FIB
The Focused Ion Beam (FIB) system is a powerful tool designed for micromachining and high-resolution imaging at the nanometer scale. Its fundamental principle involves using a highly focused beam of ions—most commonly gallium (Ga⁺)—to interact with the surface of a material. This ion beam can precisely sputter and remove material in a controlled fashion, a process known as milling, or alternatively, deposit new material through ion-beam-induced deposition. Because of its versatility, the FIB system is often integrated with a scanning electron microscope (SEM) in what is called a dual-beam FIB-SEM, enabling simultaneous imaging and site-specific modification of microstructures. In a dual-beam FIB-SEM, the ion column and electron column are fixed at about 52°. That means when you want to image the vertical cross-section wall, the SEM beam must see along that 52° line of sight.
Read more: FIB Vs IM SEM Film Cross-section ImagingIn practice, the FIB is indispensable across a range of scientific and engineering applications. It is routinely used for the following:
- Cross-section preparation, such as cutting open film interfaces or semiconductor layers for failure analysis.
- Transmission electron microscopy (TEM) sample preparation, the FIB can thin specimens down to less than 100 nm, producing electron-transparent lamellae.
- A vital tool for circuit editing, where it enables the modification of integrated circuit (IC) connections,
- Nanofabrication, where it can create intricate micro- or nanostructures.
- Additionally, FIB systems support ion imaging, providing topographic and compositional contrast through the detection of secondary electrons or ions.
Typical FIB systems operate using ion species such as Ga⁺, and in some configurations, Xe⁺ for large-area milling. The beam energy usually ranges from 1 keV to 30 keV, offering lateral resolutions of around 5–10 nm for imaging and below 100 nm for milling operations. However, the focused ion beam can cause surface damage or amorphization, particularly on sensitive materials, which is why low-energy cleaning steps are often employed to minimize such effects. Overall, the FIB remains a cornerstone of modern materials characterization and microengineering, combining precision, versatility, and direct control over material structure at the nanoscale.
IM
Ion Milling (IM), also known as Ion Beam Milling or Broad Ion Beam (BIB) milling, is a surface preparation technique designed for gentle, large-area material removal and polishing. Unlike the highly focused and localized approach of a focused ion beam (FIB) system, ion milling employs a broad, unfocused beam of inert ions, typically argon (Ar⁺), that bombards the specimen surface at a shallow angle. This grazing incidence enables the uniform thinning or polishing of materials without introducing significant mechanical or thermal damage. Because the process removes material gradually and evenly, it excels at producing smooth, artifact-free surfaces over millimeter-scale areas, though it lacks the “writing” precision of a focused beam.
The underlying principle of ion milling is based on physical sputtering—ions transfer momentum to surface atoms, gently dislodging them layer by layer. Operating at beam energies typically between 0.5 and 8 keV, the method removes material at a controlled and relatively slow rate, minimizing subsurface alteration. The result is a clean, planar surface that is ideal for high-resolution imaging and analytical techniques.
Ion milling is widely applied in materials characterization.
- In transmission electron microscopy (TEM) preparation, it serves as the final thinning step to achieve electron transparency without introducing artifacts.
- For surface polishing, ion milling removes mechanical deformation left from earlier polishing stages, yielding a pristine finish.
- It is also used in cross-sectioning, where it can expose wide, smooth interfaces across multilayer or composite structures—far superior in uniformity to mechanically fractured sections.
- Additionally, in electron backscatter diffraction (EBSD) studies, ion milling provides the final surface finish required for accurate crystallographic mapping by eliminating residual damage that would otherwise degrade pattern quality.
Because the beam is broad and unfocused, ion milling is not suitable for patterning or imaging, but its strength lies in producing uniform, low-damage surfaces. Compared with FIB, it generates minimal ion implantation or amorphization, making it the preferred technique for sensitive or heterogeneous materials. In summary, ion milling is a gentle, precision surface-conditioning method that complements other microstructural preparation techniques, ensuring samples are optimized for high-fidelity microscopic and analytical examination.
Comparison of Geometry and Imaging Concept
| Aspect | Ion Milling | Focused Ion Beam (FIB) |
| Beam type | Broad Ar⁺ beam at shallow angle (2–10°) | Focused Ga⁺ or Xe⁺ beam at steep angle (≈ 90°) |
| Cross-section created | Wedge-shaped, long and shallow | Vertical wall, directly milled |
| Imaging source | External — SEM after milling | In-situ — built-in electron or ion column |
| Interpretation | SEM scans the sloped wedge and treats it as a virtual vertical section | SEM or FIB column directly views the real vertical cross-section |
| Depth scaling | Apparent depth reconstructed (compressed by sin θ) | True geometric depth — 1:1 |
| Purpose | Gentle polishing, large-area exposure | Precise site-specific milling and imaging |
- FIB = a nano-knife and sculpting tool (precise but invasive).
- IM = a nano-polisher (gentle and broad).
How IM creates a smooth cross-section with shallow angle?
Basic Setup: Broad Ion Beam Geometry
In ion milling, you have a broad beam of inert ions (usually Ar⁺) emitted from an ion gun.
- The sample is placed at a shallow incident angle — typically ≤10–15°, sometimes as low as 2–5°.
- The beam does not focus like in FIB; instead, it covers a large area (millimeters wide).
- The gun raster or the sample rotates/oscillates to ensure uniform erosion.
Mechanism: Gentle Sputtering, Not Digging
At such a shallow angle, each ion grazes the surface and removes atoms through momentum transfer (sputtering).
Key effects:
- Low penetration depth: The ions skim the surface, not penetrating deeply — minimizing subsurface damage.
- Uniform sputtering yield: Overlapping ion tracks produce an evenly thinned layer instead of pits.
- Preferential smoothing: Peaks are hit more often than valleys → surface roughness reduces over time.
Shallow-Angle Cross-Section Formation
To create a cross-section (not just surface polish), a shadow mask or shield plate is partially covering the specimen:
- The mask edge blocks part of the beam.
- Ions only erode the exposed region.
- Because of the shallow incidence, the erosion front advances laterally under the mask, forming a wedge-shaped cross-section.
As milling proceeds, deeper layers are gradually exposed at a very gentle slope (sometimes <10°), producing a wide, smooth, distortion-free cross-section.
Why the Result Is So Smooth
| Factor | Effect |
| Shallow incidence | Reduces vertical sputter rate difference → smooth slope |
| Low ion energy (1–5 keV) | Limits damage, prevents redeposition |
| Ar⁺ inert gas | No chemical reaction, no implantation |
| Sample rotation or oscillation | Eliminates streaks and “curtaining” |
| Shadow mask geometry | Creates controlled wedge without mechanical damage |
So compared to FIB (which creates curtaining due to columnar sputtering from Ga⁺ beam), BIB/IM gives a mirror-like surface — ideal for SEM, EBSD, or even low-kV TEM observation.
Typical Parameters (for reference)
| Parameter | Typical Range |
| Ion species | Ar⁺ |
| Beam energy | 1–6 keV |
| Incidence angle | 2–10° |
| Milling time | 0.5–6 h (depending on area/thickness) |
| Cross-section width | 1–2 mm typical |
| Damage depth | <5 nm |
Summary Analogy
| Process | Analogy |
| FIB | Like a needle engraving with a fine tip (precise, but leaves tool marks). |
| IM (BIB) | Like a broad sandblaster at a grazing angle (smooths and polishes evenly). |