Chemical vapor deposition (CVD) is a technology that uses gaseous or vapor substances to react on the gas phase or gas-solid interface to generate solid deposits. Atomic layer deposition (ALD) refers to a variant of CVD that allows controlled deposition at the atomic level utilizing self-limiting surface reactions, commonly by alternating exposure to different precursors.
Gas source precursors afford the greatest versatility and control, but their production scale is limited by safety as well as volume and storage. For example, CH4 is a flammable gas, but can be used as a precursor to react with anhydrous CH3CH2OH, and then prepare boron doped diamond films together with the precursors of ammonia borane and boron oxide.
Precursors synthesized with specific reactivity, volatility, stoichiometry, and thermal behavior are often liquids or solids under standard conditions, and these are more readily used for CVD. For example, pyromellitic dianhydride and 4,4'-diaminodiphenyl ether are used as precursors, which are heated to sublimation respectively to make them fully contact and react in the vacuum chamber, and then the substrate and the viscous polyamide acid on it are heated, so that the polyamide acid is dehydrated to form a polyimide film.
The precursor of ALD usually needs to meet the following conditions:
In some instances, the ALD process can utilize the same precursors as the CVD process, but involves discrete alternating exposure to the precursors rather than simultaneous exposure. Typical ALD steps involve reaction of the precursor containing the metal or nonmetal source atom followed by a chemical reaction such as reduction, oxidation, or nitridation. Generation of adsorption and reaction sites (e.g., reactive ligands) for the following reaction is required with each step. Common precursors for ALD include hydrides (SiH4, Si2H6), halogen-substituted hydrides (e.g., SiCl4), metal halides (e.g., AlCl3, WF6, TaCl5, HfCl4), metal nitrates (e.g., Ti(NO3)4, Hf(NO3)4), and metalorganics containing alkyls (e.g., AlR3), alkoxides (e.g., Si(OR)4, Ti(OR)4), amides (e.g., Ti(NR2)4), or β-ketonates and their derivatives (e.g., Zr(thd)4, Hf(acac)4). Additionally, oxygen (H2O, H2O2 ,O2 ,O3) and nitrogen (NH3 ,N2H4 ,NHxR3-x ,HN3) sources are used to produce oxides and nitrides, respectively. Gas and liquid precursors are most commonly used, but solid source precursors can also be utilized.
With the continuous improvement of requirements in various application fields, there are new requirements for CVD and ALD precursors. For example, the combination of CVD technology and chemical fluidized bed technology can be used in many industrial fields, among which the most common field is advanced nuclear fuel. Traditional refractory metal materials can also be used as the precursor of ALD to react and produce the required films. Nowadays, many developed countries have realized the large-scale industrial application of chemical vapor deposition technology for refractory metal coatings. CVD and ALD have very broad application prospects. They can not only extend the life of materials, optimize material properties, save material consumption, but also synthesize new materials.