Towards Multi-Material Strain and Crystal Orientation Mapping with Scanning Reflectance Anisotropy Microscopy
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Author
Date
2023Type
- Doctoral Thesis
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Abstract
Recent years have seen a growth in the fields of flexible electronics and MEMS. Due to their low power consumption and versatility, these technologies are increasingly being used in a wide range of applications, e.g. from biomedical to motion sensing in smartphones. Nonetheless, the multi-material components used in these devices and the mechanical demands they face pose significant challenges for their performance and reliability, increasing the need for mechanical characterization techniques. Moreover, the prevalence of strain engineering in semiconductor systems, due to its inherent enhancement of optical and electronic properties, has also made mechanical characterization a must in the design process of industrial semiconducting technologies.
The most commonly employed non-destructive strain mapping techniques, based on electron microscopy, x-ray diffraction or Raman spectroscopy, can provide high resolution and strain sensitivity. However, they are most often limited to specific materials or a significant resource investment. Instead, reflectance anisotropy spectroscopy (RAS) is an optical technique that offers multi-material class strain sensitivity, even higher than the aforementioned techniques. By measuring the reflectance difference along two orthogonal directions, RAS is able to acquire the near-normal incidence ellipsometric response of the sample, which is related to elastic strain by means of the elastooptic effect.
This thesis demonstrates the capabilities of an RAS microscope, termed scanning reflectance anisotropy microscopy (SRAM), that builds upon previously proposed microscopy setups in the literature achieving diffraction-limited resolution and high strain sensitivity as a multi-material platform. Strain sensitivity is demonstrated for different material classes, including metals (gold) and semiconductors (crystalline and amorphous germanium), measuring the gold’s average elasto-optic constant to be 𝑃� = 0.18 − 0.30𝑖�. SRAM achieves a strain sensitivity of up to 10−4 with sub-micron resolution. This is demonstrated by analyzing complex strain distributions, created by externally straining milled structures on thin films, and comparing to FEM simulations. Furthermore, SRAM is also a highly phase sensitive technique, with phase sensitivities of up to 4.7·10−3 degrees, and can provide phase maps of, for example, metasurfaces. Plasmonic slot nanoantennas are employed as a model system to study symmetry breaking with dipolar transitions, achieving a strain sensitivity of 𝜅� = −20.9 meV/% and opening the door for strain markers in future studies. Furthermore, the influence of roughness and crystal orientation on the SRAM signal is investigated. It is found that roughness is the main limiting factor of the sensitivity of the technique and strategies for circumventing such limitation are provided.
Preliminary results also indicate a strong influence of crystal orientation, which could result in crystal orientation mapping with further research. In summary, the versatility of SRAM to study the breaking of the lattice symmetry by simple reflectance measurements opens up the possibility to carry out non-destructive mechanical and optical characterization of multi-material components, such as wearable electronics and semiconductor devices. SRAM provides an additional strain mapping technique that can cover some of the limitations of traditional strain mapping techniques. Show more
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https://doi.org/10.3929/ethz-b-000640983Publication status
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Publisher
ETH ZurichSubject
Reflectance anisotropy spectroscopy; Strain mapping; Microscopy; OpticsOrganisational unit
03692 - Spolenak, Ralph / Spolenak, Ralph
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ETH Bibliography
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