## 3 Results and Discussion  
### Structural Analysis  
Figure 1 shows the XRD patterns of Mo bilayer films prepared at different bottom layer sputtering pressures (Figure 1a) and Mo bilayer films prepared at a bottom layer sputtering pressure of 0.4 Pa and top layer sputtering pressure of 0.3 Pa annealed at different temperatures (Figure 1b), respectively. All the Mo bilayer films showed (110) plane preferred orientation, located at 2\(\theta\) of 40.3\({}^{\circ}\), which is typical for Mo films with a body-centered cubic structure (JCPDS Card No. 3-065-7442). It is ascribed to the fact that the (110) face of the body-centered cubic structure generally has the lowest surface energy and tends to grow preferentially. With decreasing bottom layer sputtering pressures and increasing annealing temperatures, the intensity of the (110) diffraction peak of Mo bilayer films increased gradually. Scherrer formula was used to calculate average grain size (D) for (110) peak as a function of bottom layer sputtering pressure and annealing temperature. As presented in Table 1, the D values  
increased with decreasing bottom layer sputtering pressures and increasing annealing temperatures, reflecting an enhancement in crystallinity. This is because at lower bottom layer sputtering pressure, the Mo sputtered particles undergo fewer collisions with Ar atoms, leading to the kinetic energy of Mo particles deposited on the substrate being larger. This promotes grain nucleation and growth of Mo bottom layer film [22]. Therefore, the Mo bottom layer film prepared at lower sputtering pressure had compact structure, larger grain size and better crystallinity, which is consistent with a previous paper [23]. However, the larger sputtering kinetic energy brings about larger compressive strain of the Mo bottom layer film deposited on the substrate; thus, with decreasing bottom layer sputtering pressures in Ar, the compressive strain increased gradually (Table 1). The Mo bottom layer films prepared at lower bottom layer sputtering pressure have higher uniformity and compactness. This resulted in higher thermal stability of bottom layer films prepared at lower bottom layer sputtering compared to those prepared at higher bottom layer sputtering pressure, which is particularly beneficial for the growth of top layer Mo films. During the bombardment of top layer particles at the same sputtering conditions, less undesirable crystallographic defects escaped from the bottom layer prepared at lower sputtering pressure, resulting in an increase of the crystal quality of the Mo bilayer film [21]. Due to the thinner bottom layer Mo film and the higher sputtering pressure, the grain growth and migration time of the bottom layer Mo film was shorter and the sputtering kinetic energy of particles was less, then leading to smaller grain size and more defects and voids. As the sputtering pressure decreased, the sputtering kinetic energy of particles increased, the corresponding grain size of bottom layer Mo film increased, and the defects decreased. However, because of thicker top layer Mo film and the lower sputtering pressure, the sputtering kinetic energy of the particles was larger and the grain growth and migration time was longer. Thus, the grain size of the top layer Mo film was larger, even twice that of the bottom layer Mo film. In addition, the increase of annealing temperature led to an increase of surface diffusibility and mobility of Mo particles between the top and bottom layer, enhancing the Mo bilayer film quality. Moreover, with increasing annealing temperatures, the energy absorbed by particles of Mo bilayer films increased gradually, and the energy used to migrate and merge the surrounding particles also increased. Therefore, the grain sizes and crystallinity of Mo bilayer films increased with increasing annealing temperatures, which is consistent with the report of [PERSON] et al [24]. Moreover, with increasing annealing temperature, the strain in Mo bilayer film was released, correspondingly, the compressive strain decreased.