- 1 1. imaging plate
- 2 2. windowless EDS detector
- 3 3. ultra-thin window (UTW) EDS detector
- 4 4. annular dark-field detector
- 5 5. energy-selection slit
- 6 6. grating
- 7 7. calorimeter
- 8 8. fluorescent screen
- 9 9. counting rate
- 10 10. detection limit
- 11 11. detection efficiency
- 12 12. detective quantum efficiency
- 13 13. high-definition image
- 14 14. photomultiplier tube
- 15 15. high-resolution photography
- 16 16. side-entry type EDS detector
- 17 17. charge-coupled device(CCD)
- 18 18. threshold value
- 19 19. silicon-intensifier-target (camera) tube
- 20 20. photo film
- 21 21. probe-current detector
- 22 22. silicon drift detector (SDD)
- 23 23. scintillator (fluorescent substance)
- 24 24. slow-scan CCD camera
- 25 25. dynamic range
- 26 26. dead time
- 27 27. minimum dose system
- 28 28. top-entry type EDS detector
- 29 29. secondary-electron detector
- 30 30. thermal noise
- 31 31. pulse-height analyzer
- 32 32. backscattered-electron detector
- 33 33. full width at half maximum (FWHM)
- 34 34. semiconductor detector (solid-state detector)
- 35 35. fiberoptic plate
- 36 36. Faraday cup
- 37 37. photodiode array
- 38 38. cathode-ray tube (CRT)
- 39 39. analyzing crystal
- 40 40. parallel detector
- 41 41. beryllium window EDS detector
- 42 42. Peltier cooling
- 43 43. point spread function
- 44 44. micro-channel plate (MCP)
- 45 45. yttrium aluminum garnet(YAG)
- 46 46. direct electron detector
- 47 47. de-scan
- 48 48. Si(Li) detector
- 49 49. pixelated STEM detector
1. imaging plate
An integration-type two-dimensional detector that utilizes fluorescence emission generated by a beam of X-rays, electrons, or neutrons. The "imaging plate" is made of a plastic film coated with microcrystlas of photostimulable phosphor (BaFX:Eu2+ (X = Cl, Br, I)). The imaging plate has excellent linear sensitivity. The plate possesses a large recording area of about 80 mm × 100 mm and a large dynamic range of 5 to 6 digits. The exposed imaging plate is irradiated with He-Ne laser light, and then blue light emitted from the plate is converted into electric signals by a photomultiplier tube (PMT). By scanning the laser light on an imaging plate, a microscope image is obtained from the recorded signals. One PMT covers a dynamic range of only 5 digits but a recent reading device which combines a PMT and a semiconductor detector covers a 6-digit dynamic range. Whether a large dynamic range can be effectively used or not is dependent on the performance of a reading device. Also, the positional resolution (pixel size) changes from 15 μm to 50 μm depending on the performance of the reading device. The maximum gray level of the imaging plate is 20 bits. The imaging plate is advantageous for both recording a low-magnification TEM image that needs to cover a large area and a diffraction pattern that has a wide dynamic range. Compared to CCD, the disadvantage of the imaging plate is that its use is limited to offline.
Related term photo film, CCD
2. windowless EDS detector
An EDS detector that does not use a window material for preventing contaminants onto its detector element. Since the windowless EDS detector suffers no X-ray absorption by the window material, the detector can analyze light elements such as beryllium (Be) and boron (B). Owing to a very high vacuum of the today’s microscope column and less severe deposition condition at Peltier cooling temperature than at liquid nitrogen temperature, the window material has become unnecessary. As a result, the windowless EDS detector has more been used in recent years.
Related term beryllium window EDS detector, ultra-thin window (UTW) EDS detector
3. ultra-thin window (UTW) EDS detector
An EDS detector that uses an organic thin film of 0.3 to 0.5 mm thickness as a window material to prevent contaminants onto a detector element. Aluminum is evaporated on the surface of the organic thin film so as to avoid charging on the window. Compared with the beryllium window EDS detector, the ultra-thin window (UTW) EDS detector better suppresses absorption of low energy X-rays. Thus, the detector can analyze elements lighter than sodium (Na).
