## 3 Results and Discussion
Figure 7 shows the microstructure of the alloy film in the deposition state and after one and five repetitions. After the thermal resistance test, the surface of the WTi alloy film was a still flattened strip structure, and the cross-section had a compact columnar crystal structure. With the increase in the number of repetitions and the longer the total time of the WTi alloy film at a high temperature, part of the internal stress was released, a few holes appeared, and the film densification decreased. In order to further understand the change in the microstructure of the WTi alloy film at a high temperature and the reason for the TCR decrease, we conducted the annealing treatment of the alloy film. Figure 5: (a) Room-temperature electrical resistivity, (b) thermal resistance curves, and (c) TCR values of the WTi alloy films as a function of Ti concentrations. Figure 6: **(a)** Thermal resistance curves and (**b**) TCR values of the WTi alloy film with 6.8 at.% Ti after cycled thermal resistance measurements. Figure 7: Surface and cross-section SEM images of the WTi alloy film with 6.8 at.% Ti after cycled-thermal-resistance measurements. The phase structure of the annealed WTi alloy with a Ti content of 6.8 at.% was characterized by XRD, as shown in Figure 8. A partially enlarged view is shown on the right side. The annealed XRD has only the characteristic diffraction peak of \(\alpha\)-W, but no characteristic peak of Ti, which indicates that the annealed alloy film still has a single-phase solid solution structure, and all Ti is dissolved in a solution in the W lattice. After annealing, the half-height width of the (110) diffraction peak increased and the grain was refined. As can be seen from the locally enlarged image on the right of Figure 8, the diffraction peak of (110) shifts to a high angle after annealing, and the compressive stress generated in the deposition process of the film is released. The microstructure of the WTi alloy films before and after annealing and at different annealing times is shown in Figure 9. After annealing, the surface and cross-section of the alloy film do not change considerably: the surface has a leveled strip structure and the cross-section has a columnar crystal structure. After annealing, the grain size of the WTi alloy film increased, a few pores appeared, and the density of the film decreased. When the annealing time increased from 30 min to 60 min, the grain size further increased and the density of the alloy film further decreased. There is compressive stress in the deposited WTi alloy film, which makes the structure of the film denser and the width of the grain boundary smaller. The surface roughness of the WTi alloy film after annealing is shown in Figure 2:0 The surface roughness of the WTi alloy film before and after annealing was about 9 nm, and the electron scattering ability of the film surface does not change. Figure 2:1 (**a**) XRD patterns of the WTi alloy film with 6.8 at.% Ti after vacuum annealing at 400 \({}^{\circ}\)C for 0\(\sim\)1 h and (**b**) expanded-scale XRD patterns around the W (110). The structure of the deposited WTi films is denser, the scattering effect of grain boundary on the electron is lower, and the resistivity is lower. After annealing, the density of the alloy film decreased and holes appeared, which increases the resistivity of the film, as shown in Figure 11a. Figure 11b shows the thermal resistance curves of the 6.8 at.% Ti alloy films annealed at different times. When the annealing time increased from 30 min to 60 min, the thermal resistance curves basically coincided and the TCR remained unchanged, indicating that the structure and thermal resistance of the WTi alloy films reach a stable state after annealing for 30 min. After annealing for 30 min, the TCR of 6.8 at.% Ti alloy film was 14 \(\times\) 10\({}^{-4}\) K\({}^{-1}\), and the linearity of the linear fitting was high, which indicates that the WTi alloy film has a good thermal resistance after annealing for 30 min.