An innovative bonding technique for optical chips using solder bumps that eliminate chip positioning adjustments

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 chips positioning for coupling purposes. The alignment accuracy increases as the solder bump diameter decreases and as the number of bumps increases. An average misalignment of 1.0 μm was obtained for specimens with four bumps 75 μm in diameter, whereas an average misalignment of 0.8 μm was obtained for specimens with 16 bumps 130 μm 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 μm 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