Multi-Material Differential Strain Mapping with Reflectance Anisotropy Spectroscopy Microscopy

The mechanical characterization at small length scales is of fundamental importance to obtain insights of deformation mechanisms. In particular, stress mapping is of great interest since it allows the study of fracture mechanics, the detection of strain fields for elastic strain engineering and the optimization of load sharing in composite materials. Conventional non-destructive lab scale stress mapping techniques are based on electron microscopy and Raman spectroscopy. Common drawbacks are the required high vacuum for electron microscopy and the need of polarizable materials for Raman spectroscopy, hindering the range of materials that can be analyzed. X-ray diffraction is also an established technique for mechanical studies, however its need for a large interaction volume poses problems for ultra thin film studies while still being limited in lateral resolution. Synchrotron X-ray diffraction bypasses these drawbacks and achieves high spatial resolution and strain sensitivity for thin crystalline materials but requires significant resource investment.<br/>In contrast, reflectance anisotropy spectroscopy (RAS) can be employed for a wide range of materials, environments and sample thicknesses to become an attractive technique for non-destructive mechanical studies. RAS is an ellipsometric technique that measures the difference in reflectance between linearly polarized light along two orthogonal directions of the sample. However, usual resolution of conventional RAS setups is in the order of millimeters. Here, we present an advanced RAS microscope based on a super continuum laser source as a non-destructive strain mapping technique. Our microscope enables insight into the electronic band structure, phase and crystal orientation.<br/>We demonstrate the capabilities of the technique by mapping the differential strain distribution in both metallic and semiconducting samples. We achieve high uniaxial strain in suspended Germanium structures by top-down fabrication, image the strain distribution with micro RAS and compare spatial resolution and strain sensitivity to measurements taken with Raman spectroscopy. We also sputter metallic thin films onto flexible substrates and employ focused ion beam (FIB) milling to modify surface topology. We then externally apply uniaxial strain on the fabricated FIB structures and in situ measure the resulting strain distribution, demonstrating the material versatility of RAS. Furthermore, we show the use of dipolar nanoantennas fabricated with FIB to tailor the RAS resonance to our microscope’s spectral range and increase the range of materials that can be analysed. The strain mapping capabilities of RAS microscopy will enable future studies on the fracture mechanics of thin films and novel materials with different microstructure and environmental conditions.