Resistivity and surface scattering of (0001) single crystal ruthenium thin films

Abstract

The resistivity size effect in nanoscale metals is of both scientific and technological interest, the latter due to its importance to interconnects between transistors in integrated circuits. In this work, the authors report the variation of resistivity with film thickness and with changes in surface scattering of ex situ annealed single crystal Ru thin films grown on sapphire substrates by sputter deposition. The room temperature deposition of SiO2 on the Ru sample surface was observed to increase the resistivity of films that had previously been subjected to annealing in a reducing gas ambient. These overcoated samples were also found to increase in resistivity as a result of an oxidizing anneal and reduce in resistivity as a result of a subsequent reducing gas (Ar + H2) anneal. To a large extent, the surface structure and electron scattering characteristics were found to be reversible between oxidizing and reducing gas anneals. The chemistry and structure of the Ru upper surface was characterized by low energy electron diffraction (prior to the SiO2 overcoat deposition), x-ray reflectivity, x-ray photoelectron spectroscopy, and resistivity measurements. The changes in surface structure and chemistry were related to the changes in the specularity of the Ru surface for electron scattering in the context of the Fuchs–Sondheimer semiclassical model of the resistivity size effect, and in this context a mostly specular metal/dielectric interface is reported.

Epitaxial metals for interconnects beyond Cu

“…8). 54 As expected for the equivalent processing up to this point (no oxide deposition), similar resistivities are observed for all four groups of samples. The next process step for the four groups of samples was room temperature sputter deposition of ~5 nm thick overlayers of the indicated dielectric materials, and this results in a significant increase in film resistivity, which is again interpreted as increased surface scattering induced by the presence of the oxides.…”

“…5, the degradation of surface specularity upon exposure to the ambient air points to the need for development of approaches enabling the controllable optimization of surface specularity to increase conductivity in nm-scale interconnects. To this end, our recent work 54 provides evidence for process-controlled optimization of these effects at SiOx/Ru interfaces, briefly summarized below as motivation for investigation of the impact of other oxides reported in the current study.…”

“…54, several key properties expected to correlate with film resistivity were measured and compared to changes in modeled values for upper surface specularity inferred from measured values of film thickness and conductivity at each of 11 sequential processing steps. 54 In this example, a ~20 nm thick Ru film was first deposited via DC magnetron sputtering onto a 2-inch diameter c-axis sapphire, (0001)Al2O3, wafer maintained at 700 °C within a 4 mTorr Ar environment. After deposition, the wafer was removed from the sputter deposition vacuum chamber and cut into several ~7×7 mm 2 coupons for further processing and characterization.…”

“…However, the average specularity, p, as well as the upper and lower surface specularity values for the lower and upper surfaces, l p and u p , respectively, are constrained by the model to be in the range of p = 0 (fully diffuse scattering) to p = 1 (fully specular scattering). To satisfy these boundary conditions, the larger range of resistivities observed for the samples with MgO/Ru interfaces require changes to the λ and l p values chosen relative to those used in the prior report 54 . Whereas upper surface Ru specularities u p calculated in the prior report 54 assumed λ = 11.0 nm, and l p = 0.0, the same parameters must be changed to 12.5 nm and 0.30, respectively, to produce physically meaningful results in Fig.…”

Resistivity size effect due to surface steps on ruthenium thin films computed with a realistic tight-binding model

“…4. Past experiments [26][27][28] report the room temperature, basal plane resistivity for single crystal ruthenium as 7.6 µΩ•cm and the c-axis resistivity to be 5.8 µΩ•cm. As mentioned previously, the number of moments N T in the KPM calculation was chosen to exactly reproduce the basal plane resistivity.…”