Effect of hydrofluoric acid in oxidizing acid mixtures on the hydroxylation of silicon surface

Silicon (100) wafers, predipped in 1:20 (vv) HF water, were treated separately with four different acid mixtures, viz., HNO3, H2 SO4 - H2 O2, HNO3 -HF, and H2 SO4 - H2 O2 -HF, for different time durations. Subsequent vigorous rinsing with deionized water rendered the wafer surfaces with hydroxyl termination. Synthesized surfaces were characterized by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), ellipsometry, contact angle and atomic force microscopy. Surfaces treated with HNO3 and H2 SO4 - H2 O2 showed increasing hydrophilicity at room temperature due to the formation of silanol (-SiOH) terminated chemical oxides, with continuous oxide growth. An increase in hydrophilicity was observed during the first 15 min of treatment with HNO3 -HF and H2 SO4 - H2 O2 -HF acid mixtures, causing a decrease in the hydrophilic character with longer incubation times. DRIFTS analysis confirmed the addition of HF in the oxidizing acid mixture controls chemical oxide proliferation, through creating a surface with mixed -SiOH and silicon hydride (- SiHx) termination. Prolonged incubation in acidic mixtures containing HF resulted in a logarithmic increase of - SiHx coverage, rendering the surface hydrophobic. Incubation for 15 minutes in each of the four acid mixture systems generated surfaces with comparable hydrophilicity, controlled oxide growth and reduced surface roughness.