When I was reading a optical coating book, most of the techniques introduced are based on PVD such as IAD Evaporation, sputtering, etc. Only a small portion is about CVD. Is PVD preferred over CVD for optical thin film coating?
PVD stands for physical vapor deposition (PVD), in PVD researchers can use a single precursor material instead of mixtures that can be coated on the target substrate without any chemical change [only changes occurred in physical properties]. For example, the researchers use WS2 or WSe2, or WTe2 precursor materials and get the same nanomaterials on the target substrate without any chemical changes.
While CVD stands for chemical vapor deposition (CVD), in CVD researchers can use more than one precursor materials which will change the properties chemically after being coated on the target substrate and the possibility to get different results instead of the target [changes occurred in physical as well as chemical properties]. For example, the researchers use WO3+(S or Se or Te) for getting WS2, WSe2, WTe2, etc. but there is the possibility to get some W or WO2 or WO3 or WO3-xor S or Se or Te, etc. nanomaterials on a substrate.
So researchers preferred PVD for coating instead of CVD techniques.
Must of pvd do under vaccum . This it will give films with out. Contamination and also pvd give good adhesive
Comparatively PVD isa simple technique, whereas CVD is a chemical process, needs a good knowledge of chemistry. CVD apparatus is expensive, and the chemicals (organometallic, metalorganic) are quite expensive. Depends on the compound you are trying to deposit in thin film form. Sometimes toxic gases are also involved. No doubt CVD is also quite successful and most preferred by industry, once the process is well optimized.
It is mainly a matter of coating uniformity. PVD using 4 m long cathodes can guarantee un areal uniformity of +/- 1% (on thickness and oprical properties). So jumbo substrates of 6 x 3,8 meters can easily be coated in a uniform way. Fir CVD this is a true challenge.
A huge part of optical coatings is for eyeglass lenses and most of these lenses are made out of organic materials. These polymers won't withstand the high deposition temperatures of a classical CVD process.
So the preferred coating method for eyeglass lenses is PVD, particularly e-beam evaporation.
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