Plasma immersion ion implantation of complex-shaped objects: an experimental study on the treatment homogeneity

Plasma immersion ion implantation is a technique which is often quoted to treat complex-shaped objects homogeneously. The present study shows that this is not necessarily the case, if the plasma conditions are not chosen in a very narrow predetermined window. It is not only important to know the dose distribution of the implanted ions across a three-dimensional object, but also the intensity of sputter etching depending on the position. Therefore, two different techniques, sensitive to these issues have been applied. The objects investigated are macro-trenches with different aspect ratios. U-shaped stainless steel sample holders with different trench widths have been lined with silicon wafer segments. Additionally a Ta metal strip, coated with a Ta2O5 layer, has been folded into the sample holders. These trenches have been treated in an argon plasma. The implanted Ar ion dose has been determined by Rutherford backscattering spectrometry of the Si pieces. The amount of sputtered material could be evaluated by determining the colour change of the Ta2O5/Ta strips. The colour of the oxide on the metal is very sensitive to the oxide thickness. It is shown that under the conditions applied, the treatment is not homogeneous. A larger amount of Ar is implanted at the trench top side compared to the trench bottom. The trench side walls are hardly affected by ion implantation. The sputter results are different from the implantation ones. It is shown that the trench bottom is most affected by sputtering, considerably more than the top side. The inner side walls of the trenches are for all aspect ratios also hardly affected by sputtering. The results are explained in terms of the sheath dimensions and the expansion of the sheath as well as by the three-dimensional geometry of the sample holders.