The development of repairable Au-Al solid phase diffusion flip-chip bonding

The authors have developed a new repairable chip-on-glass (COG) bonding technique for liquid crystal display (LCD) panels. Gold (Au) bumps on an LSI chip were bonded directly to aluminum (Al) electrodes on a glass substrate by formation of Al-Au intermetallic compounds in the diffusion layer. The developed repairable bonding technique consists of two-level bonding process. First, the chip was bonded at 250°C. Partial interconnection could be obtained at the local contact portions between the Au bump and the Al electrode. If the electrical connection failed, the bonded chip was removed. There was a distribution of the area formed Al-Au intermetallic compounds at local contact portions for 250°C bonding. Some areas formed Al-Au intermetallic compounds of the Al electrode and were sometimes removed with the chip removal, with an underlying metal layer locally exposed at the surface. Then, a new chip was bonded on the same Al electrodes under the same conditions at 250°C. After obtaining the electrical connection, the second bonding was done at 350°C. An AlAu₄ intermetallic formation was obtained by this bonding in the diffusion layer. Reliability tests of second bonded samples were carried out and the contact resistance between the Au bumps and the Al electrodes was measured by the four-probe resistance measurement. In the case that the exposed area ratio of the underlying metal layer was less than 30% of bonding area for each Al electrode, a stable electrical connection was achieved for a high temperature storage test and a thermal shock test. Thus it was confirmed that a stable electrical connection had been obtained by the proposed repairable bonding process