7.3 Chemical Vapor Deposition and Reaction Kinetics

High pressure chemical vapor deposition CVD of Tungsten is used for a Ti/TiN/W plug fill process. The geometry results from an initial low pressure deposition of a TiN barrier layer into the via. This physical vapor deposition PVD process is determined by ballistic transport of the sputtered Ti particles. For the subsequent high pressure CVD process it is assumed that W is reduced from $\mathrm{WF}_{6}$ using $\mathrm{H}_{2}$ and forming $\mathrm{HF}$ as by-product. The three gas species diffuse in the via and the reduction takes place at its surface. Depending on the diffusion coefficients and the reaction rates a steady state of the gas distribution is reached leading to a depletion of $\mathrm{WF}_{6}$ at the bottom of the via and to non-uniform deposition rates. This results in a characteristic overhang in the layer profile. The simulated structure is located at an off center position of the wafer. Thus the TiN layer, formed by sputter deposition prior to the Tungsten CVD is strongly asymmetric requiring the rigorous three-dimensional simulation of the CVD film formation.

The chemistry model is calculated on the mesh with AMIGOS [128] which provides an analytic interface for discretizing and solving differential equations. Figure 7.16 shows the mesh of the cylindrical via and the $\mathrm{WF}_{6}$ concentration. A cross-section of the mesh is depicted in Fig. 7.17. A different mesh with a highly refined region near the boundary and in the interior of the via was generated by constructing a non-uniform mesh point distribution derived from the boundary vertices (Fig. 7.18).

Figure 7.15: Flow diagram for the high pressure CVD model.

The calculated deposition rates are further used to advance the structure surface through topography simulation with ETCH3D. As shown in Fig. 7.15 the complete setup consists of several tools which are directly linked to allow a fully automatic simulation sequence for as many iterations as desired [126]. After extracting the surface of the initial geometry, a three-dimensional mesh of the gas domain above the considered structure is generated. The differential equations describing the mass transfer and the reaction kinetics are set up and evaluated with AMIGOS on this unstructured mesh. The resulting deposition rates are transfered to ETCH3D. The topography simulator controls the time step for the surface propagation which is especially important during the formation of voids. Underestimating the size of such a void is avoided by reducing a too large time step so that the first closure of the void can be observed. The surface of the resulting cellular geometry is extracted and the procedure is repeated for each time step. The parameters for the meshing tool and the description of the rate model are set up in control files and remain unchanged during all time steps. In this way the process runs fully automatic without any user interaction.

Figure 7.16: A cylindrical via, mesh with 7324 elements

Figure 7.17: A cross-section of a cylindrical via with a uniform mesh point distribution, 7324 elements.

Figure 7.18: The cross-section of the same cylindrical via meshed differently shows the non-uniform distribution of mesh points, 75720 elements.

Figure 7.19: The volume meshes used for the continuum transport model and the corresponding three-dimensional film profiles for a sequence of time steps of a Ti/TiN/W plug fill process. The profiles show from bottom to top the initial circular via, the PVD TiN layer formed by sputter deposition and the CVD tungsten layer.

The required CPU time on an Alpha-station 600/333 for the example shown in Fig. 7.19 is approximately 60 minutes for the complete, automatically controlled simulation sequence, including surface extraction, meshing, calculation of the deposition rates, time step control, void detection, and surface propagation. Depending on the size of the structure between 10.000 and 30.000 tetrahedra were used for the continuum transport model. The same model was calculated on a damascene structure and the resulting $\mathrm{WF}_{6}$ concentration is shown in Fig. 7.20.

Figure 7.20: Iso surfaces of the $\mathrm{WF}_{6}$ concentration in a damascene structure, mesh with 18148 elements.