Multi objective optimization of BEoL and fBEOL structure in flip chip package during die attach process

Semiconductor industry has recognized the need to replace traditional Al/SiO2 interconnects with Cu/Low-k interconnects in the mainstream electronics devices following the latter´s impact on power, RC delay, and cross-talk reduction. However due to lower elastic modulus, strength, and poor adhesion characteristic, reliability of the Cu/Low-k interconnects turns out to be a concern for its integration in the back-end-of-line (BEoL). Flip-chip attachment process (cooling from ~200C to room) can result in critical damage in nano-scale Cu/Low-k interconnects. The objective of this study is to improve the reliability of Cu/Low-k interconnects during die attach reflow process for a specific die to substrate size ratio by varying a group of design parameters such as substrate thickness and solder bump footprint. Preliminary parametric study has shown that the variation in the concerned design variables has a significant effect on the solder bump (fBEoL) and low-k layer damage (BEoL) [1]. However, there is a trade-off between the solder bump and the dielectric damage with bump footprint, thereby arising a need to perform a multi-objective design optimization. A simulation based multi-objective design optimization has been carried out to improve BEoL/fBEoL reliability under reflow loading by minimizing the following objective functions 1) strain energy in the solder bump and 2) peeling stress in dielectric (low-k layers). This work is of immense importance from process integration standpoint. It can provide a quantitative upstream guideline to the process/electrical team on the BEoL/fBEoL damage.