First principles-based screening method for resistivity scaling of anisotropic metals

The resistivity scaling of metals is a crucial factor for further downscaling of interconnects in nanoelectronic devices that affects signal delay, heat production, and energy consumption. Here, we present a screening method for metals with highly anisotropic band structures near the Fermi level with the aim to select promising materials in terms of their electronic transport properties and their resistivity scaling at the nanoscale. For this, we consider a temperature-dependent transport tensor, based on band structures obtained from first principles. This transport tensor allows for a straightforward comparison between different anisotropic metals in nanostructures with different lattice orientations. By evaluating the temperature dependence of the tensor components, we also find strong deviations from the zero-temperature transport properties at standard operating temperature conditions around room temperature.Comment: 25 pages, 8 figure

Resistivity scaling model for metals with conduction band anisotropy

It is generally understood that the resistivity of metal thin films scales with film thickness mainly due to grain boundary and boundary surface scattering. Recently, several experiments and ab initio simulations have demonstrated the impact of crystal orientation on resistivity scaling. The crystal orientation cannot be captured by the commonly used resistivity scaling models and a qualitative understanding of its impact is currently lacking. In this work, we derive a resistivity scaling model that captures grain boundary and boundary surface scattering as well as the anisotropy of the band structure. The model is applied to Cu and Ru thin films, whose conduction bands are (quasi-)isotropic and anisotropic respectively. After calibrating the anisotropy with ab initio simulations, the resistivity scaling models are compared to experimental resistivity data and a renormalization of the fitted grain boundary reflection coefficient can be identified for textured Ru.Comment: 12 pages, 7 figure

Cux_xAl1−x_{1-x} films as Alternatives to Copper for Advanced Interconnect Metallization

Cu$_x$Al$_{1-x}$ thin films with $0.2 \le x \le 0.7$ have been studied as potential alternatives for the metallization of advanced interconnects. First-principles simulations were used to obtain the Cu$_x$Al$_{1-x}$ electronic structure and cohesive energy to benchmark different intermetallics and their prospects for interconnect metallization. Next, thin Cu$_x$Al$_{1-x}$ films were deposited by PVD with thicknesses in the range between 3 and 28 nm. The lowest resistivities of 9.5 $\mu\Omega$cm were obtained for 28 nm thick stochiometric CuAl and CuAl$_2$ after 400$^\circ$C post-deposition annealing. Based on the experimental results, we discuss the main challenges for the studied aluminides from an interconnect point of view, namely the control of the film stoichiometry, the phase separation observed for off-stoichiometric CuAl and CuAl$_2$, as well as the presence of a nonstoichiometric surface oxide.Comment: 24 pages, 7 figure

Al3_3Sc thin films for advanced interconnect applications

Al$_x$Sc$_{1-x}$ thin films have been studied with compositions around Al$_3$Sc ($x = 0.75$) for potential interconnect metallization applications. As-deposited 25 nm films were x-ray amorphous but crystallized at 190{\deg}C with a recrystallization observed at 440{\deg}C. After annealing at 500{\deg}C, 24 nm thick stoichiometric Al$_3$Sc showed a resistivity of 12.6 ${\mu}{\Omega}$cm, limited by a combination of grain boundary and point defect (disorder) scattering. Together with ab initio calculations that found a mean free path of the charge carriers of 7 nm for stoichiometric Al$_3$Sc, these results indicate that Al$_3$Sc bears promise for future interconnect metallization schemes. Challenges remain in minimizing the formation of secondary phases as well as in the control of the non-stoichiometric surface oxidation and interfacial reaction with the underlying dielectrics.Comment: 15 pages, 4 figure

