Tik Sun

Classical size effect in oxide-encapsulated Cu thin films: Impact of grain boundaries versus surfaces on resistivity

A methodology is developed to independently evaluate surface and grain boundary scattering in silicon dioxide-encapsulated, polycrystalline Cu thin films. The room-temperature film resistivity for samples with film thicknesses in the range of 27 to 1 65 nm and different grain sizes (determined from approximately 400 to 1500 grains per sample) is compared to existing and empirical models of surface and grain boundary scattering. For the combined effects of surface and grain boundary scattering, the surface specularity parameter p is 0.6±0.2 and the grain boundary reflectivity coefficient R is 0.45±0.03. It is thereby shown that the resistivity contribution from grain boundary scattering is significantly greater than that of surface scattering for Cu thin films having Cu∕SiO2 surfaces and grain sizes similar to film thickness.

High contrast hollow-cone dark field transmission electron microscopy for nanocrystalline grain size quantification

In this paper, we describe hollow-cone dark field (HCDF) transmission electron microscopy (TEM) imaging, with a slightly convergent beam, as an improved technique that is suitable to form high contrast micrographs for nanocrystalline grain size quantification. We also examine the various factors that influence the HCDF TEM image quality, including the conditions of microscopy (alignment, focus and objective aperture size), the properties of the materials imaged (e.g., atomic number, strain, defects), and the characteristics of the TEM sample itself (e.g., thickness, ion milling artifacts). Sample preparation was found to be critical and an initial thinning by wet etching of the substrate (for thin film samples) or tripod polishing (for bulk samples), followed by low-angle ion milling was found to be the preferred approach for preparing high-quality electron transparent samples for HCDF imaging.

Surface and grain boundary scattering in nanometric Cu thin films: A quantitative analysis including twin boundaries

The relative contributions of various defects to the measured resistivity in nanocrystalline Cu were investigated, including a quantitative account of twin-boundary scattering. It has been difficult to quantitatively assess the impact twin boundary scattering has on the classical size effect of electrical resistivity, due to limitations in characterizing twin boundaries in nanocrystalline Cu. In this study, crystal orientation maps of nanocrystalline Cu films were obtained via precession-assisted electron diffraction in the transmission electron microscope. These orientation images were used to characterize grain boundaries and to measure the average grain size of a microstructure, with and without considering twin boundaries. The results of these studies indicate that the contribution from grain-boundary scattering is the dominant factor (as compared to surface scattering) leading to enhanced resistivity. The resistivity data can be well-described by the combined Fuchs–Sondheimer surface scattering model and Mayadas–Shatzkes grain-boundary scattering model using Matthiessens rule with a surface specularity coefficient of p = 0.48 and a grain-boundary reflection coefficient of R = 0.26.

Relaxation time effects on dynamic conductivity of alloyed metallic thin films in the infrared band

The behavior of nanoscale infrared antenna elements depends upon the dynamic conductivity of thin metallic films. Spectroscopic ellipsometer measurements of noble metal films show that when the product of the incident radiation frequency and the relaxation time is greater than unity, anomalous dynamic electron transport effects occur. In this regime electron scattering increases the conductivity of alloyed metallic films as demonstrated by ellipsometry measurements of films from the Au-Cu system. A binary alloy thin film was fabricated with equal parts of Au and Cu, and the dynamic conductivity was measured to be 300% larger than the high frequency conductivity of pure Au or pure Cu films at wavelengths in the 3–5 μm band. When electronic scattering is reduced, ellipsometer measurements of Au and Cu films taken near 4 K demonstrate that the IR conductivity decreases to 20% of the value measured at 300 K at wavelengths in the 3–5 μm band. Using measured dc relaxation times, a model to explain deviations fr...

Grain Growth and the Puzzle of its Stagnation in Thin Films a Detailed Comparison of Experiments and Simulations

We revisit grain growth and the puzzle of its stagnation in thin metallic films. We bring together a large body of experimental data that includes the size of more than 30,000 grains obtained from 23 thin film samples of Al and Cu with thicknesses in the range of 25 to 158 nm. In addition to grain size, a broad range of other metrics such as the number of sides and the average side class of nearest neighbors is used to compare the experimental results with the results of two dimensional simulations of grain growth with isotropic boundary energy. In order to identify the underlying cause of the differences between these simulations and experiments, five factors are examined. These are (i) surface energy and elastic strain energy reduction, (ii) anisotropy of grain boundary energy, and retarding and pinning forces such as (iii) solute drag, (iv) grain boundary grooving and (v) triple junction drag. No single factor provides an explanation for the observed experimental behavior.

