Preparing Ultra-Thin TEM Specimens for Accurate S/TEM Materials Characterization

For materials scientists, understanding the atomic structure of a material, revealing defects, or characterizing the chemical and physical processes that occur during the creation of material, are key to the development of new materials.

The ability to view individual atoms allows for a better understanding of a material’s physical and mechanical properties. For example, HR-TEM characterization can be utilized to identify unusual crystallographic defects that can be created by applied stress, chemical reactions, or thermal processes.

Nanoscale structural defects, such as stacking faults, point defects, voids, line defects (dislocations), or precipitates all impact the mechanical properties and final quality of materials. These faults and defects also adversely affect microelectronics and optical materials, as they disperse the charged particles and limit light emission.

Visualization of the atomic structure of a material requires TEM imaging. Sample preparation is critical for ensuring accurate S/TEM materials characterization. When using the samples described above, achieving the atomic resolution required in the TEM depends on a consistent and reliable sample preparation method.

For most of the materials it is required to thin the TEM sample under 50 nm, and lamella must be prepared from a specific site and with a particular orientation and axial alignment. It is only under these conditions that TEM can deliver images with atomic resolution for highly precise materials characterization. With properly prepared specimens, TEM can offer a more in-depth understanding of the material’s structure at the sub-nanometer scale.

There are many sample preparation techniques for TEM investigation; however, only FIB-SEM preparation provides the precision, final quality, and site-specific requirements for samples subject to atomic-scale TEM investigations.

(Left) TEM image of silicon sample prepared by FIB-SEM. (Right) HAADF TEM image of Si sample showing dumbbell structure in crystallographic orientation

Figure 1. (Left) TEM image of silicon sample prepared by FIB-SEM. (Right) HAADF TEM image of Si sample showing dumbbell structure in crystallographic orientation. Image Credit: Tescan Group

Materials and Methods

In this study, a Si wafer was chosen to represent a single crystal material. Although Si wafer is predominantly used in microelectronics device fabrication, these wafers are frequently employed in optical materials development as a substrate for ultra-thin layers. The properties of silicon can be altered by doping with different materials such as boron and phosphorus, which are used in solar cell production.

The TEM samples from the silicon material were prepared in the [110] orientation, allowing for the dumbbell structure of Si atoms to be viewed in HR-TEM. Clear visualization of the dumbbell structure is seen as confirmation of the FIB-fabricated sample’s quality and thickness. Both Ga FIB-SEM and plasma Xe FIB-SEM were utilized with the standard “top-down” FIB-SEM sample preparation technique.

The TEM samples were prepared in-trench and then transferred to the TEM grid using TESCAN’s OptiLift™ nanomanipulator for final polishing at high and low accelerating voltages, respectively. The TEM samples were then analyzed in a 300 keV TEM to visualize individual atoms.

TESCAN’s AutoSlicer™ was used during the entire sample preparation process. AutoSlicer™ automates in-trench preparation and final polishing post-lift-out to ensure sample preparation is conducted to the exact specifications required.

Conclusions and Outlook

TEM sample preparation is one of the most common applications for FIB-SEM. TESCAN Group continues to improve software and hardware to ensure that its sample preparation solutions meet the demand for increasingly higher-quality TEM samples. Automation is central to the improvement of user experience, making the entire process of TEM sample preparation more simplified, and creating the best quality lamellae with consistency.

TESCAN AutoSlicer™ software was created to ensure that high-quality TEM samples can be prepared from a range of materials, even by instrument users who may not possess extensive FIB-SEM operations knowledge. This enables more users to share in the sample preparation duties.

The AutoSlicer™ software houses job templates with specific settings tailored to a given materials’ properties, which enables the uniform application of milling parameters to various materials, as well as unattended operation. These templates are customizable to a particular material or final sample requirement.

TESCAN’s FIB-SEM portfolio boasts solutions that meet the demands for high-quality TEM sample preparation. TESCAN provides unique combinations of SEM and FIB technology, as well as additional hardware such as TESCAN’s OptiLift™ solution for the preparation of TEM samples with advanced geometries (planar, inverted, or rotated by 90° degrees). TESCAN AutoSlicer™ offers automated TEM sample preparation, with full automation of all FIB-SEM milling steps, as well as a guided lift-out workflow that provides support to operators with different experience levels.

This information has been sourced, reviewed and adapted from materials provided by TESCAN Group.

For more information on this source, please visit TESCAN Group.

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