Copper shrinkage results in tensile stress in the silicon while CMP stop layer shrinkage results in compressive stress in the silicon
Through Silicon Vias (TSVs), an important component of 3D chip stacking technology, typically have a “keep-out zone” around them, where transistors are not placed. This is due to co-efficient of thermal expansion mismatch between the copper TSVs and silicon, which introduces tensile stresses in the silicon and changes transistor performance. These keep-out zones are typically >7mm, which adds constraints for design and leads to die size penalties.
In this work from GLOBALFOUNDRIES, a CMP stop layer is specially designed such that it introduces compressive stresses on the silicon and compensates for the tensile stresses introduced due to copper TSVs. The result is a near-zero keep-out zone for TSV technology, that is validated with simulations as well as experiments.
[5.2. M. Rabie, et al., “Novel Stress Free Keep Out Zone Process Development for Via Middle TSV in 20nm CMOS”, GLOBALFOUNDRIES]
OOPS !…obviously keep out zones are not 7 mm