Abstract
High voltage sheaths are formed when plasmas are produced by application of high negative voltage pulses to conductive supports or components, as in Plasma Immersion Ion Implantation (PIII) treatments of materials surfaces. For parts with concave shape, as inside metal tubes, these sheaths behave quite differently according to the tube configuration and size, as well as, PIII treatment pressure of operation and pulsing parameters. In this work, an SS304 tube of 1.1 cm internal diameter and 20 cm length was pulsed typically at -0.5 to -2.6 kV, 20 μs pulse length, 500 Hz repetition rate, nitrogen pressure of 5x10-2 mbar and with one side closed configuration. Different currents (between 10 and 30 A) were used to produce plasmas with sheaths that overlapped or not, depending on the currents used. To study these sheath behaviors, a simple plasma diagnostic technique based on a bi-dimensional mapping of the deposition of sputtered materials and by etching via the plasma on a Si wafer target surface, both coming out from the tube, was used. This mapping showed clearly the border line situation between overlapping and non-overlapping sheaths in that small tube which allowed to estimate the plasma density to be around 1011 cm-3 at such a sheath condition, as previously anticipated by Sheridan. Above that border condition, nitrogen PIII was successfully obtained in such a small tube of SS304, producing TiN and Ti2N in samples of Ti6Al4V placed inside the tube, when temperatures higher than 800°C were reached there. Below the border, no significant uptake of nitrogen was possible. Using this type of experimental set-up, it is now possible to explore different hollow cathode behaviors, efficient or high temperature (above 800°C) PIII conditions and also new utilizations of the plasma ejected from the tube.
Highlights
En[0] where Øt means applied potential, no the plasma density and ε0 the vacuum permittivity
For low current experiments, sheath overlapping occurred, which resulted in low ion implantation and sputtering and no deposition on the wafer, even for nitrogen Plasma Immersion Ion Implantation (PIII) using SS304 tube with a lid
For 1.1 cmØ internal diameter SS304 tubes and low currents (
Summary
En[0] where Øt means applied potential, no the plasma density and ε0 the vacuum permittivity. For conditions of the previously presented case, sheath overlapping would be present for the small 1.1 cmØ internal diameter tube Under these conditions, the electric field applied to the inner wall of the tube by the high voltage pulser is much reduced, and it (the electric field) loses the perpendicularity with respect to the walls, leading to a total failure of desired ion implantation of nitrogen there. For tubes with a bit larger diameters, say, 4 cmØ internal diameter, inserting a central pin was tested with successful ion implantation[5,6] and confirmed by our group.[7] for tubes with diameters smaller than 2 cm, configurations with central pin become too difficult to implement due to geometrical constraints Another alternative method used to overcome the sheath overlapping in tubes is the utilization of the magnetic field in a special configuration, the magnetic bottle. For 2 cmØ internal diameter tube, either good nitrogen implantation or adherent DLC film deposition was not obtained
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