Thermal reactions of sputtered Cu films on the thermally grown silicon dioxide (SiO2) and spin-on hydrogen silsesquioxane (HSQ) layers are investigated by examining the sheet resistance, surface microstructure, phase formation, and compositional depth profiles of Cu/thermal SiO2 and Cu/HSQ structures on Si wafers after annealing in vacuum at 400-700 degreesC. The sheet resistance values of both samples decreased after annealing at 400 degreesC. The decrease in sheet resistance occurred in conjunction with growth of Cu grains and annihilation of microcracks of the as-deposited Cu films. Cu2O was first found in the Cu/SQ/(Si) samples after annealing at 600 degreesC. After annealing at 700 degreesC, the whole copper layer of Cu/HSQ/(Si) sample reacted with oxygen to form several copper oxides. On the contrary, only a minor part of copper in Cu/thermal SiO2/(Si) structure was transformed to Cu2O after annealing at 700 degreesC. The reaction and/or diffusion characteristics of the two structures upon vacuum annealing are discussed.