Recent Advances in FIB-SEM for Microstructural Characterization of Metallic Materials

1. Introduction

Since the advent of the FIB-SEM dual-beam system (1988, P. Sudraud and G. Ben Assayag) [1], the combined system of a focused ion beam (FIB) and scanning electron microscope (SEM)—the FIB-SEM dual-beam system—has become the mainstream platform for FIB technology applications. In this system, the SEM and FIB can both be focused on the same region of interest on a material: the SEM provides imaging and positioning while the FIB performs ion-beam machining. The application of FIB has thus expanded beyond semiconductors to materials science, geology, biology, and other fields, where it has flourished [2,3,4].

2. Principles of FIB-SEM

2.1. FIB

A typical FIB instrument comprises a vacuum system and chamber, ion source, ion-optical column, specimen stage, detectors, gas injection system, and control system [5]. The ion source is the key component of any FIB system; the liquid metal ion source using gallium (Ga) was the origin of practical FIB technology. Modern FIB systems have since been developed using a variety of ion species, including helium (He), neon (Ne), gallium (Ga), argon (Ar), and xenon (Xe). Based on the ion-source operating principle, FIB systems can be classified into liquid metal ion source (LMIS) systems, gas field ionization source (GFIS) systems, and plasma ion source systems.

LMIS (Liquid Metal Ion Source): Operating principle—field evaporation of a liquid metal. Ion types—metal ions (Ga⁺, Bi⁺). Beam spot size—relatively small (~5 nm). Beam current—moderate to high (10⁰–10² nA). Typical applications—general-purpose FIB machining, circuit editing, TEM specimen preparation, and nanoscale deposition.

GFIS (Gas Field Ionization Source): Operating principle—field ionization of a gas-phase atom. Ion types—gas ions (He⁺, Ne⁺). Beam spot size—extremely small (<0.5 nm). Beam current—relatively low (10⁰–10² pA). Typical applications—helium/neon ion microscopy for ultra-high-resolution imaging and nanoscale patterning.

Plasma Ion Source: Operating principle—plasma generated by gas discharge. Ion types—gas ions (Xe⁺, Ar⁺). Beam spot size—relatively large (>10 nm). Beam current—large (10²–10³ nA). Typical applications—rapid, high-throughput specimen preparation and deep etching (e.g., three-dimensional tomography).

2.2. FIB-SEM

The FIB-SEM system integrates FIB and SEM into a single vacuum chamber, combining nanoscale machining (FIB) with high-resolution imaging (SEM). In an FIB-SEM system, the FIB ion column and the SEM electron column are typically arranged at an angle of 36°–38° to each other. During operation, the FIB ion column is usually oriented perpendicular to the material being processed or characterized, as shown in Figure 1. The specimen stage of an FIB instrument is a precision multi-degree-of-freedom motion system. Its core motion capabilities include standard X, Y, and Z orthogonal linear translations (forming a Cartesian coordinate system), together with integrated rotational and tilt degrees of freedom for complex attitude adjustment.

The interaction behaviors between FIB-SEM and material atoms can be divided into those of the ions from the FIB column and those of the electrons from the SEM column, as shown in Figure 2a.

Ion–atom interactions in FIB: Incident ions undergo elastic and inelastic collisions with atoms in the material, producing elastic and inelastic scattering, respectively. Elastic collisions transfer energy from the incident particle to material atoms, triggering a cascade of secondary collisions. Some incident ions are backscattered at the material surface and leave as backscattered ions. Other incident ions, through the collision cascade, knock material atoms from their lattice sites toward the surface, creating recoil atoms. When a recoil atom has sufficient energy to overcome the surface binding energy, it is ejected from the material as a sputtered particle. Some charged sputtered particles become secondary ions (SI) after further ionization; these signals can be collected by a detector and used for imaging. The inelastic collision channel also excites plasmons and phonons while producing a large number of secondary electrons (SE), whose signals can likewise be collected and used to form images.

Electron–atom interactions in SEM: Incident electrons undergo elastic and inelastic scattering with material atoms. Elastic scattering causes large-angle deflection of electrons, generating backscattered electrons (BSE) whose yield is related to atomic number and can be used for compositional analysis. Backscattered electrons originating from a certain depth within a crystalline material, when satisfying the Bragg diffraction condition, produce characteristic Kikuchi diffraction patterns that can be used to analyze crystal orientation, phase, and strain (EBSD). Inelastic scattering transfers energy to the specimen, producing multiple signals: excitation of low-energy secondary electrons from the near-surface region forms the basis of morphological imaging (SE image); excitation of inner-shell electrons generates characteristic X-rays for elemental analysis (EDS); Auger electrons and cathodoluminescence are also produced. These signals originate from different depths within the specimen surface and together constitute the multifunctional platform of SEM for high-resolution imaging, micro-area compositional analysis, and crystal structure and orientation analysis.

When a focused ion beam scans the specimen surface, it excites secondary electron (SE) and secondary ion (SI) signals that can be collected by various detectors equipped on the FIB system. Most FIB systems are equipped with a gas injection system; combined with SiO₂, Pt, Au, W, and C gas precursors, they can realize ion-beam-assisted induced deposition for material surface protection and nano-device assembly. Combined with XeF₂ gas, assisted etching of silicon is possible; combined with H₂O gas, assisted diamond etching can be performed. Consequently, the functions of a focused ion beam system include ion exposure, ion implantation, ion etching, elemental deposition, integrated circuit fabrication, and mask repair [6,7,8,9,10], as shown in Figure 2.

Based on the above interactions of particles with material atoms in both FIB and SEM, the functions of FIB-SEM can be summarized to include morphological observation, sputter etching, induced deposition, and ion implantation [10,11,12,13]. The schematic principle diagram is shown in Figure 2, and representative application examples are shown in Figure 3.

3. Applications of FIB-SEM in Morphological Characterization of Metallic Materials

3.1. 2D Morphological Characterization

Due to the etching effect of the high-energy ion beam, the ion beam causes specimen damage while collecting signals; the extent of this damage depends on the energy and intensity (accelerating voltage and beam current) of the ion beam [13,14,15]. Therefore, continuous imaging using the ion beam as the signal source is generally avoided. However, the GFIS-based helium/neon ion microscope [10,16,17], which operates on the quantum-mechanical tunneling effect, can be used for ultra-high-resolution imaging because of its low ion energy and minimal damage; its imaging contrast and surface detail are superior to those achieved by electron-beam imaging [6]. This is attributable to an extremely short de Broglie wavelength and a highly localized interaction volume, allowing ultimate surface imaging at sub-nanometer resolution [16] and clearly revealing true atomic-scale surface steps and morphologies with almost no subsurface information interference. Hlawacek et al. [17] used a helium ion microscope to study silver alloys confined to Pt(111) surfaces, directly visualizing single-atomic-layer-high steps between terraces and measuring the periodicity of hcp/fcc patterns formed in 2–3-layer-thick Ag/Pt alloy films, as shown in Figure 4 [17].

In addition, the strong interaction between ions and atomic nuclei endows ion-beam images with excellent material and grain contrast, allowing clear differentiation of grains with different orientations in polycrystalline materials and light-element components in composite materials. The positively charged incident ions naturally neutralize the surface charge of the specimen, enabling direct ultra-high-resolution imaging of insulators (e.g., ceramics, polymers, biological specimens) without prior conductive coating. Furthermore, the technique offers an exceptionally large depth of field and is free from edge-brightening artifacts, providing a more accurate representation of three-dimensional geometry. These characteristics make the helium/neon ion microscope an indispensable and powerful tool for exploring true surface structures at the nanoscale, especially the surface properties of insulating materials.

3.2. 3D Morphological Characterization

FIB-SEM three-dimensional tomography (FIB-SEM Tomography) has gradually become a core tool for resolving the three-dimensional spatial structure of metallic materials, owing to its unique capability to acquire true three-dimensional information from moderate microvolumes (10–1000 μm³) at nanometer resolution [18,19,20,21]. In 2004, HOLZER et al. successfully reconstructed the three-dimensional structure of BaTiO₃. We also successfully employed a FIB-SEM dual-beam system to achieve slice-and-view 3D tomography, acquiring true three-dimensional (3D) information from moderate analysis volumes (10–1000 μm³) of a given material at nanometer resolution, as shown in Figure 5.

4. Applications of FIB-SEM in the Micro/Nanofabrication of Metallic Materials

Researchers have used FIB-SEM dual-beam systems, with real-time SEM monitoring, to precisely control key parameters such as ion species, energy, beam current, and dwell time, successfully achieving highly accurate three-dimensional controllable fabrication of platinum-wire nanocages, PtC nanotips, and self-assembled nanobubbles on gold films [22,23,24,25,26,27]. However, the most widespread uses of FIB-SEM in the field of metallic materials remain TEM specimen preparation, APT specimen preparation, and in situ compression experiments. These preparation techniques and the progress of their related analytical methods are introduced in detail in this section.

4.1. TEM Specimen Preparation

TEM enables nano/atomic-scale microstructural analysis of metallic materials. Under real-time SEM observation, FIB-SEM can be used to select regions of interest—such as crack tips, inclusions, and grain boundaries—and extract TEM specimens in either perpendicular or parallel orientations (Lift-out). Taking the perpendicular Lift-out method as an example, the procedure consists of eight steps: deposition, rough cutting, fine cutting, U-cutting, extraction, welding, final thinning, and cleaning. Steps 1 through 7 (deposition to final thinning) are typically performed at a machining voltage of 10–30 kV, while the cleaning step is typically carried out at 2–5 kV. A schematic diagram of the perpendicular Lift-out TEM specimen preparation process is shown in Figure 6.

