FIB + SEM/TEM : Energy beyond your imagination

Principles of Focused Ion Beam (FIB) Technology

The basic working principle of Focused Ion Beam (FIB) involves using accelerated heavy ions to bombard the target material, causing atoms to be sputtered from the target material. The efficiency of the sputtering process is primarily determined by the ion source, which must meet the following two requirements: (1) Using heavy ions to maximize momentum transfer under a given acceleration voltage (usually 30 keV); (2) The melting point and vapor pressure of the ion source material should be low. Gallium (Ga), as a low-melting-point metal with a melting point of only 29.8°C, can meet these two requirements well. Therefore, Ga metal is considered a conventional ion source. In the FIB operation process, solid Ga is heated to its melting point, and liquid Ga flows to the tip of the probe through surface tension, wetting the tungsten needle. After applying a strong electric field at the tungsten tip, liquid Ga forms a tip with a diameter of approximately 2-5 nm, where the electric field strength reaches up to $10^{10}$ V/m. Under such a high electric field, metal ions on the liquid tip's surface evaporate, producing a Ga+ ion beam (Figure 1).

Figure 1: Working Principle of the Focused Ion Beam (FIB) System

Interaction between Ga+ and the Target Material

Ga+, as a charged particle, undergoes a series of interactions with the target material similar to electrons when they come into contact. When Ga+ ions collide with the atomic nuclei in the target material, they transfer some energy to the atoms, causing them to displace or completely detach from the solid material's surface. This phenomenon is known as sputtering, and the etching function in FIB processing relies on this principle. Additionally, incoming Ga+ ions may release their kinetic energy through cascade collisions and remain stationary below the surface of the target material at a certain distance. This process is referred to as ion implantation. The non-elastic scattering of incoming Ga+ with the target material can generate secondary electrons, phonons, plasmon excitations, and X-rays. Secondary electrons are used for imaging, especially in single-beam FIB instruments, where they can be collected by a continuous dynode electron multiplier (CDEM) detector.

FIB-SEM Combined System

Assembling the ion column and electron column in the same instrument creates a device that integrates all the functions of FIB and SEM, commonly referred to as a focused ion beam scanning electron microscope or dual-beam electron microscope. Its main functions can be divided into two areas:

1. FIB etching and deposition, used for microfabrication of materials, TEM sample preparation, and metal deposition.
2. Microscale composition and morphology analysis, compatible with conventional SEM techniques such as secondary electron imaging, backscattered electron imaging, EBSD, EDX analysis, and the dual-beam electron microscope can perform transmission electron imaging at 30 kV, producing Z-contrast images with high spatial resolution.

In addition, as shown in Figure 2, the dual-beam electron microscope can also perform 3D electron backscatter diffraction, 3D cross-sectioning, 3D imaging, and 3D EDX analysis.

Figure 2: Applications of FIB-SEM Combined System

FIB-TEM Combined System

Since TEM samples need to be extremely thin for electrons to penetrate and form diffraction images, efficient sample preparation is crucial. The high-sputtering efficiency of FIB makes it commonly used for optimizing the preparation of ultra-thin TEM samples. Figure 3 illustrates the process of preparing TEM ultra-thin samples using FIB. As shown in Figure 3 (a, b), the first step involves marking the surface of the sample of interest, indicating the cutting positions, and depositing Pt. As the milling process begins, a large trench is milled in front of the Pt strip, followed by a smaller trench at the back.

The actual progress of the sputtering process is monitored by using a CDEM detector to acquire secondary electron images from the sample. After completing the large trench, milling continues, reducing the beam size and ion current. A small rectangular area is selected, and the sample is sputtered for a period based on the desired foil thickness. The pattern is then moved, and the process is repeated until a foil thickness of approximately 500 nm is achieved. The sample is then tilted about 45° relative to the ion beam, and milling continues to cut the sides and bottom of the foil, leaving only a narrow Pt strip at the top to secure it for further polishing. After tilting the sample back to its original position, milling continues using a smaller beam and reduced current until the final thickness is reached. Once milling is complete, the Pt strip is entirely cut (Figure 3c), and the sample thin slice can be placed on the perforated membrane of the TEM copper grid with the assistance of a manipulator.

