9.2.4 Chemical Solution Deposition

This process involves deposition of thin film from a solution containing required elements in desired stoichiometry. First, a homogeneous solution is prepared by mixing the stoichiometric quantities of precursors of desired substances (typically nitrates, acetates, citrates and metal alkoxides) in solvents (typically glacial acetic acid, citric acid, 2-methoxethanol).The mixing may have to be carried out at room temperature or higher temperature depending on the dissolution characteristics of precursors in the respective solvents. It is also possible that initially each precursor needs to be dissolved in separate solvents which are then mixed together later. Once the solution is made, one need to adjust the viscosity and pH by adding appropriate solutions. Use of stabilizing agents such as ethanolamine may be necessary to avoid precipitation of the ingredients into a complex. Finally, the as-prepared solution should be examined for a few says in terms of turbidity and precipitation which point towards the solution stability.

The second stage involves spin coating of solution on appropriate substrates. Substrate surfaces often require pretreatment such as cleaning with specially formulated solutions followed by ozonization (often necessary) in order to make the surface of the substrate hydrophilic to the solution and to remove any dust and organic residues. For the first coating, a few drops of the solutions are placed on top of the substrate kept on vacuum enabled chuck of a spin coater such that the substrate in completely covered with the solution. Then the substrate is spun for a particular duration (figure 9.4) so that excess solution is wiped away and a thin layer of solution remains on top of the substrates. Next, the substrate is taken of the spin coater and placed either on a hot place or an oven or furnace at a preset temperature to dry the coating to get rid of the volatile chemicals from the films without crystallizing it. These temperatures can be determined by carrying out thermal analysis of the solution. Once the coating is dried for a stipulated duration, the coating and drying procedure is repeated again to achieve a desired thickness. After achieving the desired thickness, the final film is annealing in a selected environment (typically air, nitrogen or oxygen) for a fixed period of time to achieve film crystallization. Optimization of various processing parameters is necessary to achieve a good quality films free of defects and with desired phase.