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Photograph of a FIB workstation

Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor and materials science fields for site-specific analysis, deposition, and ablation of materials. An FIB setup is a scientific instrument that resembles a scanning electron microscope (SEM). However, while the SEM uses a focused beam of electrons to image the sample in the chamber, an FIB setup instead uses a focused beam of ions. FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams. FIB should not be confused with using a beam of focused ions for direct write lithography (such as in proton beam writing), where the material is modified by different mechanisms.

Contents

Ion beam source

Most widespread are instruments using Liquid-metal ion sources (LMIS), especially gallium ion sources. Ion sources based on elemental gold and iridium are also available. In a Gallium LMIS, gallium metal is placed in contact with a tungsten needle and heated. Gallium wets the tungsten, and a huge electric field (greater than 108 volts per centimeter) causes ionization and field emission of the gallium atoms.

Source ions are then accelerated to an energy of 5-50 keV (kiloelectronvolts), and focused onto the sample by electrostatic lenses. LMIs produce high current density ion beams with very small energy spread. A modern FIB can deliver tens of nanoampers of current to a sample, or can image the sample with a spot size on the order of a few nanometers.

Principle

block diagram
the principle of FIB

Focused ion beam (FIB) systems have been produced commercially for approximately twenty years, primarily for large semiconductor manufacturers. FIB systems operate in a similar fashion to a scanning electron microscope (SEM) except, rather than a beam of electrons and as the name implies, FIB systems use a finely focused beam of ions (usually gallium) that can be operated at low beam currents for imaging or high beam currents for site specific sputtering or milling.

As the diagram on the right shows, the gallium (Ga+) primary ion beam hits the sample surface and sputters a small amount of material, which leaves the surface as either secondary ions (i+ or i-) or neutral atoms (n0). The primary beam also produces secondary electrons (e-). As the primary beam rasters on the sample surface, the signal from the sputtered ions or secondary electrons is collected to form an image.

At low primary beam currents, very little material is sputtered; modern FIB systems can easily achieve 5 nm imaging resolution (world record: 2.5 nm with Cobra FIB from Orsay Physics). At higher primary currents, a great deal of material can be removed by sputtering, allowing precision milling of the specimen down to a sub micron scale.

If the sample is non-conductive, a low energy electron flood gun can be used to provide charge neutralization. In this manner, by imaging with positive secondary ions using the positive primary ion beam, even highly insulating samples may be imaged and milled without a conducting surface coating, as would be required in a SEM.

Until recently, the overwhelming usage of FIB has been in the semiconductor industry. Such applications as defect analysis, circuit modification, mask repair and transmission electron microscope sample preparation of site specific locations on integrated circuits have become commonplace procedures. The latest FIB systems have high resolution imaging capability; this capability coupled with in situ sectioning has eliminated the need, in many cases, to examine FIB sectioned specimens in the SEM. [1]

Why Ions ?

The most fundamental difference between FIB and focused electron beam techniques such as SEM, STEM or EBID is the use of ions instead of electrons, and this has major consequences for the interactions that occur at the sample surface. The most important characteristics and the consequences for the sample interaction are :
ions are larger than electrons

  • Because ions are much larger than electrons, they cannot easily penetrate within individual atoms of the sample. Interaction mainly involves outer shell interaction resulting in atomic ionization and breaking of chemical bonds of the substrate atoms.
  • The penetration depth of the ions is much lower than the penetration of electrons of the same energy.
  • When the ion has come to a stop within the material, it is caught in the matrix of the material.

ions are heavier than electrons

  • Because ions are far heavier than electrons, ions can gain a high momentum. For the same energy, the momentum of the ion is about 370 times larger.
  • For the same energy ions move a lot slower than electrons. However, they are still fast compared to the image collection mode and in practice this has no real consequences.
  • The magnetic lenses are less effective on ions than they would be on electrons with the same energy. As a consequence the focused ion beam system is equipped with electro-static lenses and not with magnetic lenses.

ions are positive and electrons are negative

  • This difference has negligible consequences and is taken care of by the polarity of fields to control the beam and accelerate the ions.

