Control of the thermal evaporation of organic semiconductors via exact linearization

THE deposition of organic semiconductors, e.g. Pentacene (C22H12) and Alq31, becomes more and more important. Due to lower process temperatures, the manufacturing of organic electronic devices such as light emitting diodes (OLEDs) and thin film transistors (OTFTs) is less energy consuming. Additionally, these low temperatures also make it possible to use flexible substrates, e.g. plastic films, which can serve as a basis for elastic electronic devices. The crucial part of the fabrication is the deposition of the active organic layers (films) with a layer thickness between 10 and 100 nanometres. It is mostly done by thermal evaporation in a high vacuum environment. The mobility of the charge carriers within the layers and the layer morphology strongly depend on the deposition rate, i.e. the increase in layer thickness with time during the deposition process [1], [2]. The desired deposition rates range between 0.03 and 5 Angstrom per second. Usually, the deposition rate is controlled manually by an ”expert”. The result of a manually controlled deposition of Pentacene is shown in Fig. 1. The desired deposition rate is very hard to attain, the fluctuations (e.g. between 140 and 160 seconds) lead to a poor layer morphology. As a consequence of these insufficient results and the long durations of the deposition process, an automatic control of the deposition is essential. In literature only a few control methods for the deposition of metallic materials e.g. [3] and the control of the steady state evaporation of organic materials [4] are proposed. In this work the design of a new deposition rate controller is presented. It is able to cope with transient evaporation effects. Therefore, the exact input output linearization is applied to the mathematical model [5] of the given high vacuum deposition system, described below.