In general, topography simulation can be split into two parts. The first step is responsible for the assignment of the local rates, the second for the surface propagation. Thereby the local rates can be uniform, as for the isotropic and anisotropic processes from Section 3.2 or inhomogeneous, comparable with the resist development from Chapter 5. Low-pressure processes clearly belong to the second type of modeling. In contrast to resist development where the local rates are supplied by an external lithography simulation, the local rates for low-pressure processes have to be calculated within the topography simulator which has to determine the visibility conditions according to the available geometry. In consequence assigning the limits of possible particle sources with respect to the local position in the feature is extremely essential for low-pressure processes. Particle sources include sputter targets and the reactor chamber in general as well as the feature surface which is source of reflected or re-emitted particles.
For this reason, three-dimensional simulation is of significant importance for these types of processes which strongly depend on the geometry of the considered structures. Since two-dimensional simulations only allow the formulation of boundary conditions for either radially symmetric structures of infinitely long trenches, only a rigorous three-dimensional approach as presented here permits the simulation of arbitrarily complex three-dimensional structures starting with contacts of varying length/width ratios and L-shaped trenches.
In order to give a comprehensive overview of the simulation of the considered processes, the chapter starts with a general derivation of the model for low-pressure processes in Section 6.1, including transport kinetics (Section 6.1.1), distribution functions for the impinging and reflected particles in Section 6.1.2, inclusion of effects on reactor scale (Section 6.1.3), and the formulation of the rate equations (Section 6.1.4). Finally the adaptions for the surface propagation will be shown in Section 6.1.5.
The second part of the chapter is dedicated to three typical low-pressure processes (Section 6.2), namely, reactive ion etching (Section 6.2.1), sputter deposition (Section 6.2.3), and plasma deposition (Section 6.2.2).