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Create Alert. Share This Paper. Figures from this paper. Citations Publications citing this paper. Electrophoretic deposition of hydroxyapatite-hexagonal boron nitride composite coatings on Ti substrate. The beam is formed in a high-vacuum environment where selective electric potentials are used to ionize and extract gallium from a liquid metal ion source LMIS.
This beam can be directed and focused with electromagnetic lenses similar to light in a traditional, optical microscope. The beam then rasters to cover an area on the sample. With a different kind of source, an electron beam can be used for nondestructive imaging and characterization without sputtering the sample surface, much like scanning electron microscopy SEM.
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Additionally, three-dimensional information can be obtained by combining the electron and ion beam operations to perform a tomography i. Generally, conductive samples are ideal for FIB and SEM because they do not collect charge and thereby affect the pathway to imaging, milling, and deposition. However, non-conductive samples like most polymers and biological samples can be probed with the use of charge correction, conductive coating, variable pressure settings, and low energy beam settings.
Having an understanding of the basics of ion beam-solid interactions may improve the ability to achieve optimal results using an FIB system. The mechanics of ion beam-solid interactions consists in the following events: primary ions of the focused beam bombard the surface, sputter material, eject secondary electrons and implant themselves. Milling occurs due to the physical sputtering of the target. In order to understand the sputtering process, the interactions between the ion beam and the target must be explored. Sputtering takes place as a consequence of a series of elastic collisions in which momentum is transferred from the incident ions to the target atoms within a region that is called cascade region.
This process is similar to what happens when a cue ball hits the object balls when the break shot is taken. An atom on the surface of the target may be sputtered if it receives a kinetic energy that exceeds its surface binding energy SBE. The surface binding energy is the energy required to remove a surface atom from its bulk lattice. A portion of these ejected atoms might be ionized. Because of ion bombardment, inelastic interactions can also happen. These interactions produce phonons, plasmons in metals, and secondary electrons SE.
A standard FIB employs secondary electrons in order to produce an image. Deposition can also be accomplished by deploying small amounts of precursor gas molecules to the surface of the material and using the impinging ions to facilitate a chemical reaction where the material is deposited onto the surface. Though, for this study, milling and imaging are the only mechanisms covered.
Fabrication of a perforated filter from a nm thick silicon oxide membrane comparable in scale to the kidneys' endothelial cytoplasm. Figure 1 : FIB milled holes in silicon oxide membrane creating particle filter. The Focused Ion Beam is an instrument that can be used to fabricate, trim, analyze, and characterize materials on micro and nano scales. Focused Ion Beams are used in a wide variety of fields, ranging from electronics to medicine. Focused Ion Beam Systems accelerate Liquid metal ions in a vacuum to form a beam. Using a series of Electromagnetic lenses, the beam can be focused onto an area of about 10 nanometers in diameter.
When the ions from the Focused Ion Beams strike the target, some of the target material is sputtered. At Low primary beam currents, very little sputtering occurs and the beam can be used for imaging. At higher currents, Surface atoms are ejected. This allows for Site-Specific sputtering or larger scale milling of samples. Focused Ion Beam Systems create a beam of Liquid metal ions under vacuum in order to mill material from a sample or take an image of it.
The ions are accelerated through application of voltage, and then a series of Electromagnetic lenses focuses the beam on the target.
Coordonnateur : Yao Nan
The metal ions collide with the material in the sample much like a cue ball does when striking billiard balls. At low energies, a metal ion knocks away secondary electrons, which can be collected to form an image of the target surface. At higher energies, the ions may transfer enough kinetic energy to atoms in the material to overcome their surface-binding energies and scatter into the vacuum.
This is known as Sputtering. Focused Ion Beams can use sputtering to bore holes at specific sites, mill patterns onto a target, or even remove the surface layer from a sample. By repeatedly and uniformly removing a layer and the imaging the region, three-dimensional images of a sample can be constructed. A percentage of the metal ions used by the beam are implanted in the sample. After the initial impact, an ion continues to lose energy through a series of collisions until it stops inside the sample.
Chemical Vapor Deposition can also be accomplished by deploying small amounts of Precursor gas molecules to the surface of the material and using the impinging ions to facilitate a chemical reaction, wherein the Precursor gas breaks down and a portion of it is deposited onto the surface along with some of the impinging ions. Due to the accumulation of metal ions on or within the material, and scattering of secondary electrons from the surface, it is possible that charge can build up on a non-conducting target.
