Objective With the application of new generation power electronic devices, their advantages in severe working conditions are gradually emerging. For instance, SiC power devices can serve in high-temperature conditions over 300 ℃, and their packaging materials should be bonded at low temperature (≤250 ℃) and work reliably in high-temperature conditions. Sintering silver nanoparticles (NPs) technology is an effective method that can form joints with excellent electrical and thermal properties between SiC chips and metallized substrates. The traditional silver NP-sintering technology is synthesizing NPs by a chemical method and mixing them with organic components to prepare pastes. However, the adverse effects of organic components in the pastes can reduce the performance of sintered joints; thus, organic-free solutions have emerged recently. Pulsed laser deposition (PLD) with ultrafast laser as the light source is an efficient method to prepare organic-free silver-nanostructured films with large areas. Ultrafast laser with a very high peak power can ablate a target and produce ions with high kinetic energy, which can be controlled by the atmosphere to form films with unique nanostructures. In this study, organic-free silver micro-particle and NP composite film (SMNCF) was prepared by a PLD method on SiC chips. The films were used as intermediate layers for the low-temperature sintering process to bond SiC chips and metallized ceramic substrates. The size distribution of the as-prepared nanosilver films is diverse and controllable, and the sintering temperature can be as low as 180 ℃, which meets the shear strength requirements of SiC power devices.
Methods In this study, SiC chips with deposited SMNCF were mounted on silver metallized substrates to form modules. These modules were sintered at 250 ℃ with 10 MPa applied pressure for 30 min. In addition, samples in control groups were prepared with hybrid silver NP pastes. The modules were dried at 150 ℃ for 5 min and then maintained at 250 ℃ for 30 min under 10 MPa applied pressure to complete the sintering process. High-temperature storage (HTS) is a general method to verify the reliability of sintered layers using SMNCF, and it mainly focuses on the change of shear strength after tests and the influence of microstructure evolution in sintered layer. The HTS tests were performed at 300 ℃, and cross-sectional samples of sintered joints after tests were prepared to observe the microstructure of the sintered layer. Shear strength, porosity, and average pore area of sintered layer were measured. In addition, the HTS tests were divided into two categories: long-term tests in the atmospheric and vacuum environments for 2000 h (holding time of the control group using hybrid paste is 1500 h) and short-term tests at varying oxygen concentrations for 400 h. In the HTS test with varying atmosphere, three groups of the atmosphere were set for comparison: inert atmosphere using argon, which was used to confirm whether the pore evolution in the sintered layer was consistent with the samples in the vacuum environment; atmosphere with 20% oxygen concentration, which was used for comparison with the atmospheric environment; and a pure oxygen environment, which was used to observe the effect of pure oxygen on the microstructure evolution of sintered layer.
Results and Discussions Test results (Fig.3--5) show that the shear strength of sintered joints was always higher than 20 MPa in the HTS test at 300 ℃ for 2000 h, which was significantly higher than the standard MIL-STD-883K. Besides, although the shear strength of sintered joints using hybrid NP paste is also higher than the standard MIL-STD-883K, it is significantly lower than the control groups using SMNCF. This is attributable to the residual organic components in sintered layer with decomposition temperature higher than 300 ℃, which would affect the mechanical properties of sintered joints. During 0--400 h in the atmospheric environment, the pores in sintered layer gradually accumulated and led to densification of sintered layer, which increased the shear strength of the joints; during 400--2000 h, the accumulation of pores led to a continuous expansion of the pores, which increased the porosity and decreased the shear strength of joints. The vacuum environment hindered the evolution of pores in sintered layer (Fig.6). The increase in oxygen concentration in the atmosphere can accelerate the evolution process of the sintered layer during HTS tests. The influence of oxygen concentration on the mechanism of the microstructure evolution was discussed (Fig.7).
Conclusions In this study, organic-free SMNCF was fabricated by the PLD method and a low-temperature sintering process was performed to obtain porous joints between SiC chips and silver metallized substrates. HTS tests were performed in different atmospheres. The results show that the shear strength of the sintered joints is always higher than 20 MPa at 300 ℃ for 2000 h. The pore aggregation and connectivity during the tests can be observed in the atmosphere, and this phenomenon is mostly influenced by the oxygen concentration in the atmosphere, while the ambient pressure and other gas components in the atmosphere have an insignificant influence on the pore evolution. In vacuum and inert atmosphere, the microstructure of the sintered layer is relatively stable in the HTS tests, and there was no significant change in shear strength of the joints. This indicates that both vacuum and inert atmosphere are beneficial to improve the high-temperature reliability of the porous sintered joints using SMNCF.
