Characterization of In-Situ Tensile MEMS Devices for Stress Strain Measurements
O.S.R.M. Granata (TU Delft - Mechanical Engineering)
G.J. Verbiest – Mentor (TU Delft - Mechanical Engineering)
V. Papadimitriou – Mentor (DENSsolutions)
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Abstract
The mechanical, electrical, and thermal properties of two-dimensional materials such as graphene depend strongly on their applied strain and on the out-of-plane wrinkles and defects present in freestanding membranes. Quantifying the relationship between strain and structure requires applying a controlled load to a sample while simultaneously imaging it at high resolution, which is only achievable with a tensile stage small enough to operate inside an electron microscope. This thesis characterizes two such microelectromechanical (MEMS) devices, both designed for in-situ TEM tensile testing of micro- and nanoscale samples: an electrostatic device based on a comb-drive actuator, and an electrothermal device based on chevron thermal actuators. Both apply a uniaxial strain to a sample mounted across a gap and sense the resulting force through a capacitive comb structure. For each device, the actuation, sensing, and operational limits were described analytically as functions of geometry, material properties, and actuation input, and the models were compared against experimental measurements obtained through SEM imaging, Raman thermometry, and capacitive readout.
For the electrothermal device, a four-parameter model was fitted jointly to displacement and temperature data and reproduced both datasets well, with each fitted parameter retaining a physical interpretation. For the electrostatic device, the predicted pull-in displacement of 0.61 μm, shown analytically to depend only on the initial comb geometry, was confirmed experimentally, while the capacitance measurement was dominated by parasitics and could not be resolved. Two production-related effects shaped the results: the omitted molybdenum conductive layer, which caused the surrounding chip to heat substantially during thermal actuation, and incomplete deep reactive-ion etching, which restricted functional testing to the 5 μm devices.
The two devices are found to be complementary rather than competing, each covering part of the intended measurement space, and the most impactful improvements for future work are identified as incorporating the molybdenum layer, fabricating functional 20 μm