This accumulation of charge can create additional electrostatic fields that alter the beam path. One way to prevent this is by coating non-conducting samples in a conducting material such as Gold, Gold-palladium, or Carbon, before using the Focused Ion Beam System. A standard Focused Ion Beam takes an image of the sample by collecting the scattered secondary electrons from the ion interactions. For these combined systems, once the Focused Ion Beam has finished, the Scanning Electron Microscope is used to take an image of the sample. The two beams are arranged at a 54 degree angle relative to one another.
The sample must be at the focal point of both the ion beam and the electron beam. This is known as the Coincident-Eucentric Point. In the next section, we will use a Focused Ion Beam to mill a logo onto a hair in order to demonstrate the remarkable precision of the technique.
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The cone tip is small enough such that the extraction voltage of the aperture can pull Ga from the needle tip and efficiently ionize it by field evaporation of the Ga at the end of the Taylor cone, after which it is accelerated by a potential down the ion column. The source is generally operated at low emission currents of 1—3 mA to reduce the energy spread of the beam and to yield a stable beam.
At higher emission currents, the probability of the formation of dimers, trimers, and droplets increases, which both increases energy spread and decreases source lifetime. After field evaporation causes ion emission to occur, the ions begin to accelerate down the column. The current emitted from the tip is known as the extraction current.
It is regulated by both the suppressor and the extractor, as shown in Figure 1. This is particularly important to control the etching rate during milling operations. Adjusting the suppressor voltage will change the extraction current, which means that the extraction current may be regulated without changing the voltage of the extractor. This is generally the preferred method of adjustment, as changing the extractor voltage can result in spatial displacement of the Taylor cone and apparent beam drift on the sample surface.
This instability corresponds to the fact that LMIS have a highly nonlinear current—voltage relationship . Thus, the ability of the suppressor to maintain constant extraction voltage without altering the source tip is an essential requirement for FIB system stability. Field evaporation, the process responsible for ion production in the LMIS, is a physically complex phenomenon, and complete treatment is beyond the scope of this chapter. Fundamentally, however, field evaporation takes place when the potential barrier preventing evaporation has been lowered by the presence of a field and can only be crossed by the ionization of the evaporating atom on the surface of the field emitter.
It can be described analytically by first calculating the field needed to produce ions in free space, that is, in the absence of any other field. It has been found that the critical distance xc from the tungsten needle tip required for ion production is approximately 0. Introduction to the focused ion beam system 13 For comparison, we can consider one type of source of electrons in an electron beam column. In the most common configuration, a tungsten filament is heated by a large current, causing it to emit a spectrum of radiation accompanied by a number of loose electrons that have gained sufficient energy to overcome the work function of the metal and escape.
These electrons are then accelerated away from the tip by a set of electrostatic fields generated by large coils, and reduced to a relatively clean beam by an aperture below the source. The filament itself is formed into a sharp point, since this shape causes charge to cluster at the tip, giving a greater output current from the source.
The tungsten filament is also usually zirconated ZrO to increase the thermionic emission of electrons by means of the Schottky effect, in which an accelerating field for electrons exists at the surface of the filament due to an external applied electrostatic field.
Zirconated tungsten exhibits a decreased work function 2. Two other classes of enhanced electron guns are worth mentioning. The potential of rare-earth hexaboride materials, especially lanthanum hexaboride LaB6 , for thermionic electron emission was first reported in by J. Lafferty . The work function of LaB6 is lower than that of W 2. LaB6 crystals can provide considerably higher current densities, and LaB6 offers improved coherence and a smaller energy spread .
Extensive research in the s and s 14 Focused ion beam systems Tungsten needle Field emission tip First anode Second anode Primary electron beam Figure 1. The drawback to this source is mainly on its relative larger energy spread and low beam intensity for an electron probe of nanometer size.
Unlike thermionic guns, they operate at relatively low temperatures of about K and offer superior resolution and performance. FEGs are simpler in their operation — they use a pair of anodes below the tungsten tip to generate intense electric fields that extract electrons by enabling them to tunnel from the extremely sharp tip Figure 1.
As a result of the small tip size and low operating temperature, the electron beam is highly spatially coherent and experiences almost no energy spread, which limits the deleterious effects of chromatic aberration; its current density is also remarkably high . The weakness of FEGs is that they can only be under ultra high vacuum Introduction to the focused ion beam system 15 fraction of those will go into the beam itself, so a substantial increase in current is needed for a marked increase in beam brightness.
