The mechanical stresses in Cu interconnect lines arise from thermal expansion (CTE) differences, and the magnitude of the stress can be calculated based on the measured strain. In the current work, the strain (and stress) state of narrow Cu lines fabricated in oxide and porous organic spin-on dielectrics (low K) has been determined with X-Ray diffraction (XRD) during annealing. The room temperature stress along the length (X) and width (Y) of the lines are not dramatically different while the Z component is somewhat smaller with the spin-on ILD. These small perturbations in the magnitude of the Cu stress do not reflect the dramatic differences in the CTE. More insight into the materials system is obtained by studying the strain-temperature behavior, which illustrates the effect of the ILD clearly. The X strain is similar in magnitude and variation with temperature for both ILDs, supporting strain being imposed by the substrate. However, the Z strain is compressive at RT and linearly increases with temperature for Cu in low K, reflecting the lack of constraint by the ILD and the higher CTE of the ILD.