Profile Peculiarities in Semiconductor Etching: Origins & Techniques for Elimination

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As the dimensions of virtually every electronic device are pushed smaller and smaller, the ability to etch semiconductors with very small and very dense patterns becomes increasingly important. Increasing the pattern density naturally requires decreasing the lateral dimensions of the various structures etched. In order to maintain functionality at these sub-micron sizes, it is often necessary to etch structures with very large aspect ratios – on the order of 10-100. Dry etching using plasmas is the most efficient way to etch such structures; wet etch processes are crystal orientation dependent and it can be difficult if not impossible to obtain the desired aspect ratios.

Etching high aspect ratio structures with plasmas offers a unique challenge; maintaining the necessarily high degree of anisotropy without succumbing to a number of different profile defects has proven difficult. This paper will offer a review of the origins of the major profile defects observed and reported in literature thus far, as well as the strategies used to minimize their severity.

The most appropriate system for etching high aspect ratios in silicon is a high density low pressure (HDLP) plasma process, where the ion density is generally at least an order of magnitude larger than the earliest reactive ion etching systems. HDLP systems also offer the additional feature of being able to control ion density and ion energy independently by varying the source and bias powers, respectively. This ability means that the ion flux can be significantly more anisotropic than in the older RIE systems. Pressures have generally been reported in the range of 1-25 mTorr with ion densities on the order of 1012 cm-3.

Halogen-based chemistries are used in virtuall...

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...stretched, the larger sidewall area makes defect control even more difficult. Ions that might have impacted the bottom at a near-normal angle when the aspect ratio is small may end up scattering off the sidewall and contributing to microtrenching when the aspect ratio is large. Further work is necessary to investigate novel methods for controlling bowing, undercutting and microtrenching. Additional additives may be able to help balance passivation more precisely to eliminate bowing and undercut without producing slanted sidewalls or they may increase the directionality of the ion velocity distribution to reduce microtrenching. As plasma etching technology is pushed to the limit by the continuous pressure to miniaturize, improve performance and improve capacity, defect minimization and elimination will remain one of the most important topics in plasma processing.
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