Influence of Oxygen Flow Rate on the Residual Stress, Optical and Morphological Properties of Electron-Beam-Evaporated Tungsten Oxide Thin Films

This study investigates the effect of oxygen flow rate on electron-beam-evaporated tungsten oxide thin films deposited on B270 glass at 100 °C and a nominal deposition rate of 1.0 Å/s. Oxygen flow was varied from 50 to 90 sccm while the other principal deposition conditions were kept constant. The thin films were characterized using optical-constant measurements, UV–Vis spectroscopy, SEM/EDS, a Linnik microscopic interferometer, and a Twyman–Green interferometer. As oxygen flow increased, the refractive index decreased from 1.943 to 1.876 and the extinction coefficient from 4.2 × 10−3 to 3.1 × 10−3. The measured film thickness varied non-monotonically from 135.8 to 174.1 nm, indicating that the optical-constant trend cannot be explained by thickness variation alone. The apparent optical band gap in the visible region remained within 3.28–3.36 eV. The apparent O/W ratio increased from 2.94 to 3.47, while the apparent surface-feature size increased from approximately 11 to 14 nm and the root-mean-square (RMS) surface roughness increased from 5.03 to 6.03 nm. All films exhibited compressive residual stress, with its magnitude decreasing from 63.7 to 49.5 MPa. These results demonstrate a coordinated oxygen-flow-dependent variation in optical, surface morphological, apparent compositional, and residual stress properties under the electron-beam evaporation technique.