Precursor design and reaction mechanisms for the atomic layer deposition of metal films

Deposition of thin films with desired compositions, conformality and bonding to substrates is a key component in nanotechnology research. The growth of metal films by atomic layer deposition (ALD) has become an important field of study due to its wide range of applications. However, metal deposition by ALD has not been a straightforward process for most metals. Precursor design and their reactivity with surfaces, as well as their reactions with different co-reactants, are important factors in the deposition of metals. The growth of noble metals and copper by ALD are the best-established, mainly due to their favorable reduction potentials. However, due to the lack of efficient precursors and co-reactants, the deposition of other metals has been a real challenge and just few reports have been documented. This review discusses the strategies used to achieve successful metal ALD by considering in depth the current challenges associated with the development of these processes, the crucial role that ligands play in the development of new precursors, and how molecular properties can be tuned by intelligent ligand manipulation. In addition, the deposition of some metals and their reaction mechanisms are discussed in some detail.