Mechanical Characterization Analysis of a Segmented Silicon Layer on Ultra-thin Polyimide Substrates by Experiment and FE Simulation

The acceptable flexibility for ultra-thin substrate would be reached by embedding the ultra-thin substrate into the flexible polyimide and patterning the poly-silicon or silicon into square segmentations. In this contribution, results of experiments and FE simulations on mechanical reliability issues of poly- and single crystalline silicon on ultra-thin polyimide substrates are presented. Generation of cracks within the silicon and dielectric layers was then studied under controlled bending (glass cylinders with diameters of 2-10 mm, compressive and tensile stress) using specially for this purpose designed bending tools. Specimen observation was done using an optical microscope with possibility of digital recording and evaluation by pattern recognition software. The results show that the cracks appear first in the dielectric layers in-between the silicon layer segments and only at higher loads propagate or are generated within the silicon itself. The development of first cracks depends significantly on the silicon layer segmentation size which affects both the crack density and the crack width. The crack density increases sharply with the strain for early stage and then increases slightly. The crack width increases steadily. The high flexibility result can be reached that no crack be detected under the bending tests with 2 mm diameters. The maximum strain failure criterion of the ultra-thin thermal silicon dioxide layer could be reached by specific bending and tensile tests and the results of tensile and bending tests match very well. Multilevel FEM simulations were performed in order to increase understanding of the major failure processes. Results of simulations and experiments compare quite well