Bending and stretching studies on ultra-thin silicon substrates

In this contribution, results of our work on reliability issues of ultra-thin poly- and single crystalline silicon layers on thin polyimide substrates under mechanical deformations is presented. By embedding the ultra-thin silicon layers into a thin flexible polyimide substrate and patterning of silicon into square or hexagonal segmentations, an increased mechanical flexibility and resistance against cracks could be reached. Generation of cracks within the silicon and dielectric layers is studied under controlled bending (cylinders with diameters of 2-10 mm, compressive and tensile stress) and tensile loads using bending and tensile tools, being specially designed for this purpose. Specimen observation was performed, 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 initiated within the silicon itself. The development of first cracks depends significantly on the segmentation size of the silicon layer. This affects both the crack density and the crack width. The crack density increases sharply with the strain at early stage and then increases slightly. The crack width increases steadily. The high flexibility result reached here shows no crack detection under the bending tests with 2 mm diameter. A maximum strain failure criterion was established for the ultra-thin thermal silicon dioxide layer by specific bending and tensile tests