Study on the polishing mechanism of pH-dependent tribochemical removal in CMP of CaF2 crystal

Nanowear tests of calcium fluoride (CaF2) crystal against SiO2 microspherical tip at various pH solutions were conducted using an atomic force microscope to detect the atomic removal mechanism in chemical mechanical polishing (CMP). An optimized solution pH range was defined as 9–10, below which the atomic material removal of CaF2 cannot occur and above which the excessive chemical corrosion pits will form at the fabricated surface. TEM analysis of wear track demonstrates no remarkable subsurface damage in the wear region, confirming the domination of tribochemical reactions. Finally, the optimized pH was applied in real CMP and the ultimate limit of tribochemical wear in CMP was determined, i.e., a removal of Ca–F+ layer from a CaF2(111) cleavage surface.