a method is disclosed for controlling the sheet resistance of copper trenches formed on semiconductor wafers . the method includes forming a plurality of copper - filled trenches on a wafer , measuring the sheet resistance of each of the plurality of copper - filled trenches , and comparing the measured sheet resistance values to a predetermined sheet resistance value . photolithography steps performed on subsequent wafers are adjusted according to a difference between the measured sheet resistance values and the predetermined value . in one embodiment , this adjustment takes the form of adjusting a photolithographic extension exposure energy to thereby adjust the cross - section of the resulting trenches .