Integrated high voltage modulator for plasma immersion ion implantation

Abstract

Plasma immersion ion implantation (PIII) on an industrial scale requires negative high voltage pulses in the kA range with voltages up to 200 kV. Hence, the equipment to produce suitable high voltage pulses is rather expensive and shares a considerable part of the total costs of PIII equipment. Reducing the costs of equipment is therefore still a challenge to promote the commercial use of PIII. A new high voltage modulator is presented, which will meet the above requirements. The basic idea of the new modulator is that in case of two floating electrodes, immersed into a plasma and connected to both plates of a charged capacitor, the anode potential will be close to the plasma potential, whereas the cathode potential will become negative compared to the plasma potential. The new modulator works as a triode system, consisting of anode, cathode and grid, all located inside of the working chamber. The plasma is discharged between cathode and grid. As usual in PIII, the sample is immersed into the plasma and works as the cathode in the system. The grid is mounted between plasma and anode, in such a way that ions or electrons from the plasma can get to the anode by passing the grid plane only. Further a capacitor is connected to anode and cathode with both plates and the anode is additionally linked to a positive high voltage source. If the grid is negatively biased, the anode is isolated from the plasma, as electrons from the plasma are reflected by the grid bias, whereas the ions are reflected by the positive potential of the anode. As far as the grid is negatively biased the cathode is at plasma floating potential, and the capacitor may be charged by the positive high voltage source to the capacitor voltage +U0. When the grid potential is switched to ground potential electrons from the plasma can reach the anode and will shift the anode potential to plasma potential. At the same time the cathode is switched to negative high voltage potential −U0. As the performance of the integrated modulator depends on the maximum of the electron current which can be extracted from the plasma by the anode, the basic considerations concerning the anode current and the revealing design parameters of the integrated modulator are given. The experimental results of modulating 20 kV at a maximum current of 4 A confirm the theory.