# Annealing Process-Induced Microstructural Variation in NiV/B4C Multilayers
The interface roughness of the W/SiC multilayers increased after annealing. The microstructure of the W/SiC multilayers after annealing was further analyzed by X-ray diffraction (XRD). The results showed that the W/SiC multilayers crystallized when the period thickness reached 5.8 nm, and the crystallization was enhanced by further annealing. They attributed the variations in thickness and interface roughness to fluctuations in the density of the film layer. [PERSON] et al. [11] annealed Ru/C multilayers at 500 \({}^{\circ}\)C (d = 2, 5 and 10 nm). They found that the multilayers of different period thicknesses were expanded after annealing. Crystallization occurred in the multilayers with period thicknesses of 5 and 10 nm after annealing. To further study the effect of period thickness on thermal stability, [PERSON] et al. [12] annealed a Ru/C multilayer with a d-spacing of 3.5 nm at 600 \({}^{\circ}\)C. The results showed that the crystallization of the multilayers after annealing led to an increase in the period thickness, and this expansion increased the interface roughness. [PERSON] et al. [13] prepared a W/Si multilayer with a d-spacing of 3.8 nm. The reflectivity and structure of the multilayer was studied after annealing. They found that the reflectivity of the multilayer was increased after annealing at 150 \({}^{\circ}\)C. In addition, higher temperature heat treatment led to a decrease in the reflectivity of the W/Si multilayer. Previous studies showed that microstructural changes would happen during the annealing process. The changes within the layers or at the interfaces of the multilayers resulted in microstructural changes. The detailed information about the interface of the multilayer was an effective way to monitor the microstructure of the multilayer, which requires advanced testing techniques. However, traditional methods of microstructure characterization are costly and can cause damage to samples. For the analysis of interface roughness and diffusion, the grazing-incidence small-angle X-ray scattering (GISAXS) measurements were used. [PERSON] et al. showed that the GISAXS technology could achieve unambiguous interface characterization without destroying the sample [14; 15] and was sensitive to determining the correlation length [16]. In our recent work [17], GISAXS was used to study the microstructural evolution of the NiV/B\({}_{4}\)C multilayers (d = 3 nm) after annealing. We found that the amorphous NiV/B\({}_{4}\)C multilayers (as-deposited) were crystallized after annealing. The interface roughness of NiV/B\({}_{4}\)C multilayers increased with higher annealing temperatures or prolonged annealing times, while the lateral correlation length decreased. As the thickness of the multilayers increased, the metal layer transitioned from amorphous to a crystalline state. The thickness of the multilayer at which the phase transition from amorphous to crystalline occurred was known as the critical crystallization thickness [11; 17; 18]. In our recent work [17], the microstructure evolution of NiV/B\({}_{4}\)C multilayers (d = 3 nm) after annealing was analyzed by GISAXS and X-ray scattering. We found that the amorphous NiV/B\({}_{4}\)C multilayers (as-deposited) were crystallized after annealing. The interface parameters such as interface roughness and lateral correlation length were obtained by fitting the PSD function. We found that the interface roughness of NiV/B\({}_{4}\)C multilayers increased with higher annealing temperatures or prolonged annealing times, while the lateral correlation length decreased. As the thickness of the multilayers increased, the metal layer transitioned from an amorphous to a crystalline state. The thickness of the multilayer at which the phase transition from amorphous to crystalline occurred was known as the critical crystallization thickness [11; 17; 18].