Influence of plasma cleaning on silicon die back adhesion performance

Abstract

Adhesion performance between silicon die back and paste is one of major concerns in die attach process during semiconductor assembly. A peculiar phenomenon that attached dies were flied off from pads of lead frame was reported during assembling in power QFN packages with occurrence rate at ppm. level. Backside surface of failed dies are very “clean” without any paste residual. It is mostly concerned backside surface was contaminated. In this study, attach experiment was conducted on the dropped dies with/without plasma treatment as well as normal dies. Their adhesion performance was investigated through shearing test. Simultaneously, the characteristic of backside surface with/without plasma treatment including wettability, composition and roughness was analyzed. As a result, it was indicated plasma cleaning improves the interfacial adhesion performance between silicon die and paste significantly. The similar failure mode occurred on the dropped dies without plasma, but after plasma clean, paste residual can be found on die backside and pad surface with adhesion strength increased as much again. The plasma cleaning improvement is also demonstrated by wetting test and component analysis. The wetting performance was modified from hydrophobicity to hydrophilicity and carbon percentage was reduced to low level with surface cleanness. There is no significant difference for roughness before/after plasma treatment. Furthermore, the characteristic of contaminant and possible resource introduced also be addressed in this paper.