scanning electron microscope

scanning electron microscope
Name	Scanning Electron Microscope
Caption	A modern scanning electron microscope.
Acronym	SEM
Classification	Electron microscope
Inventor	Manfred von Ardenne, Charles Oatley
First built	1937 (prototype)
Related	Transmission electron microscope, Focused ion beam

scanning electron microscope. A scanning electron microscope is a type of electron microscope that produces images of a sample by scanning its surface with a focused beam of electrons. The electrons interact with atoms in the sample, generating various signals that contain information about the sample's surface topography and composition. The instrument is a fundamental tool in fields ranging from materials science to biology, providing high-resolution, three-dimensional-like images.

Principle of operation

The core principle relies on the interaction of a focused electron beam with the atoms of a specimen. Primary electrons from the beam penetrate the sample surface, where they undergo scattering events, losing energy and generating secondary signals. Key among these are secondary electrons, which are low-energy electrons ejected from the sample's surface atoms, providing topographical contrast. Backscattered electrons are primary electrons that are reflected after elastic collisions with atomic nuclei, yielding compositional contrast based on atomic number. These signals are collected by dedicated detectors, such as an Everhart-Thornley detector, and their intensity is mapped to form a pixel-by-pixel image synchronized with the beam's scan across the raster pattern.

Instrument components

A standard instrument comprises several key subsystems housed within a high-vacuum column maintained by a vacuum pump system. The electron gun, typically a tungsten filament, lanthanum hexaboride cathode, or field emission gun, generates the electron beam. This beam is focused and shaped by a series of electromagnetic lenses, including the condenser lens and objective lens, within the electron column. Beam deflection is controlled by scan coils. The sample is placed on a specimen stage inside the vacuum chamber, which often allows for precise movement and tilt. Signal detection is handled by devices like the Everhart-Thornley detector for secondary electrons and a solid-state detector for backscattered electrons, with the resulting data processed by integrated computer systems for image display.

Imaging modes and capabilities

Beyond basic topographic imaging using secondary electrons, the instrument offers multiple analytical modes. The backscattered electron mode reveals differences in atomic number, useful for distinguishing phases in alloys or minerals. Many are equipped for energy-dispersive X-ray spectroscopy, which detects characteristic X-rays to determine elemental composition. Cathodoluminescence detectors capture light emission from semiconductors or geological samples. Advanced systems may include electron backscatter diffraction for crystallographic orientation mapping. The resolution is superior to optical microscopes, typically reaching 1 nanometer or better, with a great depth of field that produces a distinctive three-dimensional appearance.

Sample preparation

Preparation is critical and varies by material. Non-conductive samples, such as biological tissues or polymers, require coating with a thin conductive layer of gold, gold-palladium, or carbon using a sputter coater to prevent charging effects. Biological specimens often need chemical fixation, dehydration, and critical point drying to preserve structure in a vacuum. Metallographic samples may require polishing and etching. The prepared sample is then mounted on a specimen stub using conductive adhesive and placed into the vacuum chamber. Specialized stages allow for the examination of wet samples in environmental SEM or samples at extreme temperatures.

Applications

The tool is ubiquitous in research and industry. In materials science, it is used for fractography, examining semiconductor devices, and characterizing nanomaterials. Geologists employ it for petrographic analysis of rocks and minerals. In the life sciences, it reveals ultrastructural details of cells, bacteria, and pollen. Forensic scientists use it for trace evidence analysis, while in quality control and failure analysis, it helps inspect integrated circuits and weld joints. Institutions like NASA have used it to analyze extraterrestrial materials, such as samples from the Moon or asteroids.

Limitations and alternatives

Primary limitations include the need for a vacuum environment, which precludes the study of most liquid or volatile samples without specialized equipment, and the requirement for samples to be electrically conductive. The high-energy beam can also cause damage to sensitive materials like some organic compounds or biological tissue. For these cases, environmental SEM allows for higher pressure around the sample. For ultimate atomic resolution or internal structure, the transmission electron microscope is used, though it requires extremely thin samples. Other complementary techniques include the scanning probe microscope, such as the atomic force microscope, which can operate in ambient conditions, and the focused ion beam system, used for site-specific analysis and milling.

Category:Electron microscopy Category:Laboratory equipment Category:Scientific techniques