Void Formation and Bond Strength Investigated for Wafer-Level Cu-Sn Solid-Liquid Interdiffusion (SLID) Bonding

Hermetic wafer-level Cu-Sn solid-liquid interdiffusion (SLID) bonding was investigated to explore the sensitivity of selected process parameters with regard to voiding and possible reduction of strength. Little or no variation was observed in the void density as a result of modifying the plated Sn thickness, the storage time between plating and bonding, the bonding tool pressure, or the thermal budget during bonding. All shear tested samples showed excellent shear strength, with an average value within 110–164 MPa. Some statistically significant differences in shear strength were found due to the variations of the process parameters. However, the differences were too small to be critical for the application. Analysis of fracture surfaces showed that the shear strengths in the lower range corresponded to fracture between the adhesion layer (TiW) and the silicon cap, while shear strengths in the higher range corresponded to fracture in the Cu3Sn formed during the bonding. The results indicate that the investigated bonding process is robust with regard to the studied process parameters.