An apparatus for plasma processing of a substrate comprising: a power supply; at least one plasma generating unit including at least two electrodes, including at least a first electrode and a second electrode, wherein the first electrode is a powered electrode coupled to the power supply; a support for positioning and supporting the substrate with a first side of the substrate facing the PGU; and at least one gas supply system and at least one exhaust port configured to provide a gas flow through a first gap between a front surface of the first electrode and the substrate and through a second gap between a first side surface of the first electrode and a side surface of the second electrode; wherein each of the electrodes has a length and a width, wherein the length of each respective electrode is at least four times the width of the respective electrode; wherein the first electrode and the second electrode are positioned relative to one another such that the second gap is less than the width of the first electrode, and wherein the first electrode is positioned such that the first gap is less than the width of the first electrode; wherein the gas supply system comprises a first gas inlet configured to inject a first gas flow into the second gap, and wherein the power supply is configured to provide an alternating current to sustain a plasma in the first gap, characterized in that the support forms a third electrode for sustaining a plasma in the second gap and the gas supply system comprises a second gas inlet located in the direction of the first gas flow downstream of the first gas inlet configured to inject a second gas flow into the plasma, wherein an injection manifold forming the second gas inlet is located within the first electrode. a power supply; at least one plasma generating unit including at least two electrodes, including at least a first electrode and a second electrode, wherein the first electrode is a powered electrode coupled to the power supply; a support for positioning and supporting the substrate with a first side of the substrate facing the PGU; and at least one gas supply system and at least one exhaust port configured to provide a gas flow through a first gap between a front surface of the first electrode and the substrate and through a second gap between a first side surface of the first electrode and a side surface of the second electrode; wherein each of the electrodes has a length and a width, wherein the length of each respective electrode is at least four times the width of the respective electrode; wherein the first electrode and the second electrode are positioned relative to one another such that the second gap is less than the width of the first electrode, and wherein the first electrode is positioned such that the first gap is less than the width of the first electrode; wherein the gas supply system comprises a first gas inlet configured to inject a first gas flow into the second gap, and wherein the power supply is configured to provide an alternating current to sustain a plasma in the first gap, characterized in that the support forms a third electrode for sustaining a plasma in the second gap and the gas supply system comprises a second gas inlet located in the direction of the first gas flow downstream of the first gas inlet configured to inject a second gas flow into the plasma, wherein an injection manifold forming the second gas inlet is located within the first electrode.