Techniques

Table of Contents

• Transmission Electron Microscope (TEM)
• Scanning Electron Microscope (SEM)
• Scanning Transmission Electron Microscope (STEM)
• Electron Microprobe
• Microanalysis in the Electron Microscope
• Energy Dispersive X-ray Spectroscopy (EDX)
• Wavelength Dispersive X-ray Spectrometry (WDS)
• Electron Energy Loss Spectroscopy (EELS)

Transmission Electron Microscope (TEM)

The TEM was developed as an optical instrument, for imaging specimen structures at high resolution beyond that possible in an optical microscope. The first instruments were developed in the 1930's. Electrons are useful in the formation of images because they have an extremely short wavelength which is dependent on their energy. The resolution limit in any optical instrument is limited by the wavelength of the imaging radiation. The wavelength of blue light is approximately 250nm, whereas the wavelength of 100kV electrons is 0.0037nm. Therefore electrons can be used to image material structures at atomic resolution (0.1 to 0.2nm). A second property of electrons that makes them useful for imaging is their ability to be deflected by electromagnetic fields. Hence it is possible to construct electromagnetic lenses using coils carrying a few Amperes of current.

• An electron source to provide the illumination. This can be either a heated filament (tungsten hairpin or LaB6) or a sharp tip, from which electrons are "pulled out" by the action of a strong electric field (field emission). Instruments in the EMS have examples of both of these types of electron source.
• A series of acceleration electrodes, which increase the energy of the electron beam. Instruments in the EMS have accelerating voltages between 100 and 300kV.
• An illumination optical system consisting of two to four magnetic lenses, controlled to give either parallel illumination or a focussed probe on the specimen.
• A specimen stage and objective lens. The specimen, which is typically a 3mm disc thinned to less than 100nm, is mounted within the field of the objective lens. The design and properties of this lens often control the resolution obtainable by the microscope. With simple coils acting as lenses it is impossible to form unaberrated images. Spherical aberration of the objective lens limits the resolution for most forms of imaging in the range 0.15-0.3nm, depending on acceleration voltage. The stage can have five axes of movement (X, Y, Z and two axes of tilt).
• Projection lens optics which control the final magnification of the image. Typically there are three or four lenses. In newer instruments the fourth one is used to ensure there is no overall rotation of the image when changing magnification.
• A detection system, historically a phosphor screen which is lifted to expose a photographic plate. Most instruments now have a TV camera and/or a CCD camera to record images digitally.

When the electron beam hits a specimen it can interact with the atoms. Some of the electrons are absorbed as a function of thickness and composition of the specimen; these cause what is called amplitude contrast in the image. Other electrons are scattered over small angles, depending on the composition of the specimen; these cause what is called phase contrast in the image. In crystalline specimens, the electrons are scattered in very distinct directions, which are a function of the crystal structure; these cause diffraction contrast in the image. Amplitude and phase contrast contribute to the formation of TEM images of non-crystalline specimens (e.g. Biological and Polymers), while phase contrast and diffraction contrast are the most important factors in image formation in crystalline specimens.

	Electronmicrograph of atrial muscle (JEM-1220, 80kV). full size image: 1024x1024 JPEG image, color space Grayscale, 1 comps., Huffman coding, 298,241 bytes
	Electronmicrograph of a gold island on an amorphous carbon film. The 0.23nm lattice fringes are visible in the island (JEM-3010, 300kV). full size image: 492x496 JPEG image, color space Grayscale, 1 comps., Huffman coding, 91,438 bytes

Electrons can also undergo inelastic events. In particular they can cause ionization of atoms in the specimen by emitting an inner shell electron. The primary electron can continue down the column, having lost some energy, which can be detected using a magnetic sector spectrometer to disperse the electron beam as a function of energy (Electron Energy Loss Spectroscopy (EELS)). The ionized atom relaxes by an outer shell electron falling into the inner shell vacancy, which can lead to the emission of a characteristic X-ray. These X-rays can be collected with an Energy Dispersive X-ray spectrometer (EDX). Both EELS and EDX can give information on the chemistry of the specimen; EELS also gives information about the electronic structure of the specimen.

Scanning Electron Microscope (SEM)

In an SEM, a fine probe is formed at the surface of a specimen. This probe is then rastered over the specimen and an image is built up on a monitor. The electrons collected can be either the inner shell electrons emitted by the atoms (secondary electrons) or primary electrons that are scattered back up the column by near nucleus collisions (backscattered electrons). Unlike specimens for transmission microscopy, the SEM specimen does not have to be thin enough for electrons to travel through, and SEMs are routinely used to look at the surfaces of bulk specimens. Although the first description of an SEM appeared in 1935, it was not until the 1960's that the first commercial instruments appeared.

