A thermal oxide film (SiO₂, linear thermal expansion coefficient about 0.5×10⁻⁶ K⁻¹) is grown on a silicon substrate at high temperature and cooled to room temperature. Taking the linear thermal expansion coefficient of silicon as about 2.6×10⁻⁶ K⁻¹, what stress develops in the oxide film due to the difference in thermal expansion?

Answer: B. Compressive stress

On cooling, silicon shrinks more than SiO₂. The SiO₂ film is made to shrink more than it would on its own, so it is squeezed in plane and develops compressive stress. The sign of thermal stress can be judged this way: if the film's thermal expansion coefficient is larger than the substrate's, the film is in tension after cooling; if smaller, in compression. The relative expansion coefficients are the reverse of those for an aluminum film on silicon, so the stress is of the opposite sign. As long as the expansion coefficients differ, stress develops whenever the temperature changes from the deposition temperature, so the choice that no stress develops is wrong. The compressive stress in thermal oxide reaches a few hundred MPa and can be determined by measuring wafer bow.