Chemical Vapour Deposition Phase Diagrams

A CVD phase diagram provides important information about the equilibrium compositions of the solid phases present under given conditions of pressure, temperature and input concentration. It is usually constructed either by the equilibrium constant method or the minimisation of Gibbs free energy method. The detailed principles are discussed below. 

For a closed reaction system the determination of the compositions can be calculated with thermodynamic methods under some constraints. The constraints include mass conservation, constant temperature and constant total pressure. Under equilibrium conditions all systems obey the Gibbs phase rale, which relates the number of the species components (n) to the number of phases present (p) and the degree of freedom (/) together. The relationship is expressed by [3] 

The Gibbs phase rule is very useful in predicting the importance of the control variables in a chemical reaction system. These variables include pressure, temperature and composition, as well as the effect on phase equilibrium. It can also be used to predict which way a given reaction will progress when the controlled variables are changed. 

Taking the Si-Cl-H system as an example [17], silicon deposition occurs with an appropriate control of the system. It is important to design and determine the control and other variables using the Gibbs phase rule first before a reliable and working system can be established. To apply the Gibbs phase rule for this system Si, Cl and H are considered as three components in the system, hence, n = 3. For this CVD process, there are only two phases (gaseous and Si solid), as a result p = 

Applying the Gibbs phase rule gives /= n = 3. These three degrees of freedom for this system include the following three control variables temperature, total pressure and the ratio of chlorine to hydrogen. 

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Some Typical Chemical Vapour Deposition Phase Diagrams Having introduced theories and the thermodynamic calculation methods it would be useful to show several CVD phase diagrams and their applications in the following section. 

Chemical Vapour Deposition Phase Diagram Collection... 

D.2 Chemical Vapour Deposition Phase Diagrams for Carbides... 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

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