High Density Metal–Metal Interconnect Bonding for 3-D Integration

3-D integration provides a pathway to achieve high performance microsystems through bonding and interconnection of best-of-breed materials and devices. Bonding of device layers can be accomplished by dielectric bonding and/or metal-metal interconnect bonding with a number of metal-metal systems currently under development. RTI has been investigating Cu-Cu and Cu/Sn-Cu interconnect processes for high density area array applications. The interconnect pad fabrication processes and the interconnect bonding conditions (pressure and temperature) required for the formation of low resistance (10´s of mΩ), high yielding (≥99.98% bond yield), and reliable interconnects are described. The effects of thermal reliability testing (aging) on electrical connectivity and mechanical strength are presented. Results from the two metal-metal interconnect bonding systems are compared in terms of ease of assembly and small pitch (sub-15 μm ) scaling. Methods for obtaining high bond yield at smaller pitches are discussed.