Thick polycrystalline silicon layers (≥1 mm) were deposited on SiO2‐coated single‐crystalline Si wafers at a wide range of temperatures by vacuum evaporation. The crystalline structure and thermal behavior of the layers were strongly dependent on the deposition temperature. These characteristics were also reflected in the mechanical strength and warpage of the polycrystalline silicon‐coated single‐crystalline Si system. A careful investigation was made to understand the thermal characteristics of the vacuum‐evaporated polycrystalline silicon layers for possible semiconductor device application such as dielectrically isolated integrated circuits.