The effect of magnetic field on plasma uniformity was investigated for a capacitively coupled plasma in a dielectric etch chamber and a tool to measure the de bias uniformity across the high-powered cathode (e.g., 1200 W) was developed. At that power, the de bias can be as high as 1500 V at 50 mT. A 5% variation in the de bias across the cathode corresponds to a 75 V potential drop across the wafer, which may in turn cause degradation or breakdown of gate oxide structures. Therefore control of the instantaneous plasma uniformity is important to minimize device damage. The primary effect of the magnetic field on instantaneous plasma uniformity is through the EXB drift force which, because of the strong electric field at the cathode surface at high powers, dominates the other magnetic held effects on the plasma. de bias measurements show that the plasma nonuniformity can be optimized by adjusting the gradient of the magnetic field, and thus the EXB drift force, across the cathode over a wide range of magnetic field strengths (10-90 G), process pressures (50-250 mT), and cathode powers (750-1100 W), for a CF4/Ar chemistry and 13.56 MHz rf power. Finally, the data analysis method used to improve signal-to-noise is discussed.