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General Description, Dimensions
The CSA 200 / CSA 300 is a cylindrical sector field analyser
operated in second order focussing. An electrostatic transfer lens (5 elements) with maximum angular acceptance is adapted to the analyser. This lens is identical for CSA 200 and CSA 300. Although the slit-to-slit distance is nearly 200mm for the CSA 200 and 300mm for the CSA 300, the dimensions of the sector field allow the instument to be operated in a very compact
mu-metal housing. The CSA 200 can be optionally equipped with a DN63CF mounting flange allowing the system integration even to small vacuum chambers.
The figure illustrates the working principle of the CSA: Having passed the entrance aperture,
the particles enter (and leave) the region between the outer and inner cylinder through a fine mesh covering exactly the inner cylinder. Within the analyser they experience the cylindrically
symmetric field (potential f (r) =f0 ln (r/r0) ). This causes a deceleration / acceleration (before
/ after the outermost point of the path), deflection and second-order-focussing into the exit aperture. Particles of the lower or higher energy are focussed at a shorter or longer distance
(energy dispersion). Owing to second order focussing the solid angle of the CSA 300 is about four times as large as the acceptance of a hemispherical analyser operated at comparable
conditions. This unique property results in a substantially higher transmission of the novel analyser.
One single control unit (NGCSA) covers the whole energy range from 0 to ± 3280eV for all electron and ion spectroscopy applications. High stability voltage supplies, in combination with
the mu-metal shielding of the analyser chamber facilitates high energy resolution applications below 50 meV FWHM in the high energy range routinely. The analyser control unit can be
operated under front panel control in conjunction with analog sweep voltage or under computer control. A dedicated interface board and software for AT-compatible is available.
The CSA 300 has the two standard working distances 10" (254mm) and 8" (203mm). Other
working distances are realized by customized adaptors. For the CSA 200 the mounting flange can be optionally a NW63CF-flange with the diameter of the shielding tube being
correspondingly reduced from 70mm to 62mm.
Analyser
The CSA 200 / CSA 300 is an electrostatic Cylindrical Sector with a slit-to-slit distance
(dispersive distance) of 200 or 300 mm respectively. This type of energy analyser for charged particles has three major advantages as compared to the more common hemispherical analysers:
- The analyser provides second-order focussing, which means that for a given energy
both the first and second derivative of the particle's path length (projected onto the cylinder axis) vs. entrance angle vanish. This has the consequence that the accepted
solid angle interval is, for the CSA, typically four times larger than for a hemispherical analyser (where the quadratic angular term is non-zero) which shows up in the analyser transmission.
- Needing only a narrow sector of the inner and outer cylinders, the CSA is extremely compact.
This is of particular importance for modern multi-purpose UHV systems, where the electron analyser must be a "space-effective" bolt-on component. Together
with the large working distance of 50 mm this facilitates integration even in existing systems.
- The CSA features a 90° deflection, which is of special interest when employing an electron spin analyser
(the Spin detector Focus-SPLEED is optional to the CSA 300). The 90°-geometry allows easy access to the spin analyser and, more important, the
90°-deflection converts a longitudinal spinpolarisation component into a transversal one which can be detected by the spin analyser. Hence, this configuration allows, e.g., the
simultaneous measurement of the in-plane and out-of-plane (perpendicular) components of the surface magnetisation in a magnetic material.
Resolution
The base resolution D E /E of the CSA200 / 300 is mainly determined by three factors,
namely the width of the analysed beam in the dispersive direction (given by the entrance and exit apertures w1 and w2), the angular divergence DQ of the accepted beam, and the width of the analysed beam in non-dispersive direction (given by the distance of closest approach rc of
the off-axis rays with respect to the cylinder axis).
The approximate analytical expression for the base resolution of the CSA200 / 300 geometry is:
The value of DQ depends upon the diameter of the entrance aperture of the lens. For high
resolution applications the angular divergence of the accepted beam should be reduced by mounting a smaller aperture (available from the manufacturer).
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a)
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CSA 200
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CSA 300
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w1 = w2
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(w1 + w2)/D
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(rc/Ri)2
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(w1 + w2)/D
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(rc/Ri)2
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1mm
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1.0%
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4.4%
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0.67%
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3.5%
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3mm
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3.0%
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4.4%
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2.0%
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3.5%
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9mm
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9.0%
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4.4%
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6.0%
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3.5%
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b)
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DQana
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2.7 (DQana/rad)3
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± 6°
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0.3%
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± 8°
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0.7%
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± 10°
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1.4%
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± 12°
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2.5%
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The three terms of eq.(1) for varous slit widths and angular acceptances.
Owing to the retardation grids in the transfer lens, the CSA is operated in the constant pass-energy mode.