In summary, ions are positive, large, heavy and slow, whereas electrons are negative, small, light and fast. The most important consequence of the properties listed above is that ion beams will remove atoms from the substrate and because the beam position, dwell time and size are so well controlled it can be applied to remove material locally in a highly controlled manner, down to the nanometer scale. [2]

Technology

Block diagram and real FIB
Block diagram and real FIB

Usage

McMaster University name and logo "tattooed" in two sizes on a hair.

Unlike an electron microscope, FIB is inherently destructive to the specimen. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Gallium atoms will also be implanted into the top few nanometers of the surface, and the surface will be made amorphous.

Because of the sputtering capability, the FIB is used as a micro-machining tool, to modify or machine materials at the micro- and nanoscale. FIB micro machining has become a broad field of its own, but nano machining with FIB is a field that still needs developing. The common smallest beam size is 2.5-6 nm.

FIB tools are designed to etch or machine surfaces, an ideal FIB might machine away one atom layer without any disruption of the atoms in the next layer, or any residual disruptions above the surface. Yet currently because of the sputter the machining typically roughens surfaces at the submicrometre length scales.[3][4] An FIB can also be used to deposit material via ion beam induced deposition. FIB-assisted chemical vapor deposition occurs when a gas, such as tungsten hexacarbonyl (W(CO)6) is introduced to the vacuum chamber and allowed to chemisorb onto the sample. By scanning an area with the beam, the precursor gas will be decomposed into volatile and non-volatile components; the non-volatile component, such as tungsten, remains on the surface as a deposition. This is useful, as the deposited metal can be used as a sacrificial layer, to protect the underlying sample from the destructive sputtering of the beam. From nanometers to hundred of micrometers in length, tungsten metal deposition allows to put metal lines right where needed. Other materials such as platinum, cobalt, carbon, gold, etc., can also be locally deposited.[3][4] Gas assisted deposition and FIB etching process are shown below. [5]

Gas assisted deposition process
Gas assisted deposition process
Gas assisted FIB etching process
Gas assisted FIB etching process


example of a 3D nanostructure that can be obtained
example of a 3D nanostructure that can be obtained
SEM image of a thin TEM sample milled by FIB.
Orsay Physics Canion 31 Plus UHV FIB on a TOF-SIMS 6600 from Physical Electronics
enhanced and selective etching
enhanced and selective etching

FIB is often used in the semiconductor industry to patch or modify an existing semiconductor device. For example, in an integrated circuit, the gallium beam could be used to cut unwanted electrical connections, and/or to deposit conductive material in order to make a connection. The high level of surface interaction is exploited in patterned doping of semiconductors. FIB is also used for maskless implantation.

The FIB is also commonly used to prepare samples for the transmission electron microscope. The TEM requires very thin samples, typically ~100 nanometers. Other techniques, such as ion milling or electropolishing can be used to prepare such thin samples. However, the nanometer-scale resolution of the FIB allows the exact thin region to be chosen. This is vital, for example, in integrated circuit failure analysis. If a particular transistor out of several million on a chip is bad, the only tool capable of preparing an electron microscope sample of that single transistor is the FIB.[3][4]

The drawbacks to FIB sample preparation are the above-mentioned surface damage and implantation, which produce noticeable effects when using techniques such as high-resolution "lattice imaging" TEM or electron energy loss spectroscopy. This damaged layer can be minimised by FIB milling with lower voltages, or by further milling with a low-voltage argon ion beam after completion of the FIB process.[6]

FIB preparation can be used with cryogenically frozen samples in a suitably equipped instrument, allowing cross sectional analysis of samples containing liquids or fats, such as biological samples, pharmaceuticals, foams, inks, and food products [7]

FIB is also used for Secondary ion mass spectrometry (SIMS). The ejected secondary ions are collected and analyzed after the surface of the specimen has been sputtered with a primary focused ion beam.

