| Abstract |
The purpose of the work presented here was the development of an ion source for low energy high flux ion implantation, based on laser ablation. Low energy stands for ion energies ranging from sub-keV up to some keV, while the term high flux refers to achieving ion doses of the order of in some minutes. The ions are ejected from the ablated target with large initial velocities in a solid angle of the order of p sterads about the normal to the target surface. The forward velocities have a broad distribution with most probable values corresponding to kinetic energy of the order of for ns pulse lasers, much larger than the thermal velocities in the plasma. These facts lead to a rapid expansion of the laser ablation plasma, which has dimensions loosely comparable to the distance of its central region from the target. The electron cloud follows the, so called, ion matrix of the plasma with random thermal velocities of the order of , much larger than those of the ions because of the large mass difference. Quasi neutrality is maintained at any time and the electron cloud density closely matches that of the ion matrix. A thin sheath of less than a mm is formed at the plasma outskirts. When the plasma is not in contact with a conducting surface, the sheath contains a negative charge density (electrons) in the outer layer and a positive charge density (ion matrix depleted from electrons) in the inner space charge layer. When the plasma is in contact with electrodes a sheath forms between the neutral plasma and the electrode surface. The polarity, size, voltage drop and current densities of this anode/cathode sheath depend on many parameters discussed in detail in the following chapters. The plasma density evolves from values characteristic of condensed matter just above the ablation spot to approximately at a distance between to from the target. All set-ups of the experiments reported here where positioned in this distance range. The high plasma density and the resulting effective shielding of external fields leave two main alternatives in trying to separate the ions from the rest of the ablated matter and direct them towards the implantation target with as high flux as possible. These are: 1. Extract the ions from the plasma and attempt to focus them on the implantation target while directing them away from the rectilinear and diverging trajectories of the neutrals. 2. Guide the plasma away from the neutrals path and use the, so called, plasma immersion ion implantation during which the quasi neutral plasma reaches within tens of hundreds of microns from the implantation target surface. The ions are then projected into the target by a pulse of electric field in the cathode sheath. We have devoted a significant amount of experimental work on the first alternative and found that it may not be employed conveniently for low energy ion implantation without sacrificing a large percentage of the ions produced by laser ablation. In the implementation of the second alternative, we have used both electric and magnetic fields to guide the plasma towards the implantation target while preventing all neutral matter in the ablation plume from reaching it. We have also attempted and succeeded to increase significantly the density of ion trajectories in the plane of the implantation target. In doing so we have taken advantage of the broad initial ion velocity distribution of laser ablation plasmas. The width of the distribution of ion arrival times at the implantation target is orders of magnitude larger than the laser pulse duration. As a consequence, the average ion flux away from the ablation target can be made as high as it is near it while the corresponding plasma densities are vastly different. The efficiency of the sources designed and tested is defined by the ratio of ions reaching the implantation target surface to the total number of ions entering the magnetic field. Our latest setup tested achieved almost , neutral free, efficiency. Furthermore, we have the experience and the measurement data necessary to significantly improve further the technique we have developed by a complete redesign of the experimental setup.
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Ιοντική εμφύτευση μεγάλης ροήςχαμηλής ενέργειας με χρήση πλάσματος αποδόμησης με Laser
