A cylindrical-mirror analyser incorporating pre-retardation

A cylindrical-mirror pre-retardation received

19 March

analyser

incorporating

1975

J H Baker* and E M Williams, Liverpool L69 38X. England

Department

of Electrical

Engineering

and Electronics,

University

of Liverpool,

P.O. Box

147,

A description is given of the design and performance of a cylindrical-mirror energy analyser incorporating a spherical grid system for the retardation of the particle flux prior to analysis. The resolution limit of the instrument is around 0.25% for an extended source of electrons. Investigations of the Auger spectra of tungsten with adsorbed oxygen are presented to illustrate the performance of the instrument.

Introduction

The current interest in electron spectroscopy for elemental and chemical analysis has prompted some considerable activity in the field of energy analysis for charged particles.’ Amongst the available designs of energy analysers the cylindrical-mirror analyser has attracted much attention, particularly in investigations of Auger electron spectroscopy of solid surfaces. The virtues of this analyser are notably its qualities of high transmission and sensitivity consistent with a resolution suitable for the detection of Auger features. Moreover, the design principles are relatively well established and tested,2-6 and its construction and operation, in comparison with other equivalent systems, offers distinct advantages of simplicity and convenience. In the present paper a description is given of the design and performance of a cylindrical-mirror analyser with a facility for the preretardation of charged particles. The introduction of retardation offers in principle a very flexible arrangement which, while retaining the basic features of the mirror analyser, allows the resolution limit to be varied as desired.’ This kind of a two stage arrangement of a high-pass filter with a cylindrical mirror analyser as a band-pass filter has also been investigated recently by Gerlachs and by Maeda and Ihara9. The present instrument was developed at Liverpool with a view to augmenting our investigations of electron stimulated desorptionl”*” with investigations of Auger electron spectroscopy of the solid-gas interface. Analyser design and construction

A schematic diagram of the analyser is given in Figure 1. The cylindrical-mirror analyser was designed to satisfy second order focussing conditions whereby electrons produced at the source are accelerated into the analyser within limits of the central ray inclined at an angle of 42” 18’ to the axis of the analyser. The angular limits in the present case were chosen to be *6”, so that the transmission of the mirror analyser is about 7 %. Retardation of the electron flux is accomplished by means of a pair of grids, G, and Gf, of spherical geometry mounted at the aperture of the analyser. The radius of the inner cylinder is 2.86 cm and * Now with V. G. Micromass, Winsford, Cheshire. Vacuum/volume

25/number

8, 1975.

Pergamon

Press LtdlPrinted

electron

beam

-

Mu-metal

zxz

Tungsten mesh Y3+a,n,ess steel

shleld

t.

Figure 1. The energy analyser.

was set at this value to allow reasonable accuracy in the manufacture and the positioning of the retarding grids yet permitting the complete analyser to be incorporated in a vacuum port of 20.3 cm (8 in.) dia. The radius of the outer cylinder of the mirror analyser is 7.15 cm which yields a relation between pass energy (E,,,,) and the potential difference VP, between the cylinders as EpapS = V,,/O.699. The corresponding value for the source to focus distance is 17.5 cm. The arrangement for positioning and insulating the inner and outer cylinders makes use of 8 BA screws incorporating ruby washers and spacers which fix the cylinders to the front and back end plates of the analyser. The outer cylinder, in turn, is held in a cylindrical cage assembly which is attached to the flange forming the seal at the vacuum port. Insulation of the outer cylinder from the support cage is achieved by means of ruby balls. The source position lies 7 mm in front of the front end plate. Distortion of the electric field due to the relative proximity of the front end-plate to electron trajectories within the intercylinder volume is minimised by means of four guard rings. These guard rings are attached to and insulated from the front end-plate by means of screws and ruby washers. Distortion of the electric field due to the entrance and exit slots in Great Britain

373

J H Baker and E M Williams:

