NOVELTY - The diffusion chamber (1) comprises a vacuum chamber (2), and two half-shells (11, 12) arranged within the vacuum chamber. A heater (31) is provided in one of the half-shells. One of the half-shells has an inlet (4) for the process gas and the other of the two half-shells a gas outlet (5). The inlet and/or the outlet are heated pipelines. Walls of the vacuum chamber are thermally insulated. A transport device (50) has a substrate carrier (41) for transporting a substrate (40). The substrate carrier is provided with a set of gas passage holes (42) on its circumference.    USE - The diffusion chamber is useful for thermal treatment of a substrate of a copper indium gallium selenide (CIGS) thin film solar cell (all claimed).    ADVANTAGE - The diffusion chamber treats the substrate at high temperature, and optimizes the temperature distribution on the substrate.    DETAILED DESCRIPTION - The diffusion chamber (1) comprises a vacuum chamber (2), and two half-shells (11, 12) arranged within the vacuum chamber. A heater is provided in one of the half-shells. One of the half-shells has an inlet (4) for the process gas and the other of the two half-shells has a gas outlet (5). The inlet and/or the outlet are heated pipelines. Walls of the vacuum chamber are thermally insulated. A transport device has a substrate carrier for transporting a substrate into or out of the diffusion chamber. The substrate carrier is provided with a set of gas passage holes on its circumference. The diffusion chamber further comprises a thermal vaporizer as a process gas source. A unit supplies inert gas into the vacuum chamber. The heater heats the substrate, and comprises a set of plate-shaped heating devices, where the plate-shaped heating devices form a heating zone. The diffusion chamber further comprises a unit for monitoring the temperature of the substrate, where the monitoring unit comprises an infrared radiation measuring device i.e. pyrometer, where the heating power of the heating device is controlled in a closed loop to achieve a homogenous temperature setting.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross sectional view of a diffusion chamber.    Diffusion chamber (1)    Vacuum chamber (2)    Inlet (4)    Gas outlet (5)    Half-shells. (11, 12)