Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) was a critical tool in my project for studying refractory multi-principal element alloy (RMPEA) and magnetic thin films. SEM allowed me to capture detailed images of the films’ surface morphology and analyze their composition, which helped me understand their structure and properties. By combining SEM with Energy Dispersive Spectroscopy (EDS), I could examine the elemental makeup at specific points or across larger areas. I also used Focused Ion Beam (FIB) milling to prepare cross-sections for SEM imaging to measure film thickness and to create thin lamellae for TEM analysis.

To begin, I prepared the RMPEA and magnetic thin films, likely deposited on substrates using techniques like magnetron sputtering, depending on the alloy composition. For SEM imaging, I didn’t need the samples to be as thin as for TEM, but I ensured they were clean and conductive to avoid charging under the electron beam. I mounted the samples in the SEM chamber, which operated at a voltage around 5-20 kV, depending on the imaging needs. SEM works by scanning the sample with a focused electron beam, detecting signals like secondary electrons to create high-resolution images of the surface. These images revealed surface features like grain structures, cracks, or roughness, which differed between the as-deposited and annealed RMPEA films, as well as the magnetic films.

For compositional analysis, I used EDS in the SEM. By detecting X-rays emitted when the electron beam hit the sample, I could identify and quantify the elements present, such as tungsten, molybdenum, or nickel in the RMPEA films, or iron, cobalt, and others in the magnetic films. I performed point analysis to check the composition at specific spots, like grain boundaries or defects, and area mapping to see how elements were distributed across larger regions. This was especially useful for confirming the uniformity of the multi-element alloys and detecting any segregation or clustering in the films.

To study the films’ cross-sections and measure their thickness, I used FIB milling within the SEM system. FIB uses a focused beam of ions, typically gallium, to precisely cut into the sample. I milled a trench to expose a cross-section of the film, then used SEM imaging to visualize the layered structure and measure the film’s thickness, which was critical for understanding deposition quality and consistency. The cross-sectional images also showed interfaces between the film and substrate, revealing any defects or intermixing that might affect performance.

Additionally, I used FIB to prepare thin lamellae for TEM analysis. This involved carefully milling a small section of the film, lifting it out, and thinning it to less than 100 nanometers so electrons could pass through for TEM. I used low-energy ion beams for the final thinning steps to minimize damage, ensuring the lamella accurately represented the film’s structure. This FIB-prepared lamella was then used in the TEM to get even finer details, like crystal structure and atomic-level features, complementing the SEM’s surface and compositional data.

The SEM process was straightforward but required precision, especially for FIB work. SEM imaging gave me clear views of the films’ surface features, EDS provided detailed compositional insights, and FIB cross-sections let me measure thickness and prepare TEM samples. These techniques together helped me compare the RMPEA and magnetic thin films, showing how processing conditions, like annealing, affected their structure and properties. The main challenge was avoiding sample damage during FIB milling and ensuring good conductivity for clear SEM images, but with careful preparation, SEM delivered valuable data for my project.