Volatile Precursors for Films Deposition: Vapor Pressure, Structure and Thermodynamics

At present the processes of chemical vapour deposition (MO CVD) and physical vapour deposition (PVD) are widely used to obtain the layers of different materials. A possibility to obtain materials of the required composition (elementary, complex, composite, etc.) is determined by the chemical nature of a precursor. By now a set of criteria for the precursors to meet the MO CVD and PVD standards have already been established (Maury, 1995). Among the parameters from which the choice of a precursor is made, the most important one is its ability to transfer to the gaseous phase at temperatures not exceeding the temperature of molecules decomposition, i.e., its volatility. The most important quantitative characteristic of volatility is the pressure of saturated vapour of the compound at defined temperature. A number of experimental data showing volatility of different classes of metal complexes with organic ligands are known. Practically all precursors crystallize in lattices of a molecular type. The relation to molecular crystals suggests that energy of intermolecular interaction is much less than the energy of intramolecular bonding. The structure of metal complexes with organic ligands is determined by different factors depending on the nature of metal and coordinated ligands. The current review summarizes the data of saturated vapor pressure and thermodynamic parameters of the sublimation process of some complexes with organic ligands as reported in the literature as well as in our works. Dependence of volatility on type of substituents in the ligand, on central atom, and on donor atom in the ligand is discussed using metal β-diketonate (Chart 1) and phthalocyanine (MPc) derivatives as examples. The volatility of metal β-diketonates and phthalocyanines is analysed from the standpoint of their molecular and crystal structure. One important aspect of chemistry of precursors for MO CVD and PVD is a possibility to predict volatility from the data of the compound structure. A description of the microscopic model created in the framework of the statistical mechanics to describe the volatility of metal complexes with organic ligands is given. It follows from the structural studies that interatomic spacing in a molecule is comparable to interatomic spacing in different adjacent molecules. With such location of the molecules of complexes in the crystal lattice, it is necessary to take into account both van der Waals and electrostatic interactions. The correlation between these interactions determines the specific features of the behavior of this class of molecular crystals.