Microscopie Électronique à Balayage

Introduction

• Scanning electron microscopy (SEM) is a type of electron microscopy that creates images of a sample by scanning it with a focused beam of electrons.
• SEM does not pass electrons through the sample—unlike transmission electron microscopy (TEM).
• SEM is primarily used to image the surface topography and morphology of a sample.
• SEM provides high-resolution images of surface features.

Principle of Operation

• A beam of electrons is emitted from a heated filament (tungsten or lanthanum hexaboride, for example).
• The electron beam is focused and scanned across the surface of the sample.
• When the electron beam interacts with the sample, secondary electrons (SE) and backscattered electrons (BSE) are emitted.
• Detectors collect these emitted electrons, and the varying signal intensity creates the image.
• The image reflects the surface topography and other material properties.

Types of Electron Detectors

• Secondary electron (SE) detector: Detects low-energy secondary electrons emitted from the surface, providing high-contrast images of surface morphology.
• Backscattered electron (BSE) detector: Detects high-energy backscattered electrons, creating contrast based on the sample's elements' atomic numbers (higher atomic number = brighter).
• Energy-dispersive X-ray spectroscopy (EDS) detector: Simultaneously identifies the sample's elemental composition.
• Cathodoluminescence (CL): Analyzes the luminescence emitted by the electron beam, revealing material defects and other relevant properties.

Sample Preparation

• Samples are typically conductive or coated to enable electron beam interaction.

Advantages of SEM

• High magnification and resolution for surface feature images.
• Enables analysis of surface topography and morphology.
• Suitable for a wide variety of samples (biological and materials).
• Non-destructive analysis.
• Comparatively low cost compared to other advanced microscopy methods.

Disadvantages of SEM

• Requires sample preparation, potentially time-consuming and complex.
• Requires a vacuum environment for analysis.
• Limited depth of field compared to optical microscopy.
• Potential for specimen damage, especially with non-conductive materials due to charging effects.

Applications

• Material science (e.g., microstructure characterization, fracture surface studies).
• Biological science (e.g., cell and tissue surface visualization).
• Nanotechnology (e.g., nanoparticle and nanostructure imaging).
• Industrial quality control (e.g., surface defect inspection).
• Forensic science (e.g., trace evidence analysis).
• Paleontology (e.g., fossil surface examination).

Image Analysis

• SEM images are examined to identify features, sizes, and morphologies.
• Image analysis software quantifies parameters.

Types of SEMs

• Variable pressure SEM (VP-SEM): Allows non-conductive samples without coating by adjusting chamber pressure.
• Environmental SEM (ESEM): Enables imaging of biological samples without drying or coating, using water vapor.