Novel deposition of amorphous aluminum oxide thin film using a direct liquid injection chemical vapor deposition (DLI-CVD) system

Aluminum oxide (Al₂O₃) stands out as a representative ceramic material thanks to its wide applicability, e.g., in electronic and optical devices, sensors, wear-resistant coatings, and catalyst support. Especially, amorphous aluminum oxide films hold great promise due to their flexibility, uniformity, and high dielectric properties compared to crystalline counterparts. Various deposition methods, such as sputtering, chemical vapor deposition (CVD), and sol-gel method have been used so far.<br/>This study introduces a noble direct liquid injection CVD (DLI-CVD) technique to fabricate amorphous Al₂O₃ thin films. DLI-CVD, unlike conventional CVD process, offers various advantages: It can avoid the unnecessary decomposition of precursors prior to deposition, by making us of a vaporization chamber (Vapbox). The liquid precursor is injected into the Vapbox and transported to the deposition chamber through the nitrogen carrier gas. Simultaneously, oxygen gas is injected into the chamber for enabling the oxide film deposition. The precursor mixture is directly injected as vapor into the deposition chamber. The decomposition occurs on the surface of the substrate, where the deposition reaction is initiated. As the precursor molecules are decomposed directly on the surface, unwanted by-products can be prevented. Furthermore, strong chemical bonds between precursor molecules and organic solvent break in advance, while minimizing the required heat energy.<br/>In this study, Al₂O₃ thin films were deposited by varying parameters such as the substrate temperature, the deposition time, and the ratio between nitrogen and oxygen gas flow. Aluminum acetylacetonate (Al(acac)₃) was selected as the aluminum precursor due to its stability, non-toxicity, and non-flammability, compared to alternatives like trimethylaluminum and aluminum chloride. X-ray reflectivity (XRR) analysis confirmed that the film thickness scaled with the substrate temperature and deposition time. Atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analysis elucidated the roughness, uniformity, and composition of Al₂O₃ films. Cross-sectional transmission electron microscopy (TEM) provided insights into the film morphology and growth mechanism. This work demonstrates the applicability of the novel DLI-CVD technique and enhances our understanding of this innovative thin film process.