The stress field around a Vickers indent was analyzed using the relative shift of the 520/cm Si-Raman line. The Vickers indent was performed with a force of 50mN, resulting in a pyramidal hole of 210nm depth and 2,75ìm diagonal size, as shown in the AFM image on the right and (5x5ìm scale).
Figure 1. AFM image.
Topography Images Around a Vickers Indent Using WITec AFM
The images below show a comparison of the topography (5x5ìm, 5nm vertical scale, left) around the Vickers indent and a subsequent stress image of the same area obtained in Raman imaging mode (10x10ìm scale, right). The area of the AFM image is marked for a clearer comparison.
Figure 2. Comparison of the topography (5x5ìm, 5nm vertical scale, left) around the Vickers indent and a subsequent stress image of the same area.
Raman Images Obtained by Rotating the AFM Objective
The Raman image is obtained by simply rotating the turret from the AFM objective to a standard optical microscope objective (Nikon, 100x, NA=0.9 NCG) without moving the sample. As can be seen, the extension of the stress field can be detected more than 5ìm from the center of the indent. The stress image (100x100=10.000 spectra) was obtained with 10mW excitation power (532nm) and only 70ms integration time per spectrum.
Stress Images Around a Vickers Indent
The position of three cross sections (labelled 1-3) are marked in the stress image on top and displayed on the right (1: top, 3: bottom). As expected, the stress field has the symmetry of the Vickers pyramid. The tensile strain (blue in the stress image) appears at the edges of the pyramid, while the compressive strain (yellow in the stress image) appears at the flat sides.
Figure 3. The position of three cross sections (labelled 1-3) on top and displayed on the right (1: top, 3: bottom).
Extreme Precision of the Stress Measurements
The cross sections show the extreme precision of the stress measurement with a standard deviation of only 0,021/cm. This accuracy was obtained by determining the peak position of a parabolic fit to the Si-Raman line of each measured spectrum. This precision was achieved with an integration time of only 70ms per spectrum.
With longer integration times, an even higher degree of accuracy is possible. As the relation between peak shift and stress for silicon is (MPa) = -435 (1/cm), an accuracy of 0.02/cm corresponds to a stress sensitivity of only 9 MPa.
This information has been sourced, reviewed and adapted from materials provided by WITec GmbH.
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