Scanning Electron Microscope (SEM)

The Early History and Development of The Scanning Electron Microscope

The earliest known work describing the concept of a Scanning Electron Microscope was by M. Knoll (1935) who, along with other pioneers in the field of electron optics, was working in Germany. Subsequently M. von Ardenne (1938) constructed a scanning transmission electron microscope (STEM) by adding scan coils to a transmission electron microscope. The first STEM micrograph was of a ZnO crystal imaged at an operating voltage of 23 kV at a magnification of 8000 times, and a spatial resolution between 50 and 100 nm. The micrograph contained 400 x 400 scan lines and took 20 min to record, because the film was mechanically scanned in synch with the beam. The instrument had two electrostatic lenses, with the scan coils placed between them. The instrument also had a viewing CRT, but it was not used to record the image

The first SEM used to examine the surface of a solid specimen was described by Zworykin et al. (1942), working in the RCA Laboratories in the United States. The electron optics of the instrument consisted of three electrostatic lenses with scan coils placed between the second and third lenses The electron gun was located at the bottom so the specimen chamber was at a comfortable height for the operator. This was a common practice in the early days. It did suffer from the slight problem however, that the specimen might fall down the column. A resolution of about 50 nm was achieved with this first SEM. By comparison with the rapidly developing TEM, this figure was considered unexciting and further development lapsed.

In the late 1940s C. W. Oatley, then a lecturer in the Engineering Department of Cambridge University, England, became interested in conducting research in the field of electron optics and decided to re-investigate the SEM as a complement to the work being done on the TEM by V. E. Cosslett, also in Cambridge at the Physics Department. One of Oatley's students, Ken Sander, began work on a column for a transmission electron microscope using electrostatic lenses, but Ken took ill after about one year and had to leave for a time. This work then was taken up by Dennis McMullan in 1948, and he and Oatley built their first SEM. By 1952 this instrument had achieved a resolution of 50 nm, but by far the most important thing about this SEM is that it produced the first micrographs showing the striking three-dimensional imaging characteristics of the modern-day SEM (McMullan (1953)).

Dennis McMullan was followed by Ken Smith (started 1952) who took over SEM1, made a number of improvements to the electron optical system, and improved the efficiency of secondary electron collection. He showed for the first time that a stable image could be formed using the true low-energy component of the total secondary emission. Ken has told me personally that during this period in the development of the SEM, microscopists in general showed virtually no interest in the instrument, and he and Oatley were continually looking for applications to promote the SEM; many which they tried were never published. However, they published the first paper setting out clearly the fields of application for the SEM (Smith and Oatley (1955)).

The third research student on the SEM under Oatley's supervision was O. C. Wells (started 1953), who built a second SEM, also incorporating electrostatic lenses; unlike SEM1, however, this instrument had the gun at the bottom of the column - a configuration considered better for experimental work. All SEMs built in the Engineering Department subsequently conformed to this layout. Wells pioneered the use of the scintillator backscattered (BSE) detector (as an alternative to the secondary electron multiplier used in SEM1) and applied his SEM to many new types of specimen including a large-scale study of fibres. He was also the first to use stereographic pairs to produce SEM micrographs with quantifiable depth information (Wells (1960)).

The next important step was taken by Oatley's fourth research student, Everhart (started 1955), who improved the secondary electron (SE) detector by using a scintillator to convert electrons to photons, which were then transmitted by a light pipe directly interfaced to the photomultiplier tube. This idea was followed up by Thornley (started 1957), and their ground-breaking work resulted in the publication of a much-quoted paper: "Wide-band detector for micro-microampere low-energy electron currents" (Everhart and Thornley (1960)). Replacement of the electron multiplier with the new scintillator/photomultiplier combination increased the amount of signal collected and resulted in an improvement in signal-to-noise ratio. Hence, weak contrast mechanisms, such as voltage contrast (discovered by Oatley and Everhart (1957)) could be better investigated. The term " voltage contrast" evolved from the fact that as the voltage applied to a specimen was changed, the image contrast changed. (Tom Everhart and Oatley were investigating this one day and Tom said: " Well, voltage contrast!" and the term has stuck to this day). Image interpretation was also improved in this early period of research when both Everhart and Wells made the first quantitative studies of the effects of beam penetration on image formation in the SEM.

It was always one of Oatley's great ambitions to produce and market a really simple low-cost SEM; he argued that most of the work undertaken by microscopists did not require high resolution. To further this idea Peter Spreadbury, his fifth student (started 1956), built a simple SEM utilising a CRT as a display unit.