Related term beryllium window EDS detector, windowless EDS detector
4. annular dark-field detector
An annular detector with an inner diameter of ～3 mm and an outer diameter of ～8 mm, which is used for obtaining a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image. The "annular dark-field (ADF) detector" of a phosphor screen or a YAG scintillator receives scattered electrons and converts them into a light signal. The light signal is directed to a PMT through a light pipe and converted into an electric signal, and the electric signal is amplified.
Related term high-angle annular dark-field scanning transmission electron microscopy, HAADF-STEM
5. energy-selection slit
The slit inserted into the energy dispersive plane of an energy filter, which selects electrons having specific energies.
Related term energy filter
The grating is an element to obtain a spectrum utilizing diffraction. It is made of a metal and many equally spaced grooves are cut on the surface. (The minimum spacing between the grooves that can be cut is ～0.5 μm.) The spacing must be suitable for the wavelength of the spectrum to be measured. In order to obtain a spectrum from a wide energy range, several gratings with different groove spacings are required. Some gratings are designed to be cut with a non-equal pitch of grooves to reduce aberrations. The grating is used for electronic structure analysis by soft X-ray spectroscopy.
Related term wavelength-dispersive X-ray spectroscopy, WDS
6-2. concave grating
A kind of reflection grating that is designed in such a way that many grooves are cut in parallel on the surface of a spherical or parabolic concave mirror. The concave grating is used for not only spectroscopy but also for focusing a beam or producing a parallel beam.
Related term Rowland circle, grating
A device that measures the total energy of a neutral particle, a charged particle or an electromagnetic wave through absorption. An XES device equipped with a "calorimeter" provides a high energy resolution (～10 eV (recent highest data is 4.5 eV)). However, the calorimeter needs liquid helium (He), resulting into large in size. The solid angle of signal acquisition for the calorimeter is ten times or much smaller than EDS. Thus, the calorimeter is used for SEM but not for TEM.
Related term X-ray emission spectroscopy, XES
8. fluorescent screen
A tool to visualize TEM images and diffraction patterns. A phosphor on a "fluorescent screen" is excited through electron collision. The emitted visible light produces "light and dark (contrast)" corresponding to the electron intensities to the screen. A fluorescent material is composed of a matrix of zinc sulfide (ZnS) and a dopant of copper (Cu), aluminium (Al) or europium (Eu). A phosphor with high luminous efficiency and with a persistence of ～100 ms is chosen for the fluorescent screen. The fluorescent screen works also as a beam shutter. When acquiring a TEM image, the screen is raised. In recent years, electron microscope images are not observed on the fluorescent screen but on a computer display.
Related term energy-dispersive X-ray spectroscopy, EDS
9. counting rate
When the average value of the occurrence of an event is measured by a counter, "counting rate" means the number of counts per unit time.
10. detection limit
"Detection limit" is defined as the minimum quantity of a detectable specific component in various analyses. If the amount of signals falls short of the detection limit or the level of disturbance of the noise, the signals cannot be detected.
11. detection efficiency
"Detection efficiency" is defined as the ratio of the output signals to incident electrons, X-rays or photons to a detector (input signals). A detector with high detection efficiency meets various requirements, such as high conversion efficiency from input to output, short dead time, and low noise.
Related term energy-dispersive X-ray spectroscopy, EDS, wavelength-dispersive X-ray spectroscopy, WDS, electron energy-loss spectroscopy, EELS
12. detective quantum efficiency
"Detective quantum efficiency (DQE)" is the ratio of (S/N)2 of the output (detection) signal to the (S/N)2 of the input signal in various analyses. In the case of the ideal detector, DQE reaches 1.
