Control of the Cu morphology on Ru-passivated and Ru-doped TaN surfaces – promoting growth of 2D conducting copper for CMOS interconnects

Prolonging the lifetime of Cu as a level 1 and level 2 interconnect metal in future nanoelectronic devices is a significant challenge as device dimensions continue to shrink and device structures become more complex. At nanoscale dimensions Cu exhibits high resistivity which prevents its functioning as a conducting wire and prefers to form non-conducting 3D islands. Given that changing from Cu to an alternative metal is challenging, we are investigating new materials that combine properties of diffusion barriers and seed liners to reduce the thickness of this layer and to promote successful electroplating of Cu to facilitate the coating of high-aspect ratio interconnect vias and to allow for optimal electrical conductance. In this study we propose new combined barrier/liner materials based on modifying the surface layer of the TaN barrier through Ru incorporation. Simulating a model Cu29 structure at 0 K and through finite temperature ab initio molecular dynamics on these surfaces allows us to demonstrate how the Ru content can control copper wetting, adhesion and thermal stability properties. Activation energies for atom migrations onto a nucleating copper island allow insight into the growth mechanism of a Cu thin-film. Using this understanding allows us to tailor the Ru content on TaN to control the final morphology of the Cu film. These Ru-modified TaN films can be deposited by atomic layer deposition, allowing for fine control over the film thickness and composition.


Description of Models:
All models in this study are based on ε-TaN in the Fe2P crystal structure. [1] The bulk geometry was optimised by relaxing the ionic positions, cell volume and cell shape yielding the following equilibrium lattice parameters which agree closely with experimental results: a = b = 5.23 Å, c = 2.92 Å, α = β = 90° γ = 120°. [2] These calculations were part of a previous study carried out by the authors. [3] Surface models of the low index surfaces (1 1 1), (1 0 0) and (1 1 0) were created from the optimised bulk structure using the Atomic Simulation Environment (ASE) package. [4] As part of the same study, we found that the (1 1 0) surface is the lowest energy surface for TaN and is used as the basis for all TaN and Ru-modified TaN models presented in this work. To accommodate large Cu clusters we use a (2 x 4) supercell with the following dimensions: a = 18.11 Å, b = 23.36 Å, c = 30.48 Å; α = β = γ = 90°. This includes a vacuum region of 12 Å. Ru doping is carried out by replacing a selection of surface Ta atoms with Ru atoms. Dopants are added only in the top layer of the slab to ensure the preservation of the barrier properties of TaN. The doping process and a study of the effects of varying the distribution of dopants in the surface were published as part of a previous study carried out by the authors. [5] The minimum distance between Cu atoms in neighbouring images when adsorbing the original Cu29 structure is 6.41 Å.
Figures S1 and S2 show the 0K and 500K structures of Cu29 on two different Ru 50 surfaces with varying Ru distribution. Ru 50-1 has Ru dopants only at site S (6 coordinate) and Ru 50-4 has dopants at both site S and F (3 coordinate).
Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2021   Figure S6 shows the changes in z-position during the geometry relaxation of Cu29 on Ru 100 .

Discussion of z-plots Ru 100
The Cu structure remains relatively flat during the geometry relaxation, as illustrated by the z-plots in Figure  S6. However, during the MD calculations, Cu atoms not only migrate into the surface layer, but Ru and N atoms also migrate out of the surface, which leads to a mixture of Cu, Ru and N instead of a clean Cu29 structure. Further, the atoms begin to associate and while there are no distinct layers as observed on other surfaces, the mixture is no longer a single layer film. This is illustrated further through the plots in Figure  S5. Here, there are many atoms that have migrated to ± 1Å from their original positions. Further, in Figure  S3, we can see the rise of Ru and N atoms out of the Ru-TaN structure, which is not reflected in the z-plot. However, a potential replacement of Ru and N atoms with Cu atoms could account for the Cu atoms that are displaced to nearly -3 Å at 800K.