The increased current can cause source instability and lower the lifetime of the source element.
Focused Ion Beam Systems: Basics and Applications
A second limitation is that of uniformity. While the electrons emitted from the zirconated tungsten are relatively evenly distributed in energy, the ions emitted by a liquid-metal ion source LMIS as described above tend to follow a Gaussian energy distribution, sometimes asymmetrical, which leads to chromatic aberration.
The energy required to evaporate ions from a liquid is dependent on local temperature, field strength, and surface tensions in the area around each atom, which can vary from point to point within the emissive region . The ion emission current is strongly dependent on the tip radius and on the tip surface condition. The sharper the tip is, the higher the field; and the higher the field, the stronger the ion emission. However, the inter-particle repulsion effect at high emission and liquid flow rates with capillary characteristics need to be balanced out in order to maintain a stable, consistent ion beam emission.
Focused Ion Beam Systems : Basics and Applications (2011, Paperback)
Electron emission is determined by the work function of the metal used and the field strength at the source, with electrons escaping as soon as they have reached a welldefined critical energy. Both sources experience an additional degree of energy spread due to mutual repulsion between the charges just beyond the source. The like charges of the ions and the electrons repel each other, imparting a small but significant random velocity that changes the overall energy profile, causing further chromatic aberration and forming the fundamental limit for the focusing ability of the system.
Variations in emission characteristics can be controlled to a degree, but before the limit of that control is reached, it is overshadowed by the aberrations arising from these mutually repulsing charged particles. A lens for an ion or electron beam can be thought of in the abstract as being almost identical to a lens for light, with a similar function and similar parameters, such as focal length and refractive index.
Standard light optics concerns thus enter the picture, including chromatic aberration, spherical 16 Focused ion beam systems aberration, and astigmatism. An additional concern specific to electron and ion beam applications is that of apparent source size, which is related to the inter-particle repulsion mentioned above. Electron and ion lenses function very much like light optics, but their construction is quite different. Instead of using a material with a certain geometry and index of refraction to bend the path of the light, electron and ion optics use magnetic and electrostatic fields to change the paths of the particles.
Electron beams, consisting of fast-moving, low-mass particles, are generally focused using only magnetic fields. The advantage of a magnetic field is that it is relatively easy to produce a uniform field over a region, and that magnetic fields do no net work on objects within them, so that the kinetic energy of the electrons is not changed. The magnetic lenses in an electron beam column are generally washer-shaped coils with small central holes, so that the field is intensified by being compressed into a smaller space.
The deflection of the beam is proportional to the distance from the axis of the lens, as in light optics, so that the field behaves just as a glass lens would. Charged particles, however, require stronger fields to focus higher-energy particles because charged particles have kinetic energy vectors that must be diverted by an applied force, while light can be focused by changing the nature of the medium through which it propagates.
The higher the kinetic energy, the higher the force needed and the higher the lens fields must be. Higher kinetic energies correspond to shorter wavelengths, which provide higher resolution. In order to achieve this higher resolution, however, everstronger fields must be produced in the focusing column. Ion beams, in contrast to electron beams, use electrostatic lenses almost exclusively. Given the higher mass of ions, their velocity is 0. Magnetic optics would need to be impractically large to provide enough focusing power for an ion beam. Electrostatic lenses, however, can be made extremely small and are capable of producing much faster response for Introduction to the focused ion beam system 17 z r Field z B Bz parallel to axis Br perpendicular to axis Figure 1.
Magnetic field lines in a magnetic lens are shown in thin lines, while the thick light lines represent two possible electron paths. The smaller inset to the left gives an approximation of the magnetic field strength along the optical axis and perpendicular to the optical axis. The narrowing helical rotation of electrons within the lens cannot be faithfully represented in two dimensions but can be visualized. After the beam is reduced to a relatively coaxial column by an aperture, it enters the top of the electron optics column and passes into the magnetic field of the first electron lens, an axial field with rotational symmetry about the axis of the column.
This field Bz has a bell-shaped distribution along the axis where it swells out from the center of the lens coil see Figure 1. As an electron travels down the column, it first encounters the horizontal component Br of the magnetic field. This causes the electron to begin a rotation about the axis along a helical path. With a nonzero angular velocity about the axis, the electron begins to feel an inward force from the vertical field component Bz that draws it toward the axis.
