Flatness-based deposition rate control of thermally evaporated organic semiconductors

The most crucial step in manufacturing organic electronic devices is the deposition of the active organic layers. This deposition is mostly achieved by thermal evaporation of special organic materials out of an evaporation cell in a high-vacuum environment. The major goal is to produce a thin layer with a well-defined deposition rate. In this study, a mathematical model of the deposition process is presented and a procedure for identifying the unknown model parameters is given. The model covers the transient evaporation of organic materials as well as the thermal behaviour of the evaporation source. A control law based on the concepts of differential flatness turned out to be superior to conventional control strategies. The controller is tested on a real world high-vacuum system using two different evaporation materials. Finally, it is demonstrated, that the proposed approach yields improved layer morphologies that are the basis of outstanding device characteristics.