Effects of Mask and Necking Deformation on Bowing and Twisting in High-Aspect-Ratio Contact Hole Etching

Abstract

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.

Plasma etching of high aspect ratio features in SiO2 using Ar/C4F8/O2 mixtures: A computational investigation

“…Bowing primarily results from the change in the acceptance angle of incident ions into the feature due to erosion of mask material and subsequent reflection from the facets of resist, diffusive particle scattering within the feature or scattering from rough surfaces. 2,6 Maintaining CDU requires minimizing pattern distortion, in which an initially circular mask opening is not preserved through the depth of the feature, evolving into ellipses or other noncircular shapes. [7][8][9] CER originates from nonuniformities in the mask due to stochastic processes during etching or from the lithography-development process.…”

“…9,10 Maintaining CDU also requires minimizing twisting, where the etch does not proceed strictly normal to the surface but deviates from the normal, and avoiding feature-to-feature variations in the total etch depth. 6,11 Many of these problems associated with CDU can be attributed to the stochastic nature of fluxes of radicals and ions incident into adjacent features. With the CDs of features now less than 10 nm and the mask opening having a commensurately smaller area, the number and variety of particles incident into adjacent features is subject to statistical noise.…”

“…Nonuniform necking was found to cause an imbalance in the ion flux to the bottom of the feature, resulting in twisting irrespective of charging. 6 Results from a voxel-slab model, developed to investigate contact hole etching in SiO 2 , indicated that physical damage was reduced by maintaining a critical thickness of the overlaying polymer. 17,34 Bowing during the etching of HAR features was found to result from excessive fluxes of F and O radicals to the sidewalls according to the results of a line-of-sight profile simulator.…”

Determination of deep hole structure for advanced semiconductor devices analyzed by transmission x-ray scattering

“…For precise control etching processes, it is strongly required to measure and evaluate etched shapes precisely. Existing semiconductor metrology methods, such as CD-SEM 4 and OCD 5 , are not suitable for measurement of HAR structures due to the nature of their probes.…”

Reduction in the Diameter of Contact Holes with a High Anisotropy and Aspect Ratio

“…They gave evidence supporting that the redeposition of particles sputtered from the mask slope on the contact-hole sidewall results in sidewall necking, and the secondary etching of the sidewall by ions reflected from the mask slope contributes to the formation of sidewall bowing. Miyake et al 17 investigated the effect of mask tapering on bowing during high-aspect-ratio contact-hole etching. They observed that bowing and mask etching rate increased with decreasing taper angle of the mask.…”