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Modelling and experiment of plasma-based ion-implanted two-dimensional target

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.