Focused Ion Beam Microscopy
Focused ion beam (FIB) microscopy operates in a similar way as a scanning electron microscope (SEM) in that a fine beam of in this case Gallium ions is scanned over the surface of the material, generating secondary signals in form of electrons or ions which can then be used to visualize the samples's morphology. However, due to the high ion mass the beam additinonally sputters material off the surface providing the opportunity to expose sub-surface structures. The ion beam can be focused down to a few nanometer allowing extremely site specifc ablation of material. By simultaneously flowing specific gases over the sample it is possible to either enhance the etch rate or conversely deposit materials such as platinum, tungsten or carbon.
Modern dual beam systems combine SEM and FIB in one instrument. The ion beam can efficiently remove volumes in the order of several ten micron cubed. By doing this in a controlled fashion the material can be exposed slice by slice and correspondingly imaged using various secondary signals with the SEM. A three-dimensional volume rendering is then achieved by stacking the 2D data (serial sectioning).
FIB has become a quintessential tool in modern transmission electron microscopy due to its unique ability to extract TEM samples from very specific regions of the material. The technique can be applied to a wide variety of materials. Modern ion columns are able to adequately focus ion beams with energies down to 1keV opening the opportunity to significantly reduce the surface damage introduced by gallium ion implantation. As a result high-quality samples thinner than 50nm can be prepared which are accessbile to atomic-resolution microscopy.
Another important application is the preparation of samples for atom probe tomography (APT, see chemical analysis tools section), which makes use of the capability to extract material, transport it to a specific holder and apply circular milling patterns to prepare a 100nm thin needle with a tip radius of curvature of a few ten nanometer.
FEI Helios Dualbeam Nanolab 650
The instrument is equipped with an Elstar field emission SEM for nanometer resolution imaging and a Tomahawk FIB column operating from 30kV down to 500V. Ion currents range from 7pA, providing a minimal beam size of 7nm, to 22nA which can be used for substantial milling tasks. The very stable piezo-driven stage can operate on an area of 150 x 150 mm.
Attached are currently a gas source for platinum deposition as well an Omniprobe manipulator that is typically used for transfer of TEM and APT samples.
Tri-BEAM (FEI Dualbeam Versa with Laser)
Under construction. Estimated release 2015.
The microscopy facility houses techniques that provide 3D structural and chemical information on various lenght scales. Very roughly categorized they provide data for volumes of 100 nm (APT), 1 micron (electron tomography, TEM) and 10 micron (FIB) edge length. The Tri-beam system aims at increasing that length towards 100 micron and more.
The new instrument, developed in the research group of Prof. Tresa Pollock in the Materials Department, augments a standard FIB with a scanning femtosecond laser. Similar to serial sectioning in the FIB high-power laser pulses consecutively ablate thin layers of material. Due to the higher milling efficiency larger volumes can be removed and the analyzed volume increased relative to FIB.
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