In sputtering and ion-assisted etching processes, the material removal rate is quantified through the use of the sputtering yield, which depends on the energy and the incidence angle of the ions bombarding the surface. The sputtering yield is defined as the number of surface atoms removed per ion impinging on the surface. The most widely used expression for the sputtering yield assumes that it is a product of separable functions of the ion energy and incidence angle and is proportional to the square root of the ion energy and f(Φ), a function that attempts to capture the dependence of the yield on the ion incidence angle, Φ, measured with respect to the surface normal. The ion incidence angle dependence of the etching yield is an important kinetic parameter that significantly affects feature profile evolution. In processes where the surface receives both energetic ion bombardment and reactive radical species that alter the cohesive energy of the atoms near the surface, the concept of the sputtering yield is adopted with the implicit understanding that the yield depends not only on the ion energy and incidence angle but also on the material composition near the surface. Such is the case in nearly all plasma etching processes where material removal is through ion-assisted etching or chemical sputtering. For example, when Si is etched by exposing its surface simultaneously to Ar⁺ ions and halogen atoms such as F or Cl, the etching yield, Y, may be modeled as a function of the halogen surface coverage, in addition to the ion energy and incidence angle. In this case, quantification of the yield requires measurement of the surface composition, which is very difficult to determine. An alternative approach is to infer the surface composition by varying the reactive radical flux and measuring the etch rate and the yield as a function of the reactive radical flux. These measurements are easier to perform but are nevertheless non-trivial and best conducted in high vacuum where well characterized beams of ions and reactive neutrals are made incident onto the substrate. Under these well-characterized conditions, the etching yield is studied as a function of the ion and neutral fluxes as well as the ion energy. The fundamental problem in using beam experiments to extract kinetic parameters is that these experiments cannot easily reproduce the composition of the reactive species flux in plasmas generated from gas mixtures used in technologically important applications. Thus, kinetic parameters derived from beam experiments may not be applicable to systems where the source of ions and reactive neutrals is different. It may be tempting to reproduce the composition of realistic plasma mixtures in high vacuum beam experiments but such studies are limited by the ability to produce beams of arbitrary radicals and ions.

Our motivation is to develop a method that allows the determination of the ion incidence angle dependence of the ion-enhanced etching yield easily in an arbitrary plasma reactor using realistic plasma conditions. In this presentation, we report a technique to measure the dependence of the etching yield on the ion incidence angle by examining cross-sectional scanning electron micrographs (SEM) of features etched under realistic plasma conditions in an arbitrary plasma reactor. The idea for the technique described herein is based on the observation that ions bombarding the surface of a semi-circular shaped feature impinge on various points along the feature at different angles that span the range from normal incidence, Φ = 0^o, to grazing incidence, Φ = 90^o. Thus, the technique is based on measuring the etch rate as a function of position along the walls of features that initially have nearly semi-circular cross sections. These initial feature shapes can be easily obtained by wet or isotropic plasma etching of holes patterned through a mask. The etch rate as a function of distance along the feature profile provides the etching yield as a function of the ion incidence angle. The etch rates are measured by comparing digitized SEM cross-sections of the features before and after plasma etching in gas mixtures of interest. We have applied this technique to measure the ion incidence angle dependence of the Si etching yield in HBr, Cl₂, SF₆ and NF₃ plasmas and binary mixtures of SF₆ and NF₃ with O₂. Advantages and limitations of this method will be discussed.