Focused ion beam technology, capabilities and ... - FEI Company

More documents

Recommendations

Info

6 Useful signals As many signals are generated simultaneously, they may be used for detection, if efficient detection is possible. In this way all charged particles can be used as an imaging signal source, but the many neutral atoms are not used. Instead, these neutrals are the main component in the sputtering process and they are removed by the pumping system. Depending on the conditions, some of the neutrals may redeposit close to the area of milling. The charged particles, both ions and electrons, can be used as an imaging signal. Since the ion beam can be highly focused and scanned over the area it can be applied to create images at high magnification. Note that the milling process itself continues during imaging but at a very low rate, because small spots and low beam currents are used for this. Although the top layer is removed continuously with every scan during imaging, in practice it is negligible in many cases. Another advantage of this gradual low rate milling is that the sample is “continuously cleaned” during imaging. The particles emitted from the surface during the irradiation with the ion beam are schematically shown in Figure 5. Figure 4: The Ga liquid metal ion source, including the reservoir. If the sample materials matrix has different alignments, as in a multi-crystal phase, ions may have a much higher or lower channeling yield depending on the local crystallographic orientation. As the ion is much larger than the electron, the sensitivity for the “projected atom compactness” is far higher. This phenomena, known as ion channeling, can be used to study the local differences in crystal orientation. It is also possible to use the (secondary) ion signal itself to create an image and in for example crystallography materials such as metals it will produce an excellent additional contrast that shows the different grains of the material. The contrast of the ion signal can be different from the SE contrast (channeling, voltage contrast) and therefore ion imaging can give additional information. Although the majority of atoms emitted from the sample are not used, the emitted ions can be used. In principle it is possible to analyze ions and determine species and quantities by secondary ion mass spectroscopy (SIMS). In this way elemental distributions are revealed during the milling of a sample, so in the third dimension as well. Substrate atoms milled from sample Collision with substrate atoms Ga + implantation Incident Ga + Beam Figure 5: Interactions of the ion beam with the sample surface. The unique control offered by beam currents and spot sizes allow use of the FIB for both nano engineering as well as for high resolution imaging using secondary electrons as well as ions. ion 10 nm Secondary electron ionization
Figure 6: High-resolution SE image by ion beam scanning of a gold on carbon sample. Also note the strong crystal grain contrast within the gold particles. Horizontal field width is 1.5 µm. FIB and SEM instrumentation As electrons and ions are both charged particles, a focused ion beam system and an electron beam system such as a SEM have much in common: vacuum, lens control, electronics, scanning and patterning facility, electron detection, PC control, stage, etc. The instruments therefore have a similar design as is shown in figure 8. Some important differences in the design of FIB and SEM are: • Continuous use of a blanking signal for the ion beam column, when the beam is not used to collect an image or to induce the milling process. This is completely interlocked and automated with the control of the system to prevent undesired milling (e.g. during imaging). For the SEM this kind of functionality is mostly applied only for e-beam lithography when unwanted exposure of e-beam resist must be avoided. • Control of the beam current in a SEM is done by selecting different demagnification factors (lens control) for the column (changing the size of the beam at the position of the aperture). Control of the ion beam current is realized by selecting different apertures (changing the size of the aperture at a fixed position in the beam). For the ion beam each of the selected apertures has an optimized performance at a certain lens setting and therefore, the more apertures, the more optimized the system will behave over the full beam current range. • The milling aspects of the FIB are so important that patterning is a standard capability of the system. For a SEM, patterning is mainly added for the sake of e-beam lithography. • The type of detectors that can be applied: SEM and FIB both have a Figure 7: Ion image of a chromium coated steel wire, showing very strong contrast of the metallic grains, due to their orientations. secondary electron detector, SEM may have a back scatter electron detector, a STEM detector and /or an x-ray detector, whereas FIB has an ion detector. Both systems can also use the sample current as a signal. • FIB can be equipped with an ion flood gun to greatly reduce the charging of insulating samples. As can be derived from the intermezzo, the charging induced by the ion beam is positive and any additional negative charge can be used to compensate this. One way that conveniently compensates the charge is the use of a flood gun: a low-energy, non focused spray of electrons that compensates the positive charge on the surface. 7
Page 1: Focused ion beam technology, capabi
Page 4 and 5: Technology 4 Electrons replaced by
Page 8 and 9: 8 Anode Lens 1 Scan & Stig Coils El
Page 10 and 11: 10 Deposition As FIB technology is
Page 12 and 13: 12 Figure 21: Cantilever with proof
Page 14 and 15: Application examples 14 The New Dim
Page 16 and 17: 16 Figure 42: FIB machined photonic
Page 18 and 19: 18 Figure 49: SE image made with io
Page 20: FEI Company World Headquarters and

Focused ion beam technology, capabilities and ... - FEI Company

Create successful ePaper yourself

Delete template?

Save as template?