• An electron source to provide the illumination. This can be either a heated filament (tungsten hairpin) or a sharp tip, from which electrons are "pulled out" by the action of a strong electric field (field emission). Most conventional SEMs have a heated filament, field emission sources are used for high spatial reolution imaging (JSM-6320F). The source will have a single stage accelerator to increase the electron beam energy to a maximum of 30kV (50kV on the JXA-733).
• An illumination optical system consisting of two magnetic lenses. These lenses are controlled to give, with the objective lens, a small focussed cross over at the specimen.
• A set of coils which allow the beam to be scanned over the specimen. Reducing the area scanned on the specimen increases the magnification of the image.
• A specimen stage and objective lens. The specimen, typically up to 100mm in diameter and 10mm high, is mounted below the field of the objective lens. The design and properties of this lens and the distance from the lens to the specimen control the resolution obtainable by the microscope. The stage can have five axes of movement (X, Y, Z, tilt and rotation).
• A detection system. Usually there are secondary (SED) and backscattered (BSD) electron detectors. These are typically below the objective lens, and limit the ability to use short working distances. On the JSM-6320F there is a secondary detector within the objective lens bore and higher resolutions are possible at low accelerating voltages with very short working distances (Semi-in-lens). The secondary detector gives very good topographical information, while a backscattered detector gives a compositional image.

Secondary electron image of cheese mold spore chains. Spores are about 3um across (JSM-6320F, 3.5kV) full size image: 1024x759 JPEG image, color space Grayscale, 1 comps., Huffman coding, 153,215 bytes

In a conventional SEM (JSM-6320F, JXA-733) the specimen must either conduct or the charge build up must be balanced (eg operating at low accelerating voltage). In many cases this is not possible and the specimen must be coated with a conducting film (carbon, gold, chromium). The coating may, however, hide the information you are looking for. Our latest microscope is a variable pressure SEM (S-3000N), in this instrument the specimen can be observed in high vacuum or in a poorer vacuum between 1-270Pa. In this low vacuum regime the incident electron beam ionizes gas atoms near the specimen which are atracted to, and neutralise any charge build up. Nearly all dry specimens can be looked at without any preparation. Because of the poor vacuum the conventional SED cannot be used due to the high voltages associated with the detector. The VPSEM primarily uses a segmented BSD which allows compositional and topographic imaging, however the microscope does also have a VPSED system.

Backscattered electron image of blood cells in dried lung tissue taken in variable pressure mode. (S-3000N, 30kV, 5Pa) full size image: 1280x960 JPEG image, color space Grayscale, 1 comps., Huffman coding, 159,561 bytes

The interaction of the beam with the specimen also gives rise to X-rays, which can be collected using an EDX detector. However, depending on the accelerating voltage, the spatial resolution for microanalysis can be as much as 1um, even if the imaging resolution is sub-10nm. In low vacuum mode the inicident beam can be significantly scattered by the gas which can degrade the spatial reolution for X-ray analysis to as much as 1mm.

Scanning Transmission Electron Microscope (STEM)

The STEM combines features of both the TEM and SEM to produce transmission images obtained with a scanning probe. In the EMS we have both a TEM/STEM (JEM-2010F) and a dedicated STEM (HB601UX). In STEM mode, the final image resolution depends on use of a high brightness source to produce a focussed probe with high current density. Both of our instruments have field emission sources, which for EDX work can produce a beam current of 1nA in a 1nm probe. The minimum probe sizes possible for imaging and EELS on these two instruments are 0.22nm (HB601UX) and 0.135nm (JEM-2010F), and these probes typically contain 15pA of beam current.

Images in the STEM are produced while scanning the beam over the specimen. Electrons transmitted through the specimen can be detected on a detector on the axis of the microscope (bright field detector (BF)), or on an annular detector sensing electrons scattered through a range of angles (annular dark field detector (ADF)). Detectors can also be fitted to look at secondary and backscattered electrons, as in a SEM, although neither of our instruments have such detectors.

The main applications of a STEM are high spatial resolution microanalysis and imaging. X-ray spatial resolution of around 1nm is possible with sufficient beam current to collect statistically significant EDX spectra in 100 seconds. The scanned probe also allows for collection of X-ray maps and line scans, although the count rates will be low. Imaging with a resolution equivalent to the minimum probe size is possible using electrons scattered through high angles onto the ADF detector. The intensity of the signal on the detector is proportional to the average atomic number under the probe, and in correctly oriented crystalline samples, it is possible to get Z contrast images at atomic resolution. Chemical and electronic information at a resolution slightly larger than the minimum imaging resolution is possible using EELS.

