Marie-Stéphane Colla

On-chip stress relaxation testing method for freestanding thin film materials.

A stress relaxation method for freestanding thin films is developed based on an on-chip internal stress actuated microtensile testing set-up. The on-chip test structures are produced using microfabrication techniques involving cleaning, deposition, lithography, and release. After release from the substrate, the test specimens are subjected to uniaxial tension. The applied load decays with the deformation taking place during relaxation. This technique is adapted to strain rates lower than 10(-6)∕s and permits the determination of the strain rate sensitivity of very thin films. The main advantage of the technique is that the relaxation tests are simultaneously performed on thousands of specimens, pre-deformed up to different strain levels, for very long periods of time without monopolizing any external mechanical loading equipment. Proof of concept results are provided for 205-nm-thick sputtered AlSi(0.01) films and for 350-nm-thick evaporated Pd films showing unexpectedly high relaxation at room temperature.

Study of creep/relaxation mechanisms in thin freestanding nanocrystalline palladium films through the lab-on-chip technology

A nanomechanical lab-on-chip set-up has been used to study the creep/relaxation response of thin palladium films with temperature. The basic idea is to use residual stresses present in a silicon nitride thin beam to load the test film after etching the underneath sacrificial layer. The main advantage of this experimental method is that we can simultaneously perform thousands of creep/relaxation tests without monopolizing any external actuating/loading equipment and without using any time consuming calibration procedures. A signature of the dominant relaxation mechanism is given by the activation volume which has been determined for different levels of plastic deformation and different temperatures. The activation volume is equal to 15-40 b3 at room temperature and tends to decrease with increasing plastic deformation. The activation volume decreases when relaxation takes place at 50 C down to 7-20 b3. These variations of the activation volume indicate the competition between two different thermally activated deformation mechanisms in the temperature range between 20 C and 50 C.