Bias voltage frequency controlled angular ion distribution in plasma processing

A method of processing a workpiece comprising:

• generating a plasma containing gas ions in a plasma chamber, the plasma forming a sheath above the workpiece, the sheath having an electric field;
• modifying the plasma sheath using a plate having an aperture disposed between the plasma sheath and the workpiece;
• generating an oscillating radio frequency bias voltage having a sequence of positive energy peaks and a sequence of negative energy peaks;
• applying the bias voltage to a workpiece holder, the workpiece holder applying the bias voltage to the workpiece to generate a workpiece bias voltage with respect to the plasma to attract ions across the plasma sheath toward the workpiece, a time duration between the positive and negative peaks being determined by the radio frequency and being great enough to allow ions to respond separately to each peak, the time duration being selected to cause a selected annular distribution of the as ions impacting the workpiece from the sheath through the aperture with a bimodal distribution corresponding to the positive peaks for a first mode of the bimodal distribution and corresponding to the negative peaks for the second mode of the bimodal distribution, the selected bimodal annular distribution determining an angle at which the ions are attracted toward the workpiece.