Renaud Vayrette

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.

Raman analysis of strain in p-type doped silicon nanostructures

In this work, 100 nm-thick boron-doped silicon beams with doping levels between 1 × 1016 and 1 × 1020 cm−3 undergoing uniaxial tensile strain are investigated by Raman spectroscopy. The structures exhibit a noticeable reduction in Youngs modulus (∼20%) compared with the value reported for bulk. The traditional Raman shift coefficients used to determine stress and strain in bulk structures are revised, and appropriate corrections are implemented to account for the observed changes in Youngs modulus. Interestingly, the Raman shift-strain relation in silicon nanostructures with strain along the [110] direction is found to be independent of size effects and doping. In contrast, the Raman shift-stress relation is found to be highly dependent on size effects. The dependency of the Fano line-shape parameters, used to fit the Raman first order peak in structures with high levels of doping, with strain is also reported. The results are shown to be crucial to accurately determine stress and strain from Raman measurements in doped silicon nanostructures and devices with size effects.