Biaxial flexure testing of free-standing thin film membrane with nanoindentation system

Abstract

The advent of microelectromechanical devices has increased the demand for biaxial flexure testing at the micro- and nanoscale. However, testing at these scales is challenging owing to difficulties in manipulating very small samples and applying highly symmetric biaxial loads to them. In this study, we developed a facile technique for on-chip biaxial flexure testing. The principle of the technique was inspired by ball-on-ring biaxial testing commonly employed for macroscopic flexure analysis of ceramics. In our technique, the specimen is tested as a circular membrane fixed at its edges to a substrate. The center of the membrane is pushed from above by a rounded conical nanoindenter (NI) until the membrane fractures. Since this method does not require microscale specimen manipulation involving the securing of microscale components and/or samples by external fixtures, the test procedure is relatively easy to perform. In addition, the load and displacement curves obtained using the NI are high resolution, enabling precise strength evaluation and application to very weak structures such as nanoscale membranes. To demonstrate the test system, we designed and fabricated polysilicon membranes with diameters of 20 μm and thicknesses of 80 nm. The most likely source of error in the system is misalignment between the indenter and the membrane center. Accordingly, its effect was numerically analyzed. The results showed that a misalignment of less than 3.0 μm causes a 0.32% error in the first principal stress. Since the NI tester can easily attain this alignment accuracy, highly accurate stress estimates can be achieved. The measured fracture strengths of the polysilicon membranes were fitted to the Weibull distribution, revealing an average strength of 8.11 ± 0.31 GPa and a shape parameter of 13.9 ± 5.4, both of which agree well with the results from previous research. The fracture origins were observed near the center of the membranes. These results confirm the viability of our concept.