contact persons: Alan Kubis (firstname.lastname@example.org);
Tim Herlihy (email@example.com)
The Focused Ion Beam (FIB) is a scanning microprobe similar
to a Scanning Electron Microscope (SEM). In both case a beam, a 30 keV Ga+
ion beam in the case of the FIB, is rastered over a surface and the
secondary electron or ion intensity is displayed on a monitor that is
synchronously scanned with the beam. The image is produced by the electron
(or ion) contrast due to differences in elements, Z contrast,
crystallographic orientation, channeling contrast, or topography. The
advantage of the FIB is that since an ion beam is used material can be
removed from the sample through ion milling. This allows shorts in
electronic circuits to be fixed as well as features to be milled into
materials for a variety of purposes.
In our lab the FIB is used for a variety of projects.
Tomographic reconstructions of three-dimensional volumes are generated at
the micro and nanoscale level allowing a direct measure of material
structures. Features are being made on silicon wafers to help direct the
growth of Germanium quantum dots (QD). These QDs are being studied as an
alternative to current methods of producing microelectronic circuits. The
FIB is also used to create printing head masters for micro-contact printing
applications. The large depth of focus allows patterning of curved as well
as planer surfaces. Transmission Electron Microscopy (TEM) foils are also
made using the FIB. The FIB is unique in that it allows a foil to be
extracted from a material with out damaging the sample except in a very
localized region. This has become a very common sampling technique in the
Schematic of the Focused Ion Beam Column
The FIB is able to image at fairly high resolution because it
uses a liquid metal ion source in which a highly focused metal ion beam is
electrostaticly extracted from a liquid metal reservoir formed at the tip of
a needle. The small reservoir forms a Taylor cone at the tip of the needle
and the ions are produced through field evaporation and accelerated to 30
kV. The very small diameter of the emission region allows for a spatially
coherent ion beam to be formed with a small energy distribution allowing
focusing of the beam to less than ten nanometers[i],[ii],[iii],[iv].
In the FEI FIB-200 the beam is focused with an electrostatic lens onto one
of a series of aperture that define the current of the beam striking the
sample. A quadrupole centers the beam in the column while a subsequent
octapole is used to stigmate and raster over the sample. Finally a second
lens focuses the ion beam onto the sample surface. This system can produce
ions beam currents from 1-11500 pA with nominal diameters of 8-500 nm
respectively. Images can be summed to reduce noise and stored for further
processing and analysis.
Secondary Electron Image of UVa seal Milled into gold surface by the FIB
The image resolution of this instrument is limited to
1024x1024 pixels. The system is also equipped with a Quadrupole Mass
Analyzer to obtain elemental information as a SIMS analyzer.
Al ion map of a Vertical Cavity Surface Emitting Laser(VCSEL) structure.
SIMS analyzer has a range of 1-200 m/z and can be set up to obtain mass
spectra, elemental depth profiles or elemental ion maps of surfaces. The
image resolution of the ion maps is limited to 512x512 pixels.
R.G. Forbes: Vacuum, 48(1), 1997, pp. 85-97.
J. Gierak, A. Septier, C. Vieu: Nucl. Instrum. Methods Phys. Res. A,
1999, 427, pp. 91-98.
K. Gamo: Nucl. Instrum. Methods Phys. Res. B, 1997, 121, pp. 464-469.
J. Orloff: Rev. Sci. Instrum., 1993, 64(5), pp. 1105-1130.