Sidewall Texture and Microstructure of iPVD Copper Seed in Narrow Damascene Trenches

Abstract

Impurities incorporated into copper during electrochemical deposition have long been postulated to inhibit grain growth in narrow interconnects. In this article we examine an alternative possible origin for the microstructural differences observed between narrow and wide lines. Specifically, the iPVD Cu seed layers in wide (1 μm) and narrow (70 nm) lines differ significantly in microstructure and texture, as analyzed by transmission electron microscopy and electron diffraction. The seed in the wide lines has a polycrystalline microstructure similar to that seen in blanket films, with strong 111 texture in the field and sidewalls, and 112 texture at the bottom of the trench. Although the field between the 70 nm lines is also strongly 111-textured, the seed on the sidewalls and bottom of the trenches are largely untextured and, in fact, not entirely crystalline. This results in a higher energy, metastable microstructure which can contribute an additional driving force for the rapid nucleation and recrystallization of polycrystalline copper in the trenches before the overburden can transform and force the creation of bamboo grains in the lines.

Rapid trench initiated recrystallization and stagnation in narrow Cu interconnect lines

“…10 Several publications, however, have suggested that in narrow lines, high dislocation density, seed layer texture, and minimization of the Cu-Ta interfacial energy cause coarsening of grains in the trenches prior to the overburden. 8,11,12 In this work, TEM analysis of samples during and after the transformation is presented supporting the trench-initiated recrystallization hypothesis, and goes further to show that the microstructure inside the trenches is stable well before the transformation in the overburden is complete.…”

“…23 In our previous work, 11 we showed that the seed texture in narrow lines is random, while the field is still 111-textured, as would be expected in films. After plating, the overburden above the trenches is also 111-textured, despite the random orientations found on the sidewall seed.…”

“…This evidence supports the hypothesis that the trenches initiate transformation of the Cu microstructure, as previously suggested by our group and others. 8,11 The crystallographic orientation of these trench initiated grains with respect to the trench sidewall can indicate a potential underlying driving force which caused them to grow. Figure 1(b) shows the crystal orientation maps of these trench initiated grains, color coded (see stereographic triangle as key) according to the crystal direction normal to the sidewall.…”

Modeling the copper microstructure and elastic anisotropy and studying its impact on reliability in nanoscale interconnects

“…Primarily, prior works (Oh et al 2013;Heryanto et al 2011;Weide-zaage 2012;Wu et al 2010;O Brien 2013;Hommel et al 2002;Paik et al 2004) have considered crystalline copper and isotropic material properties for reliability simulations. A clear dependance between the copper microstructure in nanoscale interconnects, void formation kinetics, and electromigration statistics has been established through experiments (Cao et al 2013).…”

Impurities in the Electroplated sub-50 nm Cu Lines: The Effects of the Plating Additives

“…The texture of the Cu seed on the sidewall of the narrow trenches was recently found different from the ones in wide trenches, which could potentially explain the difference in grain growth. 10 In addition, an increased incorporation of nonmetallic impurities (C, O, S, Cl) in the Cu narrow lines during electroplating has been long observed. 9,11,12 A higher impurity level was known to hinder the Cu grain growth in blanket films [13][14][15][16] and thus could potentially account for the difficulty of grain grown in narrow lines as well.…”