Micro-welding using laser-generated ultrasound

We demonstrate a novel micro-welding process in which laser-generated stress waves produced close to the bonding site assist in the formation of direct metal-metal bonds. The process has been used to attach copper bumped silicon flip-chips to silver-metallized substrates. De-bonding tests were used to study qualitatively the resulting chip-substrate bonds. It was found that generally the copper-silver welding strength exceeded the adhesion strength between bump and chip. Individual bump shear tests were carried out after de-bonding and showed shear strengths of similar to 116 MPa. In addition, scanning-electron microscope (SEM) and focused-ion-beam-scanning electron-microscope (FIB-SEM) imaging were used to analyse the test samples, revealing that the welding was direct copper-silver solid-state diffusion welding. This new process allows direct metal-metal bonding at low temperature and with localized control over the bonding parameters. Applications are envisaged in advanced electronics packaging where low process temperature or high operational reliability are required.