Line wiggling due to plasma-induced film stress and prevention by surface roughness modification

To investigate the impact of plasma-induced stress on line wiggling, pattern deformation was compared with and without hardmask exposure to the plasma during etching of porous SiOCH (p-SiOCH) with TiN hardmasks of 28 nm pitch. Wiggling occurred in patterns with low residual hardmask stress only upon plasma exposure. TiN films exposed to plasma showed a global increment in compressive stress, which explains the onset of wiggling in samples with low or tensile residual stress. These results establish the importance of controlling plasma-induced stress to reduce the risk of line wiggling at future nodes. When plasma exposure damaged the surface and roughened the hardmask, wiggling did not occur. A mechanism is proposed in which surface roughness prevented line collapse. Numerical analysis via the three-dimensional elastic finite element method (3D-FEM) demonstrates that elevated surface roughness increases the stress threshold for deformation and can prevent line wiggling. To validate the proposed mechanism, the selectivity of the p-SiOCH etch to TiN was increased to reduce plasma-induced surface damage, and wiggling was observed only with a smoother TiN surface. It is, therefore, concluded that plasma-induced stress and surface roughness are critical parameters for the selection of a back end of line hardmask material and etch package to mitigate line wiggling.