Evaluation of the Thermal Stability of TiW/Cu Heterojunctions Using a Combined SXPS and HAXPES Approach

TL;DR

This study investigates the thermal stability of TiW/Cu heterojunctions in semiconductor devices using SXPS, HAXPES, and TEM, revealing that the barrier remains stable despite titanium out-diffusion and oxide formation after high-temperature annealing.

Contribution

It provides detailed chemical and structural analysis of TiW/Cu interfaces under high-temperature conditions, demonstrating barrier stability despite titanium diffusion.

Findings

Titanium segregates to the copper surface forming oxides after annealing.

The TiW/Cu interface remains stable even after 5 hours at 400°C.

Out-diffusion of titanium does not significantly compromise barrier integrity.

Abstract

Power semiconductor device architectures require the inclusion of a diffusion barrier to suppress, or at best prevent the interdiffusion between the copper metallisation interconnects and the surrounding silicon substructure. The binary pseudo-alloy of titanium-tungsten (TiW), with $>$70~at.\% W, is a well established copper diffusion barrier but is prone to degradation via the out-diffusion of titanium when exposed to high temperatures ($\geq$400$^\circ$C). Here, the thermal stability of physical vapour deposited (PVD) TiW/Cu bilayer thin films in Si/SiO\textsubscript{2}(50~nm)/TiW(300~nm)/Cu(25~nm) stacks were characterised in response to annealing at 400$^\circ$C for 0.5~h and 5~h, using a combination of soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) and transmission electron microscopy (TEM). Results show that annealing promoted the segregation of titanium out of the TiW and interdiffusion into the copper metallisation. Titanium was shown be driven toward the free copper surface, accumulating there and forming a titanium oxide overlayer upon exposure to air. Annealing for longer timescales promoted a greater out-diffusion of titanium and a thicker oxide layer to grow on the copper surface. However, interface measurements suggest that the diffusion is not significant enough to compromise the barrier integrity and the TiW/Cu interface remains stable even after 5~h of annealing.