Electrochemically reduced polycrystalline tin oxide thin films - Surface analysis and electroplated copper adhesion

In previously reported work, a conductivity enhancement effect in polycrystalline tin oxide thin films, which are widely used for the front electrodes in flat panel displays, has been achieved by electroplating copper onto the 300 nm high sidewalls of the electrode. Recent experiments show that if an electrolytic reduction is introduced as a surface modification process just before electroplating then excellent adhesion between copper and tin oxide can be achieved. In this paper, the chemical, physical and surface morphological changes of tin oxide electrodes have been characterized by using atomic force microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and surface sheet resistance measurements by a four-point probe method. The electrolytic reduction was found to result in an increase in the intensity of the x-ray reflection from the (110) planes of tin oxide, and compositional nonuniformity of: the tin oxide surface layers, where metallic tin is trapped at the surface, and oxygen is enriched in the subsurface region. These results, taken together, suggest that the observed adhesion improvement may be due to a combination of tin oxide surface roughening due to preferential etching and the increased (110) orientation, in situ cleaning, and chemical bonding introduced by surface chemical changes.