Mechanical characterization of microelectronics embedded in flexible and stretchable substrate

Abstract

The acceptable flexibility and stretchability for ultra-thin substrate would be reached by embedding the ultra-thin substrate into the flexible polyimide/silica rubber and patterning the poly-silicon or silicon into square/hexagon segmentations. The segments interconnect by metal wires for the signal communication, different wire shapes are designed to reach more flexible. In this contribution, results of FE optimization on mechanical reliability issues of interconnect in-between segments on ultra-thin polyimide/silica rubber substrates are presented. Generation of cracks within the silicon and dielectric layers is 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 is 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 by optimizing segment size and gap. Multilevel FEM simulations are performed in order to increase understanding of the major failure processes and optimizing the interconnect geometry. Results of simulations and experiments compare quite well. Second generation samples are designed with consideration of interconnect in-between segments and avoid of silicon dioxide in-between segments.