13. high-definition image
"The high-definition image" means an image consisting of a small sized pixel, a wide dynamic range and high gray levels. Photo films for imaging have a pixel size of ～3 μm, a dynamic range of 2 digits and a gray scale of <16 bits. CCD has a pixel size of 13.5 μm, a dynamic range of 4 digits and gray scales of 16 bits. The imaging plate has a pixel size of 25 μm, a dynamic range of 5 to 6 digits and gray scales of 20 bits.
Related term photo film, CCD, imaging plate
14. photomultiplier tube
"Photo multiplier tube (PMT)" consists of a phototube that incorporates an electron multiplier. PMT detects weak light, and converts the light signal into an electric signal, then amplifies the electric signal (×105 to 106). PMT is used for a detector, such as a secondary-electron detector in SEM or HAADF detector for scattered electrons in TEM.
Related term secondary-electron detector, annular dark-field detector
15. high-resolution photography
For "high-resolution photography," photographic films were used but have been replaced by CCD cameras. Since the pixel size of the film is ～3 μm and that of the CCD camera is ～15 μm, the magnification for the film is 300,000 to 400,000× but that for the CCD camera is 600,000 to 1,000,000×. Photography at a low magnification is recommended from the view point of specimen drift.
Related term photo film, CCD
16. side-entry type EDS detector
An EDS detector which is placed on the side of the objective lens with a viewing angle of 30° or less to a specimen set horizontally. Since the "side-entry type EDS detector" can be set near the objective lens, its detection solid-angle of emitted X-rays is large or its X-ray detection efficiency is high. To improve the accuracy of quantitative analysis, the diffusion length of the emitted X-rays in the specimen is required to decrease. For this purpose, the specimen is tilted as needed against the detector so that X-rays coming to the detector take a large angle against the specimen surface. Since the X-ray intensity is sensitive to the local roughness of the specimen, the accuracy of quantitative analysis is not high enough. However, this type detector is broadly used these days as it is suitable for the analysis of a smaller specimen area because it can be combined with a high resolution objective pole-piece which has a small gap and a small bore.
Related term top-entry type EDS detector
17. charge-coupled device(CCD)
"CCD" is a two-dimensional digital semiconductor photoelectric conversion device. For electron beam detection, electrons are converted into light by a fluorescent material or a YAG crystal. When the CCD is irradiated with light, electron charges are accumulated in the depletion region (potential wells). Then, these charges are transferred to successive wells. Finally, these charges are taken out as electric signals. Since the CCD contains a dark current, the device is cooled to suppress this current (cooled to -30 ℃ by Peltier cooling). A normally-used CCD has a size of 2K × 2K (a square of approximately 3cm) with a spatial resolution (pixel size) of 14 μm (7 μm for visible light available). Compared to the imaging plate, CCD has a smaller dynamic range of 4 digits or a gray scale of 16 bits than an imaging plate. However, the most advantage of CCD is that it can be used online (only offline use in the imaging plate). CCD is used for acquisition of a high-resolution image and for detection of X-rays in WDS. Recently, a CCD with a larger area (4K × 4K) have been getting popular.
Related term imaging plate, photo film
18. threshold value
The minimum physical quantity (normally the minimum energy) that is required to cause reactions or other phenomenon.
19. silicon-intensifier-target (camera) tube
A (camera) tube for light detection using a silicon-based electron multiplication target. The "silicon-intensifier-target (camera) tube" has functions of electron-multiplication and charge-accumulation, and then operates even with a low intensity of light. To use the tube for electron microscopes, a fluorescent screen is placed at the front of the tube for converting incident electrons to light. The photoelectrons generated on the reflexible surface are accelerated to a several kV and imaged on the silicon-based target. Then, many electron-hole pairs are created, leading to a high amplification. Readout is done by sweeping an electron beam from behind the target. The advantages of the tube are high sensitivity and wide dynamic range. A disadvantage is that the surrounding part of the tube becomes dark through the lens action. Recently, the tube has been replaced with a compact and inexpensive CCD, which has unfortunately low sensitivity and narrow dynamic range but has a low noise level and does not have dark surrounding parts. Thus, the tube is not used currently for a TEM.
