Novel surface cleaning technique with chemical etching for ideal hydrogen termination: Surface chemistry and morphology of the silcon(111) and silicon(100) surfaces

Sung-Kwang Yang, Purdue University


Surface infrared spectroscopy has been utilized to characterize hydrogen-terminated Si(111) and Si(100) surfaces prepared by wet chemical etching. An ideally monohydride-terminated (atomically flat) Si(111) surface is obtained with a pH-enhanced buffered HF solution (a mixture of BHF (6:1 NH$\sb4$F:HF) and NH$\sb4$OH). Hydrogen terminations are observed to depend strongly on the pH of the HF-based solutions; ideal monohydride termination is obtained at about pH = 9.25. However, when the Si/SiO$\sb2$ interface is smooth (e.g., from thermal oxidation), a mostly ideally monohydride-terminated Si(111) surface can be obtained even only with a BHF treatment which may result in small amounts of defects. Such hydrogen terminations produced by wet etching can passivate substrates against surface oxidation and protect them against organic contaminants. It is found that Si(111) and Si(100) surfaces are most stable in anhydrous conditions. White surface deposits are seen to form in highly basic etching solutions which contain NH$\sb4$F such as 40% NH$\sb4$F or pH-enhanced BHF. From studies with surface infrared spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy, ammonium hexafluorosilicate, (NH$\sb4)\sb2$SiF$\sb6,$ is presumed to form these white deposits on the Si surface during the etch reaction of SiO$\sb2$ and BHF. A new two-step etching process is proposed to obtain an atomically smooth and impurity-free surface; this process includes a 7 min dip in 1% HF followed by a short dip ($\sim3$ sec) in pH-enhanced 1% HF solution (a mixture of 1% HF and NH$\sb4$OH, pH = 9.25). The second etching step converts a microscopically rough surface with all defects on Si(111) into an atomically smooth, ideally monohydride-terminated surface within 3 sec. Atomic Force Microscopy of the surface prepared by this new two-step chemical etching shows a remarkably flat surface (average surface microroughness of 0.3A). Similar results are obtained with a 3 sec dip in a pH-enhanced BHF solution; however, longer treatments with this solution result in the formation of white surface deposits. This two-step etching also allows studies on the initial stage of preferential etching of Si(100) substrates. In situ FTIR-Emission Spectroscopy is found to be useful in monitoring Si-H vibrational spectral features during hydrogen desorption from hydrogen-terminated silicon surfaces produced with wet chemical etching and low temperature substrate surface oxidation.




Takoudis, Purdue University.

Subject Area

Chemical engineering|Chemistry

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