A cylindrical-mirror analyser incorporating pre-retardation

at the inner cylinder is corrected by covering the slots with a tungsten wire mesh of 96% transmission. Magnetic interference at the analyser is minimised by covering the support cage and the end-plates with a mu-metal screen. The retarding grids at the entrance of the analyser are of radii 20 and 24 mm, their centres of curvature being coincident with the source position. The grids are constructed of tungsten wire mesh (96% transmission) and were formed by tensioning the mesh over a polystyrene former while spot welding the wires of the mesh to a stainless steel collar. The collars in turn serve to secure the grids at the analyser; in the case of the outer grid G1 the collar is in direct contact with the end-plate, while the collar supporting the inner grid Gz is insulated. Screws and ruby washers are again used to position and insulate the grids. The electron detector housed within the inner cylinder is a channel electron multiplier. Provision is made for intercepting the electron flux at the minimum trace width position, and the entrance cone of the multiplier can be biased so as to accelerate electrons to the detector. The electron beam employed as the mode of primary excitation is derived from an electron gun housed in a separate vacuum port mounted at right angles to the axis of the analyser. The target is so positioned that the beam is incident at about 15” from grazing incidence (see Figure 1) and the circular beam profile (l-2 mm dia) is thus deformed into an elliptical profile at the target. All components of the analyser, except where otherwise noted, are manufactured of stainless steel so that the design offers complete compatibility with ultra-high vacuum. A method of cleaning the surface under investigation is provided by electron bombardment heating with electrons derived from a filament located at the rear of the target. In the case of tungsten single crystal, for ,which investigations with Auger spectroscopy will be described later, the specimen in the form of a disc (-8 mm dia, 2 mm thick) is supported by a tungsten rod (0.5 mm dia) fitting in a hole drilled by spark erosion across the diameter. The tungsten support rod together with the heating filament is attached to a sample manipulator which enables the target to be located with precision at the source position. The electric circuit In operation without pre-retardation of the electron flux the measurement technique follows the standard procedure whereby the ramp voltage at the outer cylinder is modulated and the differentiated signal is measured by a lock-in amplifier. For this case the inner retarding grid together with the inner cylinder are connected with the end plates at ground potential. With pre-retardation of ‘the electron flux it is convenient to maintain a constant pass-energy and to sweep the retarding potential. Electrons generated at the earthed target pass thrbugh the earthed grid G, and are decelerated by the negative potential applied to the inner grid Gz. The grid Gz and the inner cylinder are connected electrically and are held at a constant potential relative to that at the outer cylinder. The variable retarding potential is derived by sweeping the potential at the outer cylinder which thus changes along with the potential common to the inner grid and inner cylinder. The ‘potential at the outer cylinder is again modulated but in this case precautions are taken to decouple the modulation signal from the inner grid and inner cylinder. 374

Investigations of the performance of the analyser The resolution of the analyser was initially investigated by observation of the full width at half maximum (FWHM) of the peak of electrons elastically reflected from a solid target. For the purpose of this investigation the output of the electron multiplier was coupled to a pulse counting system so that observations could be made of the undifferentiated peak. With no retardation the FWHM of the elastic peak of electrons of energy 1 keV was 25 eV, corresponding to an energy resolution (A/Z/E) of 2.5 %. The increase in resolution with retardation was reflected in the attainment of a FWHM of 2.5 eV at 40 eV pass energy. At this setting, which corresponded with the optimum in respect of resolution, the intensity of the elastic peak was attenuated by a factor of around 100 relative to its value in the absence of retardation. The theoretical basis for evaluating the energy resolution of cylindrical mirror analysers is discussed fully in the papers already cited in the introduction to this paper and also more recently by Risleyrz and by Bishop et aI.13 For the present design the resolution should theoretically be in the region of 0.3% for the case of a point source of electrons. In order to examine the contribution of source size to the measured resolution of 2.5% for the basic mirror analyser, the solid target was replaced by a tungsten wire 0.1 mm dia with its tip at the source position. Observations, without retardation, of the peak of electrons elastically reflected from the tip gave a resolution of 0.4%. This result lies close to the theoretical value and confirms the significance of the extended source in degrading the resolution. It is interesting to note, however, that using the wire tip arrangement, the resolution upon retarding was not improved beyond the value found with the solid target. This indicates that the finite source size is not the main factor in limiting the resolution under conditions of retardation. In this case the degradation in resolution may be associated with effects such as residual magnetic fields and the refraction of electrons at the retarding grids arising from lens effects or deviations from ideal spherical geometry.“‘ Examples of Auger spectra obtained with the analyser are presented in Figures 2-4. Figure 2 shows the Auger spectrum of tungsten single crystal (110) after heating the target to 1800 K in oxygen at lo-’ torr pressure. This result was obtained without retardation and with a primary beam current of 1 PA at 2.1 keV energy. The analyser voltage was swept at a rate around 2 Vs-’ with modulation voltage of amplitude 2V (peak-peak) and frequency of 2 kHz. The quality of the result compares favourably with the performance of other mirror analysers used in Auger spectroscopy of tungsten surfaces (Bas and Banningerl’, Joyner et a/.16, Palmberg et a1.l’). Auger spectra of tungsten surfaces obtained using retarding-field energy analysers have been published in recent years by Hass et a1.l8 by Pollard”, by Taylor*’ and by Tracy and Blakeley.*’ In all cases the principal Auger peaks are observed in the region 160-180 eV with the signal form in some cases appearing as a doublet although in two cases the same doublet is clearly resolved into a triplet peak.‘7*‘8 This result would not appear to be dependent on the crystalographic nature or the condition of the tungsten substrate, but more clearly a reflection on the resolution of the instrument employed in the study. The spectrum shown in Figure 2 contains a small peak below the lower energy peak of the apparent tungsten doublet (see arrow marked X in Figure 2) as does the data of Bas and Banninger.15 Using pre-retardation

