Studies on treatment homogeneity of plasma immersion ion implantation by an optical method

The lateral homogeneity of plasma immersion ion implantation (PIII) of three-dimensional objects has been investigated by a simple optical method. It is based on interference colours of thin oxide films of transition metals, such as titanium or tantalum. When these oxides are treated by PIII, their thickness changes as a result of sputter etching. As a result, their colour changes. It is, therefore, possible to judge the lateral treatment homogeneity at a glance with the naked eye. With the example of a model system it is demonstrated how this method works. Cylinders, either compact or with a slot, were partially covered with a strip of tantalum with tantalum oxide on top, and then treated by argon PIII. Following the changes in the colours, gradients in treatment were found on the tops and on the outer sides. In contrast, the inner walls of the cylinders with slots were hardly changed, however, the bottoms of the slots were severely affected. In some cases, the oxide was entirely lost. These findings are explained by the shape of the plasma sheath from which the ions are accelerated and by the resulting ion trajectories.