Effective methods for eliminating (NH4)(2)SiF6 powders generated on Si3N4 wafers processed by HF VPD

Abstract

Si3N4 wafers used for fabricating vertical 3D NAND flash memories are contaminated by various metallic, organic, and inorganic impurities. A process technology to effectively remove (NH4)(2)SiF6 powders generated on the surface of Si3N4 wafers processed by HF VPD was studied. It was confirmed that (NH4)(2)SiF6 powders were generated by a chemical reaction between Si3N4 and HF. In addition, the density and diameter of the powders depended on the thickness of the Si3N4 layers. The density of the powder increased from 317 to 2101 ea/mm(2) as the thickness of the Si3N4 layer increased from 30 to 70 nm, and then rapidly decreased to 85 ea/mm(2) for the 100-nm-thick Si3N4 layer. However, as the thickness of the Si3N4 layer increased from 30 to 70 nm, the diameter (approximately 10 mu m) of the powder did not significantly change, but rapidly increased to approximately 30 mu m when the Si3N4 layer thickness reached 100 nm. This was because more powders were generated by the thicker Si3N4 layer, but the powders generated on the 100-nm-thick Si3N4 layer aggregated more to reduce the surface energy. Moreover, we found that the heating and DIW cleaning processes effectively eliminated the (NH4)(2)SiF6 powders on the surface of the Si3N4 wafer processed by HF VPD. Furthermore, we determined the optimal heating and cleaning process conditions for complete decomposition: a temperature of 170 degrees C, time of 120 s, and pressure of 120 Torr in the vacuum oven for the heating process, and a temperature of 25 degrees C and time of 30 s for the cleaning process. Our proposed method for effectively eliminating (NH4)(2)SiF6 powder on Si3N4 wafers processed by HF VPD can be applied to monitor the level of impurity contamination in the fabrication process.