ALD precursor chemistry : Evolution and future challenges

Abstract
The requirements of ALD precursors differ from those of CVD concerning thermal stability, adsorption on the surface, and reactions towards each other. In the first ALD experiments in the 70s elements (Zn, Cd, S), metal halides and non-metal hydrides (H₂O, H₂S) were used. In the 80s the selection of precursors widened to metal complexes (alkoxides, β-diketonates) and simple organometallics (alkyl compounds). In the 90s both new metal (Cp-compounds, alkylarnides) as well as non-metal precursors (H₂O₂, O₃, hydrazine) have been introduced. A characteristic feature of ALD is that surface groups play an important role as reactive sites for the next precursor pulse. The development of ALD precursors is limited by the small number of groups working in the field. It seems, however, that the precuror development is diverged and tailored molecules are designed for each process.