The sources of contamination of TEM samples and the means for its reduction

Abstract

Contamination is a long standing problem in electron microscopy [1,2,3]. It can arise from poor vacuum, from sample handling and storage, or from the nature of the sample nature. We are systematically investigating candidate sources of contamination using a common TEM substrate: 10-nm thick Si 3 N 4 membrane, with typical storage methods and practices. We use a dry-pumped Hitachi H 9500 ETEM whose base pressure is1.4x10 -5 Pa without liquid nitrogen (LN 2 ). The measured residual gas is composed mainly of water vapor (8x10 -7 Pa) and N2 (about 3x10 -7 Pa), both decreasing to about 2x10 -7 Pa with LN 2 . We use electron energy-loss spectroscopy (EELS), energy-filtering thickness maps and bright-field TEM imaging in the H9500 to characterize the contamination build up, and SEM imaging in Hitachi S 5500 to image the surface morphology of the contamination dots. The Si 3 N 4 membranes were examined in the TEM as received, after cleaning using a ZONE cleaner [4] with LN 2 trap of the microscope, either cooled (ZONE-LN 2 ) or at room temperature (ZONE-RT), or without LN 2 in the cold trap and after heating the Si 3 N 4 membrane on a hot plate at 50 ºC (Hot-50C) prior to inserting in the microscope. A new Si 3 N 4 membrane was used for each experiment and the variation of the contamination rate was evaluated for 5-min increments of the cleaning process. The membrane was irradiated by a probe of 37 nm FWHM with 0.83 nA total current for 300 seconds, resulting in a 250 C/cm 2 irradiation dose. Five areas were measured for each condition and a thickness map and a bright-field image was then collected from the irradiated area using a broad illumination. For each condition, the experiment was performed on five locations of the Si 3 N 4 membrane and the volume of resulting contamination measured in nm 3 by summing over a 134x134 nm 2 thickness map, subtracting the contribution of the SiN membrane. Figure 1 shows typical thickness map for a dose D = 250 C/cm 2 , together with its profile. Figure 2 shows an SEM image of "as received sample" Si 3 N 4 irradiated at room temperature without LN 2 in the H9500, exhibiting dendritic contamination growth, possibly arising from secondary-emission-induced charging of the insulating sample during the contamination diffusion processes. Figure 3 shows the contamination rate in nm 3 /C for the as received (hereafter referred to as AR, with no cleaning), ZONE-DRY, ZONE-LN 2 and Hot-50C membranes and reveals variations around 70% in difference in the contamination rate of the AR sample. 20 minutes of ZONE cleaning reduced the contamination rate to about 70% of the AR value. The ZONE-LN 2 sample shows decrease to about 50% compared to the same sample in AR state followed by slight increased to about 60% that can be perhaps assigned to transfer between the TEM and the ZONE cleaner. The highest AR contamination rates were observed with the Hot-50C sample. Heating at 50oC reduced the contamination rate to about 44% of the initial AR rate, a value comparable to the ZONE-DRY sample in AR state. The...

Contamination mitigation strategies for scanning transmission electron microscopy

“…Removal rates reported for a system specifically designed for TEM specimen cleaning are somewhat higher. In a similar fashion to plasma cleaners, they produce an initially high removal rate, resulting a 50% reduction after 5mins cleaning, but a much slower rate of removal thereafter (Soong et al, 2012).…”

“…When used in conjunction with liquid nitrogen-cooled cold traps, much cleaner vacuum systems have resulted. Today, the majority of the contamination arrives on the specimen (Egerton et al, 2004;McGilvery et al, 2012;Soong et al, 2012).…”

“…Strategies for dealing with contamination have been developed, and include plasma cleaning (Griffiths A. J. V., 2010;Isabell et al, 1999;McGilvery et al, 2012;Zaluzec, 2001;Zaluzec et al, 1997), baking (Egerton et al, 2004;McGilvery et al, 2012;Soong et al, 2012;Williams and Carter, 1996), exposure to ultraviolet light and ozone (Hoyle et al, 2011;Soong et al, 2012) , cooling (Egerton and Rossouw, 1976;Hren, 1986) and beam showering (Egerton et al, 2004). The need for such contamination mitigation strategies has been made even more pressing with the advent aberration corrected STEM.…”