Related term CCD
20. photo film
A film that records a TEM image or a diffraction pattern. The "photo film" is directly exposed to electrons in the camera chamber of a TEM. The sensitivity characteristic of the photo film to the electron is similar to that to visual light. Since the photo film has a small dynamic range of 2 digits and insufficient linearity, it is not suitable for quantitative measurement. However, since its positional resolution is as high as about 3 μm, the photo film has been used for the photography of high-definition images. Its gray level is <16 bits. Now, the imaging plate and CCD are available as recording devices that have higher sensitivity than the photo film.
Related term imaging plate, CCD
21. probe-current detector
A detector that monitors the illumination beam current onto the specimen using a Faraday cup in EELS or EDS analysis.
22. silicon drift detector (SDD)
One of the energy-dispersive X-ray detector used for EDS. The principle, in which characteristic X-rays entering the detector element are converted to electron-hole pairs, is the same as for the Si(Li) detector. But unlike the Si(Li)detector, electrons generated from the detector element by the incident characteristic X-rays are efficiently guided to a small anode at the center of the element by a concentric electrode structure with a potential gradient. This unique electrode structure reduces capacitance, thus enabling high-speed signal response. Thus compared with the Si(Li) detector, voltage pulses are collected with higher-speed, higher-signal-to-noise ratio. As a result, SDD may not suffer the influence from dark current due to thermal noise, and works at approximately –15 °C by Peltier cooling. SDD provides an energy resolution comparable to the Si(Li) detector and also performs X-ray analysis with high count rate (＞1 x 105 cps), which is more than one order higher than the Si(Li) detector. Since it is unnecessary to use liquid nitrogen, the detector can be compactly designed. Thus SDD has more been used in place of the Si(Li) detector.
Related term semiconductor detector (solid-state detector),EDS,SSD
23. scintillator (fluorescent substance)
The scintillator is a substance that emits fluorescence by absorbing the energy of an electron beam or electromagnetic wave. A fluorescent substance for the "scintillator" is selected by taking account of the fluorescence wavelength (color) and its persistence time according to the sensitivity characteristic of a photoelectric converter placed behind the scintillator. The scintillator is used for various purposes: a fluorescent screen (yellow-green light, persistence light: ～100 ms) to visualize a TEM image, a secondary-electron detector (blue light, persistence light: ～μs) to measure the beam intensity of secondary electrons, and a TEM image detector (YAG) (blue white light, persistence light: ～μs) placed before a CCD.
24. slow-scan CCD camera
A CCD camera, whose scan speed is slower than that of a normal CCD TV camera to acquire a higher-quality image with a higher signal-to-noise ratio. A normal TV scan rate is 1/30 s per frame but the scan rate of the "slow-scan CCD camera" is 1 to 2 s per frame in the photography mode and ～0.1 s per frame in the observation mode.
25. dynamic range
A quantity that expresses a reproducibility capability of the signal intensity. The "dynamic range" is defined as the intensity range obtained by subtracting the noise level from the maximum signal level. The dynamic range of the digital signal is expressed by the number of bits.
26. dead time
"Dead time" is the required time interval between the measuring equipment receiving one signal and this equipment being able to accept the next signal.
27. minimum dose system
"Minimum dose system (MDS)" is a method to photograph a TEM image by reducing damage to the specimen due to electron-beam irradiation. MDS is effectively used to photograph biological specimens. After the field for photographing is searched, focus adjustment of the objective lens and astigmatism correction are performed at a non-photographed position. Then, the MDS sets an electron dose, a magnification and the field for photographing to minimize the electron dose and takes the image.
