Paper PS-FrM5
Effect of Charging on Twisting of Extremely High Aspect Ratio Features in Plasma Etching*

Friday, October 19, 2007, 9:20 am, Room 606

The plasma etching of extremely high aspect ratio features (eHAR) provides challenges to maintain critical dimensions. As aspect ratios approach and exceed 100, undesirable behaviors have been observed, such as extreme tapering and twisting of features. Twisting is the sometimes sudden turning of a via or trench from the vertical to a side angle. These behaviors often occur randomly. For example, of three adjacent features, only one may display the behavior. The behaviors are also sometimes associated with location on the die, such as near an open area, or location on the wafer, being more likely near the edge of the wafer. Current theories on the source of twisting and errant behavior focus on charging effects, anisotropic ion energy distributions and the randomness of the composition of the ion and radical fluxes as the opening of the feature approaches only a few tens of nm. In this talk, processes leading to twisting of eHAR features in polymerizing (etching of SiO₂ in fluorocarbon plasmas) and in non-polymerizing (etching of Si in chlorine plasmas) chemistries will be discussed using results from a computational investigation. The Monte Carlo Feature Profile Model (MCFPM) was modified to include the effects of charging by electrons and ions, including solution of Poisson's equation and conduction current through solid materials. We found that twisting largely results from a confluence of factors, including trapping of charge in polymer, randomness in the neutralization of charge due to the stochastic-like current that enters small features and location of the feature on the die. For example, trenches adjacent to open areas are influenced by charging in the open field, an effect that diminishes with distance from the open field. The twisting effects are generally diminished but not eliminated with increasing voltage.

*Work supported Micron Technology Inc., Semiconductor Research Corporation and the National Science Foundation.