SEM Component Details

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The nature of the signals collected by an SEM in order to form images are all dependent on the detector used to collect them. Thus, an understanding of what a detector is and how it produces the signal being used is a necessary step towards understanding just what in the world one is looking at in the SEM. This section covers most of the common detector types found in SEMs, as well as some of the less-common types.

The Everhart-Thornley detector
Invention of 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 (figure [E-T]). 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. 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 photomultiplier produces an output signal that is then related to the total number of electrons collected.

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. In this case it is a pure backscattered electron detector. 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. Some E-T detectors are designed to be mobile, so that the geometrical collection efficiency can be adjusted, in addition to the collection potential.

Photo of the scanning electron microscopy component details (SEM)

Detector placements inside an SEM chamber. E-T detector is the wire meshwork and assembly to the right of center, while a polepiece mounted BSE detector is just above center. The black cone surrounds the polepiece.

Backscattered Electron (BSE) Detectors
In addition to the E-T detector mentioned above, dedicated backscattered electron detectors are reasonably common.

The simplest BSE detector is simply a stripped-down version of the E-T detector. These consist of a coated scintillator coupled to a photomultiplier by a light pipe. Because there is no collection potential, the collection efficiency of these detectors is directly related to the scintillator size and proximity to the sample. Additionally, the strength of the signal is dependent on the backscattered electron energy, so a low-energy BSE will contribute less to the total signal than will a high-energy BSE.

Silicon diode
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. Again, 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. 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. A four quadrant Si diode detector of this type can be seen mounted to the bottom of the polepiece in the above figure.

Channel plates
The basic idea behind a channel plate is secondary electron emission. A channel plate (or microchannel plate) is based off a plate consisting of a large number of tubes stretching from one side to another. Each side of the plate has an electrode, the sample side at ground or slightly negative potential, and the opposite side at a substantial positive potential. The insides of the tubes are coated with some material with a high secondary electron yield.

In operation, backscattered electrons strike the inside of the tubes (or possibly the front electrode, which also produces secondary electrons), which in turn causes emission of multiple electrons by the high coating material inside the tube. These secondary and backscattered electrons are in turn accelerated down the tube by the potential between the two face electrodes, and at some point impact the tube wall, producing more scattered electrons and repeating the process. This cascade effect causes each initial signal electron to produce a measurable current which can be used as a signal.

Other Detector Types

Sample Current
A sample current detector is simply a very sensitive ammeter with a fast response time that measures the current passing from the sample stage to ground. Because the total current is a function of the backscattered, secondary, Auger, etc. electron yield at any point on the sample, this signal is sensitive to all the contrast mechanisms of the other electron signals. Because of the dominance of the backscattered and secondary electron signals to this signal, and because the secondary yield is in turn dependent on the backscattered yield, the sample current signal typically resembles the inverted backscattered electron signal.

A cathodoluminescence detector is sensitive to the optical photons emitted by some materials under beam excitation. These typically consist of some arrangement of mirrors/light guides and a photomultiplier tube.

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-ray spectrum collected from each region of a sample can provide useful information on the elemental composition of the region of the sample under 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 to approximate an ideal semiconductor, or is of high enough purity to truly be an intrinsic semiconductor. This intrinsic semiconductor is cooled so there is very little thermionic creation of charge carriers, typically by liquid nitrogen. Front and back contacts are kept at several kilovolts potential relative to each other. X-rays that pass through the front contact will tend to dissipate their energy creating 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 of the 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. WDS detectors tend to require much more space than EDS detectors, 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 for samples with many closely spaced peaks, or careful analytical work.