A review of focused ion beam milling techniques for TEM specimen preparation
Introduction
In the past few years, we have observed an increase in the use of the focused ion beam (FIB) tool for the preparation of transmission electron microscopy (TEM) specimens as witnessed by an increase of publications in this area. An FIB instrument may be simply referred to as a “fib” in common parlance. While this technique is still in its infancy, many investigators have utilized this method for preparing electron microscopy specimens from a wide range of materials including semiconductors, metals, ceramics, polymers, biological materials, and tissues.
Section snippets
The focused ion beam instrument
A FIB instrument looks and operates much like a scanning electron microscope (SEM). Both instruments rely on a focused beam to create a specimen image; an ion beam for the FIB and an electron beam for the SEM. For both instruments, the intensity of the secondary electrons produced at each raster position of the beam is displayed to create an image of the sample. In the FIB, secondary ions may also be detected and used to construct an image of the sample. Images having magnifications up to
Cross-section method
Most of the literature listed in the reference section refers to the conventional FIB method of TEM specimen preparation. A detailed description of this method is given by Stevie et al. (1995). Initial specimen preparation must be performed before the sample is placed into the FIB using the conventional TEM specimen preparation method. An area of interest is located and cut to <3 mm in length. The sample is then mechanically polished as thin as possible (e.g., <50 μm) to reduce the FIB time. It
The FIB “lift-out” technique
The lift-out technique requires little or no initial specimen preparation (Overwijk et al., 1993, Leslie et al., 1995, Herlinger et al., 1996, Stevie et al., 1998, Sheng et al., 1997, Giannuzzi et al., 1997, Giannuzzi et al., 1997a). The only requirement for the lift-out technique is that the bulk sample must fit inside the FIB specimen chamber. Insulating materials are sometimes pre-coated with carbon or chromium to prevent charging. In the lift-out technique, the electron transparent thin
FIB preparation tricks of the trade
The preparation of a TEM specimen from a Si-based integrated circuit has become routine. The overall quality of the focused ion milling will be material dependent. The electron transparent membrane walls are not perfectly parallel to each other. A cross-section of any FIB prepared TEM specimen shows broadening at the base and the angle of this broadening varies with the atomic number. This sputtering phenomenon is known as the “classic V-shape” and its shape is a function of the competition
Specimen preparation FIB induced damage
There are well known Ga+ beam induced damage artifacts that exist in FIB TEM specimens. Ga ions are implanted and mix into the specimen as a result of the sputtering mechanism. Therefore, it is possible that Ga may influence local compositions within the specimen, although the extent of this effect is not known. Hence, care should be taken when performing analytical microscopy. The Ga implantation also amorphizes the outer layer of the specimen. There is still some uncertainty in the literature
A comparison of the FIB methods
The primary advantage that is common to both methods is that the FIB tool allows for rapid production of site specific (to within ∼200 nm) TEM specimens. The disadvantages common to both techniques are the FIB induced artifacts described before.
The conventional FIB method requires significant (e.g., destructive) initial specimen preparation before insertion into the FIB, while the lift-out technique requires little or no initial specimen preparation. Thus, the lift-out specimen may be used for
Advances in FIB instrumentation
Manufacturers that produce FIB instruments which we are aware of include (in alphabetical order): FAI, FEI/Philips, Hitachi, JEOL, Metron/Seiko, Micrion, NANOFAB, and Schlumberger. Equipment manufacturers are exploring the introduction of liquid metal ion sources other than Ga. Special TEM/FIB specimen holders and instruments are available so that conventional specimens can be produced and alternated between instruments without additional handling concerns (see e.g., Gatan Inc., Hitachi).
Conclusions and final comments
An FIB tool may cost between 30 and 80% of the total cost of a TEM. This is a lot of money to spend for a specimen preparation tool, considering that the TEMs we use to analyze these specimens cost the same order of magnitude. However, materials systems are becoming more complex, and the increasingly smaller critical dimensions of features necessitate the use of the FIB. The use of the FIB is slowly expanding outside the microelectronics industry and into the “general” industrial population and
Acknowledgements
The support of the following agencies is greatly acknowledged: NDSEG Fellowship P-34862-RT-NDF, NSF Award 9703281, the I4 High Tech Corridor Council, Enterprise Florida, Cirent Semiconductor. We would also like to thank the support of Jeff Bindell, who has been a proponent of the UCF/Cirent partnership. We have been fortunate to collaborate with the following Cirent staff and UCF students: Steve Brown, Jennifer Drown-MacDonald, Rich Irwin, Brian Kempshall, Brenda Prenitzer, Teri Shofner and
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