SEM vs TEM: Understanding the Differences

This article explores the key distinctions between two powerful tools in material characterization: the Scanning Electron Microscope (SEM) and the Transmission Electron Microscope (TEM). Both utilize electrons to generate images of samples, but they do so in fundamentally different ways, leading to distinct applications and results.

Introduction

Electron microscopes have become indispensable for characterizing a wide variety of materials. Two primary types stand out: the Scanning Electron Microscope (SEM) and the Transmission Electron Microscope (TEM). While both use electron beams to create images, their methods of interaction with the sample and the information they provide differ significantly. Both SEM and TEM share some common components:

• A source of electrons
• Lenses to control the electron beam
• Electron apertures
• A chamber to maintain high vacuum conditions

SEM: Scanning Electron Microscope

How it Works

• SEM relies on scattered electrons to form an image.
• It employs a series of coils to scan the electron beam across the sample in a raster-like pattern.
• The microscope collects and counts these scattered electrons, generating an image that reflects the sample’s surface characteristics.
• SEM is primarily used to study the surface of a sample and its composition.
• It creates an image by analyzing the sample bit by bit, building a picture based on the electron scattering at each point.
• This process results in a 3D image representation.
• The magnification limit for SEM is around 2 million times.
• The resolution of SEM is approximately 0.4 nanometers.

Example

Think of an SEM image like the picture you see on a TV monitor, which is built up line by line.

Image

TEM: Transmission Electron Microscope

How it Works

• TEM, on the other hand, is based on transmitted electrons.
• In TEM, electrons are passed through the sample before being collected.
• The electron beam is directly aimed towards the sample.
• TEM is designed to look inside or beyond the surface of a sample, revealing its internal structure.
• It depicts the sample as a whole structure, showing its internal layers.
• This results in a 2D image.
• TEM offers a much higher magnification limit, reaching around 50 million times.
• The resolution of TEM is significantly finer, at approximately 0.5 Angstroms.

Example

A TEM image is similar to the image you might see on a fluorescent screen, where the electrons passing through illuminate the image directly.

Image

Key Differences: SEM vs TEM

The following table summarizes the core differences between SEM and TEM: