Electron detectors. The most used signals in a scanning electron microscope in standard conditions are the electrons and in particular the SE.

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1 Electron detectors An appropriate detector must be employed to collect and convert the radiation of interest, that leaves a specimen, into an electrical signal for manipulation to create an SEM image. The most used signals in a scanning electron microscope in standard conditions are the electrons and in particular the SE. Considering electron imaging, BSE and SE leave the specimen with drastically different properties in term of energy, fraction relative to the beam, and directionality of emission. Each specific type of radiation potentially carries information on different aspects of the specimen characteristics. This information can be contained in the number of BSE and SE, their energy distribution, and their emission directions, or a combination of all three factors.

2 General characteristics of a detector The main characteristics of an electron detector are: the detector position with respect to the beam and the specimen the dimensions of the detector sensitive area (described by the solid angle Ω) the efficiency in converting a signal incident onto the detector sensitive area in useful signal. In general, the efficiency is not constant, but it depends on the electron energy the amplification properties of the signal These widely different energy characteristics of BSE and SE present a considerable challenge to the design of a detector that can be used for both signals. Alternatively, the differences permit the design of detectors that are selective for one of the signals and thus selective for specific specimen properties.

3 Everhart-Thornley detector The electron detector most commonly used in scanning electron microscopy is the combined secondary/backscattered electron detector named Everhart-Thornley (E-T). It is so popular that it is extremely rare to find a conventional SEM without one. The rise of the SEM as a tool of broad use to the scientific community is due in considerable measure to the performance and utility of the E-T detector (large solid angle of collection/efficiency, high amplifier gain, low noise, low-maintenance performance). Because of its efficient collection of SE, the E-T detector is often mistakenly considered only a SE detector..

4 The E-T detector operates in the following manner: when an energetic electron ( 10 kev energy) strikes the scintillator material (S), light is emitted. Everhart-Thornley detector Light is conducted by total internal reflection in a light guide (LG) (a solid plastic or glass rod) to the photocatode of a photomultiplier (PM). At the photocatode the photons are converted back into electrons which are accelerated onto the successive electrodes of the photomultiplier, producing an ever-increasing cascade of electrons until the final collector is reached. The typical gain of a photomultiplier is

5 A large fraction of the BSE, that originates from incident beam with energies from 10 to 30 kev, carry sufficient energy to directly excite the scintillator, even in the absence of postspecimen acceleration. By applying a large positive potential (10-12 kv) to a thin metal coating on the face of the scintillator (S), the low-energy SE will be accelerated to a sufficient energy to generate light in the scintillator. To protect the beam from unwanted deflection and distortion by this large potential in the specimen chamber, the scintillator is surrounded by a Faraday cage (F) which is insulated from the scintillator bias. To collect the low-energy SE with higher energy efficiency than simply collecting the fraction defined by the line-of-sight solid angle, a sparate bias potential is applied to the Faraday cage, typically in range (-50 V, +250 V). This range from negative to positive provides the possibility of completely rejecting SE (-50 V) or efficiently collecting SE (+250 V).

6 When the Faraday cage bias is positive, the BSE directed towards the detector are detected; but they represent only a small fraction of all the detected electrons. So their contribution is negligible. Thanks to the presence of the Faraday cage, the Everhart- Thornley detector is a device very efficient to detect SE and for flat samples it is possible to detect nearly all the secondary electrons If the Faraday cage bias is negative, the SE are rejected and the detector collect only the BSE. Obviously, without an electric field that directs the electrons towards the scintillator, only the BSE which have the right direction are revealed. So the collecting efficiency for BSE is very low and to detect BSE other detectors are used.

7 Through the lens TTL detector The high-performance field-emission-gun SEM is equipped with a snorkel lens which produces a strong objective lens magnetic field that is projected into the specimen chamber to reach the specimen plane. Magnetic field projected out of lens Snorkel Lens TTL ET This contrasts with the pinhole lens of a conventional SEM in which the lens magnetic field is contained within the bore of the lens so that the specimen resides in a field-free region. One major consequence of the strong magnetic field is to trap with high efficiency those SE emitted from the specimen. The SE spiral along the magnetic field lines and pass up through the lens bore. In this configuration SE are detected by the TTL detector.

8 The upper and lower detectors have a different viewpoint of the specimen and so they see the specimen differently In-lens (TTL) detector gives a shadow free image with ultra-high topographical resolution. Upper SE Detector Lower SE Detector

9 Passive scintillator BSE detectors They operate on the principle that, with an incident beam of 10 kev or more, the majority of the BSE carry sufficient energy to excite the scintillator even in the absence of a postspecimen acceleration. Without such active acceleration, the SE have no effect on the scintillator, so the signal from an unbiased or passive scintillator detector will consist only of contributions from BSE. The elimination of the bias also has the benefit that the detector potential will not disturb the beam, so the detector can be placed close to the specimen for mor efficient collection. By making the scintillator of the same material as the light guide, designs that collect over much larger solid angles are possible. The detector is placed above the specimen and a hole drilled trough the material permits access for the beam. In this configuration, the detector surrounds the specimen nearly simmetrically, so the signal is integrated in all directions, nearly eliminating sensitivity to trajectory effects.

10 Solid state diode detectors They operate on the principle of electron-hole production induced in a semiconductor by energetic electrons. A solid state diode detector (p-n junction) has the form of a flat, thin wafer (typically several millimiters thick) which can be obtained in a variety of shape and size, from small square to large annular detectors. The thinness of the detector permits it to be placed in close proximity to the specimen, which combined with the large area possible, provides a large solid angle for high geometric efficiency. In general, it is mounted under the objective lens, without interference with the normal operation of the instrument. The main drawback is the long response time, with respect to the other detectors, so it doesn t permit high scan rates.