Implementation of Thick Copper Inductor Integrated into Chip Scaled Package

Implementation of thick Cu inductor in IC chips has been a highly challenging task for Far-backend packaging industry. One of the major concerns is that the thicker Cu trace wrapped by re-passivation polymer layers (Polymer-1 & Polymer-2) is often accompanied by remarkable internal stress in the overall package, which is plagued with the risk of layer-to-layer delamination and/or interface micro-cracks. In this paper, a thick Cu inductor integrated with a WLCSP-like packaging solution is demonstrated for the first time through delamination improvement, which depends strongly on both (1) via-opening (etching) and (2) post top copper treatment processes in the specific scheme. In the via-opening (etching) process, an optimized etching method is proposed to minimize the surface roughness (Ra) of polymer-1 to 2~3 nm and concurrently achieve byproducts free. On the other hand, for the subsequent post top copper treatment processes, the introduced XYZ plasma and wet clean treatment are carried out to enhance the seed layer removal efficiency. As a result, the metal residue is substantially reduced to lower than 0.1% from 1.7% according to XPS analysis. The XYZ plasma treatment also benefits the surface roughness (Ra) of polymer-1 to around 30~35 nm, which is sufficient for later polymer-1 & -2 interface adhesion. These treatment approaches will be systematically presented and discussed in detail with experimental evidence. By means of the surface modification approaches reported in this study, the interface delamination phenomenon was greatly improved. The thicker Cu on-chip inductor embedded with WLCSP-like package successfully passed multi-reflow with 10 cycles and component level reliability tests, such as MSL1+HTS500hrs and MSL1+u-HAST96hrs. Meanwhile, compared to common planar spiral inductors fabricated by Fab process, not only was the inductance boosted, but the Cu trace resistance is also greatly reduced 10-fold. Moreover, the most striking feature of this packaging solution is that one can insert some novel high-permeability materials into this novel packaging scheme for much better electrical performance simply depending on customer requirements and designs.