Extracts from the Protrain e-book series "Working With a Scanning Electron Microscope"

[Contrary to most SEM operators view of their results being solely due to secondary electrons, this presentation tries to introduce the contribution of backscatter into the operators minds.  Too few understand that backscatter enter the "so called" secondary electron detector and have a considerable influence upon the resulting image.  Simply pressing a button on the microscope marked SE does not exclude the backscattered influence!]

A number of signals are generated when an electron beam penetrates a specimen of that the following are considered in this e-book.

Inelastic scattering.  When an electron from the beam enters a material, the highest probability is that it will strike an electron in the orbit of an atom.  If incoming electron has sufficient energy to knock the orbital electron out of the atom, it becomes a free electron. This inelastic scattering reaction reduces the energy of the incident beam electron, but it may travel deeper into the specimen undergoing multiple collisions, releasing electrons until it is devoid of energy.

Elastic scattering.  An electron approaching an atom may impart energy to that atom whilst “bouncing” off the atom and changing its direction of travel through the material this is known as Rutherford scattering.  The electro-magnetic field at the nucleus retards an electron from the beam that passes near to the nucleus of an atom.  This reaction causes the incident beam electron to change direction and to emit an X-ray of an energy equal to that lost by the electron. The incident beam electrons lose very little energy through this reaction, even if scattered through one hundred and eighty degrees, but the reaction places the atom into oscillation creating heat.  Incident beam electrons suffering multiple scattering may be deflected in such a way as to be liberated from the surface of the specimen.

The volumes involved in the production of SE, BSE and X-rays, form into a teardrop or hemisphere shape within the specimen.  The depth and diameter of the teardrop or hemisphere, depends upon the accelerating voltage being used and the density of the specimen.  The higher the accelerating voltage, the deeper the beam penetration, the denser the specimen and the more the beam penetration is reduced. Secondary electrons have a very low energy level (<50eV) and are unable to travel very far through the material compared with backscatter that may be of almost the same energy as the incident beam.  Secondary electrons escape from the top 15nm of the specimen independent of the accelerating voltage where as the backscattered electrons escape from approximately 40% of the total electron penetration, best observed through a Monte Carlo simulation.

Secondary electrons have a fairly constant emission level as one moves away from the very light elements. However at very low accelerating voltages secondary electron emission varies considerably.  Backscatter signal levels continue to increase with atomic number.  With smoother specimen surfaces different elements give rise to images exhibiting "atomic number contrast", the denser the element the brighter the image.

The detection of secondary electrons is accomplished through the use of an Everhart-Thornley detector.  The negative secondary electrons are attracted toward the detector, by a positive voltage of between 200 and 400 volts, which is applied to a cage at the front end of the detector.  Once within the cage, the electrons are accelerated into a scintillator (or screen), by a potential of up to 10kV, which is applied across its face.  The acceleration of the electron into the scintillator, results in the production of a photon, which travels down a light guide to a photomultiplier.  The photomultiplier is the most efficient amplification system available, producing a current of electrons from a single photon. The detector will also accept backscattered electrons that excite the scintillator either line of sight from the specimen or through collisions with features within the specimen chamber

The electrons leaving a specimen fall into two major energy areas.  Below 50eV is the secondary electron peak and just below the accelerating voltage level is the backscattered electron peak.  The definition of secondary or backscattered electron is the 50eV point.  Less than 50eV an electron is called a secondary, above 50eV the electron is called backscatter.

During the manipulation of the microscope parameters the operator is simply manipulating how much of each signal is include within the image.  

For example raise the kV and more backscatter (sub surface detail) contributes to the image, tilting the specimen towards the Everhart-Thornley detector also increases the backscatter contribution.  Lowering the accelerating voltage reduces the backscatter contribution allowing the secondary contribution to be increased providing more of the true surface image.

There are 5 types of secondary electron contribution and three types of backscattered electron contribution to an image

Secondary Electrons Type 1.  are secondary electrons produced by the incident electron beam impinging upon the specimen surface, these electrons determine the basic image resolution.  

Secondary Electrons Type 2.   are secondary electrons produced by the backscattered electrons as they leave the specimen surface; there may be up to four times as many type 2 electrons produced than type 1. 

Secondary Electrons Type 3.   are secondary electrons produced by the backscattered electrons that strike the components of the microscope; the final lens, specimen stage, or specimen holder.  The reaction volumes that are created produce secondary and backscattered electrons, as well as x-rays that relate to the component involved. 

Secondary Electrons Type 4.  are secondary electrons produced by the incident electron beam impinging upon the final aperture, usually in instruments which do not have a variable aperture system

Secondary Electrons Type 5.  are secondary electrons produced by the backscattered electrons that strike the components of the microscope; the final lens, specimen stage, or specimen holder and bouncing back to irradiate an area away from that being investigated.  

Backscattered Electrons Type 1.  enter directly into the Everhart-Thornley detector; those that have a line of sight as they spray from the specimen surface. Backscattered electrons are of high energy and are not affected by the cage voltage; they travel in straight lines until they strike a surface.  