Related term radiation damage
28. top-entry type EDS detector
An EDS detector which is placed above the objective lens with a viewing angle of about 70° to a specimen set horizontally. Since X-rays coming to the detector take a large angle against the specimen surface, the diffusion length of the emitted X-rays in the specimen is kept small without any specimen tilt, leading to a high accuracy of quantitative analysis. However, since the detector is distant from the specimen, the solid angle of the detector against the specimen is small or its detection efficiency is not high. Since the detector takes X-ray signals from the top of the objective lens, it requires a large bore polepiece for the lens. The use of such a polepiece is unfavorable to obtain a high resolution image and a small probe size on the specimen. Due to these disadvantages, this type detector has not been used recently.
Related term side-entry type EDS detector
29. secondary-electron detector
A detector that is used for detecting secondary electrons emitted from the specimen surface by electron-beam illumination. The main elements of the detector are a scintillator and a photomultiplier tube (PMT). To efficiently collect low-energy secondary electrons (normally, several 10 eV), a positive potential (voltage) of about 10 kV is applied to the scintillator against the specimen. Accelerated secondary electrons are converted into visible light by the scintillator, and the light is guided to the PMT through a light pipe. Then, the light signal is converted into an electric signal and the electric signal is amplified. A secondary-electron image is obtained by displaying the intensity of the detected secondary electrons on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe.
Related term secondary electron
30. thermal noise
Noise that occurs in a conductor or a semiconductor material due to random thermal motions of electrons in this material. As the temperature is raised, "thermal noise" is increased. Since this noise does not have frequency dependence but shows a flat spectrum, it is also called "white noise." To suppress this thermal noise, a semiconductor detector in an EDS or a YAG single-crystal scintillator in a slow-scan CCD camera is cooled.
31. pulse-height analyzer
An instrument used for analyzing energy spectra of radial rays by counting voltage pulses output from a radiation detector. The "pulse-height analyzer" counts only the number of pulses falling within certain voltage ranges (channels), where the upper and lower limits of the pulse heights are arbitrarily specified. This analyzer is used to measure characteristic X-rays by EDS.
Related term energy-dispersive X-ray spectroscopy, EDS
32. backscattered-electron detector
A detector that is used for detecting backscattered electrons from the specimen surface by electron-beam illumination. In a TEM, a micro-channel plate (MCP) is used as the "backscattered-electron detector." (In a dedicated SEM instrument, a semiconductor detector that uses the p-n junction mechanism is adopted.) To improve the detection efficiency, an annular detector is placed just above the specimen. The MCP detects backscattered electrons and converts the electron signal into an electric signal. Since the energy of backscattered electrons is high, additional electron acceleration that is applied to secondary electrons is not required. A backscattered-electron image is obtained by displaying the intensity of the backscattered electrons on a computer monitor screen as a series of bright spots synchronized with the scan of the electron probe. A two-segment annular detector is often used for obtaining a composition (COMPO) image and a topographic (TOPO) image. That is, two signals acquired from each segment are amplified by with the preamplifier, and then processed with the main amplifier in such a way that COMPO image is obtained by addition of the two signals and TOPO image is obtained by subtraction of them. Furthermore, the use of a four-segment detector enables us to obtain a stereoscopic (SHADOW) image. To obtain SHADOW image, the composition signal from two segments and the topography signal from other two segments are synthesized.
Related term backscattered electron, micro-channel plate, MCP
33. full width at half maximum (FWHM)
A width of a spectrum measured at a half of the height of a spectral peak.
34. semiconductor detector (solid-state detector)
An X-ray detector utilizing a semiconductor silicon (Si) or germanium (Ge). The detector is used for an EDS system. The Si-Li detector has long time been used, but in recent years, it is being replaced by the silicon drift detector (SDD). For the window of the detector, three types are available: beryllium window, ultra-thin window (UTW) and windowless.
Related term energy-dispersive X-ray spectroscopy, EDS, wavelength-dispersive X-ray spectroscopy, WDS
35. fiberoptic plate
A "fiberoptic plate" is a plate bonded with bundled optical fibers with each diameter of severalμm. It is used as a tool to transfer optical signals of a TV camera system.
