A Quantitative Sputtering Method for Accurate Composition Calibration of 2.5 µm Thin Au–Sn Metallization Enabling WLP SLID Bonding

This letter presents a quantitative approach for 2.5 µm Au–Sn metallization with accurate mass calibration, which is applied to solid–liquid interdiffusion bonding of microelectromechanical systems (MEMS) wafer-level packaging (WLP) devices, particularly those that demand a micron-level gap of out-of-plane electrodes. For Au–Sn metallization, traditional deposition methods, such as electroplating, are unable to achieve micron-level thickness and accurate composition due to poor robustness to varying process conditions. In this study, we further improve the composition of the deposited Au–Sn alloy through direct mass measurement and calibration during the cosputtering process. In the experiment, a high-precision balance with a resolution of 0.1 mg was utilized to measure the mass increment of sputtered Au and Sn in the clean room. Subsequently, the sputtering rate of the Sn target was calculated and applied to calibrate the final Au–Sn composition. According to the energy-dispersive X-ray spectrum (EDS) results, the difference between the measured Au–Sn mass composition and the set value is 1.1%, which is significantly lower than the typical 5%–10% composition deviation of electroplated Au–Sn solder, demonstrating a strengthened robustness to varying conditions by the effective calibration. The test results show that the shear strength of the WLP structure reaches 31.8 MPa, and the cross-sectional EDS results of the as-bonded Au–Sn alloy are consistent with the designed Au–Sn composition. The proposed calibration method can also be applied to other alloy depositions that require precise mass composition and micron-level thickness with a better robustness to varying conditions.