Mechanical Characterization of Deformable Ultra-Thin Substrates by Experiment and FE Simulation

In this paper, results of experiments and FE simulations on mechanical issues of poly-and single crystalline silicon on ultra-thin polyimide substrates are presented. Formation and propagation of cracks within the silicon and dielectric layers are then studied under controlled bending and tensile tests using bending and tensile tools being custom designed for this purpose. The results show that the cracks appear first in the middle of the dielectric material in-between the silicon segments and then propagate into the dielectric and poly-silicon material. The development of first cracks depends significantly not only on the "segment-area-to-gap-area" ratio but also on the silicon segmentation size under the tensile and bending tests. The stiffness of samples with segments is different before and after the first crack occurs, as a consequence of the appearing cracks. The high ratio of width (or length) to thickness of components offers a challenge for proper FEM simulation. Multilevel FEM simulations are first performed to get understanding of the major failure processes. The simulated critical (mean) strain agrees with the mean tensile strain in the tested samples. The maximum local principal strains appear in the gap near the spot where during testing first fracture occurred in the oxide layers. The correspondences between simulation and measurement results provide a good base for optimization of the segment size, gap size, interconnection shape, material and structure selection. Based on the experimental and simulation results, 2nd generation optimised samples were designed with full segmentation and containing spring interconnecting lines.