Z contrast image of a grain boundary in SrTiO3. The strontium atom columns, which have a higher atomic number, show up brighter than the Ti/O columns (JEM-2010F, 200kV). full size image: 194x201 JPEG image, color space Grayscale, 1 comps., Huffman coding, 19,108 bytes

The first scanning microscope image ever obtained was a STEM image in 1938. Interest in dedicated STEM started with the development of stable field emission sources in the late 1960s, which needed ultra high vacuum. Commercial dedicated STEMs were available from 1974 to 1997. STEM attachments to TEMs have been available since the 1970's, but it is only with the recent introduction of field emission TEMs that the performance has reached the level of the dedicated instruments.

Electron Microprobe

The electron microprobe is essentially an electron beam instrument designed to detect characteristic X-radiation from a specimen. Although it developed separately to the SEM (and earlier, in the 1950s) the two instruments are very similar and share the same electron optical layout. However the requirement for higher beam currents in the microprobe restricts the imaging resolution of the instrument. The microprobe is equipped with both wavelength dispersive X-ray spectrometers (WDS) and EDX. The EDX detector allows rapid identification of the major constituents of the sample. WDS allows spectra to be acquired, serially, with much better resolution (5eV compared to 140eV) overcoming the many peak overlaps in EDX, much better peak to background ratios and better limit of detection (0.01wt% compared to 0.1wt% for EDX).

Microanalysis in the Electron Microscope

The interaction of the electron beam with the atoms in the sample can give rise to the ionization of the atom (creation of an inner shell vacancy). Energy is transferred from the incident electron beam to the electron that is ejected and the incident electron suffers an energy loss. The ionized atom can loose energy in a number of ways, one of which is for an outer shell electron to jump into the vacant inner shell, losing the excess energy in the form of a characteristic X-ray photon.

Energy Dispersive X-ray Spectroscopy (EDX)

The EDX detector was developed in the late 1960s. The typical detector consists of a single crystal of silicon up to 30mm2 in size and 3mm thick and is doped with lithium. Absorption of the individual X-rays, in the detector, leads to the generation of a photoelectron, which gives up most of its energy to the formation of electron-hole pairs. These are swept away by the applied bias (on gold contact layers either side of the detector) to form a charge pulse. This pulse is converted to a voltage pulse by a preamplifier and this signal is further amplified and shaped by a pulse processor before being displayed on a histogram of intensity against energy. The detector and preamplifier are held at liquid nitrogen temperatures and have to be isolated from the microscope vacuum, to prevent icing. Originally a thin beryllium window was used which significantly absorbed all X-rays below about 1KeV but modern detectors have an atmospheric thin window (ATW) made from a number of materials which allows X-rays down to boron to be detected.

EDX spectrum from SrTiO3. The characteristic peaks from the elements are labeled. full size image: 720x593 GIF87a image with 32 colors, 18,430 bytes

The main advantage of the EDX detector is its ability to look at all X-ray energies at once. Its disadvantages are that the count rates are restricted (due to pulse pile up), the energy resolution of the peaks is poor (>100eV) which restricts the peak to background and the spectrum can obtain artifacts from the collection process.

Wavelength Dispersive X-ray Spectrometry (WDS)

The WDS detector consists of a crystal oriented in such a way that a selected crystallographic plane is parallel to the surface. For a particular wavelength of X-ray and spacing of crystal there is an angle at which the X-rays are strongly scattered (Bragg's law). The strongly scattered X-rays are then detected by a gas filled proportional counter and the signal amplified and counted by a single channel analyzer. The intensity of the signal is recorded as the angle is varied and the angle, using Bragg's law' can be converted to wavelength. Most electron microprobes will have more than one spectrometer to cover as wide a range of elements possible with different crystals or to look at more than one wavelength at once.

The WDS detector has a better resolution than the EDX detector (equivalent of 5eV) and therefore a better peak to background, which results in a lower limit of detection than an EDX. However the crystal has to be scanned and the collection of a spectrum covering a range of wavelengths can take 10 minutes or more.

Electron Energy Loss Spectroscopy (EELS)

The EELS detector usually consists of a magnetic sector. When an electron travels through a uniform magnetic field perpendicular to its travel, its path is constrained by its velocity (energy). Hence electrons of different energies are focussed at different positions on a focussing plane. There are a number of different detectors that can be used. In our instrument the electrons are focussed onto a CCD array, and this is read out and displayed as a histogram of intensity against energy lost.

EELS spectrum from a superconductor. Characteristic edges are superimposed on a rapidly varying background. (full size image: 610x411 GIF89a image with 256 colors, 5,850 bytes)

The resolution of the information in the EELS spectrum can be better than 1eV, and depends on electron source type and current. The spectrum can give information about both composition and the electronic state of the atom; however detectability is poor as the edges are superimposed on a rapidly sloping background, and errors in quantification are greater than for X-ray analysis.

Our EELS detector also has a slit mechanism that can be inserted to select individual energy windows. Using the CCD camera, an energy filtered image can be obtained.