Finally, as the electron emerges through the bottom horizontal field component, it receives a reverse angular impulse that cancels its rotation about the axis. The constricted beam then continues to narrow toward the focal point. The electrostatic lenses found in ion beam systems operate in a simpler manner than magnetic lenses, although the underlying principle is analogous.
Positively charged particles enter the lens from the left and encounter an electrical field formed by the large voltage difference between the first two electrodes. The ions follow the field lines, receiving an impulse toward the optical axis and a boost in velocity by the increasingly negative field. As the ions pass the second electrode, they are pulled outward, but since they are now closer to the axis and have more momentum, the change in direction is less than from the first impulse.
Field lines generated by the voltages on the electrodes are shown as dotted lines, and the ion trajectories traveling from left to right are in gray. Just as in the magnetic lens, the charged particle beam exits and continues to narrow toward its focal point downstream in the column. It is worth noting that, if the potential difference between the center electrode and end electrodes were reversed, it would create not a converging but a diverging lens system commonly used in the transmission electron microscope, but not actually used in any SEM or FIB optics. A small imperfection or a fundamental limit in one lens will be multiplied by the number of lenses and cause a significant loss in resolution.
It is known that such optical aberrations in the electrostatic lenses of ion columns are also quite severe. As a result, charged particle lenses are incapable of completely faithful imaging, and to optimize their performance, it is critical to operate them in the paraxial mode. This means that, for high-resolution images, the angle of the trajectory of particles with respect to the lens axis must be kept extremely small: less than 10 mrad.
The aberrations can be categorized into three basic groups, as follows: spherical aberration, chromatic aberration, and astigmatism. Spherical aberration is the effect of a nonlinear dependence of beam deflection on radius within a lens. In an ideal lens, the larger the radius from the axis at which a particle is found, the sharper the angle of deflection: a linear relationship that holds true from the center of the lens out to the edges.
In a real lens, however, the fields necessarily experience a degree of fringing 20 Focused ion beam systems and edge effects closer to the substance of the lens, causing the beam to deflect at an angle that may not match that needed to reach the desired focus. Thus, parallel vectors traveling through the lens may be focused at different points as a result of intrinsic lens properties, and the focal length varies with radius instead of remaining constant. In magnetic lenses the mutual dependence of Br and Bz means that the variation in focal length as a particle travels through the lens further from the center is great enough to cause significant spherical aberration.
Chromatic aberration is the chief limitation on the focusing ability of electron and ion optics. This problem occurs when a spread of energies 1E is present in the beam. Since the lenses rely on the interaction between fields, charges, and velocities, according to the Lorentz force law, a slight difference in velocity owing to a difference in initial energy will result in a different focal length for a particle.
The spread in energies thus translates to a spread in focus. Particles with higher-than-expected energies will be focused beyond the surface of the sample, while particles with lower-than-expected energies will pass through focus above the sample and spread out again, producing a larger spot size than desired. The chromatic aberration so called because a similar focal length spread is observed for different energies, and thus different colors, of light is a fundamental property of the source, and represents the most serious practical limitation on the performance of current ion beam designs.
In the approximation implicit in the above equations, it is assumed that all electrons or ions have the same kinetic energy due to having gone through the same accelerating voltage. The distance of these planes from the image plane varies as the square of the distance of the point object from the lens axis and as the aperture angle. It should be noted that while chromatic and spherical aberrations are properties of the lenses and electron source and can only be corrected by apertures and the addition of other lenses with opposing aberrations, astigmatism can be dynamically corrected by the use of stigmators, a set of magnetic coils with deliberately asymmetric fields that can be used to move the two focal lines until the distortion is corrected.
This calculation is known as the quadrature method. The first major difference between electron and ion beam columns is in the emission source. As discussed previously, instead of a heated tungsten filament or a field-emission region above a tip, the ion emitter is a liquid surface drawn by electrostatic fields into a sharpened cone about 5 nm wide at the apex. It is important to note that metal ions used in a focused ion beam system have much higher mass than electrons, meaning that they travel slower at the same kinetic energy and require more force to divert.
As discussed above, this means that their lens apparatus needs to use much larger fields than in the electron column. Thus the fact that, in general, electrostatic fields are used for ion lenses, rather than magnetic fields. In order to produce the necessary forces, carefully shaped electrodes with precisely controlled potentials are used, generating electric fields that focus the slowermoving and heavier ions, as seen in Figure 1.
