Modeling of Aspect Ratio Dependent Etching in an Inductively Coupled Plasma
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From the data that is available, the overhang test structures show complete forward lag behavior (structures with lower aspect ratios etch fastest) while the via results show a mixture of forward and reverse lag (opposite of forward lag). Figure 2b also shows that the side wall angle is dependent on the aspect ratio. The trend shows that the side wall is more vertical for higher aspect ratio contacts. Figure 3a shows the post etch profile of the overhang test structure. Several features are noted as follows: (1) There is an obvious region on the bottom of the oxide surface where a hump occurs. This is due to low energy ions enhancing the polymer deposition process. Bariya 2 has shown this behavior for simple parallel plate reactors and in Monte Carlo simulation results. Oehrlein et a13 has also shown the effect exists through CHF 3 ECR plasma experiments. (2) Etching begins approximately 30 recessed from the edge of the overhang, while the ion enhanced deposition spans to approximately 60 from the same edge. This result gives the ion angular distribution which is used for the simulations presented in this paper. (3) Polymer deposition thickness is dependent upon the surface material and processing history. It is clear that the thickness is the greatest on the silicon overhang surface and the least on the bottom surface where there is no ion bombardment. Figure 4 shows the polymer deposition thickness measured from the SEMs as a function of time. From this figure, one can clearly see that deposition occurs on the poly silicon overhang as soon as the plasma processing begins. The enhanced deposition region shows a 30 second delay while the non-enhanced region shows a 60 second delay before deposition occurs. Oxide Etch Model The first step in developing a model is to describe ion enhance deposition. The mechanisms which cause the ion enhancement are unknown, but entail the breaking of bonds, activating of inert species, and the increase in activated surface sites. The total reaction probability of deposition species incorporates the probability of adsorption, spontaneous desorption, ion induced desorption, and reaction. These processes are incorporated into one term called the sticking coefficient. In theory this sticking coefficient would be dependent upon surface conditions such as the density of active surface sites, the local temperature, the surface bond configuration, the activation energies for each surface interaction, the surface roughness, etc. To illustrate the complexity of this approach, consider these simplifications: assume that (1) the surface is considerably rough that the density of active sites is a constant; (2) the surface bond configuration is completely described by the activation energy; and (3) the adsorbed species
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