The structural and stress changes in molybdenum/amorphous silicon (Mo/a-Si) EUV reflecting multilayers during annealing at 316��C were studied. The amorphous interlayers, with an Mo:Si stoichiometry of 1:2, grew during annealing. The residual stresses in each component of the multilayer also changed significantly. Through high resolution electron microscopy, selected area electron diffraction, and x-ray diffraction of the crystalline Mo, the biaxial tensile stresses were shown to increase from approximately two to about ten GPa in the lateral direction, parallel to the interface plane. The compressive strains developed in the vertical direction, perpendicular the interface plane, are consistent with a Poisson contraction calculated from bulk elastic properties. Laser deflectometry measurements of thicker (non-EUV, 0.1��m) amorphous silicon showed compressive stress relaxation in the amorphous silicon with annealing, which may also take place in the thin (EUV) silicon. The residual stress in a 40 bilayer EUV film changes from about -0.5 GPa to about +1.5 GPa. / Graduation date: 1996