Investigation of the Effect of Thermal-Induced Atomic Motion on the Conductance of Copper Thin Films

TL;DR

This study addresses how thermal-induced atomic motion affects electron transport in copper thin films, comparing pristine and roughened surfaces under finite-temperature conditions. It combines ab initio molecular dynamics with NEGF-DFT transport to compute transmission $T(E)$ and low-bias conductance $G = \frac{1}{0.2}\int_{-0.1}^{0.1} dE\, T(E)$ for different temperatures. The results show that pristine Cu exhibits reduced conductance with increasing temperature due to atomic motion and layer expansion, while surface roughness dominantly reduces conductance for roughened films; in some cases, thermal distortions can partially offset roughness-induced scattering. These insights help guide mitigation strategies for thermal effects in Cu interconnects and inform surface engineering approaches to preserve conductivity in scaled electronics.

Abstract

Decrease in the size of integrated circuits (IC) and metal interconnects raise resistivity due the amplification of electron scattering effects, which decreases the efficiency of chiplets. While previous studies have investigated the electron scattering due to a roughened surface, the effect of thermal induced atomic motion on the roughened surface remains unclear. To address this gap, we investigated electron transport in pristine and roughened Cu thin films by performing \textit{ab initio} molecular dynamics (AIMD) trajectories over 20~ps at temperatures of 218~K, 300~K, and 540~K on Cu thin film models, and then calculating the electron transport properties of the resulting snapshots at 100-fs intervals for the last 10~ps using the non-equilibrium Green's function formalism in combination with density functional theory (NEGF-DFT). As expected, higher temperatures induce larger atomic displacement from their equilibrium positions and increase atomic layer separation. We also find that increase in temperature results in increased resistance (lower conductance) for the pristine film, but less so for the roughened thin film where the surface roughness itself is the main source of resistance. This study provides insights into how pristine and roughened Cu thin films behave under thermal conditions, helping researchers design better treatments to mitigate thermal effects in ICs and their metal interconnects.

Figures (5)

• Figure 1: Illustration of the simulation cell. Panel (a) shows the pristine simulation cell with 5 Å of vacuum on top and bottom of the Cu slab with frozen regions on the left and right of the cell. Panel (b) shows the simulation cell with the roughened top surface.
• Figure 2: Transmission spectra for the pristine Cu slab at 218 K, 300 K and 540 K. The black series are the initial structures with the adjusted interlayer spacing for that particular temperature, and the gray series are snapshots taken from AIMD trajectories at their corresponding temperatures.
• Figure 3: Transmission spectra for Cu thin films with 0 % (pristine), 4 %, 50 % and 100 % surface roughness at (a) 0 K and (b) 300 K.
• Figure 4: Transmission spectra for the Cu slab with 50 % surface roughness at 218 K, 300 K and 540 K. The black series are the initial structures and the gray series are AIMD snapshots.
• Figure 5: Overall low-bias conductance of the pristine and 50 % surface roughness systems. Standard deviations are indicated on top of each bar.