Focused Ion Beam
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A focused ion beam (abbreviation: FIB ; English for "focused ion beam", German also ion fine beam system ) is a device for surface analysis and processing. If the focus is on material removal, the process is also called ion thinning . If the scanning of the surface of the object to be examined by the ion beam is primarily used as an imaging method , then one speaks of a focused ion beam microscope .
Working principle
The working principle of the FIB is similar to that of the scanning electron microscope (SEM). Ions , mostly gallium or helium , are used instead of electrons . Analogous to the SEM, the ion beam is focused in one point with the help of electrostatic and magnetic lenses and guided over the surfaces line by line. In this step the secondary electrons from the surface of which are detected and allow an image of the surface. In addition, the intensity of the beam passing through the sample and the beam reflected by the sample can also be measured.
The ions are typically accelerated with voltages of 2 to 50 kV. The beam current in the FIB can be regulated by diaphragms of different sizes (typically 1 pA up to 1.3 µA). Large currents are used for the "coarse" material removal, while small currents are used for fine polishing and imaging due to the better resolution.
Used often Gallium is because of the good producibility of the ions by means of a liquid metal ion source ( English liquid metal ion source , LMIS). Gallium is heated to the melting point using a tungsten needle and the ion beam is obtained in a field emission process. So-called plasma FIBs, which work with xenon ions, achieve very high beam currents of 1.3 µA. Helium or neon are also common.
Theoretically, a focused ion beam microscope can achieve a finer resolution than when using electrons due to the smaller De Broglie wavelength of the ions . In reality, however, the resolution of SEM and FIB is limited by the area of interaction with the sample and the beam's ability to be focused due to lens errors. The higher resolution of the helium-ion microscope cannot be explained directly by the smaller wavelength, as even the wavelength of the electrons (at 10 kV this is 12 pm) is much smaller than the achievable resolution (~ 1 nm). FIBs with gallium, which are primarily designed for material removal and thus for high maximum currents (e.g. 60 nA), achieve “only” resolutions of approx. 5–10 nm even with the smallest apertures (1 pA).
Interactions of the ion beam
Interaction with the (sample) surface
Due to the higher mass of the ions (compared to electrons), the interaction of the ion beam with the surface is significantly stronger, since much more energy is transferred to the surface atoms according to the laws of elastic collision . This is minimized through the use of light ions such as helium or (in the case of gallium, for example) used specifically to process materials on the nanometer scale. Further surface processes then occur, such as the storage of primary ions and the amorphization of the surface.
Furthermore, analogous to crystal growth structures, so-called degradation structures can form. For example, chains of screw dislocations become visible as spirals.
Interaction with process gases
If process gases are passed over the sample, e.g. B. the organic platinum complex MeCpPtMe 3 , structures can also be built. The process gases adsorbed on the surface are split into a non-volatile part (platinum in the example here) and a volatile part by the ion beam. The platinum is thus only deposited in places over which the beam scans, since the process gas does not split and evaporate again without the energy input of the beam. In addition to platinum, carbon is also deposited on the surface, which comes from the ligands of the organic platinum complex. The carbon content in the growing layer is up to 40 percent. With the additional introduction of water, the reaction between the oxygen in the water and the carbon can reduce the carbon content in the layer. Tungsten, pure carbon, silicon dioxide and many other materials can also be deposited.
Other process gases, such as water, iodine or xenon difluoride, increase the etching selectivity and allow selective etching or better removal of the materials, since redeposition (redeposition) is prevented. Aluminum can be etched with iodine and silicon oxide with xenon difluoride. Water is used to accelerate the removal of carbon. The reaction between the oxygen in the water and the carbon causes the formation of carbon dioxide, which is sucked off.
Applications
FIB technology is used in the semiconductor industry, mainly for error analysis, and in research. There, samples are prepared for further investigations (e.g. for investigations using a transmission electron microscope , TEM) or structures are produced that can be further investigated. The possibility of producing cross-sections in materials and thereby generating extremely low mechanical or thermal disturbances enables sensitive layers to be better assessed in materials research.
Furthermore, the manipulative effect of an ion beam can be targeted for ion implantation z. B. be used in semiconductor structures. One specific application is the structuring of feedback gratings on laser diodes by means of the rasterized implantation of dopants .
Cross beam or dual beam
If an FIB system is combined with an electron microscope, a “dual beam” (two-beam) or “cross beam” system (with crossed beams) is obtained, which enables materials to be observed and processed at the same time. This makes it possible to precisely prepare defects (e.g. in individual transistors of ICs) or points of interest on a sample.
See also
Web links
Individual evidence
- ↑ Plasma FIB Vion from FEI
- ↑ Functional description of the helium-ion microscope with views of the FIB ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 309 kB)
- ^ Real structure of crystals (1995), J. Bohm, 150 ff. ISBN 3-510-65160-X
- ↑ Harald König: Gain-coupled InGaAsP / InP-DFB semiconductor laser diodes based on lattice structuring by focused ion beam lithography , dissertation, Shaker 2002
