Apparatus for reactive ion etching of a substrate, comprising: - an injector head comprising ∘ a plasma etch zone including an etch gas supply and arranged with a plasma generating structure to ignite a plasma and further comprising an electrode structure arranged to accelerate the etch plasma toward a substrate portion such that ions impinge on the surface of the substrate; ∘ a passivation zone including a cavity provided with a passivation gas supply; said supply arranged for providing a passivation gas flow from the supply to the cavity; the cavity in use being bounded by the injector head and the substrate surface; and ∘ a gas purge structure comprising a gas exhaust arranged between said etch zone and passivation zone, such that the gas purge structure forms a spatial division of the etch and passivation zones; said apparatus comprising a gas bearing structure comprising a bearing gas injector arranged for injecting a bearing gas between the injector head and the substrate surface, such that the bearing gas forms one or more gas-bearings, said one or more gas bearings functioning as flow barriers and being arranged to confine said plasma etch zone and said passivation zone at at least outer perimeters thereof; the apparatus further comprising cavity walls and a pressure controller arranged to control the pressure of the one or more gas bearings and the pressures in said plasma etch zone and said passivation zone. - an injector head comprising ∘ a plasma etch zone including an etch gas supply and arranged with a plasma generating structure to ignite a plasma and further comprising an electrode structure arranged to accelerate the etch plasma toward a substrate portion such that ions impinge on the surface of the substrate; ∘ a passivation zone including a cavity provided with a passivation gas supply; said supply arranged for providing a passivation gas flow from the supply to the cavity; the cavity in use being bounded by the injector head and the substrate surface; and ∘ a gas purge structure comprising a gas exhaust arranged between said etch zone and passivation zone, such that the gas purge structure forms a spatial division of the etch and passivation zones; said apparatus comprising a gas bearing structure comprising a bearing gas injector arranged for injecting a bearing gas between the injector head and the substrate surface, such that the bearing gas forms one or more gas-bearings, said one or more gas bearings functioning as flow barriers and being arranged to confine said plasma etch zone and said passivation zone at at least outer perimeters thereof; the apparatus further comprising cavity walls and a pressure controller arranged to control the pressure of the one or more gas bearings and the pressures in said plasma etch zone and said passivation zone. ∘ a plasma etch zone including an etch gas supply and arranged with a plasma generating structure to ignite a plasma and further comprising an electrode structure arranged to accelerate the etch plasma toward a substrate portion such that ions impinge on the surface of the substrate; ∘ a passivation zone including a cavity provided with a passivation gas supply; said supply arranged for providing a passivation gas flow from the supply to the cavity; the cavity in use being bounded by the injector head and the substrate surface; and ∘ a gas purge structure comprising a gas exhaust arranged between said etch zone and passivation zone, such that the gas purge structure forms a spatial division of the etch and passivation zones; said apparatus comprising a gas bearing structure comprising a bearing gas injector arranged for injecting a bearing gas between the injector head and the substrate surface, such that the bearing gas forms one or more gas-bearings, said one or more gas bearings functioning as flow barriers and being arranged to confine said plasma etch zone and said passivation zone at at least outer perimeters thereof; the apparatus further comprising cavity walls and a pressure controller arranged to control the pressure of the one or more gas bearings and the pressures in said plasma etch zone and said passivation zone.