Two-dimensional spatial survey of the plasma potential and electric field in a pulsed bipolar magnetron discharge

Abstract

Emissive and Langmuir probe techniques have been used to obtain two-dimensional (2D) spatial maps of the plasma potential Vp, electric field E, and ion trajectories in a pulsed bipolar magnetron discharge. The magnetron was pulsed at a frequency of 100kHz, with a 50% duty cycle and operated at an argon pressure of 0.74Pa. The pulse wave form was characterized by three distinct phases: the “overshoot,” “reverse,” and “on” phases. In the “on” phase of the pulse, when the cathode voltage is driven to −670V, the 2D spatial distribution of Vp has a similar form to that in dc magnetron, with significant axial and radial electric fields in the bulk plasma, accelerating ions to the sheath edge above the cathode racetrack region. During the “overshoot” phase (duration 200ns), Vp is raised to values greater than +330V, more than 100V above the cathode potential, with E pointing away from the target. In the “reverse” phase Vp has a value of +45V at all measured positions, 2V more positive than the target potential. In this phase there is no electric field present in the plasma. In the bulk of the plasma, the results from Langmuir probe and the emissive probe are in good agreement, however, in one particular region of the plasma outside the radius of the cathode, the emissive probe measurements are consistently more positive (up to 45V in the “on” time). This discrepancy is discussed in terms of the different frequency response of the probes and their perturbation of the plasma. A simple circuit model of the plasma-probe system has been proposed to explain our results. A brief discussion of the effect of the changing plasma potential distribution on the operation of the magnetron is given.