Scanning transmission electron microscope
A scanning transmission electron microscope ( STEM ; English scanning transmission electron microscope , STEM ) is an electron microscope in which an electron beam is focused onto a thin sample and row-wise scans a particular frame. The primary electrons transmitted through the sample, whose current is measured synchronously with the position of the electron beam, are generally used as the image signal. According to the image formation, it is a sub-form of the scanning electron microscope (SEM), the examination geometry according to a transmission microscope. The same requirements are placed on the samples in terms of transparency as with the transmission electron microscope (TEM; often also referred to as conventional transmission electron microscope , CTEM , to distinguish it).
Acceleration voltages similar to those used in the TEM are used, namely around 100 to 300 kV . A dedicated scanning transmission microscope ( dedicated STEM ) is an electron microscope that is designed exclusively or primarily for operation as a STEM. But many modern TEMs also allow operation as STEM, these devices are therefore often referred to as TEM / STEM.
The first scanning electron microscope was developed and built by Manfred von Ardenne in 1938 . However, the technology was only put to practical use for application in transmission after Albert Crewe introduced the field emission cathode as a beam generator in 1964 .
The scanning transmission electron microscope does not involve the scanning tunneling microscope to be confused, used where no electron-optically generated electron beam, but the quantum mechanically explainable tunneling current between researcher and object and a mechanically guided conductive tip is measured and at the so-called scanning probe microscopes (engl. Scanning probe microscopes , SPM ) is one of .
Generation and control of the electron beam
In the STEM, the electron beam is usually generated by special field emission emitters (see section Resolving power below) and focused on the sample by a system of electron optical lenses. The last lens is called the objective. In the TEM / STEM, the objective field is often arranged almost symmetrically to the sample plane. This is necessary because - due to the design - the lens must be able to focus the beam (STEM) and image the sample electronically (TEM). In pure STEM, the objective field in the beam path is mainly concentrated in front of the sample. The deflection of the beam for the scanning process is effected by two pairs of crossed (magnetic) dipoles, so that the beam location can be pushed over the sample without changing the angle of incidence.
Signal generation
The transmitted electrons are classified according to the angular range in which they are scattered by the sample. Based on light microscopy, a distinction is made between bright field and dark field electrons ( bright field , BF , and dark field , DF ). The BF detector or detectors are located on the optical axis of the microscope and detect the electrons that are not scattered or are scattered at very small angles. The DF-detectors are generally arranged concentrically about the optical axis of the microscope, they are then referred to as annular dark-field detectors (engl. Annular dark field , ADF ). Detectors for the so-called high-angle annular dark field (HAADF) are used particularly frequently .
The HAADF signal often enables the differentiation of chemical elements simply on the basis of the signal intensity, since the scatter in the corresponding angular range is scaled approximately with the square of the ordinal number . For sufficiently thin samples, the HAADF intensity also depends approximately linearly on the sample thickness irradiated.
The ability to use several signals in parallel for imaging is one of the special properties of all scanning electron microscopes.
In addition to the BF and DF signals spectroscopies are often as energy dispersive X-ray analysis (engl. Energy dispersive X-ray analysis , EDX ) or electron energy loss spectroscopy (engl. Electron energy loss spectroscopy , EELS ) used chemical elements to determine the distribution and concentration.
Resolving power
The beam diameters in the range of 0.1 nm and below required for high-resolution examinations can only be achieved with sufficiently large beam currents if the electron source supplies sufficiently coherent electrons (improved coherence can be achieved by using small beam apertures, but only at the expense of the size of the beam current). Sufficiently coherent electrons are obtained with field emission and Schottky cathodes, but not with purely thermally emitting sources. Schottky cathodes are a mixed form of thermal and field emission sources. Moderate heating of the emitter results in the emission of electrons at field strengths below those required for field emission. Schottky cathodes are mostly used in combined TEM / STEM devices because they deliver a higher beam current than pure field emission sources, which is necessary for the TEM mode, but with less coherence. Dedicated STEM devices are equipped with pure field emission cathodes that operate at ambient temperature. The use of a monochromator can also increase the coherence, but again only at the expense of the beam current.
The smallest achievable beam diameter is determined by the aberrations of the electron-optical system for beam focusing. On the one hand, the diffraction limit for a reduction in the beam diameter in the focus requires an increase in the beam convergence angle; on the other hand, the aberrations increasingly interfere with the beam paths with increasing distance from the optical axis. This results in an optimal convergence angle and also an optimal defocus (referred to as Scherzer focus according to Scherzer ). Since the beam is limited by fixed diaphragms, a multi-stage condenser system is required here in order to be able to select precisely the portion of the beam disturbed by aberrations without contrast-reducing portions from the larger angular ranges. Modern devices have a 3-condenser system that meets this requirement. Using a sample corrector based on magnetic multipoles (dipoles, quadopoles, hexapoles, octupoles) enables spatial resolutions of better 0.14 nm at 60 kV, better 0.10 nm with 100 kV acceleration voltage and about 0.06-0.08 nm with 300 kV; In addition to the electron-optical factors, the mechanical stability of the microscope and the stability of the beam deflection also play a role. The correctors enlarge the angular range, which is less disturbed by aberrations. The larger, sensible convergence angles that are made possible by aberration correction and that are required to reduce the beam diameter (see resolution ) can improve the depth resolution (i.e. the spatial resolution in the direction of the beam).
literature
- Peter Hawkes: Recent advances in electron optics and electron microscopy . In: Annales de la Fondation Louis de Broglie . 29, Hors série 1, 2004, p. 837–855 ( PDF [accessed January 31, 2014] A comprehensive article by Peter Hawkes with a special focus on French contributions.).
Web links
- SuperSTEM Laboratory, Daresbury, UK: SuperSTEM Gallery : Collection of current results obtained in the laboratory there
- QSTEM: Quantitative TEM / STEM Simulations : Software for simulating STEM images, the page shows an animation of the beam path during the scanning process.
- LPS Orsay, France: STEM @ LPS : website of the STEM group. Particularly EELS-oriented.
credentials
- ↑ OL Krivanek, N. Dellby, AR Lupini: Towards sub-Å electron beams . In: Ultramicroscopy . tape 78 , no. 1-4 , June 1999, pp. 1-11 , doi : 10.1016 / S0304-3991 (99) 00013-3 .