Influences of the Ar/H2 plasma activation on the bonding strength and reliability of chips and substrates assembly

The plasma activation is a promised process to improve the bonding performance for materials pretreated with the plasma. In this study, the chips studded with gold bumps flipped-bonding onto alumina substrates using the thermal compressional bonding. To improve the bonding performance of gold bumps onto copper electrodes, both gold bumps and alumina substrates were pretreated to Ar/H2 plasma at 400 W for 30 s. Other parameters of plasma activating are 100 mTorr in back pressure and 120 sccm in gas flow rate of Ar/H2. After chips with gold bumps boned on copper electrodes over alumina substrates, the die-shear test was conducted to determine the bonding performance and the scanning electron microscopy (SEM) was used to examine the integrity of the bonding interface. All specimens of chips bonded on alumina substrates were subjected to reliability of high temperature storage test (HTS) at 150 °C from 200 hrs to 1000 hrs. The changes in die-shear forces and the integrity of bonding interface were used to evaluate the reliability of specimens subjected to HTS test. For alumina substrates pretreated with plasma activating, a higher die-shear force was obtained and a sound bonding interface was achieved. This observed results prove that Ar/H2 plasma was an effective scheme to enhance the bonding performance. After specimens subjected to HTS test, die-shear forces decreased with increasing the storage durations for substrates pretreated with plasma activating or not. Die-shear forces were higher than the minimum requirement stated in JEDEC specification at the storage duration of 400 hr and then die-shear forces were degraded when the storage duration extended from 600 to 1000 hrs for the substrate without Ar/H2 plasma pretreating. A obviously delamination was found at bonding interface between gold bumps and copper electrodes, and most fracture occurred at bonding interface after HTS test for substrates without Ar/H2 plasma pretreating. For substrates pretreated with Ar/H2 plasma, a clean bonding surface can be achieved and a sound bond with sufficient strength was obtained. The bonding performance and reliability of HTS are thus improved. The Ar/H2 plasma activation is an effective scheme to improve the bonding performance of chips and substrates assembly and to enhance the reliability of HTS test.