Reaction Mechanisms in Copper Atomic Layer Deposition Using Copper(II) Hexafluoroacetylacetonate and Diethylzinc via In Situ Time-of-Flight Mass Spectrometry
Minzoni et al.
Chemistry of Materials, 2025
DOI: 10.1021/acs.chemmater.5c01521
A new study in Chemistry of Materials has revealed previously unconfirmed reaction pathways during Copper (Cu) atomic layer deposition (ALD). The research, led by EMPA scientists in collaboration with TOFWERK, used the Semicon Process Analyzer G to identify distinct volatile by-products for each half-cycle. X-ray absorption spectroscopy (XAS) confirmed metallic copper films with minimal Zn and C incorporation, establishing partial and full ligand exchange as the governing mechanisms and highlighting our time-of-flight mass spectrometer (TOFMS) as a powerful tool for ALD process optimization.
Cu thin films are widely used due to their excellent electrical conductivity, thermal properties, and catalytic activity. Applications include micro- and nanoelectronics, flexible devices, catalysis, and energy storage. This recent study investigated Cu thin film growth via ALD, focusing on challenges such as island growth, nucleation inhibition, and Zn contamination from DEZ reducing agents. In situ time-of-flight mass spectrometry (TOFMS) was applied to monitor volatile by-products, providing the first direct experimental confirmation of Cu ALD surface reaction mechanisms, validating theoretical pathways, and advancing control over film quality and composition.
The Process Analyzer G was able to distinguish between structurally similar species, including Cu(hfac)₂, Zn(hfac)₂, and EtZn(hfac), by resolving small mass differences and isotope patterns. A unique fragment at m/z 221 served as a Zn-specific marker. TOFMS also revealed that both Zn(hfac)₂ and EtZn(hfac) formed in each half-cycle. Zn(hfac)₂ dominated during the Cu precursor pulse, while EtZn(hfac) was the main product in the DEZ pulse. This periodic by-product formation confirmed transmetalation and partial ligand exchange mechanisms. The absence of EtCu(hfac) fragments indicated negligible Zn incorporation into the film, aligning with XAS results. Overall, TOFMS provided direct mechanistic evidence, showing that surface reactions differ between pulses and that DEZ largely drives by-product release. The latter likely contributes to the relatively low growth-per-cycle observed.
In situ TOFMS using TOFWERK Semicon Process Solutions proved highly effective for Cu ALD by enabling real-time, high-resolution identification of volatile by-products despite mass interferences. Unique Zn fragmentation markers allowed clear differentiation between species, confirming reaction pathways and ruling out unwanted products, demonstrating the power of the Process Analyzer G for mechanistic insight and process control.
