Electroless Copper Bonding with Local Suppression for Void-Free Chip-to-Package Connections

The effect of bis-(3-sulfopropyl)-disulfide (SPS) on the void-free electroless pillar-to-pillar bonding process has been investigated. Two dome-shaped Cu pillars were joined using electroless copper deposition with the addition of a suppressor to achieve solid, compliant Cu-to-Cu bonding without high temperature or pressure. SPS was added to the electroless copper plating bath which has strong suppression to the electroless plating in order to avoid the creation of an unbonded seam between the two Cu structures being bonded. The bath suppresses the deposition of copper near the entrance of the gap between the two pillars, while allowing high deposition rate in the geometrically restricted area between the copper structures being bonded. This phenomenon is due to diffusion of SPS through the narrow gap between the pillars being bonded. At adequate concentration of SPS, the two pillars were successfully joined without any remaining seam between the joined structures, by growth of copper from the center of the gap to the outside. a fillet of material between the two components being joined so that non-planar surfaces can be joined without having to be made flat or co-planar. Excessive force would have to be applied to copper pillars, or other I/O structures if direct copper-copper bonding were used to join components. The use of electroless deposition of metal is an alternate approach to flip-chip bonding. 9-11 The yield strength of the electroless bond- ing was high enough to successfully bond pillars together; however, defects or voids in the electroless metal between mated pillars is an issue. 12 Trapped voids could be an origin of various failure modes, such as rupture during thermally induced stress, electromigration due to current crowding around the voids, and high-frequency electrical noise. Voids can be created during the joining of pillars due to in- adequate plating in the narrow gap between the two pillars surfaces. For example, pillar-to-pillar gaps as wide as 5 to 20 μm may be en- countered in flip-chip bonding. The formation of dome-shaped pillars to reduce the initial gap between the two surfaces and the use of a surfactant to stimulate plating in the narrow gap were suggested to im- prove the bonding process. 13 These actions provide better deposition of the fillet between pillars compared to flat-top pillars with unmod- ified plating baths. 13 However, there remain fundamental issues in bonding large diameter pillars due to void trapping in the electroless metal joining the pillars. If the deposition rate difference in the gap at the center of the pillars being joined is less than that of the mass transfer preferred region at the edge of the pillars, then voids can be created. Thus, it is highly desirable to control of the deposition rate of the electroless bath to suppress deposition at the spatially favored regions, such as the pillar edge, so as to maintain access to the center of the pillar as the gap is filled. Organic additives can be added to electroless baths to suppress the deposition rate. Trench filling in geometrically restricted areas has been accom- plished in copper electrodeposition through the use of accelerators act- ing at the bottom of a trench with suppression at the top of the trench. A similar approach was taken in filling through silicon vias (TSV) by use of levelers. It is of interest to extend this concept to electroless copper deposition, such as used to fill trenches and bonding of pillars.