A system including a chamber enclosing a processing region, a substrate support within the chamber and configured to retain a substrate in the processing region, a gas distribution manifold coupled to the chamber to introduce first and a second etching gases from the gas distribution manifold using first and second inlets, a first gas distribution channel coupled configured to selectively switch between directing the first etching gas along a first gas flow path towards the processing region, and directing the first etching gas towards a vent, and a second gas distribution channel configured to selectively switch between directing the second etching gas towards the vent, and directing the second etching gas along a fourth gas flow path towards the processing region, and a flow ratio controller operatively coupled to the first and second gas distribution channels and configured to direct the first or second etching gases towards the processing region.