Backscattered Electrons Type 2.  have spent a longer period of time within the specimen often escaping well away from the incident beam penetration point.  These electrons have lost more energy than those conventionally collected as backscatter, having undergone multiple collisions within the specimen. 

Backscattered Electrons Type 3.  strike the components of the microscope and are then re deflected which may be toward the Everhart-Thornley detector.  

Conventional Secondary Electron Collection. The conventional position for the Everhart-Thornley detector is within the specimen chamber.  In this case the signal variation or contrast may be changed not only as described earlier, but also through the vertical movement of the specimen within the specimen chamber.  Moving the specimen in this manner varies the signals reaching the Everhart-Thornley detector adding or subtracting the backscattered electrons influence.  Not all of this change is due to line of sight backscatter, a great deal is due to converted backscatter from components within the specimen chamber.  The high energy backscatter spray away from the specimen surface in all directions.  The backscattered electrons striking components of the chamber with energies near to the accelerating voltage react with these surfaces in the same way as the electron beam with the specimen.  Secondary electrons, backscattered electrons and x-ray are all produced from the interaction of the backscattered electrons with the chamber components.  Depending upon the ease that the electron signals “see” the Everhart-Thornley detector they will add their contribution.  Secondary electrons will be attracted into the Everhart-Thornley detector and backscattered electrons may reach the detector through direct line of sight or through multiple scattering reactions.  The denser the area of the specimen the greater the number of backscattered electrons and therefore the greater the number of converted backscatter and the greater the number of secondary electrons from the components of the microscope.  

The manufacturers detector design may also have an influence on which electrons reach the detector.  Small scintillators reduce the backscattered electron content simply by offering a smaller surface area with which to react.  However as the secondary electrons are actually attracted into the detector their number is not related to scintillator size.  Some manufactures have an open mesh collector offering no signal discrimination, whilst others have a cone or shield on the front or around the detector that constrains the detected signal.  The position of the detector will have an influence upon the signals it collects and under which conditions high levels of secondary electrons or backscattered electron collection take place.

If the microscope has a large diameter scintillator within the detector and it is not collimated in any way its ability to produce an image without the interference of backscatter is severely limited.  Placing a screen between the specimen and the detector with a very small hole (0.5mm or 1/4 an inch) in line with the scintillator is often the only "cure".  Another method for the reduction of the contribution made by backscatter is to place a carbon plate on the base of the final lens.  The plate consists of an aluminum disc with a 1cm (1/2inch) hole in its centre.  The disc is badly coated with a high quality carbon paste to produce an electron absorbing rough surface which may be fixed with double sided tape to the lens base.  This facility is also a considerable assistance for eliminating lens x-rays from a spectrum.

Double detector systems.  Since the early 1980s instruments with two Everhart-Thornley detectors have been commercially available.  The detectors are mounted one within the specimen chamber, the other being placed above the final lens.  In this configuration the final lens acts as an electron filter, the lens field and lens geometry preventing the upper detector having line of sight backscattered electron influence.

High resolution imaging of areas within large specimens had been prevented in conventional instruments as moving the large sample nearer to the final lens prevented most of the signal from reaching the Everhart-Thornley detector; the specimen was in the way!  With a detector above the final lens such problems do not exist and the following reactions influence signal collection. 

The electron beam strikes the specimen with the normal beam-specimen reactions.  The low energy secondary electrons are unable to break away from the lens axis due to the lens field being sufficient to control the beam and much too strong to allow the secondary electrons to escape.  These electrons are therefore unable to move either downwards or to one side and as a result they spiral back up the column (often helped by an extraction voltage) until they are outside of the lens field and at this point they are attracted into the upper Everhart-Thornley detector.  In this way a pure secondary electron image may be formed, but be aware that this will make the specimen more susceptible to charge! 

The high energy backscattered electron are unable to contribute to the image in a double detector instrument unless the manufacturer has a lens design that allows their influence to be incorporated in the upper detector signal.  Converted backscattered electrons, that strike the lens surfaces producing secondary electrons may be incorporated in the final image or excluded depending upon the lens configuration and the specimen position.  In the dual detector system images may be obtained from specimens less than 3mm from the final lens, longer working distances (often 7 to 9mm) offer the opportunity for the backscattered electron influence to contribute in this way.  The constructive influence of backscattered electron should not be discounted when attempting to obtain the maximum information from a specimen area, very often whilst secondary electrons offer resolution the backscattered electron offer more information.

The high resolution performance capabilities of a dual detector system should not inhibit the use of the lower detector or, available on some instruments the ability to add upper and lower detector signals together.  The lower detector will offer all the facilities and specimen manipulation that is available in a single detector instrument.  Far higher contributions from backscattered electron, therefore higher image depth and image contrast, as well as lower magnifications, will be available from this configuration. High backscattered content also reduces the possibilities of any charge effecting the recorded image.

In short, a double detector instrument offers far wider imaging variations and therefore the ability to extract far more information from the specimen; the ideal system.