36. Faraday cup
A tubular metallic electrode to directly receive an electron beam. "Faraday cup" is made to a cup shape to efficiently capture incoming electrons.
37. photodiode array
A detector on which photodiodes (photo-sensitive elements converting light signals into electric signals by optical absorption) are arrayed.
38. cathode-ray tube (CRT)
The cathode-ray tube (CRT) displays a two-dimensional image on the tube surface in such a way that an electron beam is accelerated, focused and deflected by electric voltages and magnetic fields to scan the tube phosphor surface.
39. analyzing crystal
A crystal used for the spectral analysis of characteristic X-rays in the WDS analyzer. An LiF crystal (spacing 0.4 nm) is used for analyzing heavy elements such as uranium (U Lα). An STE(Stearate) crystal (spacing 10 nm) is used for analyzing light elements such as beryllium (Be Kα).
Related term wavelength-dispersive X-ray spectroscopy, WDS
40. parallel detector
A detector that can simultaneously read signals obtained with multiple channels. Compared to the serial detector, the detection efficiency improves as much as the multiple of the number of channels.
41. beryllium window EDS detector
An EDS detector that uses a beryllium film of 8 to 10 μm thickness as a window material to maintain vacuum of a semiconductor detector. Since the window material is robust, the detector is attached to and detached from the microscope column without worry of vacuum leakage. However, the detector is hard to detect elements lighter than sodium (Na) because the beryllium film absorbs low energy X-rays. For this reason, the detector has less been used in recent years.
Related term ultra-thin window (UTW) EDS detector, windowless EDS detector
42. Peltier cooling
"Peltier cooling" uses the Peltier effect to cool substances where heat is generated or absorbed when an electric current flows to the contact region of different metals. Peltier cooling is applied to reduce thermal noise of the CCD camera for a TEM. A three-stage Peltier element can cool a substance down to -45 ℃. This cooling technique is usually used for a data-accumulation type CCD camera. The Peltier-cooling device requires water cooling of the heat generation part (air cooling provides low cooling efficiency). Recent cooling devices are made compact in size. To further reduce the temperature, liquid nitrogen is used.
43. point spread function
When light transferred from incident electrons by a thin fluorescent material is detected with CCD, the size of the emitted light is larger than that of the incident electrons due to scattering of electrons by the fluorescent particles. The size of the light can be larger than one pixel size of the CCD (～15 μm). The function that expresses the spread is called "point spread function." As a guide example, the intensity at the nearest pixel is about 1/3 of that of central pixel.
44. micro-channel plate (MCP)
A "micro-channel plate (MCP)" is a circular glass plate with a thickness of ～0.5 mm and a diameter of ～10 mm, in which cylindrical electron multipliers with an inner diameter of ～10 μm are arrayed in a honeycomb structure. The front-side surface and the backside surface of the MCP are coated with a metal. The former side acts as the input electrode (cathode) and the latter as the output electrode (anode). When a voltage is applied between the two electrodes, electrons that enter the cathode strike the inner walls of the MCP, emitting multiple secondary electrons. These emitted secondary electrons are accelerated by an electric field in the channels and repeat collision with the (totter) inner walls of the MCP. Finally, the electron flow is received at the anode as an amplified electric signal. Since the MCP has high sensitivity for not only electrons but also ions and X-rays, this plate is used as a detection element for these signals. In a TEM, the MCP is used for a backscattered electron detector attached to the TEM column.
Related term backscattered-electron detector
45. yttrium aluminum garnet(YAG)
A "YAG (yttrium aluminum garnet)" is a substance that emits fluorescence (scintillator) by absorbing the energy of an electron beam or electromagnetic wave. A YAG is placed before a CCD for acquisition of a TEM image.
