Influences of metal, non-metal precursors, and substrates on atomic layer deposition processes for the growth of selected functional electronic materials

Atomic layer deposition (ALD) is known for its self-limiting reaction, which offers atomic-level controllability of the growth of thin films for a wide range of applications. The self-limiting mechanism leads to very useful properties, such as excellent uniformity over a large area and superior conformality on complex structures. These unique features of ALD provide promising opportunities for future electronics. Though the ALD of Al2O3 film (using trimethyl-aluminum and water as a metal precursor and oxygen source, respectively) can be regarded as a representative example of an ideal ALD based on the completely self-limiting reaction, there are many cases deviating from the ideal ALD reaction in recently developed ALD processes. The nonconventional aspects of the ALD reactions may strongly influence the various properties of the functional materials grown by ALD, and the lack of comprehension of these aspects has made ALD difficult to control. In this respect, several dominant factors that complicate ALD reactions, including the types of metal precursors, non-metal precursors (oxygen sources or reducing agents), and substrates, are discussed in this review. The examination of such aspects may contribute to the further understanding of non-ideal ALD reactions. Several functional materials for future electronics, such as higher-k dielectrics (TiO2, SrTiO3), phase change materials (Ge-Sb-Te solid solution), noble metal electrodes (Ru, RuO2), and resistive switching materials (NiO), are addressed in this review. Finally, desirable directions of ALD are suggested with consideration of the uncommon and non-ideal aspects of the ALD reactions.