Blast from the Past: TEM Sample Preparation Then (1999) and Now
by Cheryl Hartfield
The semiconductor industry was and continues to be a driving force in the development of FIB-based TEM preparation techniques. The International Symposium on Test and Failure Analysis (ISTFA), held every November, is one of the premier knowledge exchange forums for characterization and failure analysts in the IC industry. Given the prevalence of topical subjects on focused ion beams and TEM sample preparation at this venue, it is interesting to revisit highlights from ISTFA 1999. This article reviews advances that were new to the state of TEM preparation technology 10 years ago, and what has happened since then.
Below is an excerpt from an ISTFA 1999 trip report:
"Advances in TEM sample prep: Length of time required to prepare TEM samples has always been a bottleneck for FA execution, so faster ways of producing samples for TEM preparation in the FIB are being developed as TEM becomes a heavily used analysis technique due to requirements for inspection of fine geometries in current and future nodes. A new system is available that is completely automated and can prepare in 20 min. or so a sample suitable for final TEM prep in the FIB, thus saving 4-8 hours of preparation time. The technique is based on micro-cleaving technology and begins with a piece of wafer which is cut to produce a 20um wide sliver, which is then automatically mounted to a TEM grid. 3 to 4 samples an hour can be prepared. The system is developed and sold by SELA. The final sample prep is then performed in the FIB, taking 1-2h depending on the sample. An alternate approach is to manually cut out the target area in the FIB, using a fine probe inside the vacuum chamber as an attachment tool to remove and attach the sample on the grid. Tom Moore developed and demonstrated one such technique within CRL in 1995. The in-situ FIB approach is more versatile; it will work on packaged samples as well as wafers. Lucent (Orlando) have just installed a Hitachi FIB with this capability, and IBM (San Jose) has just purchased an external add-on for their FEI830 FIB from Omniprobe, Inc. to enable them with this capability."
So what happened in these areas since then? The cleaving technology was a success, with many semiconductor companies purchasing micro-cleaving technology. It proved popular for preparing SEM samples and H-Bar samples for FIB-based TEM sample preparation. With the advent of "strained Si" for improved performance at the C027 semiconductor node, cleaving lost some of its oomph due to a change in wafer processing that resulted in features being rotated at 45 degrees diagonally from the traditional 110 cleavage planes. For appropriate samples, cleaving is still popular today, and SELA was acquired by CAMTEK in November 2009.
In retrospect, 1999 is the year of "in situ lift-out". This year saw the world's first sales of in situ lift-out nanomanipulators by 2 commercial entities (Omniprobe, Hitachi). The back cover of the 1999 ISTFA final program featured an ad (excerpt, left image) by Materials Analytical Service, Inc (MAS), a service lab, advertising in situ lift-out capability (MAS is now Materials Analytical Service, LLC, having sold its Raliegh and Santa Clara offices supporting semiconductor analysis to Evans Analytical Group, LLC (EAG) in December 2006). In situ lift-out is now a well known technique that has grown beyond semiconductors to be applied to the study of space materials, biological samples, fossils, catalysts, geological specimens...almost any type of sample one can think of. Its flexibility and high preparation success rate for all types of samples, including compatibility with 20nm fail site isolation, continue to drive demand. Since 1999, closed loop manipulators have been introduced that are amenable to automation, and allow saved positions for easy recall when performing manipulations. Additional developments have made it easy to lift-out a sample and turn it to an angle allowing plan-view sample preparation or sideways milling preparation, all done within the FIB quickly using only a single lift-out step (Hartfield et al., Microsc Microanal 16(Suppl. 2), 2010, pp 6-17). The Total Release(TM) method (US6570170) for cutting and extracting lift-out samples accelerated TEM sample preparation times and enabled implementation of these simple repositioning solutions, which require tilting and rotating the sample using stage movements, prior to attaching the tip for lift-out. Otherwise, besides a rotation axis on the nanomanipulator, no other special equipment is needed.
Additional ISTFA 1999 trip report excerpts and analysis will be continued in the next blog post.
Top photo courtesy nasaimages.org .
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