Prevention of thinned wafer deformation during thermocompression bonding and multi-die stacking supported by temporary bonding materials

Abstract

Process flows for memory stacking or other heterogeneous integration schemes benefit from die bonding on a thinned silicon wafer 100 μm or less. In scenarios where a thinned device wafer contains features such as microbumps or Cu pillars, a carrier and temporary bonding material (TBM) facilitate the support of the fragile landing wafer during thermocompression bonding (TCB). The landing wafer in this case is vulnerable to deformations including loss of die planarity, Si bulging, Si or low k dielectric cracking, and damage to the underlying device wafer topography. In this paper, a dual layer system for temporary bonding is presented that maintains the integrity of a thinned device wafer during and after TCB. This is achieved with TBM materials which do not reflow at typical TCB conditions. The approach is to simulate TCB conditions which demonstrate the performance between different underlying TBM materials. A method which tracks the bond head z-axis over time during a TCB cycle is described which in turn yields information on the degree of temporary substrate deformation due to TCB force and temperature. The experiments include a worst-case scenario of multiple TCB cycles in the same position to mimic multi-die stacking. Finally, the impact of process conditions on Cu pillars with solder caps embedded in a thinned wafer bond line will be discussed.