Influence of Process Parameter and Magnetic Field Strength on DC Magnetron Sputtered ZnO:Al Films From Ceramic Targets

摘要

With the demand of a low cost and high quality TCO layer in thin film solar cell applications the need for a production process of ZnO:Al films suited for large area coatings in industry became apparent. Throughout the last years strong efforts have been made to use Al-doped ZnO films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. With the rise of production capacities first sputtering coaters for large area deposition of ZnO:Al are now being set up. Nowadays cost efficient, sintered ceramic ZnO:Al_2O_3 targets with high conductivity can be sputtered by using DC power. At Fraunhofer 1ST ceramic ZnO:Al_2O_3 (1 wt.%) targets were dc sputtered using a screening experiment design, which covered a wide interval of the main process parameters like total pressure, sputtering power and oxygen partial pressure, while substrate temperature was held constant at 300 °C. In this experiment a big influence on the target erosion was shown. During sputtering from a ceramic target, the film is damaged perpendicular to the erosion area by high energetic oxygen ion bombardment. The ions receive their energy from the potential difference between cathode and floating substrate. Their impact leads to a higher resistivity, lower mobility and a higher etching rate of the deposited layer in diluted hydrochloric acid. This counteracts the necessary properties of a TCO as front contact in a-Si/μc-Si solar cells. In order to understand and minimize these problems static imprints with a standard and a strong magnet were performed additionally. A new plane ZnO: Al_2O_3 target was used. Comparing the position-dependent properties of the imprints, their height profiles remained similar but a huge impact on resistivity was observed depending on the target state, namely the race track depth. Options to diminish the damaging effect of oxygen were examined, in particular the use of stronger magnets for magnetron sputtering to reduce the applied target voltage. The observed film damage increases with higher target voltage, therefore both are reduced by applying a higher magnetic field. The results show a clear reduction of target voltage by 40 V by applying the same power using a stronger magnetron. Also less damage opposite the race tracks was observed. Resistivity is lowered by a factor of 1.5 which yields better properties of ZnO: Al as front contact of thin film solar cells. Also the crystal structure and etching behavior changed positively.