Stability of ammonium fluoride-treated Si(100)

Abstract

X-ray photoelectron spectroscopy (XPS) and contact angle analyses have been employed to investigate the chemical stability of silicon surfaces treated by an ammonium fluoride (NH4F) solution. Consistent with earlier results [Dumas and Chabal, Chem. Phys. Lett. 181, 537 (1991)], it is found that an NH4F final etch produces surfaces exhibiting lower oxygen and carbon contamination levels in comparison to the surfaces obtained with the traditional HF or buffered HF (BHF) etch. In conjunction with lower contamination levels, surfaces treated in ammonium fluoride show higher contact angles with water, indicating lower surface free energies. The Si-H surfaces produced by the ammonium fluoride etch were found to remain hydrophobic for weeks in air and showed no signs of oxidation for several days. After an induction period of about one week in air, oxidation began to occur in a more rapid fashion. The stability of the Si-H surfaces in water was also investigated, and it was found that the oxygen contamination levels measured by x-ray photoelectron spectroscopy were extremely sensitive to the final rinse time. An induction period of approximately one hour in water existed before any signs of oxidation were evident. Despite the fact that the NH4F-treated samples were initially cleaner than the HF-treated samples, both of these surfaces showed similar temporal behavior in air and water. We attribute this finding to the surface topographies produced by these treatments, as well as to the extreme sensitivity of the results to such factors as water rinse time.