Characterization of si(111) surfaces treated in NH4F and NH4HF2 solutions

Chemically treated Si(111) surfaces in aqueous NH4F and NH4HF2 solutions have been investigated using spectroscopic ellipsometry (SE), ex situ atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact-angle measurements. The measured SE spectra clearly indicate evidence of the removal of surface native oxide by these fluoride solutions. The XPS data support the removal of the native oxide upon immersing the samples in the solutions. Just after the native oxide is etched away completely, the SE data yield the spectrum of a shiny flat surface. Longer exposure to the solutions results in surface roughening; the thickness of the roughened layer immersed in the 40% NH4F (20% NH4HF2) solution for t = 600 s at 30degrees C (45degrees C), for example, is ca. 13 Angstrom (11 Angstrom). AFM observation confirms the presence of a roughened overlayer on the etched surface; however, the AFM image for the sample etched in the 40% NH4F solution for t = 600 s at 30degrees C yields rms roughness of similar to38 Angstrom that is quite larger than the SE value (ca. 13 Angstrom). This is not surprising, because, if the spatial wavelength of the roughness is longer than ellipsometric wavelength as in the present case, the SE analysis fails. The contact-angle measurements indicate that the NH4F- and NH4HF2-treated silicon surfaces are highly hydrophobic, theta ca. 74degrees (NH4F) and ca. 78degrees (NH4HF2), indicating the production of clean, hydrogen-terminated surfaces. (C) 2002 The Electrochemical Society.