A method based on the bending beam technique has been developed to measure the thermal stresses of fine lines confined by a dielectric layer. This method has been employed to determine the thermal stress of Al (2 at. % Cu) lines passivated by a SiO2overlayer between room temperature and 400 °C. The effect of quartz confinement was analyzed by matching the thermal displacement at the metal/passivation interfaces and by imposing a mechanical equilibrium condition on the structure. The analysis enables us to deduce the triaxial stress components of metal and passivation from measurements of the substrate bending parallel and perpendicular to the length direction of the lines. Results of the measurements show a substantial stress enhancement as a result of the confinement, with the stress level significantly higher than that of a passivated blanket film. Parameters that influence the magnitude of the stress components are line geometry, layer deposition conditions, and the extent of plastic deformation during thermal cycling. Results of the measurements are consistent with those determined using x‐ray techniques.