Electron Sample Interactions
When the electron beam strikes the sample, both photon and electron signals are emitted.
Elastic and inelastic scattering of electrons by positively charged nucleolus
Elastic and inelastic Interaction
• When an electron beam strikes a sample a large number of signals are generated. One possible signal could be from electrons.
• The incident electrons that are sent into the sample are scattered in different ways, namely:
– Elastic
– Inelastic.
Elastic Scattering
- As the name implies, elastic scattering results in little (<1eV) or no change in energy of the scattered electron, although there is a change in momentum.
- Since momentum, p = mv, and m doesn’t change, the direction of the velocity vector must change.
- The angle of scattering can range from 0-180 degrees, with a typical value being about 5 degrees.
- Elastic scattering occurs between the negative electron and the positive nucleus. This is essentially Rutherford scattering. Sometimes the angle is such that the electron comes back out of the sample.
- Or in elastic scattering events the primary electron comes close to the nucleus alters the path of the primary electron with a minimal loss in electron velocity.
- A primary electron whose path is altered enough to have it leave the specimen is referred to as a backscattered electron.
- Backscattered electrons will have an energy range from 50eV (electron volts) up to the accelerating potential the SEM is being operated at.
- The amount of backscattered electrons emitted from a specimen is dependent upon the atomic number of the specimen.
- This is known as the backscattered coefficient, and as a general rule, the number of backscattered electrons emitted from a specimen increases with an increasing atomic number.
- Also, increasing the beam current will excite more backscattered electrons from the specimen.
- With proper detector, the image formed from the backscattered electrons emitted from a specimen will show regions of atomic number inhomogeneity.
Inelastic Scattering
• Inelastic events are those where the primary beam electron has a collision with a nucleus or electron of an atom of the specimen.
• The primary electron undergoes a change in direction, as well as transferring energy to the specimen.
• Some signals generated by inelastic events are:
– auger electrons,
– secondary electron,
– characteristic x-rays and
– brehmstraalung radiation (continuum X-ray).
• Auger electrons are used to characterize the elemental composition of the surface of a specimen.
• Characteristic X-rays can be collected and sorted to provide elemental information of the specimen.
Secondary Electrons
- Secondary electrons are predominantly produced by the interactions between energetic beam electrons and weakly bonded conduction-band electrons in metals or the valence electrons of insulators and semiconductors.
- There is a great difference between the amount of energy contained by beam electrons compared to the specimen electrons and because of this, only a small amount of kinetic energy can be transferred to the secondary electrons.
The Everhart-Thornley detector Introduction
The Everhart-Thornley detector (E-T for short) allowed the formation of images using the secondary electron signal, which is much more dependent on the sample topography at the point of intersection of the primary beam with the sample.
• The end result is higher potential resolution using this signal. Although many people tend to gloss over the distinction, the E-T detector is actually a combined signal detector, rather than a pure secondary electron detector.
• A typical E-T detector consists of a Faraday cage in front of a scintillator in turn coupled to a light pipe leading to a photomultiplier tube.
• The Faraday cage is typically kept at a positive potential on the order of a few hundred volts so as to efficiently collect most of the secondary electrons emitted from the sample.
• When the secondary electron strikes the face (scintillator) of the detector, the electrical energy is converted to a photon.
• The scintillator typically has a thin coating of some conductor sufficient to maintain a positive voltage of several kilovolts, so that the electrons that pass the Faraday cage are accelerated into the scintillator.
• When the electrons strike the scintillator they produce light, which is in turn directed to the photomultiplier by the light pipe.
• The photon travels down a light pipe, where it enters the photomultiplier tube. In the photomultiplier tube, the photons are changed back to electrical energy, undergoing cascading events across a series of dynodes to enrich the signal. The output signal is related to the total number of electrons collected.
• The signal is further refined in the preamplifier and amplifier before being projected onto the screen of the viewing cathode ray tube.
• There is a one-to-one correspondence between a point scanned on the sample, and a pixel on the viewing screen.
• Because the scintillator is typically in direct line-of-sight with the sample, backscattered electrons (which usually are too energetic to be deflected much by a 200V potential) will also produce a signal in the detector, even if a negative potential is applied to the Faraday cage.
• By adjusting the cage potential, it is possible to ‘tweak’ the topographic contrast given by the detector. Most SEMs will have an E-T style detector, although not all will allow adjustment of the collection potential.
Backscattered Electron Detector
(look at images)
- A diode-type BSE detector is based off the fact that an energetic electron hitting a semiconductor will tend to loose its energy by producing electron-hole pairs.
- A polepiece mounted BSE detector is just above center. The black cone surrounds the polepiece.
- The number of electron-hole pairs will be dependent on the initial electron energy, so higher energy electrons will tend to contribute more to the signal.
- The simplest diode detector is a p-n junction with electrodes on the front and back of the sample. (Diodes allow electricity to flow in only one direction)
- The holes will tend to migrate to one electrode, while the electrons will migrate to the other, thus producing a current, the total of which is dependent on the electron flux and the electron energy.
- Response time of the detector can be improved by putting a potential across the diode, at the expense of increased noise in the signal.
- Diode detectors are frequently mounted on the microscope polepiece (as the greatest BSE yield is typically straight up for a horizontal surface), and it is common to break them into a number of sections which may be individually added to or subtracted from the signal.
Solid State BSE Detector
- The Backscattered Electron Detector uses a large area silicon diode specially fabricated for electron detection.
- The ability to select quadrants allows it to be used for topographical studies with the compositional information suppressed (similar to the Joel 6100 detector at SEM lab this one has four piece diode).
Use of backscattered electron signals
• Although secondary electron images are obtained most frequently with the SEM, backscattered electron images also provide important information.
• Backscattered electrons vary in their amount and direction with the composition, surface topography, crystalline and magnetism of the specimen.
The contrast of a backscattered electron image depends on
(1) the backscattered electron generation rate that depends on the mean atomic number of the specimen,
(2) angle dependence of backscattered electrons at the specimen surface, and
(3) the change in the backscattered electron intensity when the electron probe’s incident angle upon a crystalline specimen is changed.
• The backscattered electron image contains two types of information:
– specimen composition
– specimen topography
• To separate these two types of information, a paired semiconductor detector is provided symmetrically with respect to the optical axis.
• Addition of them gives a composition image while subtraction gives a topography image.
• The interaction volume for backscattered electrons is larger than that of secondary electrons, namely, several tens of nm. Therefore, backscattered electrons give poorer special resolution than secondary electrons.
• Since they have a larger energy than secondary electrons, they are less influenced by charge-up and specimen contamination.
