Semiconductor processing apparatus with compact free radical source

A semiconductor processing apparatus (1), comprising: a substrate processing chamber (158), defining a substrate support location (156) at which a generally planar semiconductor substrate (300) is supportable; and at least one free radical source (200), including: a precursor gas source (250); an electric resistance heating filament (244); a sleeve (220) with a central sleeve axis (L), wherein said sleeve defines a reaction space (222) that accommodates the heating filament (244), and wherein said sleeve includes an inlet opening (224) via which the reaction space is fluidly connected to the precursor gas source (250), and an outlet opening (228) via which the reaction space is fluidly connected to the substrate processing chamber (158), said inlet and outlet openings (224, 228) being spaced apart along the central sleeve axis (L).