New fields of application were opened up by Gary Stewart (started 1958) who fitted an ion gun to the SEM specimen chamber to allow ion bombardment of the specimen. After his research, Gary went on to the Cambridge Instrument Company to pioneer the production of the Stereoscan. The ion beam work was later extended by Alec Broers (started 1961) who improved the ion beam optics of the instrument and added a magnetic objective lens to improve resolution. He used this set-up to conduct some of the earliest experiments in electron beam microfabrication. Another breakthrough was achieved by Haroon Ahmed (started 1959) who modified the SEM built by Wells (SEM2) to enable the examination of thermionic emitters at temperatures exceeding 1000K. The first SEM in the Department to achieve a resolution of 10 nm was built by Fabian Pease (started 1960). This was an all magnetic lens SEM, the fifth to be constructed in the Group. Several of these SEMs were later manufactured in the Departmental Workshops and used by other Groups within the University.

No description of the work in Oatley's Group would be complete without mention of Les Peters, chief technician, who had a hand in the construction of all the instruments described and helped all the research students with their experimental work. For his services to the Department Les was awarded an honorary MA degree by the University in 1990. Pictured here are Les Peters and Charles Oatley together in the Board Room of the Engineering Department on the occasion of a celebration held to mark the event. The contribution of Joan Duffield, Charles Oatley's secretary, to the success of this early work must also be recognised. She organised all the paperwork for the Group, typed the technical papers and typed the majority of the research students' dissertations - in the days when five all-correct carbon copies were the norm and word processors were unheard of! Her work in those early days was invaluable.

With his appointment in 1960 to the Chair of Electrical Engineering in the Department, Charles Oatley's direct involvement in the supervision of research students came to an end; this task was taken over by Bill Nixon who joined the Group in 1959 having worked in the Cavendish EM Group with V. E. Cosslett for a number of years. So ended the first phase of the research on the scanning electron microscope at the University Engineering Department. 

The Early History and Development of The Scanning Electron Microscope. (n.d.). Retrieved from http://www-g.eng.cam.ac.uk/125/achievements/oatley/history.html. 


Introduction of Scanning Electron Microscope

A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with electrons in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. The electron beam is generally scanned in a raster scan pattern, and the beam's position is combined with the detected signal to produce an image. Specimens can be observed in high vacuum, low vacuum and an environmental SEM specimens can be observed in wet condition. The most common mode of detection is by secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons is a function of the angle between the surface and the beam. On a flat surface, the plume of secondary electrons is mostly contained by the sample, but on a tilted surface, the plume is partially exposed and more electrons are emitted. By scanning the sample and detecting the secondary electrons, an image displaying the tilt of the surface is created.

Principles & Capacities of SEM

The types of signals produced by a SEM include:

• Secondary electrons (SE)
• Back-scattered electrons (BSE)
• Characteristic X-rays
• Light (cathodoluminescence) (CL)
• Specimen current
• Transmitted electrons

Secondary electrons (SE)

• Secondary electron detectors are standard equipment in all SEMs, but it is rare that a single machine would have detectors for all possible signalsThe signals result from interactions of the electron beam with atoms at or near the surface of the sample.
• The signals result from interactions of the electron beam with atoms at or near the surface of the sample.
• In the most common or standard detection mode, secondary electron imaging (SEI), the SEM can produce very highresolution images of a sample surface, revealing details less than 1 nm in size.
• Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample.

Back-scattered electrons (BSE)

• Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering.
• BSE are often used in analytical SEM along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen.
• BSE images can provide information about the distribution of different elements in the sample.

Characteristic X-rays

• Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher-energy electron to fill the shell and release energy.
• These characteristic X-rays are used to identify the composition and measure the abundance of elements in the sample.

Application and Uses

Scanning electron microscopy can help businesses involved in the development or manufacturing of products learn more about the composition and topography of products and components. For instance, some products, like stainless steel, must be evenly coated with special chemicals for optimal performance. Scanning electron microscopy can help identify cracks, imperfections, or contaminants on the surfaces of coated products.

Industries, like cosmetics, that work with tiny particles can also use scanning electron microscopy to learn more about the shape and size of the small particles they work with. For instance, particles that are too large or jagged might not flow or mix as well as particles that are small and round. Particles that are the wrong size or shape may have an impact on the consistency or performance of the product. Scanning electron microscopy can be used to identify problems with particle size or shape before products reach the consumer.

Finally, industries that use small or microscopic components to create their products often use scanning electron microscopy to examine small components like fine filaments and thin films. If there is a problem occurring at a microscopic level, scanning electron microscopy can be used to pinpoint the problem and help find a solution.


By :-
Mohamad Hadif Iqbal Bin Mohd Radzi Cham
Muhammad Aidil Bin Abdullah Zawawi
Wan Ahmad Sabri Bin Wan Zulkifli
Muhammad Akromiin Bin Sulaiman