Characteristics of Plasma CVD

Advantages of Plasma CVD. As shown in Table 5.4, with plasma CVD, a deposit is obtained at temperatures where no reaction whatsoever would take place in thermal CVD. This is its major advantage since it permits the coating of low-temperature sub- 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

Plasma CVD tends to create undesirable compressive stresses in the deposit particularly at the lower frequencies. This may not be a problem in very thin films used in semiconductor applications, but in thicker films typical of metallurgical applications, the process is conducive to spalling and cracking. 

Another disadvantage is that fragile substrates used in VLSI, such as some III-V and II-VI semiconductors materials, can be damaged by the ion bombardment from the plasma, particularly if the ion energy exceeds 20 eV. In addition, the plasma reacts strongly with the surface of the coating as it is deposited. This means that the deposition rate and often the film properties depend on the uniformity of the plasma. Areas of the substrate fully exposed will be more affected than the more sheltered ones. Finally, the equipment is generally more complicated and more expensive. 

Overall the advantages of plasma CVD are considerable and it is used in an increasing number of applications as will be shown below. 

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