step will be greater than the throttle position set point for the current step similarly if the throttle position of the previous step is greater than the throttle position corresponding to the current step 39 s throttle position or pressure set point the preset throttle position for the current step will be less than the throttle position set point for the current step the preset throttle positions both deposition and etch can be determined by a number of methods including characterization experiments prior to running the tdm process or through the use of a closed loop feed back algorithm that adjusts the preset throttle position value during the process the preset throttle position may also be derived from the recipe specified position e g applying a multiplier and or offset to the recipe specified throttle position in the example above the preset position is held constant for the time period of the position preset hold time in another embodiment of the invention the preset position can vary during the preset hold time e g preset position is a function of time monotonically increase or decrease etc the preset hold times can also be determined by a number of methods including characterization experiments prior to running the tdm process or through the use of a closed loop feed back algorithm that adjusts the preset time value during the process fig1 shows an example of the method from fig1 applied to a two step tdm silicon etch process note the duration of the transition 1815 for the pressure response 1805 to reach set point 1810 is significantly shorter than the prior art method of position control compare the duration 1515 in fig1 to the duration.