A reaction chamber of a reactor for coating or treating a substrate by an atomic layer deposition process (ALD) by exposing the substrate to alternately repeated surface reactions of two or more gas-phase reactants. The reaction chamber is configured to generate capacitively coupled plasma and comprises a reaction space within said reaction chamber, a first inlet to guide gases into the reaction chamber and an outlet to lead gases out of the reaction chamber. The reaction chamber is configured to lead the two or more reactants into the reaction chamber such that the two or more reactants may flow through the reaction space across the substrate in a direction essentially parallel to the inner surface of the lower wall.