Figure 3: FIB-Based TEM Sample Preparation Process

Applications of FIB-SEM/TEM

5.1 Optimization of TEM Sample PreparationAs mentioned above, the preparation of Transmission Electron Microscopy (TEM) samples is a highly distinctive and important application of Focused Ion Beam (FIB). In comparison to traditional TEM sample preparation methods, FIB sample preparation has the following characteristics:

1. High precision in point and direction. It is the only method when the positioning accuracy is less than 0.5 μm;
2. Virtually no sample preparation is required;
3. Short sample preparation time;
4. High success rate in sample preparation;
5. Insensitivity to processing materials, making it possible to prepare samples of porous, brittle, and soft/hard composite materials (such as soft polymers/metal);
6. Capability to perform characteristic analysis on different regions of the same material.

5.2 3D SEM ImagingWhen studying the reaction processes of minerals, the phase sizes are close to or below the detection limits of optical microscopy, making it challenging to obtain sufficient data to accurately elucidate reaction mechanisms. However, the study of mineral reactions not only requires identifying phase structures and chemical compositions but also obtaining three-dimensional data such as the distribution, shape, and volume of different phases. Utilizing the layer-by-layer slicing and imaging acquisition of FIB-SEM facilitates the formation of 3D imaging and effectively achieves this purpose (as shown in Figure 4).

Figure 4: 3D Image After Slicing and Image Acquisition

Figure 5: 3D EBSD Image of Grain Orientation Distribution in Polycrystalline Aluminum

5.3 3D EBSDEBSD is a powerful tool for measuring the texture, grain size, and grain orientation of individual particles in a sample. Utilizing EBSD allows for the generation of phase identification and phase distribution maps. When combining FIB with a SEM equipped with an EBSD detector, it can be used to measure the grain orientation in three-dimensional samples. As shown in Figure 5, the process begins by milling trenches in the sample using FIB and cleaning the surface to form a surface parallel to the normal of the sample, while preserving the EBSD pattern. Subsequently, by sequentially milling the sample and preserving EBSD at each layer, a 3D EBSD image can be obtained.

5.4 3D Elemental Distribution MapsSimilar to 3D SEM and 3D EBSD, the combination of FIB with SEM or TEM, through sequential milling and EDX elemental acquisition, can be used to create 3D elemental distribution maps (Figure 6). The detection limit is determined by the EDX system's sensitivity.

Figure 6: Elemental Distribution Map of the Cross-sectional Area

5.5 FIB Micromachining

1. Direct formation of micro/nanostructures. Direct milling for shaping is the most commonly used mode of operation for FIB systems. In principle, FIB machining is non-selective towards processing materials, allowing for precise control of the depth at each machining point.
2. Material deposition processing. Utilizing the material deposition capability of FIB-SEM systems enables the creation of measurement electrodes for nanomaterials. As shown in Figure 7, carbon nanotubes are randomly dispersed among four 8 μm wide microelectrodes. By employing the system's Pt deposition capability, the four microelectrodes are incrementally extended, precisely covering the carbon nanotubes, for use in the electrical performance measurement of carbon nanotubes.

Figure 7: Fabrication of Carbon Nanotube Electrodes

3. Point-specific processing. FIB systems can flexibly perform point-specific processing on samples, such as modifying tips for scanning probe microscopy (SPM), such as AFM and STM. Figure 8 shows photographs before and after the modification of an AFM tip. Comparable results can be obtained regardless of whether the tip is made of Si or other materials like SiO2. Modified AFM tips can be used for specific applications, such as penetrating biological cells for detection.

Figure 8: Before and after AFM needle tip modification

References

1. Richard Wirth. Focused Ion Beam (FIB) combined with SEM and TEM: Advanced analytical tools for studies of chemical composition, microstructure and crystal structure in geomaterials on a nanometre scale. Chemical Geology 261 (2009) 217–229.