FIB imaging

FIB secondary electron image
FIB secondary ion image

At lower beam currents, FIB imaging resolution begins to rival the more familiar scanning electron microscope (SEM) in terms of imaging topography, however the FIB's two imaging modes, using secondary electrons and secondary ions, both produced by the primary ion beam, offer many advantages over SEM.

FIB secondary electron images show intense grain orientation contrast. As a result, grain morphology can be readily imaged without resorting to chemical etching. Grain boundary contrast can also be enhanced through careful selection of imaging parameters. FIB secondary ion images also reveal chemical differences, and are especially useful in corrosion studies, as secondary ion yields of metals can increase by three orders of magnitude in the presence of oxygen, clearly revealing the presence of corrosion[8]

History

History of FIB technology

  • 1975: The first FIB systems based on field emission technology were developed by Levi-Setti[9][10] and by Orloff and Swanson[11] and used gas field ionization sources (GFISs).
  • 1978: The first FIB based on an LMIS was built by Seliger et al. [12].

Physic of LMIS

  • 1600: Gilbert documented that fluid under high tension forms a cone.
  • 1914: Zeleny observed and filmed cones and jets
  • 1959: Feynman suggested the use of Ion Beams.
  • 1964: Taylor produced exactly conical solution to equations of Electro Hydro Dynamics (EHD)
  • 1975: Krohn and Ringo produced first high brightness ion source : LMIS

Some pioneers of LMIS & FIB [13]

  • Mahoney (1969)
  • Sudraud et al Paris XI Orsay (1974)
  • University of Oxford Mair (1980)
  • Culham UK,Roy Clampitt Prewett (1980)
  • Oregon Graduate Center L.Swanson (1980)

Helium ion microscope (HeIM)

The other ion source seen in commercially available instruments is a Helium ion source, which is less inherently damaging to the sample than Ga ions. As helium ions can be focused into a smaller probe size and provide a much smaller sample interaction than electrons in the SEM, the He ion microscope can generate equal or higher resolution images with good material contrast and a higher depth of focus. Commercial instruments are capable of sub 1 nm resolution[14][15].

Wien filter in focused ion beam setup

ExB Column from Orsay Physics
ExB Column from Orsay Physics

Imaging and milling with Ga ions always result in Ga incorporation near the sample surface. As the sample surface is sputtered away at a rate proportional to the sputtering yield and the ion flux (ions per area per time), the Ga is implanted further into the sample, and a steady-state profile of Ga is reached. This implantation is often a problem in the range of the semiconductor where silicon can be amorphised by the gallium. In order to get an alternative solution to Ga LMIsources, mass-filtered columns have been developed, based on a Wien filter technology. Such sources include Au-Si, Au-Ge and Au-Si-Ge sources providing Si, Cr, Fe, Co, Ni, Ge, In, Sn, Au, Pb and other elements.

diagram showing the way the masses are selected
Mass selection in the FIB column

The principle of a Wien filter is based on the equilibrium of the opposite forces induced by perpendicular electrostatic and a magnetic fields acting on accelerated particles. The proper mass trajectory remains straight and passes through the mass selection aperture while the other masses are stopped. [16]

Besides allowing the use of sources others than gallium, these columns can switch from different species simply by adjusting the properties of the Wien filter. Larger ions can be used to make rapid milling before refining the contours with smaller ones. The user also benefits from the possibility to dope his sample with elements of suitable alloy sources.