46. direct electron detector
The direct electron detector is a detector in which accelerated electrons are directly received by an image sensor of CCD or CMOS instead of using a scintillator. Those electrons are converted directly into electric signals, whereas in a usual case, the accelerated electrons are received by a scintillator, converted to light and then the light is transferred to a CCD or CMOS using a lens system or an optical fiber system. Very large electric signals are produced in the case of the direct detector because the scintillator converting electron signals to light signals is not used and the incident electrons possess a very high energy than the light signals produced in the scintillator. The direct electron detector has 10 to 100 times higher sensitivity for electron detection than the scintillator - CCD system. Taking advantage of this high sensitivity, the direct electron detector is effectively used for cryo-electron microscopy (observation of biological materials with cooling), which requires a very low electron dose to avoid specimen damage. In addition, nonuse of the scintillator enables the spatial resolution of a TEM image to be improved. Furthermore, the advantage of its high read-out rate is utilized as a detector taking scanning images. However, damage to the detector due to electron-beam irradiation is unavoidable. Thus, replacement of the sensor is necessary if the total electron dose reaches approximately 109.
Related term CCD, slow-scan CCD camera, scintillator (fluorescent substance), cryo-electron microscopy
De-scan means to bring the electron beam deflected from the optical axis back to the optical axis using a two-stage deflector coil, the deflection being caused by illumination position or illumination angle of the incident electron beam onto a specimen. De-scan is applied mainly to the following two cases. 1) When the incident electron beam is scanned over a wide area of a specimen, the electron beam can stray off the optical axis at the peripheral part of the scanned area. Then, the incidence position or the incidence angle of the electron beam to a STEM or EELS detector whose center is adjusted to the optical axis deviates from the axis. In the case of STEM; if an electron beam passing through a scan point of the specimen deviates from the STEM detector, the STEM image intensity is varied compared with the correct intensity which should be detected. In the case of EELS; if an electron beam passing through a scan point of the specimen does not run on the center of the EELS detector or runs obliquely to the optical axis of the detector, the energy value of the EELS spectrum cannot be correctly measured. To avoid such an undesirable electron-beam position or angle to the STEM or EELS detector, a two-stage deflector coil in the image-forming lens system is operated synchronously with the beam scan to always bring back the electron beam to the center of the detector and/or parallel to the optical axis. 2) In the case of precession electron diffraction, to precess the incident electron beam onto a certain point on a specimen, a two-stage deflector coil in the illumination lens system is used. The first deflector coil tilts the incident electron beam to a certain angle (up to approximately 5°), and then the second deflector coil compensates the displacement of the tilted incident electron beam from the optical axis. Then, the electron beam passing through the specimen runs oblique to the optical axis. A two-stage defector coil in the image-forming lens system is used to bring back the electron beam to run on the optical axis. A precession diffraction pattern is obtained by such operation of the two-stage defector coils.
Related term scanning transmission electron microscope (STEM) image, electron energy-loss spectroscopy, EELS, precession electron diffraction
48. Si(Li) detector
One of the energy-dispersive X-ray detector used for EDS. A lithium (Li)-doped silicon single-crystal semiconductor is used as a detector element. When X-rays enter the detector element, electron-hole pairs (their generation energy is approximately 3 eV) are generated in the detector element, whose number is proportional to the energies of the X-rays. The generated electrons are collected to the anode at the bottom of the detector element by applying an external electric field. That is, the X-ray energies are measured as electric voltage pulses. Detectable elements range from boron (B: 0.18 keV @K line) to uranium (U: 3.16 keV @M line). The energy resolution of the detector is approximately 140 eV (@Mn K line). Cooling of the detector by liquid nitrogen is required to prevent diffusion of doped Li and to reduce a dark current caused by thermal noise. Owing to the development of a silicon drift detector (SDD) which has a high count rate and can be used by Peltier cooling, the Si(Li)detector has less been used.
