FIB Sample Preparation Strategies for TEM
by Cheryl Hartfield
Today, FIB sample preparation is spreading to many industries outside semiconductor, including biology, geology, nanophotonics, nanotube and nanowire research, catalyst research, and many others. Dedicated FIB user groups exist in Europe (EFUG) and North America (DC area FIB user group) to share knowledge. Variants of the FIB preparation method have developed in response to needs for automated preparation, faster preparation, and flexible preparation to accommodate specific goals. Key strengths of the different methods are summarized here, while detailed information is available in the references.
Historically, TEM sample preparation of bulk materials was achieved by thinning a solid piece of material by mechanical means such as sawing, polishing, or microtoming. With the advent of the focused ion beam (FIB) microscope in the early 1990's, for the first time non-mechanical thinning with site specificity to 20nm could be performed. The method of FIB sample preparation, with its tremendous advantages in success rates, wide range of material compatability, quality of results, and speed, was rapidly adopted by the semiconductor industry and became a routine method for preparation of TEM samples by the late 1990's.
There are 3 fundamental strategies for FIB sample preparation of specimens destined for TEM inspection: H-Bar sample preparation (left image), ex situ lift-out (EXLO) preparation (center image), and in situ lift-out (INLO) preparation (right image).
(Center image courtesy David Su, TSMC)
Differences Between Lift-out Methods (INLO and EXLO) and the H-Bar Method:
- Generally, lift-out methods are significantly faster than H-Bar preparation (which can take 6-8 hours per sample);
- Lift-out methods do not require mechanical pre-thinning, saving not only time and material, but also extending the reach of analysis to fragile samples and enabling sampling from multiple locations on the same specimen;
- Lift-out methods do not limit the TEM analysis area by producing samples with blocked fields of view.
Advantages of INLO over EXLO methods:
The best lift-out results (success, quality and throughput) are achieved with an optimized nanomanipulator finely tuned for lift-out operations. With in situ lift-out methods, the nanomanipulator operates within the vacuum chamber of the FIB to complete the sample separation and transfer to TEM grid (or other specimen support, such as atom probe posts). This has numerous advantages over EXLO, which typically uses a long glass needle on a benchtop and optical visualization to monitor sample manipulation effected by electrostatic forces:
- Final thinning is performed after securing the sample to the grid, resulting in a cleaner sample
- Ability exists to easily rethin samples, if needed, to provide an optimum sample (for example, EELS samples should generally be 20-30nm thick, and angstrom resolution imaging may require for some materials 10nm thick samples)
- The large depth of field afforded by electron imaging, combined with controlled attachment processes (in situ welding vs. EXLO electrostatic forces) yield high success rates, precise control, conservation of sample orientation, and rapid placement. The entire lift-out and grid attach process is generally completed in 15-30 minutes per sample.
- The success rate of transfer from bulk sample to grid is very high for INLO, making it ideal for unique, "one of a kind" samples, and ideal for any environment (humidity does not impact INLO success, as it does with EXLO success).
- A number of tools and methods have evolved to efficiently change the INLO sample orientation in order to meet demands for higher sample quality, better results and different imaging views.
- INLO not only assists fabrication of TEM samples, it also improves EDX and EBSD data, as well as facilitating atom probe sample preparation.
- The in situ nanomanipulator can also be used for nanomanipulation of nanowires, nanotubes, small particles and other small structures that must be visualized within the electron microscope.
EXLO methods are not without their own merits. EXLO manipulation using vacuum instead of electrostatic forces was developed to achieve more controlled handling that meets the needs of industry for large volume process control sampling. The vacuum-integrated EXLO solution for industry interfaces with automated FIB thinning of multiple samples. FIB thinning can be performed overnight, unattended, and then samples removed from the FIB for ex situ lift-out. The FIB is thus immediately free to process the next batch of samples. Generally, EXLO handling requires robust samples that are thicker than those typically achieved with the INLO method. Vacuum-integrated EXLO is typically implemented where routine large volume STEM imaging is required.
- R.J. Young, P.D. Carleson, T. Hunt, J.F. Walker, Proc. 24th ISTFA Conf. (1998) p. 329.
- L.A. Giannuzzi et al., in Analysis Techniques of Submicron Defects, 2002 Supplement to the EDFAS Failure Analysis Desktop Reference (ASM International, Materials Park, Ohio, 2002) pp. 29-35.
- L.A Giannuzzi, F.A. Stevie, Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques, and Practice (Springer, New York, 2005)
- J.Mayer et al. TEM Sample Preparation and FIB Induced Damage. MRS Bulletin 07 V32 (2007) pp. 400-407
- Langford RM, Rogers M. In situ lift-out: steps to improve yield and a comparison with other FIB TEM sample preparation techniques. Micron. (2008 Dec V39:8) pp.1325-30