The composition, microstructure and morphology of SiOxNy barrier and Al layers deposited by a modified Ar ion beam evaporation system were investigated as a function of substrate temperatures and gas flow rates. Furthermore, the impact of defects and stress on the moisture permeation in the inorganic SiOxNy/Al stack were studied for their correlation. Al films deposited at 60 oC exhibited a columnar structure with grain size 50-100 nm. Since pinhole defects and grain boundaries were likely introduced in the Al electrode during low-temperature deposition, an amorphous SiOxNy passivation layer was deposited onto Al electrode. It was found that Al hillock was formed even at low temperature, such as 60 ℃ because of its high purity of Al films and small grains. During the deposition of SiOxNy passivation layer, the energy imparted by evaporated atoms and molecules raised the Al surface temperature to 250-361 oC resulting in further Al grain growth. Moreover, SiOxNy
films prepared under high substrate temperature and high flow rate showed the lowest Al hillock density, but possessed the tallest Al hillock height. Excellent correlation between hillock density and moisture permeation through SiOxNy/Al samples was established, indicating Al hillock was the culprit of moisture permeation in the passivation layer.
A model of hillock formation in SiOxNy/Al was proposed in this thesis to illustrate the causes for their difference in hillock density and height. The hillock density related to the initial Al grain size, while the hillock height was primarily derived by SiOxNy stress. The tallest height of hillock affected greatly on the numbers of barrier layers for complete sealing of defects. Therefore, high substrate temperature and high N2 gas flow rate deposition process yielded barrier films with less hillocks and improved moisture resistance if single layer of passivation was employed, while
lower deposition temperature and low N2 gas flow rate were preferred for least number of layers if multiple-layer structure was adopted. In summary, defect control of Al hillocks was essential for controlling the moisture barrier performance in the practical OLED stack.
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