Novel selective cyclic etching of SiN over SiO2 via the formation and desorption of ammonium fluorosilicate was developed. The formation of ammonium fluorosilicate was observed using X-ray photoelectron spectroscopy after hydrofluorocarbon-based radical exposure. Etching of SiN was observed after ammonium fluorosilicate was removed by thermal annealing. Cyclic etching tests were carried out by repeated radical exposure and thermal annealing. The etching depth increased on increasing the number of cycles. It was found that the cyclic etching is self-limiting because the etching depth does not depend on the radical exposure time but on the number of cycles.

The hypothetical mechanism of bottom profile degradation, such as distortion and twisting in high aspect ratio feature etching, was verified based on the pattern transfer observation of etched pattern. The authors mainly focused on trench pattern sample to make the investigation easier, that is, direct observation of the sidewall roughness, using an atomic force microscope, as well as analysis of the depth dependence of pattern deformation in high aspect ratio trench etching. Using Fourier transformation analysis for the trench sidewall roughness, it was found that lower spatial frequency component of the mask's sidewall roughness is amplified at the bottom region of the trench and that higher spatial frequency component of over 10 μm−1 disappears. However, the higher spatial frequency component is transferred directly to the upper sidewall of the trench. The observation of the pattern deformation profile, as a function of etch depth, revealed that the ratio of line width roughness to line edge roughness decreases linearly with increasing etch depth. This indicates that the deformation mode changes from “roughness” to “wiggling” as a function of etch depth. Also, the twisting depth, as determined by the cross section images, was found to be shallower when using lower wafer bias etching condition. Based upon these results, and previously proposed mechanisms, the authors conclude that the mask deformation should be considered as one of the root causes when bottom deformation occurs. This is due to the imbalance between ion flux and deposition amount created by mask asymmetric profile. In addition, the etching profile and etching direction should be affected by this phenomenon under polymer-rich process condition. In order to verify our assumed mechanism, the relationship between mask deformation degree, and bottom distortion and twisting amount was also evaluated by using different pitch hole patterns. As a result, the authors determined that the distortion is improved by 22% and the twisting is improved by 20% when the mask deformation is reduced by 50% with using a wider pitch pattern.

The effects of mask characteristics on high-aspect-ratio contact hole (HARC) etching profiles were investigated. The evaluation of etching profiles produced with different taper angle masks confirmed that the bowing amount and mask selectivity worsened with decreasing mask taper angle. The relationship between mask taper angle and distribution of scattered ion flux on the sidewall of a tapered mask was calculated. The scattered ion flux was heavily concentrated in the upper part of the sidewall in the case of a tapered mask, and this was considered to be the main cause of the bowing formation. Direct observation of an etched sidewall by atomic force microscopy (AFM) revealed that the roughness of the necking was strongly related to the roughness of the bottom part of the etched sidewall. To evaluate the dependence of twisting on nonuniform necking, the incident ion flux in a circular hole was calculated. As a result, in the case of nonaxisymmetric necking, an imbalance of ion flux in the bottom of the hole appeared and broke the etching symmetry in the bottom part of the hole, causing twisting. In addition, the probability of twisting was found to increase with increasing necking growth rate irrespective of mask electrification. Therefore, mask deformation and nonuniform necking in the upper part of the sidewall during HARC etching are considered the main factors causing bottom degradation. Accordingly, a vertical and nondeformed mask is very important for a smaller critical dimension (CD) and HARC etching.

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