Scanning Electron Microscope

The Scanning Electron Microscope is another form of Electron Microscope which was first demonstrated by Max Knoll and whos principle inventor is considered to be Manfred von Ardenne, who unlike Knoll, first managed to exceed a magnification of 1:1.

Scanning Electron Microscopy as a technique is realized in many forms, including as accessories for the Transmission Electron Microscope. In both cases, the basic principle is the demagnification of the electron source crossover which is then moved over the the specimen, usually in a TV scan like pattern (line by line).

The magnification is thus the ratio of the scan area on the specimen to that of the view / recording screen and or other means of recording and presenting the data. The intensity of the scanned signal on these screens is modulated by the output of a multitude of detectors which measure various interactions of the primary electron beam with the specimen.

The over all resolution is there for limited by the Minimum size of the electron spot which the microscope is capable of producing. The final resolution is further limited or rather deteriorated from the theoretical Limit by the interaction which is to be used to modulate the Scan (see interaction volume). Additionally the current density also limits the users ability to correct for Astigmatism as well as focus the electron spot. This is due to the demagnification decreasing the available electrons and thus decreasing the signal output from the detectors.

Signal Sources

Secondary Electrons

The secondary electron detector is the most commonly found detector used for scanning electron microscopy. It functions by means of collecting the Secondary Electrons which the specimen emmits upon being struck by the high energy Primary Beam. Typically, though depending on the density and workfunction of the specimen, the amount of secondary electrons produced by each primary electron is several times more that of a single electron, thus achieving amplification.

The Images produced by this methode feature a greater brightness at edges, as well as the appearence of a directional light source, due to the secondary electrons being emmited in greater numbers from the side of a specimen due to the interaction volume. The directional lightning effect, which is also one means by which false color images may be produced from a scanning electron microscope, is due to electrons emmited with directions which not always reach the detector. This technique of False color was invented by David Scharf.

The SED (Secondary Electron Detector) is commonly found in 2 general designs. Both of which are based on the detector first described by Everhart and Thornley, after which it is named.

Cage Type

The Cage Type SED detector is the design first proposed by Everhart and Thornley, it uses a positively charged metal grid surounding a scintilator plate or hemisphere which is made conductive and held at a high positive portential. This Scintilator is optically coupled to a Light Pipe, which in turn is coupled to the face of a Photomultiplier Tube.

The function of the Grid is to attract the slow moving Secondary Electrons to the detector, where they are then further accelerated onto the Scinitlator plate, thus causing it to emit light. The amount of light is relative to the speed and quantity of electron hitting the Scinitlator, and is typically many times that of the incoming electrons. Thus further amplifying the SE Signal. The light output is then converted back to electrons via the photocathode of the Photomulitplyer where its Dynode structure further amplifies the signal by means of Secondary Electron Emmision.

The Advantage of having the Photomultiplyers vacuum not be coupled to that of the specimen chamber is rather easy to see, as a sealed glass bulb pumped down to a high or ultra high vacuum tents to remain as such, with no or little contamination happening within, thus allowing long opperational lifespans as well as the ability to more great amplify the signal therein.

Pipe Type

This detector geometry functions along the same lines as that of the tradiational Everhart and Thornly design, however it ommits the usage of a collector grid, instead using the geometry of a grounded pipe and the highly positively charged Scintilator surface to produce the low voltage attraction potential needed to collect seconardy electrons. This detector in theory has a higher collection efficiancy due to not having obstructions from the collector grin.

Back Scatter Electrons

The Back Scattered Electrons for which this signal source is named, are primary electrons from the scanning beam which have been reflected back out of the specimen by mans of Elastic and Inelastic Scattering within the atomic structure of the specimen.

The amount of Back Scattered Electrons is largely dependent on the Atomic Weight of the specimen, thus producing what is commonly reffered to as Z-Contrast (Z being the Element number).

The BSE (Back Scattered Electron) signal is typically picked up by 3 types of Detectors, of which the Semiconductor type has become the domminent methode.

Scintilaton BSE Detector

This one of the 2 main methodes of detecting the back scattered primary electrons. It is usually avalible in all electron microscopes by simply turning off the collection voltage of the Secondary Electron Detector. Thus primary electrons which have been scattered in the direction of the SED are then used to modulate the scan Signal.

