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3.2.2 Anisotropic Etching and Deposition

For anisotropic processes the etch or deposition rate depends on the growth direction. This is the case for reactive ion etching as well as for PVD and CVD processes which require detailed modeling and calculation of the locally varying etching and deposition rates. Therefore they will be considered in own chapters later. For less complex dry etching or deposition processes with some directionality of the incident particles, simplified models are sufficient. Under the context of morphologically based physical modeling, such processes with a non-isotropic but uniform rate are called ``anisotropic''. The structuring element for such processes will take the shape of an ellipsoidal. The sphere used for the isotropic process from above represents a special case of the ellipsoidal. Due to the anisotropic behavior of the growth or etching velocities the contour of the rate vectors originating from a certain surface position will form an ellipsoidal whose diameters reflect the growth velocities which vary according to the orientation.

Figure 3.10: Structuring element for anisotropic etching.

Fig. 3.10 shows an anisotropic structuring element applied under the same conditions as in Fig. 3.8. The anisotropic model is suited for the simulation of an etching step where constant vertical and lateral etching rate can be assumed. The resulting structuring element is a rotational ellipsoid with a vertical axis of rotation.

With the same selectivity of the etchant and the same initial geometry as used for the simulation shown in Fig. 3.8, the underetching for such kind of process is less pronounced than for the isotropic case. This can be observed in Fig. 3.10, where the simulation which started with one structuring element on the left hand side is completed by applying the ellipsoidals to the complete surface. The lower underetching and the reduced lateral broadening is the main reason, why dry etching processes are preferred for transferring mask patterns to the underlying layers.

Figure 3.11: Structuring element for anisotropic deposition.

Fig. 3.11 shows the analog procedure for an anisotropic deposition step. If a vertical directionality of the deposited particles is assumed, the vertical film growth is faster than the lateral one. This is mirrored in the structuring element emerging as rotational ellipsoidal, as depicted on the left hand side of the figure. The final structure on the right hand side, comparable with the example for isotropic deposition in Section 3.2.1, shows the lower sidewall coverage for the anisotropic process. If the goal is to close the feature, a longer deposition time will be necessary for the anisotropic process with respect to the isotropic one and the film thickness at the top surface at the time of feature closing will be significantly larger than for an isotropic process.

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