Related Term EDS, energy-dispersive X-ray spectroscopy, silicon drift detector, SDD
49. pixelated STEM detector
A STEM detector that records a diffraction (CBED) pattern formed on the detector as a two-dimensional (2D) image with a high-speed frame rate in synchronism with the scan of the incident electron probe. To take a two dimensional CBED pattern during the stay time of the STEM probe at one pixel, a direct electron detector of CCD or CMOS, which has a very high-speed frame rate of a few thousands of fps or more, is required. At present, using such a direct electron detector, a synchronous STEM scan with taking a 256 x 256 pixel CBED pattern is achieved. A conventional STEM detector and a pixelated STEM detector are compared in Fig. 1.
Fig. 1 Conventional single-channel STEM detector and Pixelated STEM detector (a) Conventional single-channel STEM detector. The electron beam is converted to light using the scintillator and the light is guided to PMT. Then, the light creates electrons in the PMT. Finally they are measured as the output voltage signal. It should be noted that the signal is obtained as an integrated value from a CBED pattern area determined by the shape of the scintillator. (b) Pixelated STEM detector. The electron beam enters the direct electron detector (CCD or CMOS) and converted to electric signals. The detector records a CBED pattern as a 2D image unlike the conventional single-channel STEM detector. In the conventional STEM detector, the electron beam is converted to light with a scintillator and the intensity of the obtained light is measured as the output voltage of a photomultiplier tube (PMT) (Fig. 1(a)). All or part of the light signals from a CBED pattern formed on the detector plane are detected with a single-channel PMT, and then a 2D STEM image is acquired by displaying the output voltage as the function of the incident probe position. By selecting the acquisition region of the scattered electrons with the use of a circular or annular scintillator (detector), BF-, ABF-, or HAADF-STEM image can be obtained. It is noted that angle-resolved information is not available because the detector is single-channeled, thus the intensity of the CBED pattern is integrated. In the pixelated STEM detector, the CBED pattern is recorded as a 2D image (Fig. 1(b)). Furthermore, the electron probe is two-dimensionally scanned, and then the acquired final data becomes 4D data. It is emphasized that, the angle-resolved information about the diffraction intensity, which is lost in the conventional STEM detector, can be effectively used. For example, a variety of STEM images are created by flexibly changing the integration area of the CBED pattern. Fig. 2 shows five sort of STEM images created from the final 4D data of SrTiO3  taken at an accelerating voltage of 200 kV. It is seen that different information can be obtained by changing the integration area of the CBED pattern. The other applications of the pixelated STEM detector include creation of an electric-magnetic field map utilizing positional shift of the CBED pattern, and reconstruction of a phase image of a specimen by utilizing ptychography image processing.
Fig. 2 Various STEM images of SrTiO3  taken at an accelerating voltage of 200 kV, by changing the integration area of the CBED pattern. Those STEM images are created from the final 4D data. The integration area of each CBED pattern is shown in insets (lower right of each STEM image), indicated by translucent red and blue colors. Acquisition conditions are as follows: Number of pixels of the STEM image: 256 x 256 pixels, Frame rate of the pixelated STEM detector: 4,000 fps (dwell time 250 μs), Data acquisition time: approx. 16 s. (a) STEM image created by integrating the whole area of the CBED pattern. Sr columns and Ti+O columns are observed as dark. O columns are not seen. (b) BF-STEM image. Sr columns are observed as dark, but O columns as bright. (c) ABF-STEM image. All of Sr-, O-, and Ti+O columns are observed as dark. Compared with the BF image, the ABF image enables intuitive interpretation of the atomic columns. (d) e-ABF (enhanced ABF) image. The image is created by subtracting the intensity of the area B from that of C. O columns appear more clearly compared with those of the ABF-STEM image. (e) LAADF-STEM image created by integrating the area except for the transmitted wave disk. Sr columns and Ti+O columns are observed as bright. O columns are not seen. (f) Conventional HAADF-STEM image acquired using a circular single-channel STEM detector.