J H Baker and E M Williams: A cylindrical-mirror

analyser incorporating pre-retardation

Figure 2. Auger spectrum of tungsten with adsorbed oxygen.

d WIfI t

z-

vv

/I

-4-J

I

163

I II

1;s

179

Figure 3. The principal peaks of the tungsten spectrum (with preretardation).

d

Xlfl

t dE

45

Figure 4. The oxygen KLL spectrum (with pre-retardation). this energy region can be investigated in greater detail, and in Figure 3 the spectrum obtained with pre-retardation to a pass energy of 60 eV is shown. For this measurement the beam current was increased to 2.5 PA with a modulation voltage of amplitude IV (peak-peak) at a frequency of 2 Hz and a voltage ramp rate of 2 Vs-‘. From this measurement the triplet character of the peak is clearly resolved. Hass et al. attribute the triplet character, which is common to many elements from

period-6 transition metals, to the transitions N,N,N,, Ns NsN,, N,N,N,. Figure 3 shows the oxygen KLL transitions observed under conditions of pre-retardation similar to those employed for the data of Figure 3. The solid arrows indicate the three principal peaks of oxygen. 17.22 Two further peaks present in the spectrum are indicated by the dashed arrows. Similar structure is seen in the work of Basset et a1.23 with oxygen spectra from MgO investigated using an energy analyser of resolution around 0.1%. In conclusion, the observations of Auger spectra from tungsten and oxygen confirm the resolution capabilities of the instrument as judged from observations of elastically reflected electrons. The incorporation of retardation clearly increases the versatility of the cylindrical mirror analyser, enabling the attainment of a resolution which is comparable with that of the best four-grid, retarding-field energy analyser. The findings of this work follow, in general, the observations of Gerlach* and of Maeda and Ihara9 using similar arrangements of a cylindrical mirror analyser with pre-retardation. With the instrument of Maeda and Ihara the resolution limit was improved beyond 0.1 ‘A, although at the expense of a larger overall analyser size and with a significantly lower semiangular aperture (2”). The beam current employed with the present analyser ranged in value up to a few PA, providing a current density at the target of a few tens of pA/cm2. This relatively low current density is clearly of advantage in reducing the perturbing influence of the electron beam at the surface, particularly as in the present application of Auger electron spectroscopy to the study of gas processes at solid surfaces. Acknowledgements The authors are grateful to the Science Research Council for the award of a grant both to construct the equipment and to support a post-doctoral appointment (J H B). The authors acknowledge also the interest of Professor J H Leek in this work, and the contribution of members of the technical staff of the Department whose skills enabled the instrument to be constructed. 375

J H Baker and E A4 Wiliiams: A cylindrical-mirror

analyser incorporating

References

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376

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