Bjorn Debecker

Chip package interaction (CPI): Thermo mechanical challenges in 3D technologies

The residual stresses generated during different processing steps and during thermal cycling of 3D stack packages, mimicking its service life, are quantified by Finite Element Modeling (FEM) together with measurements of dedicated FET arrays used as CPI sensors. Thermo-mechanical deformation of the package can be directly transferred to the Cu/low-k interconnect, inducing large local stresses to drive interfacial crack formation and propagation. The test vehicle used in this work is an imecs proprietary logic CMOS IC on top of which a commercial DRAM is stacked. Different test structures contained in the chip, allow monitoring thermo-mechanical stresses and electrical characteristics of TSVs and micro-bumps. It is shown that FET current shifts can be used to measure the stress in the surface of the chip. The use of standard FEM approach is insufficient to simulate the CPI due to the large dimensional difference between the packaging and interconnects structures. Due to size and speed limitations of commercial computers, a 3D thermo mechanical model of a 3D package cannot contain all the details from the package and at the same time simulate the small structures such as metal and dielectric layers in the BEOL. For this reason, multi-scale simulations are the best choice for identifying the critical regions of the package where high stresses and/or delamination failures are expected to occur. We have shown the methodology to follow to study the CPI.

Delamination in BEOL: Analysis of interface failure by combined experimental & modeling approaches

It is demonstrated that the widely used equation to derive the adhesive strength from the external load in a 4 point bending test has a certain error by taking only the substrates elastic parameters into account. Next to this, failure location analysis gives insight on the measured variation of adhesive strength for the same interface. Subsequently, with numerical simulations, the adhesive strength can be separated in its different energy modes, allowing a better understanding and characterization of the experimental observations (e.g. higher cohesive energy for increasing low-k stiffness). Finally, the competing failure mechanisms within the BEOL are identified as necessary qualifiers for BEOL failure assessment, where currently adhesion strength is mainly used for ranking the performance of different BEOL interfaces.

Localization by Acoustic Emission in Transversely Isotropic Slate

A method for localization by acoustic emission in transversely isotropic media is developed and validated. Velocities are experimentally measured and then used to calculate a database of theoretical arrival times for a large number of positions. During an actual test, positions are assigned by comparing measured arrival times with the databases arrival times. The method is applied during load tests on slate samples and compared with visual observations of fractures. The localization method allowed for a good identification of the regions of fracturing at different stages during the test.