Mechanical Characterization Analysis of flexible and stretchable Ultra-Thin Substrates by Experiment and FE Simulation

Initial studies have demonstrated that specially designed and fabricated microelectronics embedded in flexible substrates can maintain functionality when subjected to stretching as well as bending. The acceptable flexibility and stretchability for ultra-thin substrate could be reached by embedding the ultra-thin substrate into flexible polyimide and patterning the silicon into square or hexagon segmentations. 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. Generation of cracks within the silicon and dielectric layers are then studied under controlled bending and tensile tests using bending and tensile tools being specially designed for this purpose. Specimen observation can be done using an optical microscope with possibility of digital recording and evaluation by pattern recognition software. The crack onset and the propagation of cracks are characterized. The results show that the cracks appear first in the middle of the dielectric material in-between the silicon segments. Only at higher loads they propagate or are generated within the silicon itself. The development of first cracks depends significantly on the silicon segmentation size and gaps between segments. The highest flexibility result can be reached such that no cracks are detected under bending tests on cylinders with 2 mm diameter. The segment size and gap size have influence on the tensile deformation. The stiffness of samples with segments is different before and after the first crack occurs, as a consequence of the appearing cracks. Multilevel FEM simulations are performed in order to increase understanding of the major failure processes. The maximum principle strain as found from simulation and from the experiment compares quite well. The maximum local principal strains appear in the gap near the spot where first fracture occurred during testing in the oxide layers.