7/28/2019 What is a SEM

1/5

What is a SEM?

SEM stands for scanning electron microscope. The SEM is a microscope thatuses electrons instead of light to form an image. Since their development in theearly 1950's, scanning electron microscopes have developed new areas of studyin the medical and physical science communities. The SEM has allowedresearchers to examine a much bigger variety of specimens.

The scanning electron microscope has many advantages over traditionalmicroscopes. The SEM has a large depth of field, which allows more of aspecimen to be in focus at one time. The SEM also has much higher resolution,so closely spaced specimens can be magnified at much higher levels. Because

the SEM uses electromagnets rather than lenses, the researcher has much morecontrol in the degree of magnification. All of these advantages, as well as theactual strikingly clear images, make the scanning electron microscope one of themost useful instruments in research today.

How does a SEM work?

7/28/2019 What is a SEM

2/5

Diagram courtesy of Iowa State University SEM Homepage

The SEM is an instrument that produces a largely magnified image by usingelectrons instead of light to form an image. A beam of electrons is produced atthe top of the microscope by an electron gun. The electron beam follows avertical path through the microscope, which is held within a vacuum. The beamtravels through electromagnetic fields and lenses, which focus the beam down

toward the sample. Once the beam hits the sample, electrons and X-rays areejected from the sample.

7/28/2019 What is a SEM

3/5

Detectors collect these X-rays, backscattered electrons, and secondary electronsand convert them into a signal that is sent to a screen similar to a television

screen. This produces the final image.

How is a sample prepared?

Because the SEM utilizes vacuum conditions and uses electrons to form animage, special preparations must be done to the sample. All water must beremoved from the samples because the water would vaporize in the vacuum. Allmetals are conductive and require no preparation before being used. All non-metals need to be made conductive by covering the sample with a thin layer of

conductive material. This is done by using a device called a "sputter coater."

The sputter coater uses an electric field and argon gas. The sample is placed ina small chamber that is at a vacuum. Argon gas and an electric field cause anelectron to be removed from the argon, making the atoms positively charged.The argon ions then become attracted to a negatively charged gold foil. Theargon ions knock gold atoms from the surface of the gold foil. These gold atomsfall and settle onto the surface of the sample producing a thin gold coating.

What are the radiation safety concerns?

The radiation safety concerns are related to the electrons that are backscatteredfrom the sample, as well as X-rays produced in the process. Most SEMs areextremely well shielded and do not produce exposure rates greater thanbackground. However, scanning electron microscopes are radiation-generatingdevices and should be at least inventoried. The Indiana State Department ofHealth requires that the machines be registered with their office using State Form

7/28/2019 What is a SEM

4/5

16866, Radiation Machine Registration Application. It is also important that theintegrity of the shielding is maintained, that all existing interlocks are functioning,and that workers are aware of radiation safety considerations.

The main reasons for developing a SEM safety plan are:

to keep accurate inventory of all SEM's on campus (manufacturer/model,

serial number, location, contact person and phone number) to warn workers of the risk of interfering with any safety devices

(investigator needs to have permission to override any interlocks orwarning devices)

to make sure shielding is not compromised (exposure rate not greater

than 0.5 mrem/hr at 5 cm from any surface of machine) to let workers know who to contact in an emergency or if they have any

questions

X-Ray Detectors:

X-ray detectors are useful for examining the X-ray spectrum emitted

by the sample under the influence of the beam electrons. Because

most elements emit easily measurable characteristic X-rays, the X-rayspectrum collected from each region of a sample can provide useful

information on the elemental composition of the region of the sampleunder the electron beam.

Energy Dispersive X-Ray Detector (EDS)

The EDS detector (sometimes called EDX detector) is essentially a

large single crystal semiconductor that has either been treated toapproximate an ideal semiconductor, or is of high enough purity to

truly be an intrinsic semiconductor. This intrinsic semiconductor iscooled so there is very little thermionic creation of charge carriers,

typically by liquid nitrogen. Front and back contacts are kept atseveral kilovolts potential relative to each other. X-rays that pass

through the front contact will tend to dissipate their energycreating electron-hole pairs in the intrinsic region; because each

electron-hole pair has a characteristic creation energy, the total

number of charge carriers created is proportional to the energy ofthe incident X-ray. Thus, by measuring the charge pulse that is

created for each X-ray, the energy of the X-ray can be determined.A computer keeps track of the number of counts within each

energy range, and the total collected X-ray spectrum can then be

determined.

Wavelength Dispersive X-Ray Detector (WDS)

A WDS detector uses X-ray diffraction to separate the different X-ray energies (and therefore wavelengths) emitted from the sample.

7/28/2019 What is a SEM

5/5

WDS detectors tend to require much more space than EDSdetectors, as well as higher probe currents and long collection

times (due to lower collection efficiencies). WDS resolution is far

superior to EDS resolution, making it the detector of choice forsamples with many closely spaced peaks, or careful analytical

work.