Practical Electron Microscopy and Database

While single-axis tomography is the most common method, there are other techniques that utilize multiple axes of rotation to overcome some limitations of single-axis tomography. The types of electron tomography rotation schemes are:

• Single-Axis Tomography:
  • The sample is tilted around a single axis, usually from -70° to +70° or similar ranges.
  • This method is straightforward and widely used, but it can lead to the "missing wedge" problem, where certain regions of the sample are not imaged because the tilting cannot cover the entire 180° range.
  • Missing wedge artifacts can result in incomplete reconstructions and loss of detail in certain directions.

• Dual-Axis Tomography:
  • In dual-axis tomography, the sample is tilted around two perpendicular axes.
  • After collecting a tilt series along one axis, the sample is rotated by 90°, and a second tilt series is collected.
  • This approach reduces the missing wedge to a "missing pyramid," which still leaves some gaps but provides a more complete dataset than single-axis tomography.
  • One example of holders for dual-axis tomography had been presented by dual-axis tomography et al. [1]. The holder used in the experiments is a specialized 360° rotation holder designed for use in both Focused Ion Beam (FIB) systems and Scanning Transmission Electron Microscopy (STEM). This rotation holder allows for full 360° rotation about the pillar's axis (denoted as θ) and provides additional tilt control (denoted as α) as shown in Figure 0093a.

    

    Figure 0093a. Schematic illustration of the rotation holder and pillar-shaped specimen in the FIB (a) and in the STEM (b). The rotation holder allows 360° rotation of the specimen (θ), while the microscope stage controls the sample tilt (α). [1]

    
    The key features of the holder are:
    • 360° Rotation Capability: The holder allows the sample to be rotated a full 360° around its axis, which is crucial for eliminating missing wedge artifacts in tomography.
    • Dual-Axis Tilt Control: The sample can also be tilted along a secondary axis (α), from +90° to -90°, providing more flexibility in aligning the sample to the electron beam for accurate crystallographic alignment. That is, the sample can be tilted along a different axis while also rotating, which allows for dual-axis control.
    • Compatibility with FIB and STEM: The same holder can be used in both the FIB system (Hitachi FB-2100) and the STEM, enabling seamless transfer of the sample between instruments without losing alignment or requiring repositioning.
    • Pillar-Shaped Sample Preparation: The holder is particularly suited for pillar-shaped samples, allowing uniform thickness across all angles during rotation, which is ideal for EELS tomography.
    The holder's primary rotation occurs around the θ-axis, allowing the sample to rotate around its central axis and enabling the collection of a full set of projection images over a 360° range. This comprehensive rotation is crucial for capturing detailed data from all angles, which is essential for accurate 3D reconstruction. Additionally, the holder provides a secondary tilt along the α\alpha α-axis, which permits more complex and complete sampling of the 3D structure. This secondary tilt significantly enhances the tomographic process by potentially reducing or even eliminating the missing wedge artifacts that are commonly encountered in single-axis tomography, thereby improving the overall quality and accuracy of the reconstructed images.

    For the W-to-Si contact sample [1], the FIB was initially used to prepare a conventional 200-nm-thick 2D cross-section of a semiconductor device, which was then placed on the sample stub of the rotation holder. STEM images of this cross-section were subsequently utilized to identify the specific W-to-Si contact of interest. The FIB was then employed to remove the excess material from either side of the W contact, leaving behind a 200 × 200-nm-thick pillar-shaped sample (Figure 0093b). This process ensured that the rotation axis (θ) of the holder was approximately aligned with the W-to-Si contact of interest. This example demonstrates how the FIB can be effectively used to prepare a pillar-shaped sample of a targeted nanostructure.

    

    Figure 0093b. A W-to-Si contact from a semiconductor device, prepared as a 0.2 × 0.2 × 6 µm pillar-shaped sample and mounted on a rotation holder using FIB microsampling [2]. The images show (a) a SEM image, (b) a HAADF STEM image, and (c) a bright-field STEM image of the sample. The W-to-Si contact is centrally positioned within the pillar-shaped sample and mounted close to the rotation axis of the holder to facilitate 4D STEM-EELS acquisition. This pillar-shaped geometry, combined with the rotation holder, enables STEM imaging and analysis over a full 360° range with a nominally uniform projection thickness.

• Tri-Axis or Multiple-Axis Tomography:
  • In this method, the sample is tilted around three or more axes, further reducing or nearly eliminating missing wedge artifacts.
  • Multiple-axis tomography provides even more comprehensive data, but it is more complex and requires more advanced instrumentation and computational techniques.

• Conical Tomography:
  • Conical tomography involves rotating the sample around an axis while it is simultaneously tilted, tracing out a cone shape.
  • This technique can help to address the missing wedge issue by providing data from a broader range of angles.

[1] Konrad Jarausch, Paul Thomas, Donovan N. Leonard, Ray Twesten, Christopher R. Booth, Four-dimensional STEM-EELS: Enabling nano-scale chemical tomography, Ultramicroscopy 109 (2009) 326–337.
[2] P. Hawkes, The Electron Microscope as a Structure Projector. In: J. Frank. (Ed.), Electron Tomography: Three-Dimensional Imaging with the Transmission Electron Microscope, 1992.