The ions undergo a slight acceleration in the process, which must be accounted for when considering the impact energy of the beam on the sample. High electrostatic fields are easier to create than magnetic fields, and in general make for a more stable lens. Other than replacing the magnetic term in the focal length equation with the radial component of the electric field the only component that exists, if edge effects are disregarded, which is a reasonable approximation for such strong fields and high energies , there is functionally no difference between ion optics and electron optics, as far as the physics of operation goes.
Since the aberrations depend on the same factors, it should be noted that many ion systems contain stigmator coils even though astigmatism is not as much a concern in ion beams. It uses the focused beam of gallium ions and rasters the surface of the material of interest. The small amount of material sputtered from the surface during this process may form secondary ions and electrons which are then collected and analyzed as signals to form an image on a screen. This allows high-magnification microscopy with the FIB system.
In both machines a source emits charged particles that are focused into a beam and rastered over small areas of the sample using deflection plates or scan coils. The SEM uses magnetic lenses to focus its beam of electrons; however, since ions are much heavier and, therefore, much slower with a lower corresponding Lorenz force, magnetic lenses are less effective. The FIB system is instead equipped with electrostatic lenses shown schematically in Figure 1. Both the SEM and the FIB form high resolution images by collecting the secondary electrons SE that are emitted from the interactions between the beam and the surface atoms, although images may also be formed from 24 Focused ion beam systems backscattered electrons BSE or secondary ions SI.
SE detection is the chief method, however, and two main detector types exist: multi-channel plate and electron multiplier. A multi-channel plate is generally mounted directly above the sample, and, as a result, offers negligible topographical information.
The Everhart—Thornley electron multiplier detector is the most common design used today for secondary electron detection. Also known as a scintillatorPMT photomultiplier tube detector, it consists of three main parts. Secondaries are attracted toward the wire mesh screen by a potential of several hundred volts, and most of them continue to be accelerated into the scintillator. Not unlike a fiber optic cable, a light pipe extends from the scintillator and internally reflects due to its high index of refraction the photons to the photomultiplier tube PMT.
The PMT is a highly sensitive visible photon detector that consists of a sealed glass tube containing a high vacuum. At the entrance to the PMT, the incoming photons strike a lowwork-function material that comprises the photocathode, liberating valence electrons that are subsequently accelerated as photoelectrons toward the first of a series of usually eight dynode electrodes. Each dynode is biased positively with respect to the photocathode, and each of them is also biased — V positively with respect to the preceding one.
The photoelectrons generate secondary electrons at the first dynode, and these secondaries are then amplified by a factor of about after they have completed striking the remaining dynodes. Recalling that each SE originally produced at the specimen surface generated about photoelectrons, the overall magnification of the scintillator-PMT detector can be as high as , depending on the applied dynode voltage.
Thus, although it may seem unnecessarily complicated to convert SEM secondary electrons into photons, then into photoelectrons, and finally back into secondaries, the high amplification and low electronic noise — versus a simple metal plate to absorb the electrons — in fact fully justifies the system. Backscattered electrons can also be detected by a scintillator-PMT if the bias on the first grid is made negative instead of positive, therefore repelling lower-energy secondaries but not BSE, which retain most of their kinetic energy after impact. Raised areas of the sample hills produce more collectable secondary electrons, while depressed areas valleys produce less, thereby creating a contrast that is interpreted by the machine and at the same time intuitively Introduction to the focused ion beam system 25 understood by the operator as light and shadow.
Also, to increase the secondary electron yield, the whole sample is often tilted away from the horizontal plane and toward the detector in order to increase the SE signal without interfering with the contrast-based topography. A viewing monitor synched to the scan coils controls the beam so that as it scans across the sample surface, the image of the sample is reproduced on the screen, with a magnification inversely proportional to the area of scan.
Images obtained from secondary ions can be below 10 nm resolution and show topographic and materials information complementary to that obtained from an SEM image. Although material contrast arising from differences in specimen chemistry can be significant in FIB secondary electron images, it is most readily observed in secondary ion images, where it is often the dominant effect.
While SE images provide uniformly good depth of field, SI images reflect more selective depths that depend on different materials and sample structure. Information about the grain size and crystal orientation can also be obtained using an FIB because of the dependency of the ion—atom interaction upon the crystal grain orientation; this is known as channeling contrast.
