AN INNOVATIVE BONDING TECHNIQUE FOR OPTICAL CHIPS USING SOLDER BUMPS THAT ELIMINATE CHIP POSITIONING ADJUSTMENTS

This paper describes an innovative bonding technique for optical chips using solder bumps. The valuable feature of this technique is self-alignment of the optical chips due to the surface tension of the molten solder. This eliminates the need for painstaking adjustments of chip positioning for coupling to optical waveguides/fibers on the optoelectronic circuit boards. The major emphasis of this paper is on alignment accuracy, focusing on correlations relating to solder bump diameter and number. The alignment accuracy increases as the solder bump diameter decreases and as the number of bumps increases. An average misalignment of 1.0-mu-m was obtained for specimens with 4 bumps 75-mu-m in diameter, whereas an average misalignment of 0.8-mu-m was obtained for specimens with 16 bumps 130-mu-m in diameter. The long-term stability of the alignment accuracy was tested by subjecting the specimens to thermal cycling. Alignment fluctuations of less than +/-0.5-mu-m were found after 1000 h (over 300 cycles). These results suggest that the solder bumps are suitable for such applications as the bonding of photodetectors to optoelectronic circuit boards. Hybrid integrated optoelectronic circuit boards using this technique ate expected to be widely applied in future optoelectronic equipment.