Abstract: The invention relates to a heat treatment inner chamber (3) for thermally processing a substrate (20), having walls (10) which enclose an inner space (24) of the heat treatment inner chamber (3), having a mounting apparatus (8) for mounting the substrate (20) during the thermal processing and having an energy source (11) for introducing energy into the inner space (24) of the heat treatment inner chamber (3), at least one part of the inner sides of the walls (10) being formed in order to reflect power introduced by the energy source (11), wherein the at least one part of the inner sides of the walls (10) consists of a material which is highly reflective at least for infrared radiation.