Self-Actuating Isothermal Nanomechanical Test Platform for Tensile Creep Measurement of Freestanding Thin Films

Microelectromechanical systems (MEMS) enable potent methods for nanomechanical testing. In many MEMS-based test platforms, high force polycrystalline silicon chevron-type thermal actuators are used as on chip actuators. Here, we implement thermal actuators using the refractory metal tantalum (Ta) as a new micromachined structural material in such a way that it can be integrated to test mechanical properties of a wide variety of metals. Because Ta’s coefficient of thermal expansion is more than double that of silicon, actuation is accomplished simply by raising the temperature. An isothermal condition is attained, while no force is lost to a heat sink. The platform is batch fabricated by conventional surface micromachining processes and is especially well suited to measure creep of thin films because the load spring is soft compared to the specimen uniaxially loaded in tension. This in turn gives rise to a slow stress relaxation and a large displacement range. Specimen alignment is excellent and handling procedures for thin films are simple and robust. As a validation of the apparatus, uniaxial Au tensile specimens of 110 nm thickness are fabricated and tested. Young’s modulus is measured and compares well with literature values, while creep rate is lower than other reported values for nanocrystalline Au, indicating that steady state creep is approached. This work demonstrates a powerful drift-free nanomechanical test platform for mechanical property measurement of freestanding thin films subjected to uniaxial tension. [2021-0171]