Comparative Study of Surface Energies of Native Oxides of Si(100) and Si(111) via Three Liquid Contact Angle Analysis

The effects of crystal orientation and doping on the surface energy, γ^T, of native oxides of Si(100) and Si(111) are measured via Three Liquid Contact Angle Analysis (3LCAA) to extract γ^T, while Ion Beam Analysis (IBA) is used to detect Oxygen. During 3LCAA, contact angles for three liquids are measured with photographs via the Drop and Reflection Operative Program (DROP™). DROP™ removes subjectivity in image analysis, and yields reproducible contact angles within < ±1°. Unlike to the Sessile Drop Method, DROP can yield relative errors < 3% on sets of 20-30 drops. Native oxides on 5 x 10¹³ B/cm³ p^- doped Si(100) wafers, as received in sealed, 25 wafer teflon boats continuously stored in Class 100/ISO 5 conditions at 24.5°C in 25% controlled humidity, are found to be hydrophilic. Their γ^T, 52.5 ± 1.5 mJ/m², is reproducible between four boats from three sources, and 9% greater than γ^T of native oxides on n^- doped Si(111), which averages 48.1 ± 1.6 mJ/m² on four 4 Si(111) wafers. IBA combining ¹⁶O nuclear resonance with channeling detects 30% more oxygen on native oxides of Si(111) than Si(100). While γ^T should increase on thinner, more defective oxides, Lifshitz-Van der Waals interactions γ^LW on native oxides of Si(100) remain at 36 ± 0.4 mJ/m², equal to γ^LW on Si(111), 36 ± 0.6 mJ/m^2, since γ^LW arises from the same SiO2 molecules. Native oxides on 4.5 x 10¹⁸ B/cm³ p⁺ doped Si(100) yield a γ^T of 39 ± 1 mJ/m², as they are thicker per IBA. In summary, 3LCAA and IBA can detect reproducibly and accurately, within a few %, changes in the surface energy of native oxides due to thickness and surface composition arising from doping or crystal structure, if conducted in well controlled clean room conditions for measurements and storage.