Atomic layer deposition (ALD) is a special CVD that can be deposited at the atomic level. It can form very smooth, uniform thickness, highly dense, and alternating layers of different materials with minimal defects. CVD / ALD processes are highly attractive, through which thin films that meet specifications and are uniform and with precise thickness control can be grown. CVD is suitable for manufacturing optical storage media and manufacturing semiconductor devices. With more precise control of film formation, ALD is becoming more and more attractive for depositing thin films through microelectronic device applications, such as ferroelectric memory, integrated circuits, MEMS, thin-film capacitors, and new high-k gate dielectrics. They are also essential for advancing the technology of electroluminescent devices. CVD and ALD Precursors.pngFig 1. Applications of atomic layer deposition and chemical vapor deposition for perovskite solar cells. (Raiford J. A, et al. 2020) Process conditions have a great influence on the properties of ALD/CVD materials, and the correct selection of precursors is very important to obtain the required materials. Initially, CVD precursors included metal hydrides and halides, but nowadays a large number of metal-organic compounds are used. The precursors are volatile and thermally stable, so they will not decompose during the vaporization process, and are preferably soluble in an inert solvent or liquid at room temperature. The prerequisite must have preferential reactivity to the substrate and the growing film. It is also important that the precursor has self-limiting reactivity with the substrate and film surface. Certain metal-organic precursors may cause the accidental incorporation of oxygen and carbon into the film, and therefore must also be considered.