Impact Of Surface And Grain Boundary Scattering On The Resistivity Of Nanometric Cu Interconnects

This work addresses the resistivity increase in Cu interconnects with decreasing line width. Recent published resistivity data for Cu interconnect lines is found to be accurately modeled by a fixed surface scattering specularity parameter, p = 0.52, and a grain boundary reflection coefficient, R = 0.43. In this model, the resistivity contribution from surface scattering at the line top and bottom surfaces, the contribution from surface scattering at the line sidewalls, and the contribution of grain boundary scattering from within the line are simply summed with the phonon and impurity resistivity contributions following Matthiessens rule. The more recent line resistivity reports were found to have reduced impurity contributions to resistivity and larger grain sizes than earlier reports. It is concluded that a significant mitigation of the Cu resistivity increase with decreasing line width is possible, if the grain size can be maintained larger than the electron mean free path (39 nm at room temperature).

Evolution of nanoscale roughness in Cu/SiO2 and Cu/Ta interfaces

Synchrotron x-ray scattering was used to study the evolution of interface roughness with annealing for a series of Cu thin films. The films were encapsulated in SiO2 or Ta/SiO2 and prepared by sputter deposition. Specular x-ray reflectivity was used to determine the root mean square roughness for both the upper and the lower Cu/SiO2 (or Cu/Ta) interfaces. The lateral roughness was studied by diffuse x-ray reflectivity. Annealing the films at 600 °C resulted in a smoothing of only the upper interface for the Cu/SiO2 samples, while the lower Cu/SiO2 interfaces and both interfaces for the Ta encapsulated films did not evolve significantly. This difference in kinetics is consistent with the lower diffusivity expected of Cu in a Cu/Ta interface (compared to a Cu/SiO2 interface) and the mechanical rigidity of the lower Cu/SiO2 interface. As a function of roughness wavelength, the upper Cu/SiO2 interfaces exhibited a roughness decay with annealing that was only 12.5% of that expected for classical capillarity dr...

Grain growth and void formation in dielectric-encapsulated Cu thin films

Grain growth in 40-nm-thick Cu films encapsulated by overand under-layers of SiO2, Al2O3, Si3N4, and MgO was investigated. The films were magnetron sputter deposited onto cooled SiO2/Si substrates in an ultrahigh vacuum purity environment. Ex situ annealing was performed at 400 and 800 °C in 1 atm reducing gas. Films deposited at −120 °C exhibited more extensive grain growth after annealing than films deposited at −40 °C. Films annealed at room temperature had grain sizes less than 35 nm. All films exhibited some void formation after annealing at 400 and 800 °C, but the films encapsulated in Al2O3 exhibited the lowest area fraction of voids. The mean grain sizes of the Al2O3-encapsulated films, as measured by the linear intercept method, were 86 and 134 nm after annealing at 400 and 800 °C, respectively.

Grain boundary and surface scattering in interconnect metals

This work addresses the classical size effect in interconnect metals and presents the theoretical background and quantification of the contributions of grain boundary and surface scattering to the observed resistivity increase in Cu. The results of experimental studies of Cu films and lines are reviewed. The extent to which the experimental data supports the theoretically expected interactions between surface and grain boundary scattering mechanisms will also be discussed.

Resistivity Size Effect in Encapsulated Cu Thin Films

The work addresses the resistivity increase in Cu interconnects with decreasing linewidth. The surface and grain boundary scattering parameters in Cu thin films encapsulated in SiO2 with and without Ta barrier layers are quantified, and grain boundary scattering is shown to be much greater than surface scattering. 21 samples and 17,882 Cu grains were measured to provide the grain size data. This work indicates that significant mitigation of the Cu resistivity increase with decreasing linewidth is possible, if the grain size can be maintained larger than the electron mean free path (39 nm at room temperature).