Ion-Shading Effects During Metal Etch in Plasma Processing

Self-consistent computations of electric fields (E-fields) and ion orbits inside trenches were done in order to verify Hashimoto's hypothesis of damage that is induced during plasma processing. In his well-accepted theory, Hashimoto proposed a mechanism for electron-shading damage, whereby the photoresist at the tops of trenches and vias collects a negative charge from the thermal electrons, creating an E-fleld, which prevents electrons from reaching the trench bottom, where collector is located. The sheath E-field accelerates the ions and drives them straight into the trench where they impinge on the collector, and charge it positive if it is isolated. In the computations presented in this paper, it is shown that ion orbits depend only on the E-fields at the entrance and are sensitive to changes in the shape of the photoresist layer there. In addition to the electron-shading mechanism, there is an "ion-shading" effect that protects part of the trench walls, and the number of ions that strike the wall is too small to cause any deformation of the walls.