As an example for the UPS mode, we estimate the resolution for 1mm slit width and ± 8°
analyser acceptance (typical value) for the CSA300. According to the table D E/E » 0.7% + 3.5% + 0.7% = 4.9%. At a pass-energy of 4eV this corresponds to an absolute value of D E »
196meV base resolution of i.e. 98meV FWHM.
In the XPS/AES-mode a typical setting is 3mm slit width and ± 10° acceptance, yielding
2.0% + 3.5% + 1.4% = 6.9%. At a pass energy of 20eV this corresponds to 0.7eV FWHM,
being approximately equal to the bandwidth of non-monochromatised Al or MgKa radiation.
The pass energy Epass and the potential difference U between the outer and inner 'cylinder'
are related by the spectrometer constant K:
This constant depends on the geometry of the analyser. The value can be re-calibrated by
using the positions of sharp XPS or Auger-lines or a primary peak of an e-gun. The spectrometer constant is correctly set when line positions do not appear shifted when the pass energy is changed.
For beam corrections inside the analyser, e.g. due to residual magnetic fields, two correction
electrodes are mounted in the region of the longitudinal fringe field corrections.
Additional features of the CSA 300:
- Electron spin polarisation analysis
The CSA 300 can be equipped with an electron spin polarisation detector for spin- resolved
spectroscopy, e.g. photo- or Auger electron emission from ferromagnetic systems. The CSA deflects the electrons by exactly 90°. This facilitates an analysis of all three magnetisation
components by a simple sample rotation (refer to the SPLEED- detector).
The CSA 300 can be equipped with an array of channeltrons at the position of the exit slit to
accumulate the counts for adjacent energy channels in parallel. Due to second order focussing the acceptance angle of the analyser allows a high input angle of ±15° for the five element lens
system. This input angle can be varied by exchangeable front-end apertures to allow angle resolved experiments in XPS, UPS and ISS.
The combination of the lens system, special low noise electronics and mu-metal shielding of
the analyser allows high energy resolution measurements better than 10 meV FWHM.
With independently variable entrance and exit slits (optional in-situ change) optimisation of
both energy resolution and signal for any given experiment can be achieved. The entrance slit selects the analysed area on the sample (e.g. for small area XPS) and both the exit slits in
combination with the pass energy determines the resolution.
Entrance Lens
The CSA 200 / CSA 300 is equipped with a transfer lens to match the analyser to various
configurations of the experimental chamber. We designed the lens for a working distance of 50 mm in order to have enough space around the sample. Furthermore, the lens adapts the
kinetic energy of the electrons / ions kinetic energy (0..3280eV) to the fixed pass energies of the analyser. For this purpose, the lens contains a pair of spherical retarding grids at its
entrance. The two grids are concentric to the centre of the sample so that the trajectories of the charged particles are almost unaffected by retardation or acceleration.
It is a common experience that concentric spherical grids with fine mesh allow a retardation by
a factor of 10 without significant aberrations. Given a maximum kinetic energy of 3280eV the kinetic energy is reduced to a constant value around 300eV between the first (grounded) grid
G1 and the second grid G2. The subsequent transfer optics can thus be kept at fixed potentials with respect to G2 during the energy sweep. The lens potentials are only switched
beween fixed preset values when varying the pass energy of the analyser. The advantage of this method of retardation is that the overall transmission is practically constant during the
energy sweep. This is very useful for a quantitative evaluation of peak intensities. Since the analyser is always operated at a fixed pass energy, its transmission enters only as a
constant factor in the overall transmission.
Electron Detection
For both electron and ion detection the CSA employs a Channeltron (Galileo) that is capable of high count rates
(up to 13Mcps). The voltage can be adjusted for gains between 104 and 108.
Electronics and Software
The kinetic energy scan voltages for the CSA are generated by the NG CSA control unit and
driven by an interface card. Careful design of this unit has resulted in exceptionally low noise levels allowing an analysed resolution of better than 10 meV FWHM.
The pulse counting path down from the channeltron to the data system handles mean peak
count rates up to 13 Mcps (20 MHz peak). This enables fast experiments with high count rates to be carried out thus exploiting the full capability of the CSA analyser.
Four kinetic energy ranges can be selected:
- 0..65 eV with 1 meV step width (jumpable)
- 0..320 eV
- 0..1600 eV
- 0..3200 eV (ideal for survey scans in XPS and AES)
An optional noise filter offers high resolution UPS capability. Reverse polarity mode for ISS is
standard in all energy ranges. The following pass energies can be selected:
1, 2, 4, 8, 16, 32; 10, 20, 40, 80, 160 and 320 eV.
Data Acquisition Package DAT 300
The data acquisition package provides computer controlled experiments with the CSA
analyser. It comprises of a PC interface card and spectroscopy software, giving complete control of parameters as well as data acquisition, processing and display. A PC 486 with MS-
Windows XXX is recommended.
