Crossatron Modulator Designs And Performance For Plasma-based Ion Implantation Systems

Abstract

electrons produced by ion bombardment of the cathode surface are Plasma Ion Implantation (HI) is a proven process to improve trapped in a CUSP magnetic field localized near the cathode, and the surface characteristics of a broad spectrum of manufacturing travel in cycloidal ExB orbits around the cylindrical anode due to and tooling products. Tooling such as punches, drills and dies the radial electric field in the switch and the axial component of the have all shown significant lifetime improvements. In the PI1 magnetic field prior to being collected by the anode or grids. The process, the object to be implanted is placed in a weakly ionized long path length of the electrons near the cathode surface enhances plasma and pulsed to a high negative voltage. Ions in the plasma ionization of the hydrogen background gas, and reduces the are accelerated through the sheath at the plasma boundary into the pressure and plasma density (compared to thyratrons) at which the material, changing the surface chemical and physical composition. switch operates so that grid controlled interruption is possible. PI1 requires high voltage (2100 kV), high average-power The switch is closed by pulsing the control grid positive (>>lo kW) modulators to process relatively large-size parts in a relative to the cathode to initiate the plasma, and then allowing the reasonable amount of time. The load impedance of the PI1 pulse plasma to diffuse into the anode gap. Interruption of the plasma modulator is highly variable, and can change over an order of current is achieved by pulsing the control grid negative with magnitude during the 10-to-20 psec pulse period. The modulator respect to cathode potential. The flow of plasma from the must also accept different impedances due to changes in the PI1 production region near the cathode through the grid apertures is system which occur during process development and optimization. then blocked by the formation of a growing Child-Langmuir sheath A flexible and efficient modulator system for PI1 should have a in the apertures that eventually electrostatically closes the hole. switching device that is capable of providing the high current-surge The switch opens as the plasma erodes from the gap between the characteristic of the beginning of the pulse with low series control grid and the anode. The maximum interruptible current is switching losses, and provide some manner of fault protection determined by the plasma density in the control-grid apertures during target arcing. The CROSSATRON switch is a device well required to conduct the peak-current, the diameter of the apertures, suited to this requirements. A 100-kV CROSSATRON modulator and the bias applied to the grid. If fault currents in the switch built for the PI1 program and operated at Hughes has provided exceed a critical level, for example, they cannot be interrupted hard-tube-like modulation at voltages of over 100 kV, peak because the overly-dense plasma in the control grid region currents of up to 1 kA, and pulse repetition frequencies (PRFs) of electrostatically shields the grid. over 1 kHz. Various CROSSATRON circuit topologies as they CROSSATRON switch operation has been demonstrated in a apply to the PI1 process are examined. The performance of wide variety of application^.^ The performance of existing modulators is presented, and various fault modes, CROSSATRON switches can be optimized for different protection networks, and grid drive circuits are discussed. applications by changing the switch geometry, and the operating parameters for several versions of the switch for hard-tube-type applications are summarized in Table 1 . Depending on the target material and size selected for the PI1 process, one of the switches