A Process Model for Thin Film Deposition by Sputtering: Study for Bottom Coverage of Sub-Micron Contact Holes
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C. C. Fang and V. Prasad Process Modeling Laboratory, State University of New York at Stony Brook, NY 11794-2300.
ABSTRACT Titanium (Ti) is widely used to improve the contact resistance with the underlying substrate in contact or via filling applications. As the critical dimensions of the electronic devices fall below 0.5 micron and the aspect ratio of the contact holes increases, it becomes more difficult to obtain a good coverage at the bottom of the contact. In our continuing effort to develop a better technology for the metallization of sub-micron liners [ 1, 2] using the sputtering process, we have constructed a feature scale model and have initiated an extensive study on the coverage of the contact liners. Our objective is to search for an alternative process technique to replace the current, widely-used collimated sputtering with the help of a realistic model that accounts for all important deposition phenomena such as the effect of the working pressure and the ion bombardment. Since the mean free path of the deposited atoms, particularly in a low pressure system, is comparable to the surface feature of the substrate, particle methods are more appropriate for the modeling of thin film formation phenomena and predictions of microstructures than the continuum approach. This is specially true in the case of sputterdeposition. In this paper, a simple two dimensional molecular dynamics model is used to predict the growth of the thin film. It allows us to study the effects of the physical parameters such as working pressure and ion energy as well as the influence of the geometric configuration, e.g., distance between the target and the substrate and the size of the contact hole. Simulations for long-throw/low-pressure sputtering and ionized magnetron sputtering are presented together with the experimental results for collimated sputtering.
INTRODUCTION In semiconductor manufacturing, titanium liners are widely used to improve the contact resistance, while TiN layers are used as a diffusion barrier to prevent the attack of WFE on the underlying substrate layer. As ULSI fabrication processes become highly integrated, the width of the contact/via hole shrinks and the aspect ratios of these contact holes become large. Under such conditions, it becomes extremely difficult to obtain a good coverage at both the bottom and the sidewall. The use of collimated sputter-deposition to produce a sufficient step coverage on a 0.25 micron contact was demonstrated by Joshi and Brodsky [3]. However, since the collimated sputtering is a filtering process, it is slow and inefficient. As demonstrated by Hsieh and Joshi [1] and Fang et al. [2], the use of a collimator with aspect ratio of 2 can result in a 90% reduction in the throughput. The collimators can also get partially clogged with the deposited material which can further reduce the deposition rate requiring that they be removed for cleaning. Particle performance such as flaking from the collimator is also a major concern. Recently, long throw/low pressure [4,5] and ionized m
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