Development of Au-Al solid phase diffusion flip chip bonding

The authors have developed a new chip-on-glass (COG) bonding technique for liquid crystal display (LCD) panels. The technique, named Au-Al solid phase diffusion flip chip bonding, was developed as follows. The gold bumps on the LSI were bonded directly to the aluminum electrodes on a glass substrate by formation of Au-Al diffusion layer. The examined bump pitch was 80 μm. After bonding, the mean contact resistance was nearly 30 mΩ, and the shear strength was 2.7 kgf/chip (4.5 kgf/mm²). A stable electrical connection has been proved through reliability tests. The Au-Al diffusion layer in the contact interface was analyzed by the X-ray diffraction (XRD) method and the Auger electron spectroscopy (AES) method using test samples which have 0.4 μm thick Al layer and 20 μm thick Au layer. After annealing, the intermetallic compounds of the diffusion layer were identified as “Au5Al2” and “Au4Al”. The intermetallic compounds gradually change from “Au5Al2” to “Au4Al” over time. We estimated that the longer the annealing time is, the thicker is the diffusion layer in the contact interface. We estimated the relationship between the reliability of this interconnection and the condition of the contact interface. If bonding conditions are higher temperature, longer time, and higher pressure, the reliability is enhanced. These bonding conditions suggest that the diffusion layer of the contact interface is thick and the formation ratio of “Au4Al” compounds is large. Prototype TFT-LCD panels with 80 μm pitch-driver ICs were successfully developed. This new bonding method can be applied to TFT-LCD panels and many other kinds of electronic equipment