This disclosure relates to a surface processing apparatus for use in the surface processing of a substrate. The surface processing apparatus comprises a plasma source including a wall defining a plasma chamber and an excitation source adjacent the wall and a processing chamber in which a substrate having a predetermined maximum lateral dimension is mounted in use, the processing chamber being operatively connected to the plasma source. A transmission plate for the transmission of plasma in use is arranged between the plasma source and processing chamber, the transmission plate comprising a plurality of apertures. The apertures follow a non-rectilinear path through the transmission plate such that there is no line of sight in use between a substrate with the predetermined maximum lateral dimension mounted in the processing chamber and the most intense region of the plasma in the plasma chamber.