Microwave processing techniques for high density interconnects and hybridization

Microwave Bonding Instruments, Inc. (MBI) has been investigating die-to-die (Level-1) substrate hybridization. Through high-density interconnect indium bumps, MBI applies microwaves to selectively heat the interconnect metal and bond the substrates. MBI conducted experiments to bond 81,920 (320x256) indium bumps on infrared-detector devices made of GaAs to the CMOS readout indicator array on silicon substrates. This selective heating technique has doubled the bump-bump bond strength (0.2g/bump), minimized the processing time (less than 15 seconds), at an overall low-substrate temperature (60 °C). The thermal mismatch between substrates is less of an issue in this bonding process. Higher density interconnect (e.g. third generation infrared focal plane arrays have sizes in excess of 1024 x 768) will benefit from this as the high applied pressure requirements used in other techniques are not needed here. MBI will be testing this technology for a staring array, with at least 750,000 interconnects per die pair on 15 mm x 15 mm. This microwave technique selectively heats up thin film metals that are lithographically patterned on semiconductor substrates. Gold-to-gold bonds have also been demonstrated using the same process. In the future, this type of process will have applications far beyond the IR market.