Reaction Mechanisms in Copper Atomic Layer Deposition Using Copper(II) Hexafluoroacetylacetonate and Diethylzinc <i>via</i> <i>In Situ</i> Time-of-Flight Mass Spectrometry

The development of high-quality copper (Cu) thin films by atomic layer deposition (ALD) remains challenging due to a limited understanding of the reaction mechanisms that govern film growth and composition. To address this gap, in this study, time-of-flight mass spectrometry (TOFMS) was used to perform the first comprehensive in situ investigation of the surface reactions during Cu ALD, using dehydrated Cu(II) hexafluoroacetylacetonate (Cu(hfac) 2) (CAS 14781−45−4) and diethylzinc (DEZ, ZnEt 2) (CAS 557−20−0). The high sensitivity and mass resolution of TOFMS enabled the identification of previously only hypothesized surface reaction volatile by-products, revealing two different mechanisms in each ALD half-cycle. During the Cu(hfac) 2 pulse, both Zn(hfac) 2 and EtZn(hfac) volatile by-products were detected in varying amounts, while the DEZ pulse predominantly yielded EtZn(hfac). X-ray absorption spectroscopy (XAS) of the as-deposited films confirmed the formation of metallic copper with minimal zinc (Zn) and carbon (C) incorporation. The results underline simultaneous full and half-ligand exchange mechanisms during the precursor pulse, resulting in a surface saturated with hfac ligands. However, the reaction mostly happens in the DEZ half-cycle, which is dominated by a partial ligand exchange pathway, leading to the saturation surface being composed of Zn species. This study provides the first experimental evidence of the mechanistic pathways for copper deposition and demonstrates the value of in situ TOFMS for monitoring ALD reaction mechanisms, a critical aspect for process development and optimization.