ICMCTF 2022 Session H1-1-MoM: Spatially-resolved and In-Situ Characterization of Thin Films and Engineered Surfaces I

In situ TEM experiments are typically recorded either in real space or diffraction space. However, it would be ideal to have information from both for when transient events occur that cannot be repeated exactly (ie- defect generation or irreversible phase transformations). 4D-STEM can come close to providing simultaneous real-space imaging and diffraction analysis during in situ testing, making it possible to perform orientation and strain mapping and defect and phase identification via diffraction pattern analysis during in-situ deformation in a TEM. This talk will highlight recent in situ 4DSTEM nanomechanical deformation experiments that explore transient events where both information from diffraction space and real space are used. The diffraction patterns are used to identify different phases, defects, orientations and relative strain, while the images formed by using virtual apertures provide microstructural context for the analysis. Example experiments include defect generation and fracture in multi-principal element alloys and in situ heating and cooling of materials going through phase transformations.

Noble-metal ultrathin films, with nominal thickness smaller than ~15 nm, are ubiquitous in a wide range of plasmonic devices and other optoelectronic applications. Silver (Ag) layers have recently gained interest as alternative transparent conductive electrode (TCE) candidates for flexible photovoltaics to currently used indium tin oxide, which is inherently brittle and suffers from high cost and poor sustainability. However, Ag films obtained by conventional physical vapor deposition have the natural tendency to self-assemble into 3D agglomerates on weakly interacting substrates, resulting in the formation of rough surface profiles. Therefore, strategies to produce fully continuous, ultrathin and ultrasmooth Ag layers without compromising their electrical conductivity are needed. Among them, the use of gaseous additives, such as N₂ or O₂, appears as an efficient route to shift the continuous film formation thickness to lower values [1,2]. However, to understand the full evolutionary growth regime requires the implementation of in situ and real-time diagnostics.

In the present work, the impact of N₂ addition on the morphological and structural evolutions of ultrathin Ag layers is investigated by coupling complementary in situ and real-time diagnostics. Lab-scale studies include wafer curvature, surface differential reflectance spectroscopy and electrical resistivity to determine the morphological transition thicknesses (percolation and continuous formation thickness) [3] as a function of N₂ partial pressure. These are augmented by real-time X-ray synchrotron studies (SIXS beamline at SOLEIL) in which the diffraction and reflectivity signals are simultaneously recorded, together with stress evolution. This enable us to explore the influence of N₂ on island shape, texture and stress development, as well as relaxation mechanisms during growth interruptions.

1. Yun, J. et al. An unexpected surfactant role of immiscible nitrogen in the structural development of silver nanoparticles: An experimental and numerical investigation. Nanoscale12, 1749–1758 (2020).

2. Jamnig, A. et al. 3D-to-2D Morphology Manipulation of Sputter-Deposited Nanoscale Silver Films on Weakly Interacting Substrates via Selective Nitrogen Deployment for Multifunctional Metal Contacts. ACS Appl. Nano Mater.3, 4728–4738 (2020)

3. Colin, J. et al. In situ and real-time nanoscale monitoring of ultra-thin metal film growth using optical and electrical diagnostic tools. Nanomaterials10, 2225 (2020)

The transition of thin films into isolated particles at a temperature below the melting point of the bulk material is known as solid-state dewetting. So far, most of the literature is limited to dewetting studies on pure metal and binary alloys thin films. The experimental data on dewetting of compositionally complex alloys is rather missing.This work studied the solid-state dewetting behavior of precious metal high entropy alloy thin film deposited on a (0001) single-crystalline sapphire substrate and annealed in a range of temperatures. The combination of dedicated imaging, composition mapping, and diffraction techniques is used to investigate the interplay of grain growth, phase transformations, and dewetting kinetics with the process of in-situ annealing in the TEM.Both X-ray diffraction and the transmission electron microscopy observations revealed the FCC (single-phase) →FCC1+FCC2 (double-phase)→FCC3 (single-phase) phase transformation sequence during annealing. In addition, ex-situ annealing experiments have been performed in the same temperature range to assess the film's dewetting and phase transformation kinetics quantitatively.

With the constantly increasing needs in microelectronics, photovoltaic cells, and other high-tech components in specialized industries, characterizing the growth of epitaxial (epi) silicon on silicon or evaluating the quality of Si/Si bonding becomes more and more critical. For instance, managing the epi Si layer paved the way to new generations of devices such as the Metal Oxide Semiconductor devices (MOS, CMOS and MOSFET) [1] as well as high performance photovoltaic cells [2]. Therefore, in order to address these increasing needs, existing characterization tools provide constantly improved performances, and new characterization tools regularly emerge to provide a technological breakthrough that offers more challenging features.

Some of the existing characterization tools that meet the needs are destructive, such as Scanning and Transmission Electron Microscopy, or semi-destructive such as Secondary Ion Mass Spectrometry. Only a few characterization tools can measure the thickness of epi Si on Si and evaluate the quality of Si/Si bonding after the fabrication process and non-destructively. Among these techniques is the emerging Picosecond Ultrasonics (PU) [3]. This communication will present the principle of operation of this technique and describe some of the potentialities regarding the characterization of Si-based samples. First, we will see how PU can measure the thickness of a layer of epi Si on a Si substrate and describe the related advantages of this technique. Second, we will focus on Si/Si bonding applications with an illustration of PU possibilities to evaluate the quality of the silicon direct bonding.

[1] Skibitzki, Oliver. "Material Science for high performance SiGe HBTs: Solid-Phase Epitaxy and III-V/SiGe hybrid approaches." (2013).

[2] Hamon, Gwenaëlle, et al. "Plasma-enhanced chemical vapor deposition epitaxy of Si on GaAs for tunnel junction applications in tandem solar cells." Journal of Photonics for Energy 7.2 (2017): 022504.

[3] Thomsen, C., et al. "Coherent phonon generation and detection by picosecond light pulses." Physical review letters 53.10 (1984): 989.