Abstract
In situ transport measurements on epitaxial 7.6-nm-thick Co(0001)/Al2O3(0001) films with and without Ti and TiN capping layers during O2 exposure are used to investigate the effects of surface chemistry on electron scattering at Co(0001) surfaces. The Co sheet resistance Rs increases with increasing thickness dTi and dTiN of the Ti and TiN capping layers, saturating at 8% and 31% above the uncoated Co(0001) for dTi > 0.2 nm and dTiN > 0.1 nm, respectively. This increase is attributed to electron scattering into local surface states, which is less pronounced for Ti than TiN. In situ resistance measurements taken during a continuously increasing O2 partial pressure from 0 Pa to 40 Pa indicate a relatively steep 24% increase in Rs at an exposure of ∼50 Pa s, which can be attributed to Co surface oxidation that leads to atomic level roughness and a decrease in the electron scattering specularity p. Ti and TiN cap layers with dTi ≥ 0.5 nm and dTiN ≥ 0.13 nm exhibit no resistance change upon air exposure, indicating suppression of Co oxidation. These results indicate a promising Co–Ti interface with an electron scattering specularity of p = 0.4–0.5, which is retained during oxygen exposure, while, in contrast, electron scattering at the Co–TiN interface is completely diffuse (p = 0), suggesting that Ti barrier layers facilitate higher-conductivity Co interconnects than TiN barriers, as long as the Ti layer is sufficiently thick (dTi ≥ 0.5 nm) to suppress Co oxidation.
Highlights
We scitation.org/journal/adv have recently reported the growth of epitaxial Co(0001) layers and shown that oxygen exposure causes a resistance increase, which is attributed to a transition from partially specular (p1 = 0.55) to diffuse (p1 = 0) surface scattering upon oxygen exposure,33,34 similar to the reported effects on Cu(001) surfaces
We report on the electron scattering at the Co(0001) surface including the effects of surface oxidation, Ti capping layers, and TiN capping layers on the resistivity
All Co layers in this study have the same nominal thickness of 7.6 nm and are epitaxial Co(0001) layers as confirmed by X-ray diffraction (XRD) methods5,28,40,41 including ω–2θ and φ scans of the Co 0002 and 101 ̄0 reflections, indicating the epitaxial relationship: Co[0001]∥Al2O3[0001] and Co[101 ̄0]∥Al2O3[112 ̄0] as we have previously reported and described in detail in Ref. 33
Summary
Electron transport in metallic conductors at the nanometer scale is an area of considerable interest to the semiconductor industry because the resistivity of narrow interconnect wires increases signal delay and power consumption in integrated circuits. At wire dimensions near or below the bulk electron mean free path, electron scattering at surfaces, grain boundaries, and surface roughness cause the resistivity to increase well above the bulk value. The resistivity size effect in Co is of particular interest because Co is currently used in the first few metallization levels for the narrowest interconnect lines by multiple integrated circuit manufacturing companies, replacing the previous Cu and W metallization schemes due to a number of advantages, including (1) a lower melting point in comparison to W, allowing defect healing and grain growth at feasible annealing temperatures, (2) a larger electromigration resistance in comparison to Cu, and (3) thinner and more conductive barrier/adhesion layers, allowing greater volumes of metal fill and lower vertical via resistances.. We. scitation.org/journal/adv have recently reported the growth of epitaxial Co(0001) layers and shown that oxygen exposure causes a resistance increase, which is attributed to a transition from partially specular (p1 = 0.55) to diffuse (p1 = 0) surface scattering upon oxygen exposure, similar to the reported effects on Cu(001) surfaces.. Scitation.org/journal/adv have recently reported the growth of epitaxial Co(0001) layers and shown that oxygen exposure causes a resistance increase, which is attributed to a transition from partially specular (p1 = 0.55) to diffuse (p1 = 0) surface scattering upon oxygen exposure, similar to the reported effects on Cu(001) surfaces.7,27–29 These results suggest that the scattering specularity of the Co surface may be intentionally modulated with appropriate capping layers, which motivates the study presented here. TiN caps protect Co oxidation but result in a 55% resistance increase in comparison to bare Co due to a transition to diffuse scattering at the Co–TiN interface, indicating that Ti liners provide a conductivity benefit over TiN liners for Co metallization
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