Thickness dependence of the resistivity of Platinum group metal thin films

We report on the thin film resistivity of several platinum-group metals (Ru, Pd, Ir, Pt). Platinum-group thin films show comparable or lower resistivities than Cu for film thicknesses below about 5\,nm due to a weaker thickness dependence of the resistivity. Based on experimentally determined mean linear distances between grain boundaries as well as ab initio calculations of the electron mean free path, the data for Ru, Ir, and Cu were modeled within the semiclassical Mayadas--Shatzkes model [Phys. Rev. B 1, 1382 (1970)] to assess the combined contributions of surface and grain boundary scattering to the resistivity. For Ru, the modeling results indicated that surface scattering was strongly dependent on the surrounding material with nearly specular scattering at interfaces with SiO2 or air but with diffuse scattering at interfaces with TaN. The dependence of the thin film resistivity on the mean free path is also discussed within the Mayadas--Shatzkes model in consideration of the experimental findings.Comment: 28 pages, 9 figure

First-principles modeling of alumina and aluminates high-k dielectrics for non-volatile memory applications

Portable electronic devices call for reliable storage of increasingly large amounts of data. In this framework, the development and optimization of solid state non-volatile memories is crucial for further progress. The predominant non-volatile memory is based on the NAND Flash technology, i.e. Floating Gate. So far, its basic architecture has remained unchanged since its first introduction. This era of “happy scaling” of the physical dimensions is now coming to an end, as Floating Gate devices cannot shrink further due to the severe physical limitations. Two different routes are being followed. On the one hand, the introduction of high-k/metal gate structures in place of traditional SiO2/poly-Si stacks, as it happened for logic, has been proposed for Flash memory. This replacement together with charge trapping medium concept (i.e, the TANOS cells) is considered to be a viable alternative candidate for scaling the NAND Flash technology below the 30-nm node. Naturally, these “higher-k” oxides ought to offer the same properties in terms of stability, controllability, data retention and low defect concentration as SiO2. For this transition to occur, it is hence of prime importance to get a detailed understanding of the properties of the possible material candidates and of their related defects. On the other hand, extensive research efforts are devoted to develop a successor of the Flash technology and to drive the scaling limits beyond the 10-nm node. Among the possible alternatives to Flash technology, resistive switching based memories are currently the object of intense investigation. More particularly, the conductive bridging random access memory (CBRAM) is a very attractive concept. This type of memory could revolutionize the current memory market. By combining the speed of static random access memory, the density of dynamic random access memory and the non-volatility of Flash memory, this emerging technology has the potential to merge the benefits into a single standalone product for future ICT applications. However, this technology still needs maturing in terms of reliability, performance, and production costs to compete with the present technologies. A thorough investigation of the physical mechanisms on which the device relies has to be performed. In order to overcome these concerns, first-principles (ab initio) modeling techniques are choice tools to provide insights into the very nature of these materials, their associated defects the high-k dielectrics in TANOS cells and the detailed driving mechanism of the conduction bridge-formation in CBRAM.(FSA 3) -- UCL, 201

Ab initio screening of metallic MAX ceramics for advanced interconnect applications

The potential of a wide range of layered ternary carbide and nitride Mn+1AXn [an early transition metal (M), an element of columns 13 or 14 of the periodic table (A), and either C or N (X)] phases as conductors in interconnect metal lines in advanced complementary metal-oxide-semiconductor (CMOS) technology nodes has been evaluated using automated first-principles simulations based on density-functional theory. The resistivity scaling potential of these compounds, i.e., the expected sensitivity of their resistivity to reduced line dimensions, has been benchmarked against Cu and Ru by evaluating their transport properties within a semiclassical transport formalism. In addition, their cohesive energy has been assessed as a proxy for the resistance against electromigration and the need for diffusion barriers. The results indicate that numerous MAX phases show promise as conductors in interconnects of advanced CMOS technology nodes

First-principles modeling of intrinsic and extrinsic defects in γ-Al2O3

The electronic properties of a set of intrinsic and extrinsic point defects in gamma-Al2O3 are investigated using quasiparticle calculations within the G(0)W(0) approximation. We find that the electronic signature of atomic vacancies lie deep in the band gap, close to the top of the valence band edge. The introduction of C, Si, and N impurities induces defective levels that are located close to the conduction band edge and near the middle of the band gap of the oxide. The comparison with electrical measurements reveals that the energy levels of some of these defects match with the electronic fingerprint of the defects reported in gamma-Al2O3 based nonvolatile memories. (C) 2010 American Institute of Physics. [doi:10.1063/1.3507385