Discussion of z-plots Ru 50
Similarly to Cu29 adsorption on the passivated surface, at 800 K the atoms begin to interact at the periodic boundary, forming the beginning of a pseudo 1D ribbon structure. Some migration into surface recesses is observed, but no third layer is formed. Figure S7 shows how the displacement of atoms along the Z axis changes with increasing temperature. The larger number of atoms in the central area (coloured in blue) shows that the majority of atoms do not move away from their original location. This confirms that the structure remains as a two-layer system. As temperature increases, there are several atoms displaced by around 1.5 Å from their origin, while 2 and 3 atoms migrate into a recess and are displaced by around -2.0 Å.

Discussion of z-plots Ru 25
At 800 K, there are strong distortions to the structure of copper and a tetrahedral Cu4 appears to split off from the remainder of the structure, which remains as a two-layer structure with two copper atoms having migrated upwards into the S+2 layer and 3 atoms burying into one of the recesses created by the surface Ru. This is visible in the z-plots (see Figure S8), although it is difficult to distinguish S+2 layer atoms from S+1 layer atoms. These are best identified visually (see Figure S3).

DOS plots and Bader Charges
In general, atoms bound to the surface are more strongly oxidized than what was observed for any of the other surfaces, with computed charges ranging from 10.5 to 10.9 electrons. Cu atoms solely bound to other Cu atoms once again remain metallic. Those atoms that are slightly elevated from the surface layer, but not fully in a S+1 layer remain metallic. Atoms incorporated into the surface layer are found to be more oxidized than atoms on the surface, with computed charges of 10.4 to 10.6 electrons. DOS plots, Figure S9, and Bader charge analysis show that 1 ML Ru-modified TaN is metallic, and copper also remains metallic. Copper atoms that are directly bound to the surface are partially oxidised, with computed Bader charges between 10.6 and 10.9 electrons. Atoms in the second layer of the cluster are metallic, with computed Bader charges of 11.0 electrons. For the 1ML passivated surface Bader analysis shows that Cu atoms remain metallic. This indicates that there is no charge transfer between the Cu adatoms and the Ru atoms in the passivation layer.
The DOS of Ru 25 and Ru 50 , as shown in Figure S9, shows a very weak contribution from the Cu d orbitals, in contrast to the much stronger contribution from the Ru d orbitals. This is unexpected, as there are less Ru atoms in the system than Cu atoms and as the number of Ru atoms is doubled for Ru 50 . Similarly, to the bare surface, the atoms bound to the surface are slightly oxidised with computed Bader charges of around 10.8 electrons, while atoms in the second layer have remain fully metallic.

Possible Effect of Thermal Expansion
To explore if there is any potential impact as a result of thermal expansion of TaN and RuTaN at the AIMD temperatures, we carried out NPT ensemble AIMD calculations of (3x3x3) supercells of bulk TaN and 11% Ru doped TaN. The latter was achieved by replacing one in every three Ta atoms in one layer of the bulk with Ru (as a model system). The AIMD calculations were run at a temperature of 800 K with 1 bar of pressure for up to 2 ps. The original lattice parameters (based on the experimental crystal structure of TaN available in the Materials Project Database at https://materialsproject.org/materials/mp-1279/), were: a = b = 5.23 Å, c = 2.93 Å, α = β = 90°, γ = 120°. Throughout the AIMD run the lattice parameters for Ru-doped TaN varied from the original as follows: a = -0.38 % to -3.60 %, b = -0.74 % to -3.38 %, c = +0.34 % to -5.58 %; please note that the a and b parameters contract and do not show any expansion. The corresponding changes in lattice vectors for undoped TaN are quite similar: a = -0.76 % to -3.70 %, b = -0.52 % to -3.38 %, c = +0.34 % to -5.58 %. From the geometry we observe that as on the Ru-doped TaN surfaces, small cavities form around the dopants due to the difference in ionic radius of the two metals.
However, the overall decrease in lattice parameters indicates that Ru-doped TaN should have a negative thermal expansion coefficient and shrink at higher temperatures, which rules out the possibility of thermal expansion causing further cavities that impact the diffusion barrier properties of the material.