An apparatus for processing substrates is provided. A first gas source is provided. A first gas manifold is connected to the first gas source. A second gas manifold is connected to the first gas source. A first processing station has a first gas outlet, wherein the first gas outlet is connected to the first gas manifold. A second processing station has a second gas outlet, wherein the second gas outlet is connected to the second gas manifold. A first variable conductance valve is between the first gas source and the first gas outlet along the first gas manifold. A second variable conductance valve is between the first gas source and the second gas outlet along the second gas manifold.