Settings for the pass energy, kinetic energy range , polarity, channeltron and X-ray source are
all controlled via the computer interface. The DAT 300 data processing functions include Shirley background subtraction, 3 or 5 point smoothing, integration or differentiation of
spectra, a de-spike routine, FFT smoothing, de-convolution and peak fitting. All information is stored in ASCII format and can be easily be imported into other software packages. XPS
peaks in a spectrum can readily be identified by using the peak identification database for elemental analysis. For multichannel detection up to five counting channels can be provided.
The software was written in Turbo Pascal. It provides a user interface which allows to link personal subroutines to control other experimental parameters.
Energetic Considerations
The sample is normally at (or near) earth potential. The kinetic energy of Auger- or
photoelectrons is thus measured with respect to ground. Electrons leaving the sample with a certain kinetic energy Ekin are accelerated or decelerated by a retarding voltage Uret to the
selected pass energy Epass. Then according to the energy diagram:
where W is a constant arising from the work function of the analyser materials and from a possible bias on the sample.
The CSA acts as a narrow pass filter letting through only electrons within a small energy interval of width D E around Epass.
For Auger lines Ekin is independent of the energy of the exciting photons or electrons. The
Auger process gains its energy through an electronic transition of an outer-shell electron to an inner-shell vacancy. This process does not depend on how the inner vacancy has been produced.
In contrast a direct phototransition in XPS or UPS is governed by the energy relation:
where hn is the photon energy, EB is the electron's binding energy referenced to the Fermi
level, and f is the work function of the sample (i.e. the minimum energy required to bring an electron from the Fermi level of the sample to the detector placed at a macroscopic distance
away from the sample).
Energy Level Diagramm (schematic)
Principle of an Auger transition
Transitions in UPS (dashed) and XPS (full arrow)
Ultraviolet Photoelectron Spectroscopy (UPS)
For UPS typical photon energies are hn =21.22 eV (for He I), hn = 40.8 eV (for the He II
resonance wavelength) or synchrotron radiation in the vacuum ultraviolet (VUV) region. The CSA200 / 300 is operated in the low-energy mode:
Ekin = 0 to 163.8 eV (step resolution 2.5 meV)
(or 0 to 65.5 eV with step resolution of 1 meV) Epass = 1 eV, 2 eV,4 eV, 8 eV, 16 eV or 32 eV
In order to directly observe work-function changes, e.g. due to absorption or contamination
processes, it is desirable to scan the spectrum down to the low-energy cut off Ekin = 0. To
obtain this cut-off correctly, it is advantageous to apply a small negativ bias voltage (typically a few V) to the sample which facilitates a better handling of the low-energy electrons. Note,
however, that thia bias voltage shifts the kinetic energy scale accordingly and a ripple on the bias voltage (as well as a ripple on the ground potential) fully shows up in an increased peak
width, i.e. a reduced energy resolution.
Note that a bias voltage reduces the angular resolution. The electrons are accelerated in the
(normally field free) region between the sample and the lens front end. This distorts the initial starting angle of the electrons.
X-Ray Photoelectron Spectroscopy (XPS)
For XPS typical photon energies are hn =1486.6 eV (for Al Ka ) or hn =1253.6 eV (for Mg Ka ) or synchrotron radiation with energies beyond the UPS range.
The CSA200 / 300 is operated in the high-energy mode:
Ekin = 0 to 1638 eV (step resolution 25 meV)
(or 0 to 655 eV with step resolution of 10 meV,
selectable by jumper setting) Epass = 10 eV, 20 eV, 40 eV, 80 eV, 160 eV or 320 eV
For XPS a sample bias is usually not required.
Auger Electron Spectroscopy (AES)
The energy of an Auger line is independent of the initial excitation process (photon- or
electron-induced) and can be found in Auger-tables. For photon-induced AES the operation of the CSA is identical to XPS, see preceding section. In case a tuneable photon source (e.g.
Synchrotron radiation) or a twin-anode (Al/Mg Ka ) X-ray source is used, an easy distinction
between Auger and XPS lines in an electron spectrum is possible by changing the excitation energy (e.g. switching from Al to Mg). All XPS photoemission lines will shift in Ekin accordingly
by the photon-energy difference, whereas the Auger line positions will remain fixed.
In electron-induced AES an electron beam of typically 3 to 10 keV energy is used to create
the primary core hole. The deexcitation via Auger-electron emission then happens as in the first case. An essential difference is, however, that here the Auger lines are riding on a
substantially higher background of secondary electrons, released by the primary beam. The sensitivity of Auger-line detection can be strongly improved by a differentiating techique. The
optimisation of the position and size of the primary beam focus on the sample usually requires a few cycles of steps 6 to 9 thereby gradually improving the settings of the x/y-deflection of the
electron beam and the focus of the electron gun together with optimisation of the sample position and orientation. After accumulation of the spectrum, the differentiation is done
numerically. Alternatively, the Auger lines can be displayed after background subtraction without differentiation. This way is often preferred for a quantitative analysis of Auger line intensities.
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