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 AI-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 degrees 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 AI-Au intermetallic compounds at local contact portions for 250 degrees C bonding. Some areas formed AI-Au intermetallic compounds of the Al electrode were sometimes removed with the chip removal. And an underlying metal layer was locally exposed at the remained surface. Then, a new chip was bonded on the same Al electrodes under the same conditions at 250 degrees C. After obtaining the electrical connection, the second bonding was done at 350 degrees 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 A] electrode, the stable electrical connection has been kept for a high temperature storage test and a thermal shock test. It was confirmed that a stable electrical connection had been obtained by the proposed repairable bonding process.