(A) study on epoxy die attach films(DAFs) for chip stacking

Abstract

The increasing demand for high performance integrated circuit devices has been leading the development of 3-D chip stacking technology. The wire-bonding based 3-D(WB-3D) chip stacking is performed by stacking dies using thermally curable polymer adhesive and electrical interconnection by wire bonding. The adhesives used in the WB-3D chip stacking are applied in two forms; pastes and films. Due to the benefits of thickness uniformity or easy processing, film type adhesives are widely used in market. There are three types of DAFs for the WB-3D chip stacking; a spacer, a die to die / die to substrate, and a film over wire types. DAFs are inevitably exposed to high temperature during the wire bonding process for electrical interconnection and the curing process of epoxy molding compounds. Furthermore, the moisture penetrating through packages is able to be absorbed in the DAF. Absorbed moisture in DAFs causes voids or delamination resulting in electrical failure by the degradation of adhesion strength. Therefore, to have better reliability of stacked dies assemblies using DAFs, DAF materials should have not only stable adhesion strength between upper and lower dies even at the high temperature but also lower moisture absorption to prevent voids or delamination. Therefore, adhesion strength at high temperatures and moisture absorption of DAFs should be investigated. It is reported that the thermo-mechanical properties of an adhesive such as modulus and $T_g$ are closely related to its adhesion strength at high temperature. Therefore, it is possible to improve the adhesion strength by adjusting the modulus and $T_g$ of DAFs. In this study, the thermo-mechanical properties of cured DAFs for die to die bonding have been investigated by modifying materials formulation such as multi-functional epoxy, acrylonitrile butadiene rubber (NBR), and silica filler. As the content of multi-functional epoxy in DAFs increased, modulus increased and $T_g$ moved to higher te...