Massive Ta diffusion observed in Cu thin films but not in Ag counterparts

Abstract

This letter presents an extensive investigation by means of microscopic and chemical analyses finding Ta diffusion in Cu films but not in Ag films. This difference in Ta diffusion persists in all samples containing either Cu/Ta or Ag/Ta interfaces, wherein both a driving force for diffusion and point defects for mediation of atomic movement are present. By referring to atomistic simulation results in the literature, it is plausible that the subtle difference between the Cu/Ta and Ag/Ta interfaces plays a crucial role in differentiating them in making Ta available for diffusion. The energetically favored binding between Cu and Ta assists in liberating Ta atoms from being strongly bound by surrounding Ta atoms, as the bond strength of Cu-Ta is about one third that of Ta-Ta. Hence, the formation of the much weaker Cu–Ta bonds acts as an important intermediate step. Such a mechanism does not exist for the Ag/Ta interface.

Electromigration behavior of Cu metallization interfacing with Ta versus TaN at high temperatures

“…At 600 C where the annealing was performed, the amorphization or Ta-Cu interaction at the Cu/Ta interface is fully developed. 11,20 As we have observed earlier, 4,5,11 the liberated Ta atoms will diffuse through the Cu film via the GBs in amounts readily detectable by RBS. In Cu, GB diffusion starts around half the melting point, i.e., around 400…”

“…Such a grain growth was also observed in the symmetric combinations including both TaN/Cu/TaN and Ta/Cu/Ta (results not shown) as well as in Cu films without any cap. 11 Apart from grain growth leading to a clearly discernable difference in grain structure, no other morphological change in the symmetric combination TaN/Cu/TaN could be concluded from RBS analysis even after annealing at 700-800 C; 4 thus, this symmetric combination is especially interesting for the EM studies below.…”

Designing an anisotropic noise filter for measuring critical dimension and line edge roughness from scanning electron microscope images

“…One of the analysis procedures is to employ image processing techniques by which the boundaries of features are detected and compute the CD and LER from the boundaries. [5][6][7][8][9][10][11][12][13][14] Since SEM images tend to be noisy, it is essential to reduce the noise level before the boundary (edge) detection is carried out. The noise filtering has a direct effect on the accuracy of boundary detection, and therefore, it is critical to employ a noise filter optimized for the detection of feature boundaries in SEM images.…”

“…The noise filtering has a direct effect on the accuracy of boundary detection, and therefore, it is critical to employ a noise filter optimized for the detection of feature boundaries in SEM images. 6 One of the characteristics specific to typical SEM images, which may be taken into account in designing a noise filter, is that features in a circuit pattern tend to have a certain spatial orientation as in a line/space (L/S) pattern. The power spectral density (PSD) of the SEM image of such features shows a different distribution in the direction of orientation, compared to the direction normal to the orientation.…”

“…A fixed filter, e.g., median or spatial averaging filter, may be considered 5 but would not be able to consider the abovementioned characteristics properly. In a recent study, 6 a method of designing an isotropic Gaussian filter of which the cutoff frequency and size are adaptively determined based on the power spectra of signal and noise in a given SEM image was proposed and tested with L/S patterns. A requirement in this design is that the signal and noise powers passed through the filter are equalized.…”