EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The progressive miniaturisation of feature sizes in microelectronic devices has increased the probability of higher local power densities and consequently temperatures. Complex multi-layered metallisation schemes, often implemented in devices, can suffer from detrimental interdiffusion phenomena between adjacent layers, compromising device reliability, when subjected to such severe local conditions.

The challenge of interdiffusion is particularly persistent in power semiconductor devices due to the high-power loads. Copper metallisation schemes in combination with titanium-tungsten (TiW) diffusion barriers are used, where the TiW barrier is required to isolate Cu from the silicon substructure, but local high temperature events can induce the diffusion of titanium out of the barrier and into the top Cu metallisation layer. This loss of Ti can ultimately lead to the degradation and failure of the diffusion barrier. Additionally, oxygen accumulation at the TiW/Cu interface can occur, adding an additional reliability challenge as this can promote delamination at the interface.

Here, device-relevant Si/SiO2/TiW and Si/SiO2/TiW/Cu thin film stacks were characterised using a combination of soft and hard X-ray photoelectron spectroscopy (SXPS/HAXPES). Combining the two techniques provides the opportunity to non-destructively study both the titanium diffusion mechanism and the oxidation behaviour of TiW at multiple sampling depths. Annealing for varying durations at 400°C under forming gas is used to simulate temperature stress on the device stacks and systematically follow occurring processes across the multi-layer structures. A clear dependence of the titanium surface enrichment and oxidation behaviour on the annealing duration is observed and can provide a detailed explanation of the degradation mechanisms associated with the TiW/Cu heterostructures. Overall the SXPS/HAXPES characterisation approach delivers an insight of the depth dependent behaviour of metallisation schemes under thermal stress and can be extended to other multi-metallic systems.