Continuous development of thin film deposition technologies is essential for fabricating materials that meet the targeted requirements of specific applications. In this context, the present dissertation focuses on atomic layer deposition (ALD) of metallic copper, aiming to deepen the understanding of Cu ALD and establish a groundwork for future application-driven process development. Specifically, the Cu ALD process investigated in this work was based on copper(II) hexafluoroacetylacetonate (Cu(hfac)2) and the metal-containing reducing agent diethylzinc (DEZ). To date, despite extensive efforts, the development of reliable metal ALD processes remains challenging due to several limitations, the most critical of which are: long nucleation delays, islands formation, and incomplete understanding of the surface reaction mechanisms. To address these issues, the reaction mechanism of Cu(hfac)2 and DEZ during ALD was first examined using in situ time-of-flight mass spectrometry (TOFMS). The investigation focused on the qualitative analysis of the surface reactions occurring during steady-state growth. 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). The majority of the reaction occurred during the DEZ half-cycle, suggesting the presence of an abbreviated ALD cycle. Building upon these mechanistic findings, copper deposition was successfully extended to various substrates, specifically on oxide, nitride, and flexible materials. The resulting Cu films exhibited compact but island-like morphology, consistent with a Volmer-Weber growth mechanism assisted by surface diffusion of copper species. Interestingly, no growth was observed on metallic surfaces such as Au and Pt, highlighting the inherent selectivity of this specific Cu ALD process. This selective behaviour was further explored using patterned metal-oxide and metal-nitride substrates, demonstrating inherent selective ALD (AS-ALD) on micro-scale device geometries. Cu deposition was also attempted on high-aspect-ratio (HAR) structures, demonstrating initial conformality, but requiring further development to achieve uniform coverage. TOFMS investigation was further extended by monitoring 400 Cu ALD cycles to provide insights into possible changes in the reaction pathway and by products yields from the early stage to the steady state regime. Initial experiments focused on oxide substrates, while future work will extend these studies to other surfaces to assess the substrate influence. In conclusion, by linking mechanistic insights with process optimization, this work enabled controlled and reproducible Cu ALD on diverse substrates. The preliminary Cu depositions on technologically relevant substrates, such as flexible materials, gas diffusion layers for catalysis, and dental implant materials, highlight the practical relevance of this study. Overall, these findings lay the groundwork for future application-driven Cu ALD processes.