The treatment capability of a plasma based ion implanting two-dimensional trench-shaped target has been discussed through particle-in-cell simulation. By analysing time-dependent expansion of the plasma sheath and the ion implantation parameters during a high voltage pulse, we found that the sidewall of the trench could not be implanted effectively. Trenches with the same depth and three different widths were simulated in this paper. Simulation results show that the plasma sheath conforms to the target better in the case of a wider trench, but it tends to expand as a plane for the majority of the pulse. The ions accelerated in such a sheath would impinge vertically on the upper surface and the bottom of the trench with the converging at the convex corner and diverging at the concave corner, but neglecting the sidewall. After the short initial stage of the pulse when the sheath is conformal to the target well (about 1/10 of the pulse duration), the ions impact the sidewall of the trench with grazing angle, lowest ion impact current, and lowest ion impact energy. All the above three factors would induce a severe sputtering effect, reduce ion implantation depth and decrease retained dose. Increasing the width of the trench would alleviate this effect, but to a slight extent. The plasma based ion implantation process in trench cases is similar to ion beam implantation with a large beam and would not get the effective implantation for all the surfaces of the trench.
A prismoidal-shaped target with trapezoidal section containing four different convex corners was implanted with nitrogen using plasma-based ion implantation (PBII) in order to study the effect of target shape on the retained dose and its distribution with depth. Nitrogen was implanted into a silicon wafer clamped on the side wall of the sample holder, and Auger electron spectroscopy was employed to obtain the nitrogen depth distribution and the retained dose. Both a former simulation and the present experimental analysis exhibit dependence of the dose on the target shape but with a reversed trend. A method that combines a fluid dynamic model to simulate plasma sheath expansion during a high voltage pulse and the Monte Carlo method of the TRIM code to simulate the incident ion distribution in the solid was presented to predict the concentration depth profile after PBII. When establishing the model, the mechanism of the resulting lower retained dose near the corner with the higher density of ion impact flux was discussed. It was found that the oblique impact of the ion flux reduces the retained dose of the modified layer in three ways and changes the form of the profile remarkably. The continuous distribution of ion impact energy and the low N+/N2 ratio in the plasma shift the N depth profile nearer to the surface, which reduces the implantation depth significantly. In addition, the oblique impact near the edge of the convex corner decreases the reduction in ion range and retained dose and should account for the gradient in the retained dose distribution on the target surface. The model presented can give a good prediction and explanation for the experimental results.
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