# Annealing Process-Induced Microstructural Variation in NiV/B4C Multilayers
## 3 Results and Discussion
### Grazing-Incidence Small-Angle X-Ray Scattering Analysis  
The core principle of GISAXS measurement is to collect spatial distribution information on scattered light intensity that deviates from the reflection direction. In actual multilayer structures, the complex surface and interfaces contribute to the observed scattered light distribution, which is the result of the combined effect of these interfaces. To accurately analyze the X-ray scattering information, researchers typically use the distorted wave Born approximation (DWBA) theory [22]. According to the DWBA model, the transformation of the X-ray from an incident wave (initial state) to a scattered wave (final state) at any interface is primarily driven by surface roughness or perturbation potential due to diffusion within the material. The perturbation theory treats the scattering amplitude as a power series of roughness height, making it possible to associate the scattering amplitude with the power spectral density (PSD) function without making any assumptions about the roughness height distribution. The average interface statistics can be determined using the integral method of perturbation theory, and the specific formula derivation can be seen in our previous work [17]. The GISAXS measurements of the NiV/B4C multilayers are shown in Figure 8a. The most obvious feature of GISAXS is the Bragg sheet [14] caused by the correlated interface roughness [24]. As the annealing temperature increases, the shapes of the Bragg sheets for the NiV/B4C multilayers slightly broaden with no changes in the number of Bragg sheets. However, annealing at 340 \({}^{\circ}\)C, the third Bragg sheet disappeared, indicating that the structure of the multilayer had changed. To investigate the process involved in this change, multilayer samples annealed at 340 \({}^{\circ}\)C for different times were tested. We found that after annealing at 340 \({}^{\circ}\)C for 10 min, the third Bragg sheet almost disappeared, and it disappeared completely at 60 min. When the annealing time reached 120 min, the third Bragg sheet reappeared and showed stronger scattering intensity. In Figure 8b,c, as the annealing temperature increases and the annealing time extends, the number of Bragg-like peaks of the NiV/B4C multilayers remains at five. However, the peak shapes and positions of the multilayers have changed, indicating that the thickness of the multilayers had changed. Combining the GIXR results, we found that the thickness of the NiV/B4C multilayers gradually increased throughout the annealing process. This result showed that it was not purely physical expansion; a chemical reaction took place at its interface [23]. In Figure 8d, an enhancement of the scattering outside the Bragg sheets can be seen [25; 26], which is more apparent in Figure 8a. In Figure 8d, the small peaks on both sides of the annealed sample correspond to the density fluctuation distribution. The line extraction of the sample before annealing has no visible peak outside the qx = 0 center, indicating that the sample does not have density fluctuations. Note that in the W/Si multilayer structure, similar density fluctuations are observed within the Si layer, which is attributed to the formation of crystals in the Si layer [26]. Figure 9 showed the PSD functions of the NiV/B4C multilayers. The interface structure of the NiV/B4C multilayers remained relatively stable as the annealing temperature increased, but the interfaces changed progressively with extension of the annealing time at 340 \({}^{\circ}\)C.  
The parameters for interface roughness and lateral correlation length were obtained by quantitatively fitting the interface PSD function. The PSD functions and fitting curves of the NiV/B4C multilayers before and after annealing are shown in Figure 9. As seen in Figure 9a,b, the PSD of the multilayers increased gradually with the rise in annealing temperature. As seen in Figure 9c,d, the PSD of the multilayers remained stable when the annealing time was less than 120 min, with an inflection point occurring at 120 min.