With the continuous advancement of microelectronic technology, electronic components are developing toward higher integration and higher operating frequencies, which imposes requirements for electronic devices to withstand harsh service conditions such as high temperatures and high-frequency vibrations. In core application scenarios of high-power chips including radar and microwave communication, aerospace equipment, power grid inverters, and electronic control systems for new energy vehicles, extremely stringent reliability standards are imposed on power devices. Nowadays, such devices must not only support higher current carrying capacity but also operate stably under extreme environments with high temperature and high humidity, bringing brand-new and tough challenges to practical applications. Third-generation wide-bandgap semiconductor materials represented by silicon carbide (SiC) and gallium nitride (GaN) emerge as ideal solutions, featuring outstanding merits including excellent chemical stability, high breakdown voltage, high power density, high electron mobility and low dielectric constant. These advantages not only greatly boost the operational efficiency of SiC chips but also enable stable operation under severe service environments, delivering a more reliable solution to meet the demands of high-power chip applications.

Nevertheless, higher requirements are raised for the packaging technology of power chips. Devices are expected to maintain structural stability under high melting points and repeated temperature cycling, alongside superior mechanical strength, electrical conductivity and thermal conductivity. Multiple interconnection technologies have been proposed to satisfy these demands, such as high-temperature alloy brazing, transient liquid phase bonding and low-temperature silver sintering.

Silver sintering technology exhibits prominent advantages over traditional alloy brazing fillers. Composed of microscale and nanoscale silver particles, silver atoms can diffuse at low temperatures owing to high surface free energy, achieving full densification. Benefiting from this characteristic, silver sintering delivers exceptional thermal conductivity, electrical conductivity and mechanical properties, providing robust support for stable operation of high-power devices.

During electronic packaging, the surface cleanliness of substrates exerts a critical influence on solder joints. However, multiple factors in manufacturing, such as fabrication processes and packaging materials, may degrade surface cleanliness. For instance, the silver sinter paste adopted in this study consists of organic solvents and silver particles of various sizes. Volatilization of organic solvents during preparation and application will leave contaminants on the backside of substrates. To save space and realize miniaturization of electronic products, double-sided packaging is widely adopted, which adversely affects bonding on the opposite side of the substrate. Therefore, it is of great significance to investigate organic contaminants generated from silver sintering and the microscopic mechanisms that degrade the bonding quality between gold-plated substrates and chips.

Effects of Plasma Cleaning on Surface Contaminants

Contaminated substrates are placed in the vacuum chamber of a plasma cleaner and treated with mixed oxygen-argon plasma (power: 980 W, duration: 300 s). GC-MS detection identifies various organic contaminants on untreated substrates, including alkanes, alcohols and aldehydes. After plasma cleaning, alkanes and other organic pollutants are almost completely eliminated. As shown in Figure 1.1, the chromatographic peaks of organics are drastically reduced or even disappear, indicating a remarkable drop in the category and content of organic contaminants on sample surfaces. This demonstrates that plasma bombardment induces surface oxidation and activation, decomposing most organic pollutants into small molecules for volatilization and removal. The surface chemical composition becomes cleaner, and surface energy is elevated, facilitating interfacial interaction between silver sinter paste and substrates in subsequent sintering procedures.

Figure 1 Total ion chromatograms (TIC) of gold-plated substrates: (a) Contaminated substrate; (b) Plasma-cleaned substrate

Sintering Quality of Sintered Joints after Plasma Cleaning

Plasma cleaning significantly optimizes the bonding strength of Ag-Au sintered joints. Substrates treated by plasma cleaning are sintered with semi-sintered silver paste. Shear strength testing, fracture surface analysis and microstructure characterization are conducted on sintered joints to evaluate bonding performance. As illustrated in Figure 2.1, the average shear strength of untreated contaminated samples is only 24.4 MPa, while plasma cleaning raises the shear strength to 34.2 MPa (an increase of 40.2%), nearly matching the performance of uncontaminated reference samples.

Comparison of fracture morphologies between contaminated and cleaned samples (Figure 2.2) reveals that both samples present mixed failure modes (failure at the upper interface + cohesive failure within sintered silver). However, the sintered silver region accounts for 70% of the fracture area for plasma-cleaned samples, further verifying stronger interfacial adhesion between Ag and Au layers. Microstructural analysis further confirms the optimization effect of plasma cleaning on sintering quality. As shown in Figure 2.3, a large number of irregular voids exist at the Ag-Au interface of untreated samples. Statistics via ImageJ software show the interfacial bonding ratio is merely 38.2%, and silver particles are loosely stacked (Figure 2.3a). After plasma cleaning, the interfacial bonding ratio rises to 60.3%. Silver particles form dense sinter necks via surface diffusion, and a continuous Ag-Au interdiffusion layer is generated at the interface (Figure 2.3d). Such morphological evolution arises from the effective removal of adsorbed hydrocarbons by plasma cleaning, which eliminates physical barriers between silver particles and the gold coating, and promotes intimate contact between Ag particles and the Au layer.

Figure 2.1 Shear strength of Ag-Au sintered joints

Figure 2.2 Fracture surfaces and corresponding elemental distribution maps of Ag-Au joints: (a) Contaminated sample; (b) Plasma-cleaned sample

Figure 2.3 Cross-sectional SEM images of Ag-Au joints: (a, b) Contaminated sample; (c, d) Plasma-cleaned sample