Visualizing stress in silicon micro cantilevers using scanning confocal Raman spectroscopy

Abstract

We report on the determination of surface stress in a resonantly oscillating silicon micro cantilever using confocal Raman spectroscopy. Focusing on the optical phonon line of silicon allows one to determine the lateral distribution of mechanical stress. However, the Raman shift caused by mechanical stress can be concealed by thermally induced Raman shifts and line-broadenings. Both effects are intrinsic to the micro Raman measurement which uses a strongly focused laser beam in a confocal microscope. In order to unravel the different contributions, we use a practical analytical method for the compensation based on reference measurements on a heated silicon wafer of the same crystal orientation. As an example, the structure of the micro cantilever was specially weakened by introducing a rectangular hole in the center of the lever. After compensation of the thermally induced shift, the true mechanical stress can be visualized as shown for a stress maximum of the cantilever driven at its second flexural eigenmode.