Thus, from a materials science standpoint, secondary ion imaging is an invaluable capability. SI imaging is also superior for insulating materials when used in conjunction with a charge neutralizer electron flood gun. Although ions move slower than electrons, they still move faster than the image can be collected. It is important to note that, since the sample is continually sputtering during the FIB imaging process, small beam currents 26 Focused ion beam systems Electron beam Ion beam 52 degrees tilt Sample Y X Translation axes Rotation axis Figure 1.
Alternatively, a direct ion-imaging detector such as the resistive anode encoder RAE can be used to capture the ion images. The resolution in microscope mode imaging is limited by the electric field strengths of the electrostatic lenses to about 1 mm. In addition, the stage can be tilted, allowing changes in the sample-beam orientation. Similarly to the FIB system, integrated software with a single user interface controls the two-beam. Introduction to the focused ion beam system 27 The two-beam system provides new advantages that simplify as well as improve nanoscale imaging, analysis, and fabrication as detailed in the following chapters.
One such advantage is in imaging. Whereas FIB imaging has high contrast abilities but can cause damage to the sample, SEM images have relatively lower contrast, but provide a higher resolution and do not damage the sample. The result is a more complete set of data. A study done at Portland State University showed that using this combination of beams could provide a more comprehensive imaging and characterization of carbon nanotubes .
Also the reconstruction of three-dimensional structure and chemistry of a sample can be simplified using the two-beam system to interpolate two-dimensional SEM and FIB images and ionassisted SIMS chemical maps of layers that have been exposed using the milling feature of the ion beam . The charging effects of ion and electron beams are worth consideration.
One of the great challenges in the use of electron beam for imaging is that if the sample is not fairly conductive, the electrons from the beam will build up charge in the material. This charge will then distort both the incoming beam and the outgoing secondary and backscattered electrons, producing electron artefacts and distortions in the data.
While the ion beam does to some degree increase local charge at the point of impact, the nature of the ions themselves, generally metals, causes the imbalance to be quickly rectified. This can be exploited to assist in electron-beam imaging of the sample, by using the ion beam at a low setting over a large area to reduce local charging effects and increase surface conductivity . The complementary nature of the negatively charged electrons and the positively charged ions also eliminates the charging problem found in the single beam FIB.
The accumulation of charge would hurt the resolution of the image, but because of the availability of both charges, this is no longer a problem with the two-beam system. Not only does the combined system produce a better and more extensive collection of data, but it also allows for precise monitoring of FIB operation through the SEM.
By using the slice-and-view technique to observe the progress of the ion beam cross section, the operator can stop the milling process at a precise point in order to obtain local information. Also, the twobeam system allows for the use of both the ion beam and the electron beam simultaneously without interference, doing away with the necessity to switch back and forth. The sample can be imaged in real time with the SEM while 28 Focused ion beam systems the FIB is in use, providing for higher levels of accuracy in the creation of cross sections . It can also be very useful in the localization of integrated circuit failures, so that more damage is not caused to the sample than is necessary, and the system can combine ion milling, deposition, and SEM imaging to characterize the failures .
The two-beam system also improves the deposition of metal or insulating layers. In the case of insulating layers, ion beams often leave the layer with poor insulating properties due to the incorporation of gallium ions in the deposition; however, in the two-beam system, the SEM beam can be used to induce deposition, ensuring a high insulating quality. One such case is that of SiOx, where the resistance of the layer is two orders of magnitude higher than when an ion beam is used for the deposition .
One more advantage of using the two-beam system is the creation of smaller diameter holes. Using the FIB system, a hole can only be accurately milled to a diameter of 10 nm without having material along the sides partially fill in the hole. The ability to have smaller diameter holes has potential applications for single molecule studies, DNA sequencing, and ultra-high-resolution single atom doping. The FIB completes electron transparent samples by thinning out a region of a bulk material, making samples as thin as possible in significantly less time than older methods of sample preparation.
When an SEM is used in addition to an FIB the basic techniques of lift-out and micropillar sampling are vastly improved. In addition to machining and TEM sample preparation, we also look into the many imaging capabilities of the FIB in the two-beam system. Whereas electrons in an SEM beam generate far less damage to the material and provide much better resolution, the ion beam of the FIB offers much better sensitivity to details such as crystal orientation and grain structure as well as better contrast.
The two-beam system is then an exceptionally practical machine, combining the crisp nondestructive imaging of the scanning electron microscope with the milling capabilities of the focused ion beam. Threedimensional material information is also available through the two-beam system by shaving off thin layers and imaging them with the SEM, obtaining a series of useful two-dimensional illustrations. Graphs and images provided Introduction to the focused ion beam system 29 only by the combined SEM and FIB systems can then be easily interpolated with the two-dimensional data.
