The FEI Quanta 3D scanning electron microscope is a high resolution dual beam device. Dual beam means that besides the electron source it has an ion source as well. The electron and the ion beams are suitable for taking microscopic images while the ion beam allows tailoring the sample surface at the same time.
The image formation in a scanning electron microscope is different from that in a conventional optical microscope. In the SEM a focused electron beam scans the sample surface producing various "products" from the surface. These are secondary electrons, backscattered electrons and X-ray photons. These products are collected by dedicated detectors and using their signals an intensity map is constructed on the screen: this is the microscopic image. Due to the image formation in SEM the sample can not be thinned as it can be made in TEM.
As the energies of the emerging products are different, they provide information from different depths.
The secondary electrons are specimen electrons, knocked out of the surface by inelastic collisions with the beam electrons and their majority are emitted with energies of a few electron volts (3 – 5 eV). This is why they provide only surface information. In this case, the resolution is determined by the size of the focused electron beam. The ultimate resolution in case of secondary electrons is ~ 1 nm.
The backscattered electrons are beam electrons experiencing large angle (or multiple) elastic scattering. The order of magnitude of their energy is some tens of kilo electron volts (10 – 30 keV). Owing to their energy range they provide information from deeper layers. Accordingly, in this case the maximum resolution is smaller, ~ 2 – 4 nm.
The energy of X-ray photons is characteristic of the atoms they originate from, thus they can be used for elemental analysis. The analysis can be point or surface analysis (elemental mapping).
The extraordinary feature of Quanta 3D microscope is that insulating samples and biological objects can be examined without any special sample preparation.
The charge delivered by the electron beam can be accumulated on the surface of an insulating sample, making further examination of the surface impossible. This charge effect is usually avoided in a conventional scanning electron microscope by coating the surface with a thin gold (or carbon) layer, converting the sample surface into conductive. In many applications, such as in case of nano objects, the surface coverage is not an eligible solution because it can substantially modify the properties of the sample. Quanta 3D microscope has a so called low vacuum operating mode, which eliminates the need for coverage of the surface since the gas ionised by the electron beam (water molecules) is capable to neutralize the surface charges.
Another exceptional feature of the Quanta 3D microscope is that biological samples sensitive to vacuum and water loss can be examined directly. This is the so-called environmental operating mode where the measurement is performed in wet gas.
Therefore, through its beneficial properties, the Quanta 3D scanning electron microscope is suitable for the convenient examination of the nano-size range (1 nm – 100 nm).
This versatile device is suitable for backscattered electron diffraction (EBSD) measurement as well. The diffraction image measured point by point enable to characterise the crystal structure and orientation in a small vicinity of the measured point. Having been measured all points of a given area, orientation map can be constructed. Using the orientation map grain structure and texture can be studied.
The apparatus has also a transmission operating mode (STEM mode). If the sample is thin enough, bright-field and dark-field images can be taken by the STEM detector, information content of which corresponds to those of TEM images. The ultimate resolution at this operating mode in ideal circumstances is 0.9 nm.
The second beam of the dual-beam microscope is a focused Ga ion beam (FIB), with maximum energy of 30 keV. The surface of the sample can be sputtered by the ion beam, and thus the sample surface can be manipulated by high efficiency with this method. FIB is applicable for several function: manufacturing sample of cross section, thin TEM lamella or nanolithography etc.
At the Chemical-, Physical-, Biological-, Earth Science Institutes of Eötvös University several groups are engaged in nanotechnology oriented materials science, chemical, biological and interdisciplinary research. Some examples in which scanning electron microscopy can be involved as a basic tool: special features of nanopowders, nanocomposites, carbon nanostructures, structure change due to deformation, preparation and characterization of nano- and micro-sized samples, hydrogen storage in nanostructures, ceramic materials, micro- and nano-size biological objects.
Besides researchers and research groups from our university guests from other universities, research institutes, or from other research organizations are welcome in our laboratory. Both mutually beneficial cooperation and contract work are not without precedent.
If you wish to learn more, read the detailed description of the SEM/FIB system.
