Reactant Purity and Contamination

The requirements placed on the performance and reliability of CVD coatings are continuously upgraded. For one thing, this means the need for an ever increasing degree of purity of the precursor materials since impurities are the maj or source of defects in the deposit. The purity of a gas is expressed in terms of nines, for instance, six nines, meaning a gas that is 99.9999% pure, which is now a common requirement. It is also expressed in ppm (parts per million) or ppb (parts per billion) of impurity content. 

There are several methods used to purify gases catalytic adsorption, palladium diffusion, gettering, chemisorption, and filtration. 

Catalytic Adsorption. This method can reduce impurities, such as H2, O2, CO, and hydrocarbons, to less than 10 ppb. The catalyst converts these impurities into CO2, H2O, and other species that can then be removed by molecular sieves and cryogenic adsorption. 

Palladium Diffusion. Palladium is very permeable to hydrogen but not permeable to other gases. As a result, it is a useful hydrogen purifier. A palladium membrane, heated to 400 °C, purifies hydrogen to 10 ppb but requires a high pressure differential for net diffusion to take place at reasonable rates of hydrogen supply. 

Gettering. Gettering materials, such as zirconium or titanium alloys, are heated to 400°C. At that temperature, they react with the impurities in the gas stream such as O2, H2O, N2, H2, CO, CO2, and hydrocarbons. Total impurities can be reduced to 100 ppb. 

---

Handbook of Chemical Vapor Deposition 3.2 Reactant Purity and Contamination... 

---