If secondary ion mass spectrometry SIMS is then performed to yield the elemental composition of the material, the set of data can be complete. In summary, the two-beam FIB has immense relevance as one of the most important tools in the study of nanotechnology today. References  V. Rocketry, 5 , 73— Krohn and G. Orloff, M. Utlaut and L. Handbook of Microscopy for Nanotechnology, ed. Yao and Z. B, , — Gerlach and M. SPIE Int. Van Doorselaer, M. Van den Reeck, L. Van den Bempt et al. Testing and Failure Analysis , pp. Futamoto, M. Nakazawa and U. Williams and C.
Arimoto, T. Morita, E. Miyauchi and H. Feynman, R. Leighton and M. The Feynman Lectures on Physics, 2 , 29— Giannuzzi and F. Stevie New York: Springer, , pp. Dong, J. Jiao, D. Tuggle and S. Hull, D. Dunn and A. Gnauck, U. Zeile, W. Rau, G. Benner and A. Micron, 30 , — Soden and R. IEEE, 81, 5 , — Lipp, L.
Frey, C. Lehrer et al. B, 14 , —3. A fraction of the particles are backscattered from the surface layers, whilst the others are slowed down in the solid. The collision induces secondary processes such as recoil and sputtering of constituent atoms, defect formation, electron excitation and emission, and photon emission. Thermal and radiation-induced diffusion contributes to various phenomena of mixing of constituent elements, phase transformation, amorphization, crystallization, track formation, permanent damage, and so on.
All those processes are interrelated in a complicated way and several processes have to be included for the understanding of individual phenomena. Therefore, it is necessary to quantitatively understand the experimental observations and to have stringent design abilities for sophisticated applications of these versatile processes in the field of nanotechnology aiming at material modification, deposition, implantation, erosion, nano-fabrication, surface analysis, and so on.
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- Focused Ion Beam Systems : Basics and Applications - ecejyredagij.ml.
This chapter is composed of basic processes and outline of theoretical models, ion implantation and defect formation, sputtering, and surface morphology. It is focused on the recent experimental findings in the field of interaction of ions with matter and theoretical models including various simulation codes explaining the complicated experimental phenomena. These collisions basically contain a complicated many-body problem because of the atomic composition of a nucleus and many electrons. Fortunately in the interaction of an ion with an atom, the collision between the ion and the nucleus can be treated separately from that of the ion and the electrons because of the large mass difference between the nucleus and the electron.
The former collision is called an elastic or nuclear collision and the latter is an inelastic or electronic collision. Kinetic energy and momentum should be conserved during the nuclear collision by which the incident ion recoils the target atom and is scattered at the same time. The electronic collision results in excitation and ionization of the constituent electrons in the atom.
When the kinetic energy of the incident ion is not high enough to go deep inside the atom, the nuclear charge is screened by the inner-shell electrons. In this case, the electrons should be included in the nuclear collision for taking into account the screening of the nuclear charge. This screened interaction potential has been one of the fundamental debates for the understanding of the nuclear collision process.
This subsection first treats interatomic potentials, binary scattering process, recoil energy, and scattering cross section concerning the nuclear collision process dominating in a low energy region. Then, nuclear and electronic energy losses are introduced based on several models. Finally, theoretical models and simulation codes which will be presented in this chapter are listed.
The energy loss by the electron-related collision is called electronic or inelastic energy loss and the kinetic-collision induced energy loss is called nuclear or elastic energy loss. The former and the latter respectively work mainly at a high and a low velocity range. The term of stopping power is also used instead of the energy loss. At the high velocity range the ions lose their energies by the process of photon generation and by inducing nuclear reactions. In the following the nuclear and electronic energy losses will be presented.
The shell correction term is important in a relatively low velocity range where the ion cannot ionize nor excite inner shell electrons. In a keV energy range, the velocities of ions, especially of heavy ions are very low and the Bethe—Bloch formula is not applicable any more.
The ions capture electrons from the constituent atoms and as a result the screening of the nuclear charge becomes important. The nuclear collision competes with and further predominates over the electronic collision with decreasing ion velocity. Lindhard, Scharff, and Schiott LSS derived the universal electronic and nuclear energy loss formulae based on the Thomas—Fermi atomic model at a low velocity range .
