apparatus is provided for plasma treating a substrate . this has a chamber and a plasma generator which forms a plasma from one or more gases flowing within the chamber so as to produce one or more species for interacting with a substrate placed within the chamber . a guide is provided for directing the gas flow containing the species towards the substrate . when in use , the width of the plasma is greater than that of the substrate by an amount defining an outer region of plasma . the guide is adapted to direct the species from at least substantially all of the outer region of the plasma towards the substrate . a corresponding method of plasma treatment is also disclosed .