Copyright © 2005 Casa Software Ltd. http://www.casaxps.com
Modern XPS instruments combine technology from a range of fields including electron optics, X-ray optics, digital/analog electronics, mechanical engineering and software engineering. The complex mixture of these disciplines is challenging to manage at the best of times but the ever-increasing demands of a maturing technique coupled with the need to satisfy both research and application-orientated customers (from a comparatively small market) means that state-of-the-art instruments must come at a relatively high price. It also requires users to be relatively sophisticated in order to exploit the technique to its full potential since they too must appreciate many of the details wrapped up in the systems. With this in mind, exploring the features of a typical system helps to gain some insight into how measurements are performed and what the results mean.
The logical components of an XPS instrument are shown in
Figure1. X-rays illuminate an area of a sample causing electrons to be ejected
with a range of energies and directions. The electron optics, which may be a
set of electrostatic and/or magnetic lens units, collect a proportion of these
emitted electrons defined by those rays that can be transferred through the
apertures and focused onto the analyzer entrance slit. Electrostatic fields
within the hemispherical analyzer (HSA) are established to only allow electrons
of a given energy (the so called Pass
Figure 1:Logical layout for an XPS Instrument.
Electrons of a specific initial kinetic energy are measured by setting voltages for the lens system that both focus onto the entrance slit the electrons of the required initial energy and retards their velocity so that their kinetic energy after passing through the transfer lenses matches the pass energy of the hemispherical analyzer. To record a spectrum over a range of initial excitation energies it is necessary to scan the voltages applied to these transfer lenses and the prescription for these lens voltages is known as the set of lens functions. These lens functions are typically stored in some configuration file used by the acquisition system.
The efficiency with which electrons are sampled by a spectrometer is very dependent on these lens functions and without properly tuned lens functions the performance of an instrument can be severely impaired. Even with a well-tuned system the collection efficiency varies across the many operating modes and it is necessary to characterize an instrument using a corresponding transmission function for each of the lens modes and energy resolutions.
A hemispherical analyzer and transfer lenses can be operated in two modes, namely, Fixed Analyzer Transmission (FAT), also known as Constant Analyzer Energy (CAE), or Fix Retard Ratio (FRR) also known as Constant Retard Ratio (CRR). In FAT mode, the pass energy of the analyzer is held at a constant value and it is entirely the job of the transfer lens system to retard the given kinetic energy channel to the range accepted by the analyzer. Most XPS spectra are acquired using FAT. The alternative mode, FRR, scans the lens system but also adjusts the analyzer pass energy to maintain a constant value for the quantity “initial electron energy” / “analyzer PE”. This mode is typically used for Auger spectra since the energy interval accepted by the detection system (i.e. resolution) increases with kinetic energy and recovers weak peaks at high kinetic energies while restricting the intense low energy background that could do damage to the detection system.
Figure 2: Monochromator Schematic.
A number of factors influence the energy resolution achieved within a spectrum. The diameter of the analyzer, the pass energy and the spread of energies in the X-ray source play a major role in determining the full width half maximum (FWHM) for a given photoelectric line. Sample dependent considerations are also important where localized charging may broaden lines regardless of the precision built into the instrument and therefore effective charge neutralization is an important part of any system.
An achromatic X-ray gun relies on narrow resonance peaks in the X-ray spectrum for the anode material and limits the energy resolution possible for a photoelectric line. Monochromatic X-ray sources provide improved energy resolution by filtering a narrower band of X-rays from the resonance peak and this is achieved by exploiting a quirk of nature. X-ray diffraction through a quartz crystal (Figure 2) allows only certain wavelengths to be reinforced into a spot and so monochromatic X-rays can be directed at a sample. The crystal spacing is such that X-ray wavelengths that are multiples of the Al K Alpha X-ray resonance are reinforced by virtue of the Bragg relationship for X-ray diffraction. The fact that multiples of this wavelength are reinforced by the quartz crystal means that other anode materials are possible sources for monochromatic X-rays, such as Ag and Cr. Commercial X-ray monochomators have been offered using both two (Ag) and four (Cr K Beta) times the wavelength of the Al X-rays line.
Electrons are dispersed through the hemispherical analyzer so that different energy electrons arrive at different positions in the radial direction; further, they are also spatially dispersed around the circumference of the sphere. This relationship has been exploited by the Scienta ESCA300 and SPECS delay-line-detector systems, where images can be recorded that show both energy dispersion and spatial information in the form of sets of line-scans. VG ESCALABS 220i instruments use additional lenses at the entrance slit and before an imaging detector, which perform a Fourier transform and the inverse operation to allow an energy-resolved stigmatic image to be recorded through a hemispherical analyzer whilst operating in deflection mode.
Spatially resolved images can also be recorded by restricting the field of view for the transfer lens system, using a combination of magnetic and electrostatic lenses plus apertures, and then scanning the field of view using deflection plates. This technique has been used by Kratos Analytical in their Axis HS/Axis165 range and has since been enhanced by use of an additional hemispherical analyzer operating as a mirror analyzer rather than in deflection mode. Stigmatic energy-resolved images are acquired through the mirror hemispherical analyzer and offer a fast XPS image acquisition mode. The introduction of delay-line-detector technology has further enhanced the Axis Ultra/Nova systems to permit pulse-counted and therefore quantifiable chemical state XPS images.
An alternative means of acquiring XPS images is to restrict the X-ray source and raster the small spot of X-rays over the sample. This probe-oriented method is used by Physical Electronics in their Quantum range of instruments, where the monochromator is used to produce a small spot of X-rays. The X-rays are rastered over the sample by scanning a focused electron gun across the X-ray anode material whilst compensating for any changes in photon energy through the control system.
XPS images provide the means for assessing a sample, but quantitative analysis is performed using spectra, even if these spectra derive from stacks of images. Small area analysis using direct spectroscopy acquisition modes requires the use of imaging techniques applied to a single pixel. Matching the type of imaging for a sample can be an important factor in the choice of system. Many samples degrade under exposure to X-rays and there are costs and benefits associated with both field-of-view verses probe oriented methods. The probe method allows X-rays to be pointed at a specific location and therefore damage to the surrounding sample surface is reduced. On the other hand, stigmatic imaging involves irradiating a larger area but parallel sets of spectra are possible and charge compensation operates in a global context.