Lindhard et al. However, obtained experimental values of energy loss depend on incident ion and energy, and the target atom. Ziegler et al. Section, subsection Name of code 2. On the basis of the binary-collision approximation BCA Sigmund proposed a linear collision cascade model, which has been successfully applied in a low-energy density collision .
In order to include complex situations depending on individual ion—solid combinations many researchers developed simulation codes written by Monte Carlo method as listed in Table 2. Those models were developed in the framework of BCA and resultantly it is difficult to reproduce experimental phenomena appeared in a high-energy density collision. In this situation, various molecular dynamics simulation codes using sophisticated potentials can work well and are listed, too, in Interaction of ions with matter 39 Table 2. Results predicted by these models and their comparisons with experimental findings are presented in the respective related subsections.
Defect formation following ion implantation plays, in general, unpleasant roles in the above applications but sometimes can be used effectively. This section presents recent experimental and theoretical findings on ion implantation and defect formation. The formation of these nanostructures is closely related to the nanotechnology. In the high-density energy deposition caused by very heavy ions and cluster ions, nonlinear effects on defect production were found.
To understand these experimental facts, a combined code of Monte Carlo MC , diffusion, and molecular dynamics MD simulation codes were developed. In addition, it is necessary to localize defects when one fabricates nanostructures with a focused ion beam. Diffusion lengths were measured and an idea to localize the defects was presented. This chapter includes the above experimental and theoretical results.
Then, many atomic defects are generated along the passage of the ion. This defect distribution is also important for the application of energetic ions to material modifications. For amorphous targets, the range and the range straggling can be calculated from the energy loss formulae given in Section 2. The more reliable 40 Focused ion beam systems values of the projected range, range and defect distributions can be obtained by consulting the SRIM code developed by Ziegler et al.
In the case of crystalline targets, energy loss and range of an incoming ion depend strongly on an incident angle against the crystal axis and plane. In a channeling condition, that is, when the ion is incident along the crystal axis or plane, the ion passes through the crystal with large impact parameters and as a result the energy loss is very low compared with that for the amorphous target of the same constituents.
Then, the ion goes through deep inside the crystal. Therefore, in order to avoid the trouble caused by the channeling phenomena, ion implantation is usually carried out at an incident-angle condition of off-axis or off-plane, or random incidence. Ion implantation induces mixing of constituent elements, alloy formation, and clusterization and crystallization in nanometer size in matrix. These processes are very complex and depend not only on ion-related parameters of species, dose, and energy but on a high-temperature chemical reactivity between the implanted and matrix elements.
Related to the nanotechnology, formation processes of nanoclusters and nanocrystals are one of the most exciting and promising topics. In special conditions implanted atoms assemble at interfaces of different materials and form nanostructures by themselves. A comprehensive review was made by F. Gonella et al. Figure 2. The introducing criterion of cluster formation of implants in SiO2 Interaction of ions with matter Ion implantation Self-assembling and nanocluster formation induced by thermal- and radiation-induced diffusion Figure 2. Reprinted with permission from Nucl. B, — , —9. Copyright Elsevier Science B.
The corresponding cuts through the NCs perpendicular to the plane of view are shown in b0 , c0 , and d0. B, , —9. A more general approach is due to Hosono et al. Strobel et al. By applying annealing procedure pre-implanted atoms X form nanocrystals. Then, implantation of a different type of element Y induces collisional mixing of atoms. The arrows indicate some of the two-fold clusters made of a Au-enriched alloy and Cu2O: c SAED selected area electron diffraction diffraction pattern from the circular region of part b.
Reprinted with permission from Phys. Copyright The American Physical Society. Mattei et al. They found that the sequential alloying directly forms alloyed Au-Cu colloids caused by an enhanced diffusion of copper in small gold clusters during implantation. Annealing of the formed Au-Cu alloy nanoclusters in oxygen atmosphere leads to separation of gold and copper by the possible reason that Cu is extracted from the nanoclusters by chemical interaction with the incoming oxygen Figure 2.
Another dealloying is caused by irradiation with light ions, which forms vacancies in the nanocrystal and 44 Focused ion beam systems resultantly Au diffuses preferentially. A similar procedure of subsequent implantation of two elements and post annealing was successfully applied by White et al.
Klimenkov et al. The crystallization process depends on irradiation dose and dose rate. Irradiation with a dose above 3. For an irradiation dose rate above this value, the nanocrystals are fragmented into twinned and multiple particles.