In this process, precursor compounds are heated to convert them into vapour phase and are introduced into the heated reaction chamber where a source of O₂ is present. The reaction vapours and gas are flowed over a substrate, usually heated between 400-1000°C, produce the film species which subsequently deposit on the substrate as shown by Figure 9.5. The residual gases and volatile products are pumped away from the system. Deposition temperature in CVD is limited by the temperature at which chemical precursors dissociate and react to form the film compound. However its further innovations like metallo-organic chemical vapour deposition (MOCVD) and plasma enhanced MOCVD (PE-MOCVD) processes offer advantages of depositing films at relatively lower temperatures as compared to those employed in PVD processes. This is achieved either by the use of a plasma or metallo-organic precursors which can react at relatively low temperatures. CVD processes also have the advantage of depositing films over large area substrates. However these processes are complex in nature because of many process parameters to be controlled. Some other problems with these processes are control of composition, accurate knowledge of phase diagrams, toxicity of precursors. Also precursors for MOCVD process are particularly expensive which also limit the application of the process on a wide scale. There are several reviews existing on the CVD deposition of the high T_C superconductors^27.28 and ferroelectric thin films.^29.30