ABSTRACTS FROM THE MICROSCIENCE 2002 CONFERENCE

FIB Specimen Preparation: methods, problems and artefacts
S. B. Newcomb, Materials & Surface Science Institute, University of Limerick, Limerick, Ireland
Focused ion beam (FIB) microscopes have made a significant impact over the last few years and their industrial usage is now paralleled by the activities of a number of academic laboratories. FIBs have found widespread use in the preparation of TEM samples, for example, and this has led to a diversification of the applications of TEM itself. Some of the ways in which samples for both cross-sectional and plan-view TEM examination can be prepared using a FIB will be described and the range of problems that can be usefully addressed will be illustrated with examples taken from recent TEM studies. Whilst FIB will be shown to have a number of advantages over traditional methods for TEM sample preparation, some of the problems that can occur will also be described and techniques by which artefacts can be minimised discussed.

Optimising Focused Ion Beam Performance
Phil D. Prewett, Research Centre for MicroEngineering and Nanotechnology, School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

The ultimate probe resolution and current of a focused ion beam system are determined by the demagnification of the focusing column and the brightness of the source. However, in most applications the system is operated in the chromatic aberration dominated mode with the probe performance determined by the chromatic aberration coefficient of the lens and the chromatic weighted angular intensity of the source. The energy spread depends upon the ion equivalent of the electron Boersch Effect broadening as described by the work of Knauer1,2 . This predicts two modes of dependence on source current, depending on whether the ion flow is wholly laminar or contains crossing trajectories and has been confirmed experimentally by Prewett et al3. The normal assumption of a Gaussian round beam is a simplification and, in practice, there is evidence of a halo effect surrounding the central high intensity portion of the probe. This is of particular significance in FIB microfabrication applications, being responsible for a resolution-limiting "proximity effect". One of the most important applications of FIB microfabrication is the repair of photomasks and reticles for optical lithography of integrated circuits, for which the probe resolution and staining due to gallium ion implantation are key issues. The latter has largely been overcome for current technologies through the use of carefully designed scan algorithms and etch enhancing gases. However the challenges of 193nm are still being addressed and the future use of 157nm lithography with new mask materials will require further development and optimisation of FIB tools and processes.

1. W. Knauer, Optik 54, 1979, 211
2. W. Knauer, Optik 59, 1981, 335
3. P.D. Prewett, D.J. McMillan, D.K. Jefferies and G.L.R. Mair, Proc SPIE 393, 1983, 120

FIB specimen preparation: The lift-out method

Lucille A. Giannuzzi, Mechanical Materials and Aerospace Engineering, UCF/Cirent Materials Characterization Facility, University of Central Florida, 12443 Research Parkway, Suite 305, Orlando, FL 32816, USA

The focused ion beam instrument (FIB) has been used for site-specific specimen preparation for transmission electron microscopy (TEM) and other analytical instruments. The lift-out (LO) technique consists of preparing a thin membrane via FIB milling from a bulk sample and then transporting the specimen to e.g. a TEM for further analysis. The LO technique may be further categorized into either the "ex situ" method or the "in situ" method depending on whether the FIB prepared specimen is removed outside of the FIB vacuum chamber, or from within the FIB vacuum chamber. Details on the LO techniques will be given, advantages and disadvantages of the techniques will be summarized, and numerous examples from a wide range of materials from both the physical sciences and biological sciences will be presented.

Damage layers in III-V semiconductors following Focused Ion Beam Milling

P.R. Munroe and S. Rubanov, Electron Microscope Unit, University of New South Wales, Sydney NSW 2052, Australia

The structure, thickness and chemistry of the damage layers created in transmission electron microscope specimens prepared by focused ion beam milling in several III-V semiconductor materials, including GaAs, InAs and InP, have been determined. Sidewall damage on cross-sectional TEM specimens consists of films, typically ~20 nm in thickness, amorphous in structure and containing low concentrations (1-2%) of implanted gallium. These layers cause minor hindrance to HREM imaging. However, the damage layers in plan-view specimens are up to 60nm thick and include microcrystalline regions. These crystallites are formed through recrystallization of the amorphous films, associated with local heating from the ion beam. In general, the experimentally determined thicknesses of these damage layers are slightly larger than those theoretically predicted by Monte Carlo simulations. This is attributed to greater penetration and knock-on damage through pre-existing amorphous films, created by the ion beam during initial milling, rather than through a fully crystalline substrate.

Final polishing of FIB-prepared specimen by in-situ ion milling in a Transmission Electron Microscope (TEM)

Claus Burkhardt1, Peter Gnauck2, Erich Plies3 and Wilfried Nisch1, 1 NMI Natural and Medical Science Institute, Markwiesenstraße 55, 72770 Reutlingen, Germany, 2 LEO Elektronenmikroskopie GmbH, 73446 Oberkochen, Germany, 3 University of Tübingen, Institute of Applied Physics, 72076 Tübingen, Germany

We have developed a low voltage focused ion beam system (LVFIB) to be used for localized ion milling in the specimen stage of a TEM at low energies. The main parts of the system are an ion source, transfer optics, deflection unit, power supply and electronics. The system is controlled by GUI Software, including control of the ion gun, scanning and deflection, image registration and image storage. By recording a secondary electron image of the region of interest, the user may precisely select small areas on the specimen for further ion milling.
The performance of the system was tested with the 200 kV TEMs JEOL 2000 FXII and LEO 922 OMEGA. The system proved to be suitable for controlled thinning of selected specimen areas, specimen cleaning and removing of oxide layers inside the TEM. A particular advantage of low energy ion milling is that there is less amorphization and implantation of gallium. Therefore in-situ milling is well suited for final polishing of FIB-prepared specimen. The build up of an amorphous layer can be reduced by employing low ion energy (< 5keV) and operating at low angle of incidence (<10°) under direct observation in the TEM.

We thank BAL-TEC AG for financial support.

 

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