The latter property has found great interests in the investigation of magnetic materials and devices. Khizroev and Litvinov have shown, with the help of magnetic force microscopy (MFM), that there is a critical dose of ions that a magnetic material can be exposed to without experiencing a change in the magnetic properties. Exploiting FIB from such an unconventional perspective is especially favourable today when the future of so many novel technologies depends on the ability to rapidly fabricate prototype nanoscale magnetic devices. [17]

References

  1. ^ "Introduction : Focused Ion Beam Systems". http://www.fibics.com/fib/tutorials/introduction-focused-ion-beam-systems/4/. Retrieved 2009-08-06.  
  2. ^ FEI Company (2006). Focused ion beam technology, capabilities and applications.  
  3. ^ a b c J. Orloff, M. Utlaut and L. Swanson (2003). High Resolution Focused Ion Beams: FIB and Its Applications. Springer Press. ISBN 0-306-47350-X.  
  4. ^ a b c L.A. Giannuzzi and F.A. Stevens (2004). Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques and Practice. Springer Press. ISBN 978-0-387-23116-7.  
  5. ^ Koch, J.; Grun, K.; Ruff, M.; Wernhardt, R.; Wieck, A.D. (1999). Creation of nanoelectronic devices by focused ion beam implantation.  
  6. ^ Principe, E L; Gnauck, P; Hoffrogge, P (2005). "A Three Beam Approach to TEM Preparation Using In-situ Low Voltage Argon Ion Final Milling in a FIB-SEM Instrument". Microscopy and Microanalysis 11. doi:10.1017/S1431927605502460.  
  7. ^ "Unique Imaging of Soft Materials Using Cryo-SDB". http://www.quorumtech.com/Applications/Cryo_Apps_Library/cryo-SDB.pdf. Retrieved 2009-06-06.  
  8. ^ "FIB: Chemical Contrast". http://www.fibics.com/MS_FIBApp_SII_SteelCorrosion.html. Retrieved 2007-02-28.  
  9. ^ Levi-Setti, R. (1974). "Proton scanning microscopy: feasibility and promise". Scanning Electron Microscopy: 125.  
  10. ^ W. H. Escovitz, T. R. Fox and R. Levi-Setti (1975). "Scanning Transmission Ion Microscope with a Field Ion Source". Proceedings of the National Academy of Sciences of the United States of America 72 (5): 1826. doi:10.1073/pnas.72.5.1826.  
  11. ^ Orloff, J. and Swanson, L., (1975). "Study of a field-ionization source for microprobe applications". J. Vac. Sci. Tech. 12: 1209. doi:10.1116/1.568497.  
  12. ^ Seliger, R., Ward, J.W., Wang, V. and Kubena, R.L. (1979). "A high-intensity scanning ion probe with submicrometer spot size". Appl. Phys. Lett. 34: 310. doi:10.1063/1.90786.  
  13. ^ C.A. Volkert and A.M. Minor, Guest Editors (2007). "Focused Ion Beam: Microscopy and Micromachining". MRS Bulletin 32: 389. http://www.nanolab.ucla.edu/FIB/pdf/MRS_Bulletin_2007_FIB_machining.pdf.  
  14. ^ "Carl Zeiss press release". 2008-11-21. http://www.smt.zeiss.com/C1256A770030BCE0/WebViewAllE/F4BF4E46C9379912C1257508002B9F7C. Retrieved 2009-06-06.  
  15. ^ "The Southampton Nanofabrication Centre: Helium Ion Microscope". http://www.southampton-nanofab.com/orion.php. Retrieved 2009-06-06.  
  16. ^ Orsay physics work on ExB mass filter Column, 1993  
  17. ^ Khizroev S.; Litvinov D. (2004). "Focused-ion-beam-based rapid prototyping of nanoscale magnetic devices". Nanotechnology 15: R7. doi:10.1088/0957-4484/15/3/R01.  

Further reading

  • Mackenzie, R A D (1990). Nanotechnology 1: 163. doi:10.1088/0957-4484/1/2/007.  
  • J. Orloff (2009). Handbook of Charged Particle Optics. CRC Press. ISBN 978-1-4200-4554-3.  

External links

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