Prev: 7.3.1 Plug-Fill Process

7.3.2 L-Shaped Trench

The deposition of tungsten into an L-shaped trench with an aspect ratio of 1.5:4 illustrates the analysis of a transition between reaction and transport limited process conditions. In the first example given in Fig. 7.7(a) and Fig. 7.7(b) the simulation is performed with the experimental parameters from (7.7). The reaction velocity at the wafer surface is very low, therefore only a small fraction of ${\rm WF\hspace*{-0.2ex}_6}$ is extracted from the gas phase. The diffusion velocity is high enough that the consumed ${\rm WF\hspace*{-0.2ex}_6}$ is immediately replaced by the molecules coming from the plasma above the wafer. At any time there is more ${\rm WF\hspace*{-0.2ex}_6}$ present than can be transformed by the surface reaction given by (7.6). Therefore the film thickness is uniform over the whole structure as can be seen in Fig. 7.7(a) for an intermediate time-step. Due to the larger width of the trench in the corner, it closes last, but no void is formed in the final structure Fig. 7.7(b).

Figure 7.7: Different process conditions for CVD of tungsten into an L-shaped trench. (a) and (b) represent an intermediate and the final profile for reaction limited process conditions, (c) and (d) are analog time-steps for a diffusion limited process.

A completely different situation is given in Fig. 7.7(c) and Fig. 7.7(d) where the reaction rate is artificially increased in the same way as described in Section 7.3.1. The fast reaction causes all ${\rm WF\hspace*{-0.2ex}_6}$ transported to the wafer surface to be immediately reduced to tungsten. Diffusion of ${\rm WF\hspace*{-0.2ex}_6}$ from the reactor chamber is too slow to compensate the ${\rm WF\hspace*{-0.2ex}_6}$ consumption of the surface reaction resulting in the depletion of ${\rm WF\hspace*{-0.2ex}_6}$. This effect leads to a deposition rate which decreases downwards to the bottom of the feature. The resulting ${\rm WF\hspace*{-0.2ex}_6}$ depletion is also observable in Fig. 7.4. An intermediate time-step for the filling of the L-shaped trench [Fig. 7.7(c)] already shows the beginning overhang formation. As can be observed in Fig. 7.7(d) this situation leads to the formation of a void which closes last in the corner region of the trench with the top of the void almost reaching the initial wafer surface.

These simulation results conclude the presentation of the three-dimensional model for the feature scale simulation of arbitrary, multiple chemistry, high-pressure CVD processes in the continuum transport regime. The rigorous three-dimensional approach has been realized by a combination of a cellular surface movement algorithm with an automated three-dimensional unstructured mesh generation and a three-dimensional FEM solver, allowing a very flexible formulation of the involved differential equations and process chemistries. Simulations for different geometries at different wafer positions and for varying process conditions have revealed that special care has to be taken when large wafer sizes lead to non-uniformities of the film profiles. For processes with fast surface reactions leading to diffusion limited deposition conditions the effect of overhang profiles leading to void formation is especially pronounced. Special emphasis has been put on the time-step control when the closure of voids is observed. Formation, size, shape and especially the location of the top of the void are of special interest, when following process steps such as etching or chemical mechanical polishing (CMP) may lead to opening of the vias thus resulting in low quality or failure of the device.

Prev: 7.3.1 Plug-Fill Process