THE INFLUENCE OF SUBSTRATE TOPOGRAPHY ON ION-BOMBARDMENT IN PLASMA-ETCHING

The reactive ion etching of small features in a planar substrate produces a wide range of profiles which depend on many factors. Of underlying importance is the energetic positive ion flux incident on any substrate exposed to the plasma. Therefore, the nature of this directed ion flux is of interest when trying to understand the microscopic nature of dry etching where ion-enhanced surface chemistry and physical sputtering contribute to the etching performance. This paper uses a simple model to examine the influence of the local electric fields at the substrate on ion trajectories. It is found that the presence of trenches in the substrate distorts the local electric field, and the nature of this field is such that positive ions arriving normal to the substrate surface are deflected towards the sidewalls of the trench. The deflection is largest for those ions arriving near the trench sidewalls, the magnitude of which is calculated to be a few degrees. From the results of this investigation a new technique is proposed where in principle the positive ion flux can be influenced by externally applied potentials in an attempt to control the ion trajectories.