However, this high angle signal is usually rather weak in comparrison to the low angle signals avalible when the detector is placed more in line with the scanning beam. This can be acomplished by either a Scintilator disk furnished with a hole through which the primary beam may pass, mounted directly beneth the Objective lens. Or by means of a Scintilator Disk mounted at an angle to the primary beam, which subtends a large solid angle of the specimen.

In both cases the construction is that of the Everhart and Thornly detector, but without the collection potentials.

Semiconductor

These detectors are typically PIN diodes of large area, which produce a current proportional to the electrons impacting them. These are typically mounted as a ring around the Objective Lens, and my be made of single diode, or more commonly a greater number, allowing the angle of detection to be varied or several different angles to be processed differently, thus producing signals such as Topographic contrast, or be used as Color signals for False color imaging.

Faraday Cup

The Faraday Cup is likely the simplest and most obvious detector design, as it consists of a capacitor plate, or cup (see Faraday Cup), on which the electrons impact, imparting a charge. This charge can be measured and used to Modulate the Scan Signal.

One advantage over the aformentioned methodes of Scintilation and Semiconductor detectors is the accuracy of the signal. If correctly designed, the signal output is directly proportional to the amount of electrons impacting the detector, thus allowing precise measurments to be carried out. However this comes at the cost of increased complexity to the electronics used, as well as typicall having a greater amount of noise then compared to the Scintilation and Semiconductor Detectors.

Auger Electrons

Auger Electrons are those electrons whos energy is greater then 100eV but less then the primary beams energy. The energies observable are directly corelated to the Element from which they originate, thus allowing the discrimination of different elements and their quantity within the sample.

Typically Auger Electron Detectors are not featured on normal Scanning Electron Microscopes as they require a much better vacuum to function then is normally avalible within the SEM. Thus a special class of microscopes called the Auger Electron Microscope exists which are specially designed to meet the requirments of Auger Electron detection.

Detection is done by using Energy Analyzers to filter and or scan the energy range, producing a spectrum or, if tuned to a specific energy, an image modulated by the amount of the coresponding element within the sample.

For further details see Auger Electron Microscope

Electron Back Scatter Defraction

This technique relies on the same interaction of elastic and inelastic scattering within the atomic latice of the specimen. Unlike the Backscatter Detectors described above, the EBSD (Electron Back Scatter Defraction) detector uses not a single element detector, but instead uses a 2D sensor array. Typically this is acomplished by means of a scintilator plate and a video camera which images this plate.

The resulting image shows the so called Kikuchi Lines which give insight over the crystal orientation at the position the electron beam is impacting.

There exist 2 general means of obtaining the Kikuchi line images, these are:

True Kikuchi / 2D Scintilator

This detector is simply a Scintilator disc which is being observed by a 2D camera. It is usually mounted off axis to the main beam, thus requiring the specimen be tilted toward it. When using this type of detector, the scan is usually stopped and a single spot is illuminated by the electron beam.

Psudo Kikuchi

This detector works not by observing the real Kikuchi lines, but instead uses the scanning system of the microscope, with the addition of a third set of scan coils, to incline the electron beam around the observation spot. This produces a Back scatter signal which is analegous to that of the true Kikuchi signal by means of Electron channeling.

Electron Channeling

Electron Channeling is a technique by which crystal planes produce a steep contrast change by fully or more considerably absorbing the incident electrons. It is detected like in the Psudo Kikuchi technique by means of a Back Scatter Detector.

In order to achieve this contrast, the specimen must be oriented mechanically in such a way that the crystal plane i in line with the electron beam.

Electron Beam Induced Current

EBIC (Electron Beam Induced Current) is the current signal originating from the sample its self. It represents the amount of electrons which have remained in the specimen and where not backscattered.

It further allows the detection of semiconductor junctions as well as observing their function. (describe more when time permmits)

The means by which this signal is obtained is simply to measure the total charge at each spot of the scan, and or its current. This is done by very high gain current meters / Electrometers. The signal to noise ratio is thus infirrior to that of the amplifying type detectors mentioned previously.

Further the specimen, or parts there of may be charged up, producing various effects.

Zero Loss Back Scattered Electron

(fill this in when time permits)