| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Monday Sessions |
| Session TF1-MoM |
| Session: | Precursors and Surface Reactions |
| Presenter: | Tania Sandoval, Universidad Técnica Federico Santa María |
| Authors: | T.E. Sandoval, Universidad Técnica Federico Santa María T-L. Liu, Stanford University R. Tonner, Philipps-Universität Marburg S.F. Bent, Stanford University |
| Correspondent: | Click to Email |
Atomic layer deposition (ALD) relies on the adsorption of metalorganic and inorganic molecules to create thin and conformal films for semiconductor device fabrication. Aluminum precursors, particularly trimethylaluminum (TMA), have been used for decades to deposit thin dielectric films. The reaction mechanism for this process is well known and reported in literature. However, with continued advances in semiconductor nanofabrication and thin film deposition, the ability to select precursors that meet different processing requirements is important. For this purpose, it is necessary to fundamentally understand the effect that the precursor structure can have on nucleation and growth. Thus, in this work, the adsorption of a series of Al-X molecules (X= –CH3, –Cl, –C2H5, and –C3H8) on SiO2 were explored. In the series, TMA is used as a model system and benchmark for comparison.
Density functional theory (DFT) calculations suggest interesting trends regarding the adsorption of Al-X molecules on SiO2. Results indicate that when the number of Cl ligands on Al is increased, the dative bond that forms between the precursor molecule and the SiO2 surface becomes more exothermic with respect to TMA. This increase is a result of electronic contributions to the total energy, which can be attributed to inductive effects caused by the addition of the Cl ligand. Additionally, it is observed that increasing the number of carbon atoms in an alkyl ligand stabilizes the dative bond; however, these surface adducts are primarily stabilized by dispersion interactions, which could be related to the ligand size. After initial adsorption of Al-X, either the alkyl or chorine ligand exchange reaction can proceed. Our DFT results suggest that exchange of the alkyl ligand is more thermodynamically favorable than that of the Cl ligand, which correlates well with the higher dissociation energy observed for Al-Cl versus Al-C.
Preliminary X-ray photoelectron spectroscopy (XPS) results for ALD using dimethylaluminum chloride and aluminum trichloride as precursors show chlorine at the silicon surface, suggesting only a partial ligand exchange reaction. These experimental results agree with our theoretical findings which show Cl-ligand exchange to be less thermodynamically favorable than the precursor state (dative bond). These results provide interesting insights into both the fundamental aspects of the adsorption chemistry of organometallic compounds on semiconductor surfaces and the practical aspects of designing precursor molecules for ALD growth.