部位开始进行的。经40%NH_4F处理后，Si（111）表面变得原子平坦。原子力显微镜清晰地观察到台阶宽度为几百nm的双层原子阶梯结构。覆盖4nm厚热氧化物的Si（111）表面的AFM图像与氧化前的表面形貌几乎相同，并且在氧化物上仍然可以观察到阶梯结构。稀释HF去除氧化物后的Si表面与初始表面相似。这表明Si（111）表面的氧化是通过一层接一层的机制进行的，这可能也是Si（100）表面的情况。此外，在厚度小于25A的范围内，由Si-O-Si拉伸振动产生的LO声子模式引起的红外吸收峰出现了相当大的红移。这种红移现象可以很好地解释为界面附近压应力的存在。

It was found that the wet chemical treatment in a pH-modified BHF(pH=3.8)solution or a 0.1%HF+1%H_2O_2 solution significantly reduces the surface microroughness of Si(100)wafers as demonstrated by a sharp SiH_2 stretching vibration peak accompanied with the weak SiH and SiH_3 peaks in FT-IR-ATR spectra. The suppression of anisotropic etching of Si with OH ions in a BHF solution or the selective removal of chemically reactive sites on the surfaces by the oxidation with H_2O_2 followed by oxide etching with HF is thought to be effective to produce the flat Si(100) surface. Taking into account the fact that the native oxidation rate of the flat Si(100)surface is significantly suppressed in the early stages of the oxidation compared with the case of rough Si(100)surfaces, it is likely that the native oxidation starts to proceed from the chemically reactive site such as microfacets and step edges on the surface.The Si(111)surface becomes atomically flat by 40%NH_4F treatment. The bilayr atomic step structure with a terrace width of a few hundreds nm is clearly observed by AFM.The AFM image of the Si(111)surface covered with a 4nm-thick thermal oxide is almost identical to the surface morphology before oxidation and the step structure is still observable on the oxide. Also, the Si surface after the removal of the oxide by dilute HF is similar to the intial surface. This suggests that the oxidation of the Si(111)surfaces proceeds through a layr-by-layr mechanism and this might be the case also for Si(100)surfaces. Furthermore, it was found that the infrared absorption peak due to the LO phonon mode originating from the Si-O-Si stretching vibration shows a considerable red-shift in the thickness range below 25A.This red-shift is well explained by the existence of the compressive stress near the interface.