Electrical resistivity size effect in compound conductors

Zhang, Minghua
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Sundararaman, Ravishankar
Palermo, Edmund
Lu, Jian Qiang
Gall, Daniel
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Materials engineering
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This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute (RPI), Troy, NY. Copyright of original work retained by author.
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The downscaling of modern integrated circuits is facing a major challenge posed by the resistivity size effect in interconnect lines which is mainly due to electron scattering at surfaces and grain boundaries. The resistivity increase and associated signal delay can be mitigated if a suitable material can be found that has a small resistivity size effect and therefore conducts better than Cu at reduced dimensions. Thus, a lot of experimental and theoretical efforts have been made to reduce electron scattering at small dimensions or decrease the thickness of barrier layer. Nevertheless, there is still no clear winner on replacing Cu in the back-end-of-line process and resistivity scaling of many materials are still unknow. Therefore, in this thesis, I perform a series of experiments dedicated to quantifying the resistivity size effect of various compound conductors to pave the road for the prospective barrier-free interconnect integration.The resistivity of metal can greatly increase once its dimension reaches below the phonon-electron scattering mean free path λ of 39 nm in Cu. The increase resistivity including surface scattering and grain boundary scattering is proportional to the ρoλ product where ρo is the bulk resistivity of the material. Therefore, we are going to search for material with lower ρoλ to achieve smaller resistivity increase at small metal pitch. ρoλ is a intrinsic material’s property determined by its electronic structure and changes inversely related to the effective electronic states near to the Fermi level. In principle, the smaller ρoλ would result in a better conductivity at small dimensions. However, we have to make sure the resistivity of the new material is comparable with the know metals, based on which we are mainly searching for material with smaller electron mean free path to reduce the electron scattering contribution from both surface and grain boundaries. In the field of compound conductors, I first focus on Ti4SiC3(0001), a MAX-phase material with a previously predicted ρoλ= 3.1 × 10 16 Ωm2 that is more than two times smaller than that of Cu and exhibits a 2.5 times larger cohesive energy. Magnetron co-sputtering from three elemental sources at 1000 °C onto 12-nm-thick TiC(111) nucleation layers on Al2O3(0001) substrates yields epitaxial growth with Ti4SiC3(0001) || Al2O3(0001) and Ti4SiC3(101 ̅0) || Al2O3(2(11) ̅0), a low and thickness-independent surface roughness of 0.6  0.2 nm, and a measured stoichiometric composition. The room-temperature resistivity ρ increases slightly with decreasing thickness, from ρ = 35.2  0.4 to 37.5  1.1  cm for d = 92.1 to 5.8 nm, and similarly from 9.5  0.2 to 11.0  0.4  cm at 77 K, indicating only a minor effect of electron surface scattering on ρ. The curve fitting based on semiclassical model yields a low λ = 1.1  0.6 at 293 K and λ = 3.0  2.0 nm at 77 K. This provides a great potential for MAX phase materials to be used for high-conductivity narrow interconnect lines in spite of its relatively high resistivity ρo = 35.1  0.4 cm. I also conduct some experiments on isotropic intermetallic compounds. The resistivity ρ as a function of thickness d of epitaxial CuTi(001) and CuAl2(001) layers is measured to quantify the resistivity size effect of these compound conductors and evaluate their promise as a replacement material for Cu in highly scaled interconnect lines. The layers are deposited by magnetron co-sputtering onto MgO(001) substrates and their epitaxy is confirmed by x-ray diffraction -2 scans, ω rocking curves and φ-scans. The surface morphology is quantified by x-ray reflectivity and atomic force microscopy, and the composition measured by photoelectron spectroscopy and Rutherford backscattering. Data fitting of the measured ρ vs d yields room-temperature electron mean free paths λ = 12.5 and 15.6 nm for CuTi and CuAl2, respectively, and bulk resistivities ρo = 19.2  0.8 and 7.7  0.4 μΩ·cm. The overall analysis yields ρoλ benchmark values of 24 × 10-16 and 12× 10 16 Ωm2, respectively, indicating that the conductivity advantage of the evaluated compound conductors against Cu can only be realized if their higher cohesive energies and stability can be exploited to achieve liner-free lines. In my third research thrust, I study the directional conductors with suitably anisotropic Fermi velocity distributions such that they can achieve superior conduction along specific axis. Thus, epitaxial VNi2 layers are deposited onto MgO(001) and their resistivity ρ measured as a function of layer thickness d = 10.5-138 nm to quantify the resistivity size effect. A cube-on-cube epitaxy of the fcc parent structure on MgO(001) leads to two possible layer orientations for orthorhombic VNi2(010) and VNi2(103), resulting in considerable atomic disorder at domain boundaries. In situ ρ vs d measurement yield a bulk resistivity ρo = 46  2  cm and a benchmark quantity ρoλ = (160  10) × 10 16 Ωm2, where λ is the bulk electron mean free path. These values are considerably higher than theoretically predicted, which can be attributed to the two possible atomic arrangements, causing extra electron scattering at domain boundaries. The superior predicted conduction along a specific crystalline direction could not be experimentally verified. The superior anisotropic high conductivity can only be achieved with a single crystalline orientation which necessitates the suppression of any domain boundaries between the two nearly identical orientations. Lastly, a polycrystalline CuTi is fabricated to study the stability of compound conductors on dielectric SiO2. With similar deposition condition with epitaxial layers, polycrystalline CuTi layers show a preferred out-of-plane orientation of 001 direction and a random in-plane orientation. The resistivity scaling of CuTi is only comparable with elemental metals if it facilitates barrier-free interconnect lines. Accordingly, the stability study includes interdiffusion reliability test, thermal stability test and interfacial adhesion test. The overall results indicate that CuTi has a much better stability on SiO2 than Cu and enhances the potential for compound interconnect to achieve a reduced barrier thickness or even barrier-free interconnect.
School of Engineering
Dept. of Materials Science and Engineering
Rensselaer Polytechnic Institute, Troy, NY
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