Scanning electron microscope

A scanning electron microscope, or SEM, is a powerful scientific instrument used to observe the surfaces of tiny objects. Unlike a standard light microscope that uses photons, an SEM uses a focused beam of electrons to create images. This allows scientists to see details at an incredible scale. Some SEMs can achieve resolutions better than 1 nanometer. The magnification range is also much larger than light microscopy. An SEM can magnify an object from 10 times up to more than 500,000 times. This is about 250 times the limit of the best light microscopes.

The mechanism of an SEM relies on the interaction between electrons and the atoms of a sample. The microscope scans a focused electron beam across the specimen in a pattern called a raster scan. When the beam hits the surface, it triggers several different signals. These signals contain data about the topography, or surface shape, and the chemical composition of the material.

The position of the beam is combined with the intensity of these detected signals to construct a digital image. Because the electron beam is so narrow, the resulting micrographs have a large depth of field. This gives the images a characteristic three-dimensional appearance, which is very helpful for studying complex surface structures.

Different signals provide different types of information about the sample. The most common mode uses secondary electrons (SE). These are low-energy electrons, around 50 eV, that are emitted from the very top few nanometers of the surface. Because they only come from the surface, SE imaging provides very high resolution for seeing fine details.

Another signal comes from back-scattered electrons (BSE). These are primary beam electrons that are reflected from the sample through elastic scattering. Because they have higher energy, they emerge from deeper within the specimen. The intensity of the BSE signal is strongly related to the atomic number (Z) of the elements in the sample. This allows researchers to see how different elements are distributed across a surface.

Beyond electrons, the beam can also produce X-rays and light. When the electron beam removes an inner shell electron from an atom, a higher-energy electron moves to fill the gap. This process releases energy in the form of characteristic X-rays. Scientists use techniques like energy-dispersive X-ray spectroscopy (EDS) to measure these X-rays. This allows them to identify specific elements and map their distribution within the sample. Some SEMs can even detect cathodoluminescence, which is the emission of light from the sample.

The history of the SEM is marked by several key developments in the 20th century. While Max Knoll produced early images showing channeling contrast, Manfred von Ardenne is credited with inventing a high-resolution SEM in 1937. He used a finely focused electron beam to scan a small raster. He aimed to surpass the resolution of the transmission electron microscope (TEM) and avoid problems like chromatic aberration. In the same year, Cecil E. Hall built the first emission microscope in North America at the University of Toronto. Later, research groups in Cambridge, including Charles Oatley, made significant progress in the 1950s and 1960s. This work led to the first commercial SEM, the "Stereoscan," which was sold to DuPont in 1965.

Preparing a sample for SEM imaging is a complex process because of the microscope's environment. Most conventional SEMs operate in a high vacuum, which means samples must be completely dry or cryogenically cooled. Biological samples often require chemical fixation using substances like glutaraldehyde to stabilize their structure. To prevent the sample from collapsing during drying, scientists use organic solvents or critical point drying with liquid carbon dioxide.

Furthermore, non-conductive specimens can collect an electrostatic charge when scanned. This charging causes scanning faults and artifacts in the image. To prevent this, researchers often coat non-conductive samples with a thin layer of metal, such as gold, platinum, or iridium.

Modern SEM technology has expanded to handle many different types of materials and conditions. Variable pressure or environmental SEMs (ESEM) allow for imaging in low vacuum or even wet conditions. This is useful for samples that cannot survive a high vacuum. There are also specialized instruments that can operate at a wide range of cryogenic or elevated temperatures. In industrial settings, SEMs are used for defect analysis in semiconductor wafers. Some large-scale instruments can even tilt a 300 mm wafer by 45 degrees and rotate it 360 degrees to examine every part of the surface.