Practical Electron Microscopy and Database

TEM sample preparation methods can differ depending on the type of tomography holder used. The design and capabilities of the tomography holder often dictate the specific requirements and constraints for sample preparation. One example of dual-axis tomography is [1]:

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  Figure 0004a. 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-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 0004b). 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 0004b. 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. [1]

  Another examlpe of sample preparation is that the ZnO thin-film sample was carefully prepared using FIB to facilitate STEM analysis of electronic properties across various crystallographic orientations, all utilizing a single axis of rotation. The preparation process included the following steps:

  • The FIB was first used to create a 2D planview sample of the ZnO thin film.
  • This planview sample was then fixed onto the sample stub of the rotation holder.
  • A secondary electron (SE) image and a HAADF STEM image were captured with the electron beam aligned parallel to the c-axis, the growth axis of the film, as shown in Figure 0004c (c) and (d). The bright, faceted structures observed in the HAADF image are Au nano-dots embedded within the 200-nm-thick film, which served as markers in this study.
  • The FIB was subsequently employed to remove the excess ZnO film from both sides of the region of interest, resulting in a 150 × 150-nm-thick pillar-shaped sample (Figure 0004c (c) and (d)).
  • A STEM nano-beam diffraction pattern (inset in Figure (d)) was used to confirm the c-axis orientation of the film.
  • This innovative FIB preparation allowed the rotation of the pillar sample in the STEM, enabling the electron beam to transition from being parallel to being perpendicular to the film’s c-axis. This transition corresponds to a rotation from a planview projection (with the electron beam parallel to the c-axis or growth axis) as seen in Figure 0004c (c) and (d), to a cross-sectional projection (with the electron beam parallel to the film’s a-axis) as depicted in Figure 0004c (e) and (f).

  

  Figure 0004c. FIB preparation of a 200-nm-thick ZnO thin film with embedded Au nano-dots into a 4.0 × 0.15 × 0.15 µm pillar-shaped sample. The sample was initially prepared as a planview containing the ZnO thin film. STEM images show (a) a secondary electron and (b) a HAADF view of the planview sample with the electron beam aligned parallel to the c-axis, or growth axis, of the thin film. After FIB milling, the planview sample was shaped into a 4.0 × 0.15 × 0.15 µm pillar, as shown in the STEM images (c) secondary electron and (d) HAADF. The diffraction pattern inset in (d) confirms that the electron beam remains parallel to the c-axis of the thin film. Following a 90° rotation of the pillar-shaped sample, STEM images (e) secondary electron and (f) HAADF were taken in the cross-sectional orientation. The diffraction pattern inset in (f) confirms that the electron beam is now parallel to the a-axis of the thin film. This innovative FIB preparation enables STEM probing of the ZnO film’s electronic properties across various crystallographic orientations using a single axis of rotation. [1]

[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.