Dottorato in MODELLI MATEMATICI PER L'INGEGNERIA, ELETTROMAGNETISMO E
NANOSCIENZE

Titolo della tesi: Non-destructive techniques for strain characterization and failure analysis in microelectronic devices down to the nanoscale: spotlight on Tip-Enhanced Raman Spectroscopy (TERS)

The semiconductor industry is undergoing a period of transformation, driven by the relentless pursuit of miniaturization. This downsizing trend, while advancing information processing technology, is also expanding applications and capabilities across multiple domains. The industry is constantly confronted with demands for higher performance and energy efficiency, which are critical factors shaping its evolution. The dominant technology, complementary metal-oxide-semiconductor (CMOS), uses metal-oxide-semiconductor field-effect transistor (MOSFET) inverters. However, shrinking MOSFETs below 10nm faces challenges such as high gate leakage, maintaining a high Ion/Ioff ratio, short channel effects and high power dissipation. In response to these challenges, micro- and nanoscale strain is emerging as a critical factor in modifying Si lattice parameters and Si band structure, leading to improved electronic mobility. Strain engineering and surface bonding techniques are increasingly employed, with silicon-germanium, particularly in the context of strained silicon technology, being a promising approach. This technology, which involves stretching silicon atoms in the layer beyond their normal interatomic spacing, is becoming an integral part of the manufacture of next generation semiconductor devices, impacting computing, communications and consumer electronics. In parallel, the PV industry is focusing on developing low-cost, high-efficiency solar cells using thinner silicon substrates (<100 μm) to reduce costs. Ultra-thin c-Si wafers offer higher conversion efficiency, flexibility and lower weight, making them suitable for building-integrated PV. However, there are concerns about mechanical yield loss due to micro and nano cracks during handling and processing of thin wafers. The implementation of stress control and stringent process control, similar to modern electronics manufacturing, is critical to maintaining reliability and lifetime in thin Si wafer and solar cell production. The increasing demand to determine the local value of the strain tensor and to control the properties of micro-nanomaterials and semiconductor devices underlines the need for advanced high-resolution characterization techniques. In today's micro- and nano-electronics fabrication facilities, there's a growing need for faster and non-destructive characterization tools, especially for strain monitoring during fabrication stages and real-time quality control through in-line monitoring. While Transmission Electron Microscopy (TEM)-based techniques have traditionally been the reference standard for nano-deformation analysis, they are destructive, expensive and time-consuming. This research explores the application of advanced spectroscopic and non-destructive characterization techniques, including µ-RAMAN Spectroscopy, Photoluminescence Spectroscopy and Tip-Enhanced Raman Spectroscopy (TERS). In addition, these techniques are validated with High Resolution X-ray Diffraction (HRXRD). µ-RAMAN spectroscopy provides insight into strain values and crystallinity defects, but its lateral resolution is limited by the Abbe diffraction limit. Consequently, the focus is shifting to the implementation of TERS, which combines the high spatial resolution of a scanning probe microscope (SPM) with the high chemical/molecular sensitivity of a Raman spectrometer, making it an excellent solution to these needs, with the potential to achieve nanoscale resolution and well placed to become a reliable, accurate technique with real-time capacity configurations. The key concept of TERS is the nanometric tip-induced plasmonic resonance, which acts as an antenna to enhance signal collection from the surface layer of the sample. The research involves evaluation and comparison of commercial metal (Ag/Au) TERS tips with innovative material-coated (TiN) TERS tips, aiming for compatibility with production environments (clean room) and improved reliability. The activities of this thesis have been carried out in the framework of the European project Challenges (Real time nano CHAracterization reLatEd techNloGiEeS -https://cordis.europa.eu/project/id/861857) and the results obtained are the outcome of a strong collaboration between the partners, in particular with Cea-Leti (where part of the thesis work of the PhD project was carried out) and Scansens GmbH, which further strengthens the interdisciplinary approach to the challenges of the next generation of electronic devices. In this context, the main objective reported in this thesis is to demonstrate the potential and reliability of the TERS technique to be integrated as an in-line technique for real-time monitoring of advanced semiconductor devices. The analyses focused on devices selected within the context of the semiconductor industry and Si photovoltaic applications. The objective was to demonstrate the potential for achieving nanometer lateral resolution, obtaining accurate nanoscale deformation data, generating strain field maps and identifying specific defects in these devices. As well as contributing to the advancement of characterization techniques, the research is also in line with the broader objectives of the Challenges project, providing insights into the development of nanoscale metrology for real-time applications in semiconductor manufacturing.