Field Emission Scanning Electron Microscope
Introduction
FESEM analysis is a technique used to examine the surface characteristics and external morphology of various samples. It belongs to the family of Scanning Electron Microscopes (SEM) and uses an electron beam with specific energy and wavelength to scan the sample’s surface. By collecting data from sensors that detect backscattered electrons from the sample surface, valuable information is obtained about the surface and even atomic-level imaging.
Measurement Definition
FESEM is an advanced technique in chemical material analysis used to study morphology, composition, and surface structure on a nanoscale. It is known for its high resolution, attributed to its electron gun (often tungsten-based), which accelerates electrons using an electric field.
The electrons generated are faster than those from thermal guns, leading to a shorter wavelength and better imaging resolution (smaller spot size). Once emitted, electrons are accelerated through magnetic lenses and hit the sample. The interaction generates several signals, including:
Backscattered electrons: Produced by elastic scattering, with slight changes in kinetic energy.
Secondary electrons: Produced by inelastic scattering when incident electrons transfer energy to sample atoms, causing electron ejection.
These signals are collected to create a surface topography image. FESEM is ideal for detailed study of materials, including nanomaterials, making it a powerful tool in R&D.
Both secondary and backscattered electrons are image-producing signals in electron microscopy. While they originate from different areas in the material, X-rays and Auger electrons are also generated, enhancing elemental studies.
Secondary electrons, due to their low energy, emerge only from the near-surface layer (5–50 nm deep), whereas backscattered electrons come from deeper regions. The interaction region resembles a pear shape, illustrated schematically in Figure 2.
FESEM surface contrast is expressed through variations in emitted electron energy due to surface geometry differences. Images resemble 3D representations of surface features. Two main effects produce contrast: trajectory line effect and electron count effect.
Trajectory effect: Electrons from surface areas facing the detector are collected more efficiently and appear brighter. Non-facing surfaces appear darker due to poor collection.
Electron count: Some surface points (like particle edges) emit more electrons, appearing brighter.
Although both types of electrons contribute, secondary electrons are the primary source of surface contrast. Only directly backscattered electrons contribute significantly to this contrast.
As noted, signals from electron–sample interactions include emitted electrons and electromagnetic waves.
Emitted electrons are classified into three main types:
1. Backscattered Electrons (BSE)
2. Secondary Electrons (SE)
3. Auger Electrons (AE)
In FESEM, backscattered electrons are those reflected from the surface by the same incoming beam.
SE and AE originate from the surface atoms. These three types enable comprehensive imaging and data acquisition about sample characteristics.
When electrons are emitted, their vacancies are filled by other electrons, and electromagnetic radiation is emitted. This radiation is used to identify the chemical elements in the sample.
Measurement Benefits
Imaging of various sample surfaces, especially nanomaterials.
3D imaging of samples.
Analysis of morphology for different sample types (powders, solids, etc.).
Determining particle sizes of powders in the nanometer range.
Imaging insulating samples without coating.
Imaging biological and polymer samples at low voltage with surface info retrieval.
Fracture surface study.
Semi-quantitative analysis using EDS for elements above boron, unknown samples, and quantitative surface chemical composition.
Performing localized (spot), line, and map analyses.
Preliminary identification of unknown materials and their elemental composition.
Ability to perform both qualitative and quantitative analysis with excellent lateral resolution.
High-speed, high-accuracy elemental mapping using X-ray digital mapping.
Measurable Sample Types
Powder samples: Coated with gold even if metallic, as oxidation or contamination may insulate the surface.
Solid samples: Surface must be prepared to study microstructure and grain size. Good preparation is key, and coating is often needed.
Cross-section surfaces: Studied for some samples. Preferably prepared by the researcher, or fractured in liquid nitrogen if thin (extra fee applies).
Liquid samples: Wet/biological samples can be imaged with E-SEM, but not with regular SEM.
Measurement Conditions
Samples must be fully dry; no wet/greasy samples accepted. Use ESEM for wet samples.
Samples are gold-coated unless otherwise requested.
Polymer samples must be prepped and fractured in liquid nitrogen by the researcher to avoid extra charges.
Cross-section imaging and thickness measurement count as separate tests.
For EDS, specify elements or all will be reported.
If you don’t want gold peaks in the spectrum, note it in the request.
A total of 8 microscopic images are taken.
Note: EDX analysis is available in the elemental analysis section.
Results Analysis
When analyzing FESEM images, if multiple phases are present, backscattered imaging is recommended along with secondary electron imaging. Contrast in SE imaging is based on surface shape, while BSE contrast depends on atomic number. Thus, BSE imaging helps distinguish phases—those with higher average atomic number appear lighter; lower ones appear darker.
To analyze elemental composition, Energy Dispersive X-ray Spectroscopy (EDAX) is used. Three main methods include:
1. Point Analysis: Determines elemental ratio at a specific point (min. region ~2 μm).
2. Line Scan: Similar to point analysis but over a defined line. Useful for coating thickness, penetration depth, and particle size in alloys.
3. Mapping: A large surface area is analyzed, with colored dots representing element distribution—ideal for distinguishing and identifying specific phases.
Note: EDAX accuracy for second-period elements (C, N, O) is low and often has large error margins.