Multi-layer devices based on thin films in Large Area Electronics have different intrinsic and extrinsic properties that must be optimised to produce the correct physical shape of the device, in addition to having acceptable electrical characteristics. For instance it is quite easy to produce individual layers optimised for electrical performance that will physically “pull off’ underlying films, or result in poor step coverage. The factors that must be considered include mechanical stress, etching rates and profiles, thickness and stoichiometry uniformity, and thermal budgets, as well as electrical characteristics. This paper gives an example of silicon nitride optimisation for use in a-Si:H TFT projection displays. Three different silicon nitride layers were included to give a storage capacitor, together with controlled etch profiles for step coverage. The layers were optimised with respect to several different parameters in the minimum number of depositions by the use of experimental design techniques.