MEMS based nanomechanical testing method with independent electronic sensing of stress and strain

We report significant improvements in the sensing scheme of a microelectromechanical system (MEMS) based nanomechanical tensile testing technique that has been previously demonstrated to allow direct microstructural observations inside a transmission electron microscope (TEM) at high magnifications while simultaneously measuring the stress and strain in the sample electronically. The particularity of the MEMS device is the presence of two capacitive sensors on either side of a specimen gap (across which nanostructures like nanowires and thin films can be manipulated and clamped), allowing independent measurement of applied load (stress) and crosshead displacement (strain). The improvement in the sensing technique lies in the independent, separate measurements of the signals from the two capacitive sensors (as opposed to the previous technique based on the differential measurement between the two sensors). The new technique (called technique 2 in this paper) provides independent electronic sensing of stress and strain without making any assumption about the material behavior and opens up the possibility of doing force controlled tests. The new sensing technique is demonstrated and compared to the previous one (called technique 1 in this paper) by performing ex-situ monotonic and stress relaxation tests on freestanding 100-nm-thick Au films. Important practical matters such as specimen clamping and signal drift are also discussed.