## 3 Results and Discussion
### Morphological Analysis  
Figure 2 shows SEM images of bottom layer Mo films prepared at different sputtering pressures (Figure 2a-e), Mo bilayer films prepared by RF/DC sputtering at different bottom layer sputtering pressures (Figure 2f-j), and Mo bilayer films with a bottom layer sputtering pressure of 0.4 Pa and top layer sputtering pressure of 0.3 Pa annealed at different annealing temperatures (Figure 2k-o). It is showed that the bottom layer Mo films prepared at different sputtering pressures consisted of spindle nanoparticles. With increasing bottom layer sputtering pressures, the grain sizes of bottom layer Mo films decreased, the uniformity of grain distribution decreased, agglomeration occured, gaps and voids increased, and the RMS roughness increased gradually (Table 1). As mentioned in the above XRD results, the Mo particles of bottom layer Mo films prepared at higher sputtering pressure underwent more collisions with Ar atoms and had less kinetic energy, which weakened grain nucleation and growth of bottom layer Mo films. All the Mo bilayer films prepared by RF/DC sputtering at different bottom layer sputtering pressures also consisted of spindle nanoparticles with uniform distribution. The length of spindles were about 120 nm, the main changes were the width of the spindles. With decreasing bottom layer sputtering pressures, the width of the spindles of Mo bilayer films increased gradually, and the voids and the root mean square (RMS) roughness decreased (Table 1). As mentioned above, the lower bottom layer sputtering pressure caused higher kinetic energy of Mo particles and then promoted the diffusion and growth of Mo particles, resulting in larger grain sizes, denser surfaces and lower RMS roughness and lower microvoids and/or vacancies in the bottom layer Mo films. This resulted in higher thermal stability of bottom layer Mo films prepared at lower sputtering pressure compared to those prepared at higher bottom layer sputtering pressure, which is particularly beneficial for the growth of top layer Mo films and the lattice matching between the bottom layer and the top layer is better, which is more conducive to the growth of Mo bilayer 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 and grain sizes of the Mo bilayer film. SEM images of Mo bilayer films with a bottom layer sputtering pressure of 0.4 Pa and top layer sputtering pressure of 0.3 Pa annealed at different temperatures are illustrated in Figure 2k-o. All the Mo bilayer films showed a uniform distribution of nanoparticles, with increasing annealing temperatures, the width of the particles increased, gaps and voids decreased. This is because, as the annealing temperature increased, the energy absorbed by Mo particles increased, the diffusibility and mobility enhanced, the combining ability with adjacent grains and nucleation ability of Mo grains increased. Therefore, with increasing annealing temperature, the width of the particles increased, the surface was more uniform and compact, and RMS roughness decreased gradually. Figure 2: Scanning electron microscope (SEM) images of bottom layer Mo films deposited at different bottom layer sputtering pressures: (**a**) 0.4 Pa, (**b**) 0.6 Pa, (**c**) 0.8 Pa, (**d**) 1.0 Pa, (**e**) 1.2 Pa, Mo bilayer films deposited at different sputtering pressures: (**f**) 0.4 Pa, (**g**) 0.6 Pa, (**h**) 0.8 Pa, (**i**) 1.0 Pa, (**j**) 1.2 Pa and Mo bilayer films annealed at different temperatures: (**k**) 100 \({}^{\circ}\)C, (**l**) 150 \({}^{\circ}\)C, (**m**) 200 \({}^{\circ}\)C, (**n**) 300 \({}^{\circ}\)C, and (**e**) 400 \({}^{\circ}\)C.  
Figure 3 shows the grain size (width of the particle) distribution as a function sputtering pressure and annealing temperature, which were obtained by nano measurer software. Most of the grains size distribution of Mo films showed normal distribution.