AXIS Ultra - silver/aluminum monochromator

Introduction

High energy silver (AgL

) X-rays have several advantages over those produced by the more commonly employed aluminum (AlK

) source. A photon energy of 2984.3eV means that a silver source can generate higher energy core levels and Auger series, and has a greater excitation volume. A further advantage over other high energy sources is that the condition for Bragg "reflection" (Bragg's Law) of the silver L

line is satisfied via second order diffraction within the confines of the conventional AlK

monochromator body, with only minor modifications.

We have now for the first time implemented a silver monochromator as a modification to the standard aluminum monochromator on a modern magnetic lens based electron spectrometer, the Axis ULTRA. The anode of the aluminum monochromator has been adapted to have both a conventional Al (metal) face and an additional Ag face, thus a changeover from one source to the other may be made with only minor adjustment and without breaking vacuum. The great improvement gained in photoelectron collection efficiency by employing magnetic lens technology improves the sensitivity and ultimate usability of the silver source making it a viable alternative for the measurement of deep 1s core levels and Auger parameters.

The results presented here show that the silver source gives a resolution better than 0.9 eV FWHM for a silver sample and a sensitivity greater than 4,300 CPS on gold at a resolution of 1.3eV.

Table 1: Higher energy X-ray sources
used for XPS [2]

Line	Energy (eV)	Width (eV)
Si K	1739.4	1.2
Zr L	2042.4	1.6
Au M	2122.9	2.15
Mo L	2293.2	1.9
Ag L	2984.2	2.6

Theoretical

Conventional commercial spectrometers are limited to a maximum ionisation energy of 1486.6eV, the photon energy of the AlK source. Higher energies are useful in a variety of analyses and many sources, some of which are listed in Table 1, have been employed previously, with varying degrees of success. Ordinarily the AgL would be too broad to be of interest to the analyst: fortunately, however, the wavelength is such that the line width can be reduced by a suitably configured aluminum monochromator.

Ag L; Ep = 2984.2eV
Wavelength, = 4.1544Å, approx. 1/2 Al (8.3393 Å)

Therefore the Bragg relationship can be satisfied:

n = 2d sin (2nd order diffraction, n=2)

In this way a source is created with a reduced photon line width and a wider range of accessible energies.

New core levels can be excited for:
Al, Si, P, S, Cl, Br -> Ru and Tm -> Pt, Au, Hg, Pb, Bi.

As well as exciting the deep core levels and Auger transitions, the analysis depth is also increased, because of the greater photon energy, a useful attribute for those seeking "non destructive" depth profiling.

Figure 1: X-Ray Monochromator Geometry

Implementation

In practical terms it is necessary to run a modern photoelectron spectrometer with better than 90% of uptime. To switch between aluminum and silver monochromated sources it is unacceptable to vent the system and so lose valuable instrument time. Therefore we have developed a twin anode monochromatic source. The anode is provided with both aluminum and silver coatings and two electron sources are used to generate Al or Ag X-rays as required - see Figure 1.r

Figure 2: Gold survey spectrum using Ag mono

Results

Performance

Silver standard:

>10000cps at 1.4eV FWHM
>1000 cps at <0.9eV FWHM

Gold foil (cleaned, survey spectrum: Fig. 2).

Au 4f 7/2 @1.3eV FWHM 4,500cps (Edgell et al 25cps)

Figure 3: Al 2p regions: Al mono (a), and Ag mono (b) sources

Greater analysis depth

The Al 2p spectra shown (Figure 3) were acquired with (a) Al and (b) Ag sources. Spectrum (b) is a clear illustration of the greater analysis depth of the Ag source as the element : oxide ratio is greater than that acquired with the Al source. An oxide thickness of 1.8nm indicated here was confirmed by independent angle resolved measurements.

Figure 4: Si 2p (a) and Si 1s (b) regions, using Ag mono

More information with higher photon energy

Depth profiling can be performed by analysing photoelectrons of different binding energies. In this case the Ag source is used to analyse the Si 2p and Si 1s core levels from a thin native oxide on Si. Clearly the higher BE (lower KE) Si 1s spectrum has a greater oxide ratio (spectrum (b)) due to the reduced analysis depth when compared to the lower BE (higher KE) Si 2p spectrum, (a).

Figure 5: Si 1s and Si KLL Auger series from Ag mono source.

Higher energy Auger series and core levels

The higher photon energy of the Ag source may be used to generate higher energy series. In this case Si KLL and Si 1s spectra were acquired from a native oxide on Si.

Conclusions

The silver monochromatic source implemented within the geometry constraints of a classical aluminum monochromator has been shown to offer viable, narrow line, high energy excitation. The unique combination of the two sources in the same physical unit means that the silver source is also a practical option for routine high energy analysis. In addition, improved performance of modern magnetic lens instruments means that although the Bragg equation (for silver) is only satisfied by a second order diffraction, useable count rates and realistic acquisition times are achieved. It is expected that the introduction of the dual anode monochromatic source will lead to more studies of higher energy core levels and non-destructive depth profiling.

References

[1] Edgell et al., J. Electron Spec. & Rel. Phen. 37 (1985) pp 241-256

[2] K. Yates and R. H. West, Surf. Interf. Anal. Vol. 5, No. 4. (1983) pp 133-138

This paper was presented at AVS '99, the 46th International Symposium, Seattle, WA, 26th October 1999.

	Copyright © Kratos 2003 Last Update:03/08/12 Kratos Analytical is a wholly owned subsidiary of the Shimadzu Corporation.

For comments, suggestions, contributions or an Instrument Demonstration e-mail to: xps@kratos.com

A Dual Anode Monochromatic X-ray Source