This technique continues to be updated and extended. We proposed a multi-voltage FIB-SEM Lift-out technique for extracting cross-sectional TEM specimens from diamond and Zr alloys [28,29]. TOMUS et al. [30], JUBLOT et al. [31], and WANG Xueli et al. [32] successfully developed FIB-SEM Lift-out techniques for preparing plan-view TEM specimens suitable for crack analysis. Deng et al. [33] proposed a thin-film-embedding-strengthened FIB (FeS-FIB) technique, based on FIB-SEM, to successfully extract TEM specimens from both spherical microparticles and nanosheet-type structures, and successfully analyzed the lattice structure of Ni and Fe double hydroxide (Ni,Fe-LDH) nanosheets.

4.2. APT Specimen Preparation

APT enables three-dimensional atomic-scale chemical compositional analysis of metallic materials. Under real-time SEM observation, FIB-SEM can be used to select regions of interest—including precipitate phases [34,35,36,37,38,39,40,41], grain boundaries [42,43,44,45,46,47,48,49,50], phase interfaces [51,52,53], and other regions of interest [54,55,56]—and extract APT specimens in either perpendicular or parallel Lift-out orientations. MILLER et al. [57,58] adapted the TEM specimen preparation concept to propose the Lift-out method for APT, which involves Pt protection, cut-and-extract, welding to microstubs, and final annular milling to achieve precise targeting of regions of interest. After continuous refinement of technical details by subsequent researchers [59,60], the Lift-out method has become the most universal standard approach for APT specimen preparation. A schematic diagram of the perpendicular APT Lift-out specimen preparation process is shown in Figure 7.

These methods have been widely applied to metallic materials. Takahashi et al. [61] used APT analysis to investigate micron-scale spherical oxide inclusions in titanium-containing weld metal, revealing non-uniformly distributed multi-layered oxide shells of varying composition formed on an oxide core; the outermost oxide layer was rich in titanium and was surrounded by a manganese-depleted zone approximately 30–40 nm thick. Lee, Y. et al. [62] employed atom probe tomography (APT) to determine the temporal evolution of composition after heat treatment, in order to resolve the uncertainty in the Pt-Pd phase diagram, particularly the proposed miscibility gap. Woods et al. [63] built on this foundation to explore APT specimen preparation techniques for various metals, oxides, supported frozen liquids, and battery materials at low temperatures, successfully characterizing materials such as nickel-coated AAO membranes.

5. Other FIB-Based Correlative Characterization Techniques

Precise control of crystallographic orientation is also an important requirement for FIB-SEM micro/nanofabrication of metallic materials. GAO et al. [64] proposed an FIB-SEM-EBSD combined TEM specimen preparation technique to prepare TEM specimens of Widmanstätten structures in γ-TiAl alloys with specific zone axes, thereby successfully resolving the atomic structure of the special interface (Σ-11 CSL) between Widmanstätten and lamellar structures. COJOCARU-MIRÉDIN O et al. [65] proposed an FIB-SEM-EBSD-PPMS combined APT specimen preparation technique, enabling correlative characterization of crystal orientation, physical properties, and atomic-scale composition.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS), which enables ppm- to ppb-level detection of all elements (from hydrogen H to transuranium elements) at solid material surfaces, has attracted the attention of materials researchers and has been integrated with FIB-SEM. Shin-ichi et al. [66] have realized the application of FIB-SEM-TOF-SIMS in all-solid-state batteries (ASSBs) using a Bi-ion-source FIB-SEM.

The mechanical behavior of metallic materials is also of major interest to researchers. UCHIC et al. [67] used micropillar compression to study the plastic flow behavior of pure nickel and superalloys at the micrometer scale. XUE et al. [68] proposed an FIB-SEM TEM specimen preparation technique based on in situ SEM tensile testing, studying the microstructural evolution in the heat-affected zone (HAZ) of laser-welded DP1180 dual-phase steel joints. KIENER et al. [69] proposed a quantitative in situ TEM tensile testing technique using FIB-prepared nano-specimens to study the plastic deformation mechanism of copper single crystals at the sub-micrometer scale (100–200 nm).

6. Conclusions and Outlook

This paper systematically reviews the latest research advances of FIB-SEM technology in the field of metallic materials science. The principles and basic functions of FIB-SEM are introduced, together with detailed descriptions of its applications in morphological characterization and TEM and APT specimen preparation, as well as its integration with other characterization techniques. Currently, FIB technology is advancing along two parallel trajectories: the concurrent development of multiple ion-source technologies (e.g., Ga⁺, Xe⁺, and other plasma sources) and their deep integration with multimodal characterization techniques. In the future, FIB technology is expected to combine with artificial intelligence and big data analysis to enable real-time, correlative measurements of multidimensional physical, chemical, and mechanical properties at the micro-scale, thereby providing powerful technical support for constructing a complete "materials genome" map.