Effect of Chloride Ions on the Chemical Mechanical Planarization Efficiency of Sapphire Substrate

Abstract

Sapphire is widely used in solid lasers, semiconductor chip substrates, light emitting diodes substrates and other high-tech fields for its excellent mechanical and optical properties. At the same time, the chemical mechanical polishing(CMP) performance and quality need to be improved for different crystal plane sapphire substrate. In this paper, in order to improve the removal rate NaCl was used as an additive in sapphire slurry. From the CMP experiment results of a- and c-plane sapphire, it was found that higher material removal rate (MRR) and lower surface roughness (Sq) were obtained by adding NaCl with the concentration of 0.2wt% in sapphire slurry. In order to reveal the action mechanism of NaCl, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) measure methods were used. From the SEM measure results it can be seen that there were microscopic corrosion pits on substrate surface, which indicates that chemical reaction took place between NaCl and substrate material. From XPS measure results it was verified that sodium tetrachloroaluminate (NaAlCl4) was formed during CMP. However, during the experiment, it was found there was a very small amount of silica sol crystalline particles after NaCl was added to the slurry. After analyzing the formation mechanism of crystalline particles, KCl was selected to replace NaCl. The same polishing effect was obtained and there were no crystalline particles found in the slurry tank. From the results it is found chloride ions played an important role in improving the removal rate and surface roughness of sapphire substrate.