Heteroleptic Precursors for Atomic Layer Deposition

As the atomic layer deposition (ALD) method is based on sequential, self-limiting surface reactions the precursor chemistry is the key to a successful processing of conformal high quality thin films. ALD precursor chemistry has traditionally been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides and alkoxides. However, these precursors may have drawbacks such as possible halide contamination and low thermal stabilities in the case of the alkylamides and alkoxides. Consequently, heteroleptic precursors have been investigated as alternatives to the existing homoleptic counterparts, leading to the development of several advantageous processes.

In the recent years, increasing research effort has been dedicated in development of novel ALD processes focusing especially on transition metals and their oxides using heteroleptic precursors. Interestingly, heteroleptic compounds often show different properties than their homoleptic counterparts. One type of ligand can provide reactivity and another stability, thus widening the process window with comparable film growth rate.

In this presentation heteroleptic precursors for ALD processes of transition metals and their oxides will be reviewed. Special focus is given to oxides of the rare earths and groups 4 and 5. Trends in the properties of heteroleptic precursors are discussed. Several examples of our recent results are shown, including introduction of novel processes based on amidinate-cyclopentadienyl complexes for ALD of rare earth oxides.