Improving Carbon Analysis with Plasma Cleaning

For a number of reasons, examining carbon by EDS in an electron microscope is tough. The Carbon X-ray K line is of low energy and can easily be absorbed. Its low energy means that only the Carbon X-rays from the specimen’s surface can be measured. Carbon X-rays are also easily absorbed by the X-ray detector windows. Moreover, there can be a significant Carbon background signal, since hydrocarbon [HC] contamination can be accountable for a rapid growing C peak when the beam is placed in the spot mode.

Hydrocarbons from the chamber surfaces, sample surface and vacuum pumps travel and react with the electron beam to develop a black spot that is rich in carbon. Here the EM analysis needs to be conducted. The Hydrocarbon background also obstructs with the detection of the Carbon X-rays, as an oil film can collect on the surface of the detector window. Figure 1 illustrates condensed oil on a detector snout and collimator. This oil file on an Ultra-Thin Window (UTW) can greatly decrease the transmission of C and N rays. The analytical struggle extends to N, whose X-ray peak is partly overlapped by the growing adjacent C peak and powerfully absorbed by the carbon layer that builds up at the window surface.

Oil condensed on EDS detector snout and collimator.

Figure 1. Oil condensed on EDS detector snout and collimator. Oil on EDS detector window adsorbs C X-rays and interferes with quantitative analysis. Evactron cleaning removes this oil without damage to the UTW window

The Evactron Anti-Contaminator [A-C] eliminates HC contamination from the SEM vacuum and from the sample surface [1]. The Evactron A-C comprises of a small plasma device, Oxygen Radical Source [ORS], which mounts onto an appropriate and available chamber port and a RF Generator/Controller. The ORS creates O radicals in a plasma from air metered through a control valve during system pump down. The O radicals are moved from the ORS via the chamber by pressure differential, and then they ash HC into CO, H2O and CO2 to be pumped away. Subsequent to Evactron cleaning, the chamber is pumped down to high vacuum for sample analysis.

Evactron cleaning rapidly removes light surface layers of contamination for the SEM chamber. Although a complete clean-up of a contaminated long-used microscope will not be fully cleaned in a few minutes, the subsequent results reveal that the Evactron A-C makes low level carbon analysis possible just after installation. Total cleaning may require repeated, brief Evactron cleanings to keep the HC away for long longer periods. Monitoring of HC levels is the proposed way of establishing when Evactron cleaning needs to be repeated. Monitoring C peaks by EDS is one promising monitoring technique.

A pure Cu polished sample was utilized to test the C contamination level enhancement. The microscope, a Gemini 982, was equipped with a Noran thin window EDS. The sample was prepared using the best standard practice recognized by the operator to give the least pollution: polishing, ultrasonic cleaning and final cleaning with pure ethanol. Spectra were obtained at 15 kV and the EDS system used to measure net intensity peaked on the elements discovered: Cu, La and C.

The spot was left static for 15 minutes and spectra were run for 50 seconds. The first spectrum was obtained instantly, followed by three others after 3, 6 and 15 minutes. An Evactron Model C Anti-Contaminator was employed for the tests. It was mounted to a port level with the sample stage. It was operated at the manufacturer suggested operating conditions of 10 Watts of RF power and at 80 Pa pressure. These conditions enhance the manufacture of O radicals and the cleaning mechanism.

Results

The first series of spectra were obtained before Evactron A-C cleaning. The spectrum showed a C peak instantly, and this carbon peak rapidly grew up from a relative C/ CuLa intensity ratio of 0.76% to 5.06% after 15 minutes of static beam. This can be viewed on the spectrum in Figure 2. The Evactron A-C was then operated for 4 minutes to remove hydrocarbons. The first spectrum attained after cleaning still exhibited a small carbon peak ratio of 0.61% that increased up to 1.39% after 15 minutes.

This improvement was remarkable but not enough: Evactron does not expect to clean a sample and a large chamber that had been used for a number of years in four minutes. Thus, a second 4-minute cleaning was performed, followed by a third one two hours later. This allowed time for the captured hydrocarbons to diffuse back and be converted and eliminated. After the third run, the EDS system did not find any carbon in the spectrum for the spectra acquired immediately and after 3 minutes. The spectrum obtained after 6 minutes and 15 minutes exhibited a small increase of the C intensity that allowed the software to detect C.

EDS spectra without cleaning and after third cleaning

Figure 2. EDS spectra without cleaning and after third cleaning. The Moxtek, Inc. window transmission curve versus energy is added to the Evactron A-C cleaned spectrum.

Background Hump in the C Peak Area

Although not identified as C, the spectrum revealed a hump that was very close to a carbon peak shape. The Moxtek UT Window is built with a 3000 Å polymer with 300 Å Aluminum film [1]. The window transmission is not linear and this hump firmly follows the window transmission curve as illustrated in Figure 2. When carbon was located, the software did not distinguish the C contribution from the background discontinuity. This explains the increase in the intensity that the software finds when C is found to be present, as shown in Figure 3.

Measured C intensity versus time with no cleaning, after the first and the third cleaning

Figure 3. Measured C intensity versus time with no cleaning, after the first and the third cleaning. The measured intensity is zero if the peak is not found and integrates the background hump when C is found.

Conclusion

The Evactron A-C excellently cleaned the SEM chamber within a short period of time, enhancing carbon quantification by SEM-EDS analysis. Both sample and chamber were cleaned properly within 12 minutes by the Evactron A-C run to allow a stable, undetectable C peak for up to three minutes of static spot. After 15 minutes, the C intensity was smaller than the peak found instantaneously after positioning the beam on a sample prepared the traditional way. The sample was sufficiently clean to reveal the background hump that should not be mistaken for a small carbon peak.

The Evactron A-C displayed, within a few minutes, a substantial improvement in cleanliness of a large chamber microscope in use for several years and sample surfaces. It was obvious that no oxygen peak appears in the spectrum, although Cu is effortlessly oxidized: Evactron A-C cleaning is efficient in eliminating labile hydrocarbons without being destructive to the sample surface.

References

[1] N. Sullivan et al, Microscopy and Microanalysis 8 (suppl. 2)(2002) 720CD

[2] C. Roberts, Moxtek, Inc. Application report: Improvements to Light element X-ray Windows for Si(Li) Detectors, Moxtek, Inc. Orem, Utah 2002

This information has been sourced, reviewed and adapted from materials provided by XEI Scientific.

For more information on this source, please visit XEI Scientific.

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