Electron Microscopy Sample Preparation Techniques

High-quality material observation and characterization requires an artifact-free surface, which can be difficult to achieve with traditional polishing techniques like grinding or mechanical polishing. The Thermo Scientific CleanMill Broad Ion Beam System offers a comprehensive ion polishing solution for SEM applications in materials science. It enables pristine surface preparation, particularly for sensitive materials. 

The CleanMill System, designed as a cross-polisher (BIB-CP), addresses applications where sample preparation is as crucial as the result. It comes standard with an ultra-high-energy argon ion source (with a maximum accelerating voltage of 16 kV) for efficient ion milling without compromising surface quality. When ultra-fine surface polishing is needed, the CleanMill System can be configured with an ultra-low-energy ion source to achieve the finest surface finish.

Sample surface quality is a critical consideration in numerous SEM and FIB techniques. High-resolution SEM imaging and EBSD analyses, for instance, demand exceptionally clean, artifact-free surfaces. Traditional methods like mechanical polishing are time-consuming and often fall short in ensuring the necessary sample quality. Additionally, locating specific features on the sample surface for lamella preparation or 3D data collection can be challenging, especially after exposure to air or when removing thin coating layers to access the desired structure.

The Spin Mill technique offers a valuable solution by utilizing the extended negative tilt range of Thermo Scientific PFIBs and Hydra DualBeam FIB-SEMs. It exposes the sample surface to the ion beam at a glancing angle while rotating the stage, effectively polishing surfaces up to 1 mm in diameter. Spin Mill is fully automated and integrated with the Thermo Scientific Auto Slice & View 5 Software package, providing a quick and efficient method for preparing large-area surfaces of the highest quality, even at low FIB voltages.

A basic application of FIB-SEM instruments is creating cross sections, a fundamental application providing insights into materials' microstructure. By selectively removing material with a FIB, hidden structures beneath the sample surface can be examined with SEM or EDS, revealing details about structural properties, composition, and failure mechanisms. 

Key factors for achieving high-quality cross-section preparation include the FIB column's ability to efficiently focus ions for fast and clean cuts. Complex or non-uniform samples may pose challenges, but the "rocking mill" technique, a software feature in all Thermo Scientific DualBeam FIB-SEMs, can aid in producing smooth surfaces without the need for additional hardware. Furthermore, fully automated preparation of numerous cross sections is a standard feature in Thermo Scientific AutoTEM 5 Software, streamlining operator and tool efficiency.

Sample preparation for (scanning) transmission electron microscopy ((S)TEM) is a critical task in materials characterization labs. FIB-SEM is the primary method for creating site-specific, artifact-free samples of controlled thickness and is widely employed in research.

Sample quality significantly impacts (S)TEM results, prompting researchers to invest time and effort in mastering preparation techniques. Key requirements for a good (S)TEM sample include achieving the appropriate thickness (typically 10nm to 100nm), maintaining sample cleanliness and structural integrity, and ensuring the presence of features of interest. Often, this requires preparation of planar samples that are parallel to the sample surface, or inverted lamellae.

Atom probe tomography (APT) achieves atomic-resolution 3D compositional analysis by ejecting surface atoms as ions and analyzing their identity using a mass spectrometer. Focused ion beam (FIB) milling in DualBeam FIB-SEMs is essential for precise, high-quality, site-specific sample preparation for APT characterization. DualBeam FIB-SEMs make it possible to create sharp tips of the shape required for ion ejection, while monitoring the process in real time using SEM. The fundamental criteria for a good APT sample are:

• Site-specific sample preparation capability
• Needle-shaped specimen with tip radius typically less than 50 nm
• Uniform, circular cross section of the tip to produce a radially symmetric electric field
• Correct taper angle for significant evaporation events to occur
• Minimal damage introduced to the tip during specimen preparation (apex region of needle should represent original sample in terms of microstructure and composition)

Thermo Scientific DualBeam FIB-SEMs, such as the Helios 5, Helios 5 PFIB and Helios Hydra DualBeam FIB-SEMs, allow top-notch APT sample preparation. Gallium FIB provides the best FIB resolution for ultimate control of the sample shape, while Xe plasma is essential for preparing samples sensitive to Ga. Finally, the Helios Hydra DualBeam FIB-SEM makes use of different plasma ion species (oxygen, argon, nitrogen, or xenon) in the same instrument, optimizing milling for the most challenging samples. Additionally, Thermo Scientific AutoScript Software allows you to automate the final thinning step of the process, alleviating the need for repetitive sample preparation tasks.