An apparatus for introducing gas into a processing chamber comprising one or more gas distribution tubes having gas-injection holes which may be larger in size, greater in number, and/or spaced closer together at sections of the gas introduction tubes where greater gas conductance through the gas-injection holes is desired. An outside tube having larger gas-injection holes may surround each gas distribution tube. The gas distribution tubes may be fluidically connected to a vacuum foreline to facilitate removal of gas from the gas distribution tube at the end of a process cycle.