Morphological characteristics and formation mechanism of latent scratches in chemical mechanical polishing

Latent scratches are extremely shallow scratches inevitably introduced during chemical mechanical polishing (CMP), which deteriorates the performance of functional crystals. Although latent scratches can currently be detected by some advanced microscopes, there is still a lack of in-depth understanding of them. In this paper, we describe the morphological characteristics of latent scratches using atomic force microscopy and develop a mathematical model to explain the formation mechanism of latent scratches with YAG crystals as an example. The observations showed that the distributions of the depth and width of latent scratches were always Gaussian-like. And the pressure was positively correlated with the number of latent scratches but did not significantly affect the depth and width. The proposed mathematical model describes the contact state between the particles and the crystal surface, and identifies the formation of latent scratches resulting from plastic deformation of the crystal surface, a conclusion further supported by transmission electron microscopy observations. The model also explains that the distribution of depth and width is mainly determined by the material properties and that pressure affects the number of particles in contact with the workpiece but does not significantly change the contact stress between individual particles and the workpiece. There is an upper limit to the contact stress, resulting in scratches always below 1 nm in depth. The analysis of latent scratches is of great significance to removing latent scratches and manufacturing the ultra-smooth surface in the CMP process.