Ion orbits in plasma etching of semiconductors

Fabrication of high-speed semiconductor circuits depends on etching sub- micron trenches and holes with straight walls, guided by sheath-accelerated ions which strike the substrate at a normal angle. Electrons accumulate at the non- conductive entrance of each trench, charging it negatively and preventing the penetration of electrons to the bottom of the trench. This "electron shading" effect causes an ion charge at the bottom which is well known to cause damage to thin oxide layers. In addition, the deflection of ions by electric fields in the trench can cause deformation of the trench shape. To study this effect, we have calculated the ion orbits self-consistently with their charging of the trench walls. We find that a) the orbits depend only on the electric fields at the entrance and are sensitive to changes in the shape of the photoresist layer there; b) there is an "ion shading" effect that protects part of the wall; and c) the number of ions striking the wall is too small to cause any deformation thereof.