**András E. Vladár, K. P. Purushotham and Michael T. Postek
National Institute of Standards and Technology (NIST), 100 Bureau Dr. Stop 8212, Gaithersburg, MD 20899

Paper presented at SPIE Advanced Lithography February 2008 San Jose, CA Abstract 6922-41, page 75

**Dr. Vladár is the leader of the SEM Metrology Project at NIST. He is an expert in scanning electron microscope-based dimensional metrology and one the best known research scientists and a technical leader of this field. He received his M.S. (1977) and Ph.D. (1984) at the Technical University in Budapest, Hungary. From 1995 - 1999, he was a metrology engineer and member of the technical staff of the Hewlett-Packard ULSI Research Laboratory, Palo Alto, CA. Since 1999 he works at NIST as a permanent member of the staff.

Electron beam-induced contamination is becoming one of the most bothersome problems of the scanning electron microscopes (SEMs). Even in clean-vacuum instruments it is possible that the image gradually darkens because a polymerized hydrocarbon layer with low secondary electron yield is deposited. This contamination layer can get so thick that it changes the size and shape of the small structures of current and future state-of-the art ICs. This greatly disturbs the measurement process and the erroneous results can lead to wrong process control decisions. Unfortunately, there is no specification or requirement that a well-working SEM must meet. NIST has developed cleaning procedures and a viable contamination specification that offer an effective solution for this problem.

The sources of the contamination are the sample itself, the vacuum system and the sample stage in the sample chamber. Unless special measures are taken, all these contribute to the problem to some extent. The newly developed NIST contamination specification calls for the following procedure: select a sample with amorphous Si patterns on a Si wafer, set the instrument parameters (landing energy, beam current, etc.) to the values that are used for the best resolution images, sharpen the electron beam, go to a new location and take one image at 100 000 times magnification. Save the image, and go up to 200 000 times magnification and continuously image for 10 minutes, and finally, go back to 100 000 times magnification and take another image. If there is any visible darkening, frame, any structure beyond the sample itself in the middle of the last image, the instrument fails to meet this specification.

If the specification was not met, first clean the sample in the mixture of 3:1 concentrated sulphuric acid to 30 % hydrogen peroxide solution (acid piranha solution). This ferocious oxidizer will clean all hydrocarbon residues from the sample in less then 30 minutes. If the instrument -with the clean sample- fails the test again, it needs to get cleaned with a low-energy plasma cleaning process.

For low-energy plasma cleaning of the sample chamber and stage the Evactron* Model C was used. It was found that at the beginning an overnight cleaning was needed to meet the contamination specification. Later, as the instrument gradually cleaned up, 1 hour or even only 15 minute long cleaning procedures were sufficient. It is important to point out that the nascent (ionized atomic) oxygen generated by the plasma cleaner oxidizes many materials, but the process advantageously is very effective on hydrocarbon residues. It is recommended to use the minimum, but sufficient time and plasma current. This calls for a reliable and regular monitoring of the contamination performance of the SEM. It was found that for this purpose Si wafer or chip samples with amorphous Si patterns on them were suitable, and this type of sample withstands the piranha solution cleaning very well, even for overnight cleanings.

The newly developed NIST cleaning procedures and a viable contamination specification offer an effective solution for the electron beam-induced contamination problem, and can eliminate erroneous results that can lead to wrong process control decisions.

*Certain commercial equipment is identified in this report to adequately describe the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the equipment identified is necessarily the best available for the purpose.