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Focused Ion Beam

 

 

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contact persons: Alan Kubis (ajk7s@virginia.edu);  Tim Herlihy (tjh5z@virginia.edu)

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 microelectronics industry.

 

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.

 The 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.


 

[i] R.G. Forbes: Vacuum, 48(1), 1997, pp. 85-97.

[ii] J. Gierak, A. Septier, C. Vieu: Nucl. Instrum. Methods Phys. Res. A, 1999, 427, pp. 91-98.

[iii] K. Gamo: Nucl. Instrum. Methods Phys. Res. B, 1997, 121, pp. 464-469.

[iv] J. Orloff: Rev. Sci. Instrum., 1993, 64(5), pp. 1105-1130.