Investigation into the multi-scale evolution and mechanisms of the chemical mechanical polishing process for AlN

Single-crystal AlN is a key substrate for deep-UV optoelectronic and high-power electronic devices, whose surface integrity critically affects epitaxy and device performance. However, PVT-grown AlN exhibits local heterogeneities, such as dislocation-density fluctuations and non-uniform intrinsic stress, leading to uncertain machining behavior. The present paper investigates the multiscale evolution of polar Al surfaces during the chemical mechanical polishing (CMP) process and proposes an explanation based on stress release to account for the observed phenomena. CMP reduced surface anisotropy and yielded sub-nanometer roughness across the wafer (Sa < 1 nm). Nevertheless, Raman stress analysis reveals that morphological homogenization does not necessarily correspond to mechanical homogenization. The region with initially higher defect density exhibited a transition from a highly compressive stress state to a low tensile stress state of 22.1 MPa after polishing, whereas the region with superior initial crystallinity retained a higher residual tensile stress of 26.5 MPa, indicating different stress relaxation behaviors associated with crystal quality. Based on the correlation between crystallographic quality and residual stress evolution, a plausible dislocation-mediated stress relaxation hypothesis is proposed to account for the observed phenomena. These findings provide insights into the relationship between crystal heterogeneity and surface integrity evolution, offering guidance for low-damage ultra-precision machining of PVT-grown AlN substrates.