The monochromatic x-ray system in Axis 165 uses a high power (up to 450 W) Al anode x-ray gun and a large backplane quartz crystal monochromator. The monochromatic x-ray system offers narrower x-ray line widths, the removal of x-ray satellite lines and reduction of the Bremsstrahlung radiation.

Improvement in Energy Resolution

The line width of the incident x-ray is reduced and therefore, a higher energy resolution in XPS spectrum is possible compared to that using a non-monochromatic Al x-ray. The line width (measured as the full width at half maximum of the peak intensity, or FWHM) of AlKa_1,2 is 1.0 eV while the monochromator can produce a line width of about 0.3 eV for Al. The observed width of a particular XPS core level is reduced in accordance to the following expression:

w_o² = w_i² + w_x² + w_s²

where w_o² = observed FWHM of the XPS core line

w_i² = intrinsic width of XPS core level being studied

w_x² = FWHM of x-ray

w_s² = spectrometer widening term

To compare the effect of using a monochromatic and a non-monochromatic x-ray source, we will calculate the expected XPS core line widths for the two cases assuming that w_i is 0.3 eV wide, w_s is 0.2 eV, and w_x for the monochromatic source is 0.3 eV and for the non-monochromatic source is 1.0 eV. Substituting these values we get

w_o for monochromatic source = 0.47 eV, and

w_o for non-monochromatic source = 1.06 eV

Removal of X-ray Satellites

Characteristic x-rays are generally accompanied by satellite lines, that is, both the Mg and Al x-ray sources produce not only the primary Ka_1,2 line but also a series of lower intensity lines which arise from less probable transitions. For example, the Kb line arises from the valence band ® 1s transition while the Ka_3,4 line arises from transitions in a multiply ionized atom. A summary of the observed x-ray lines from Mg and Al are given below. The most troublesome is the Ka_3,4 lines since they are about 10% of the primary Ka_1,2 excitation and they tend to confuse or obscure other details of the XPS spectrum. Using a monochromatic source, we can see from the plot of the O 1s spectra below that the Ka_3,4 contribution in the spectrum is removed.

Separation and relative intensities of satellite x-ray lines of Mg and Al

Monochromatic and non-monochromatic O 1s spectra of SiO2

Removal of Bremsstrahlung or “White Radiation”

The Bremsstrahlung or “white radiation” is due to the slowing down or “braking” of the primary electron beam as it excites the anode material. This is a continuous background radiation that is of no use in XPS but simply adds up to the background in the XPS spectrum. Because monochromatization removes this Bremsstrahlung radiation, a better signal to background ratio is obtained and low intensity peaks which otherwise would have been lost in the background become detectable. The plot below shows the Bremsstrahlung region of the x-ray spectrum of Al excited by 15 keV electrons. Notice that the Bremsstrahlung background extends up to the electron beam energy.

X-ray spectrum of Al excited by 15 keV electrons

Principle of Monocromatic X-ray Operation

The figure below shows the actual mounting arrangement of the monochromatic source on the Axis 165 SAC. The x-ray gun is of the microfocus type and typically produces a short line focus at the anode. The Al anode is at a high positive voltage and the thoriated tungsten filament is at ground potential. A deflection shield around the filament is positioned to optimize the focusing of the electron beam on the target anode. The target is water cooled by a high pressure coolant system.

Axis Bench

Monochromatic x-ray source in Axis 165

The Al x-rays are diffracted by the quartz crystals mounted on a formed curve in conformity with the Johann geometry as shown below. The crystals have a toroidal form which gives focusing in two planes. To monochromatize the AlKa line, the quartz crystals are cut parallel to the 1010 planes whose d-spacing is 4.255 Å. The corresponding Bragg angle for 1^st order diffraction for the 1487 eV Al Ka_1,2 x-ray is therefore 78.5˚ as shown in the calculations below.