The Effect of Plasma Surface Activation Treatment on Capillary Flow Wettability of the Underfill Adhesive in the Standoff Gap of Si/Underfill/Si Bonded Structure in 3D ICs

The activation effects of mixed 02/ Ar plasma on the underfill capillary flow wettability in 100 µm sub-gap layers using radio-frequency capacitively coupled plasma (RF CCP) and microwave electron cyclotron resonance plasma (MW ECR) are systematically investigated in this study. To mimic the die/underfill/substrate structure in actual 2.5D/3D integrated packaging, the Si/underfill/Si adhesion structure is designed. Results demonstrate that as the proportion of O2 in mixed 02/ Ar plasma increases, the surface energy at the substrate center rises steadily. Moreover, the enhancement of surface energy is greater using MW ECR than RF CCP, enabling rapid wetting and dispensing of underfill on both die and substrate layers. Surface energy components measurements reveal that the contribution of mixed 02/ Ar plasma to the surface energy of substrate is primarily due to an increase in the polar component. Analysis results of X-ray photoelectron spectroscopy (XPS) surface chemical compositions of the substrate center after plasma treatment show a decrease in the carbon contamination peak and the generation of specific polar functional groups (C=O, O-C=O, Si-OH). Subsequent underfill dispensing and curing after plasma activation treatment result in reduced delamination in the Si/underfill/Si structure, along with a decrease in porosity. This comprehensive characterization of plasma surface activation treatment brings a better understanding of plasma modification, plasma equipment, and the enhancement of reliability in advanced packaging.