Grain Engineering and Surface Passivation of Electroplated Copper for Direct Copper-to-Copper Bonding

Three-dimensional integrated circuits (3D ICs) have emerged as the mostpromising solution to address the physical limitations of Moore’s Law. By leveraging thevertical dimension, 3D IC involves the vertical stacking of silicon chips, utilizingmicrobumps and through-silicon-via (TSV) technologies. The current bondingtechniques rely on Cu bumps or pillars capped with solder materials, but issues arise infiner-pitch packages. Recently, direct Cu-to-Cu (Cu-Cu) bonding has attractedattention because of its potential to replace solder joints using tiny direct Cu-Cuconnections, thereby providing higher interconnect density to enable advanced packagingtechnology with pitches of 10 μm and below. However, challenges such as high bondingtemperatures and extended processing times pose significant hurdles for Cu-Cu bondingin large-scale industrial production. The high thermal budget can lead to device damage,particularly affecting temperature-sensitive components, and may result in problemslike wafer warpage, misalignment, and thermal stress due to varying thermal expansioncoefficients of materials. In addition to the inherent properties of copper, another criticalissue is the rapid formation of surface copper oxide during long processing time afterelectroplating and before bonding. This copper oxide layer acts as a barrier to successfulbonding, especially under low-temperature conditions with limited driving forces.To reduce the thermal budget for Cu-Cu bonding, research has explored grainengineering with (111)-oriented nanotwinned (nt) Cu or nanocrystalline (nc) Cu. The nt-Cu joints still exhibit a sharp interface and weak bonding strength when bonding below200 °C, requiring heating above 250 °C to improve cross-interface bonding quality. Onthe other hand, the rapid recrystallization characteristics of nc-Cu enable grain growthacross the bonding interface, yet the self-annealing effect of grain growth at roomtemperature limits its practicality during extended processing times. Therefore, thisproject aims to fabricate a metastable copper material that remains stable duringprocessing at room temperature while enabling substantial grain growth during bondingat elevated temperatures. We propose two structures to achieve metastable Cu, nc-Cuelectroplated onto nt-Cu and composite structure combining nc-Cu and nt-Cu.Moreover, to address Cu surface oxidation, we propose developing a DCB (dynamiccovalent bond) chemical coating material with real-time responsiveness to externalstimuli, providing temporary passivation to prevent oxidation and allowing for easyremoval before bonding. The objective of this project is to provide a promising andpractical approach to attain superior bonding quality, lower operating temperatures, andincreased throughput for direct Cu-Cu bonding in advanced electronic packaging.