NOVELTY - Substrates (610) having a copper and indium composite structure are transferred to a furnace. A gaseous species including selenide species and carrier gas are introduced and thermal energy is transferred to the furnace to initiate formation of a copper-indium diselenide film. The temperature is maintained for a period of time, and residual selenide species are removed. A sulfide species is introduced into the furnace and temperature is increased while the substrates are maintained in an environment including a sulfur species.    USE - Fabrication of copper-indium diselenide semiconductor film. Uses include but are not limited to photovoltaic device (e.g. solar cell) using copper-indium diselenide species or copper-indium-gallium diselenide species, photovoltaic modules, flexible sheets, building or window glass, and automotive components.    ADVANTAGE - The method enables maintenance of structural integrity of substrate materials, and ensures uniformity and granularity of thin film materials.    DETAILED DESCRIPTION - Substrates (greater than 5) having a copper and indium composite structure are transferred to a furnace, each substrate being provided in a vertical orientation with respect to a direction of gravity. A gaseous species including a selenide species and a carrier gas are introduced into the furnace and thermal energy is transferred to the furnace to increase temperature from temperature (I) to temperature (II) (350-450 degrees C) to initiate formation of a copper-indium diselenide film from the copper and indium composite structure on each substrate. The temperature is maintained at temperature (II) for a period of time, and residual selenide species are removed from the furnace. A sulfide species is introduced into the furnace and temperature is increased to temperature (III) (500-525 degrees C) while the substrates are maintained in an environment including a sulfur species to extract out one or more selenium species from the copper-indium diselenide film. Each substrate is maintained in a planar configuration free from warp or damage.    DESCRIPTION OF DRAWING(S) - The drawing shows a thin film copper-indium diselenide device.    Device (600)    Layers of n-type semiconductor material (602,604)    Semiconductor layer (606)    Layer comprising film of molybdenum (608)    Substrate (610)