Design and Fabrication of Micro-System Embedding in Flexible Substrates

The possibility to conform microelectronic circuits onto non-planar or even high-curvature surfaces is of increasing interest for future applications. A promising flexible substrate concept based on vertical thinning/lateral partitioning and the substrate transfer technology is developed. Test structures for flexibility testing which consist of a partitioned ultra-thin poly-silicon layer sandwiched between 300 nm thermal and 500 nm PECVD silicon dioxide layers were designed and fabricated. These test structures were embedded into a soft material layer (polyimide). The results of bending tests around cylinders showed that the flexible structures could easily withstand the bending around a cylinder with diameter of 2 mm. The onset of cracks was detected in the dielectric layers (silicon oxide) region between the silicon segments, therefore second generation samples were designed with the silicon oxide between the segments removed. The wave-shape flexible interconnects with different parameters were also designed for signal communication between the functional silicon segments. The resistances of vertical and parallel paths were measured during the tensile tests with a custom designed setup which also allows optical observation. Multi-level FEM simulations were performed in order to increase understanding of the major failure processes. The correspondence between simulation and measurement results provides a good knowledge base for the further optimization of the embedded flexible structures.