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An ultrahigh vacuum electron spectrometer for surface studies
Joournal of Electron Spectroscopy and ReEated Phenomena, 3 (1974) 241-261 @ Elsevier ScientificPublishingCompany, Amsterdam - Printed in The Netherlands
AN ULTRAHIGH STUDIES
VACUUM
ELECTRON
C. R. BRUNDLE, M. W. ROBERTS School of Chemistry, University of Bradford, Bradford,
SPECTROMETER
Yorkshire 807
FOR
SURFACE
IDP (England)
D. LATHAM and K. YATES Vacuum Generators Ltd., East Grinstead, Sussex (England)
(Received 10 September 1973)
ABSTRACT A high-resolution electron spectrometer has been constructed for the study of clean metal surfaces and their interactions with gases. In its present form it is most suitable for the study of evaporated films but may also be used for ribbons, single crystals, or powders. Electron spectra induced by three sources, X-ray, vacuum ultraviolet, or electron impact, may be recorded sequentially. Electron energy analysis is normally performed by setting the analyzer (10 cm radius, hemispherical) to pass electrons of a fked energy and applying a scanning retarding potential to the sample. The limiting instrument absolute resolution is about 25 meV FWHM at 5 eV analyzing energy, as judged by an He I gaseous argon spectrum. Resolution can be traded for sensitivity by analyzing at higher electron energies (up to 100 eV), or by means of an external slit-width adjustment mechanism. The spectrometer carries a preparation chamber which has a variety of demountable sample preparation, cleaning, and reaction monitoring devices. Spectra from gold, silver, and molybdenum films. are presented by way of illustration. I. INTRODUCTION Photoelectron spectroscopy, as developed over the past five years, has become one of the most useful techniques for studying electronic energy levels. The study of valence electron energy distributions with vacuum UV radiation (5-50 eV, but generally Q 21.2 eV) was termed Molecular Photoelectron Spectroscopy (MPS or UPS) where gases were studied’ and Photoemission for solids’. The study of core level energies (usually solids) with X-radiation (up to 20 000 eV, but generally < 1500 eV) acquired the name Electron Spectroscopy for Chemical Analysis (ESCA)3. Within the last three years improvements in instrumental sensitivity have facilitated
242 the study of valence ievels4 and gaseous studies5 by ESCA. It has been the exception rather than the rule that these complementary studies (X-ray and UV) were performed in the same spectrometer. It had been recognised initially that both techniques were surface sensitive23 3 but the extent of this sensitivity was perhaps not fully appreciated for some time (for example, see Eastman 6 and Delgass et al.‘). There is now available a considerable to indicate that the depths probed are similar (within a factor of body of data’-l’ 2 or 3) to those generally accepted for electron impact Auger spectroscopy, and that the major controlling factor in determining this depth in all three types of spectroscopy is the kinetic energy of the escaping electron being analyzedi2. Energy Loss Spectroscopy, the inelastic scattering of an incident electron beami3, makes a fourth electron energy analysis technique which has been shown to be surface sensitive14. Energy loss spectra are obtained at low resolution in conventional electron impact Auger spectra, and at high resolution by using monoenergetic incident electron beams14. We believe that the instrument described here was the first serious attempt to combine the above techniques. The advantages of such a system for clean surface work are considerable and obvious. A reactive clean surface may become completely contaminated in a few seconds at lo- 6 torr pressure, necessitating base pressures of less than lo-’ torr for their study over any extended period of time. Such systems are expensive and the incorporation of four techniques into one UHV chamber affords a considerable cost saving. Since all the techniques involve the analysis of electron energies, one electron energy analyzer suffices, again reducing cost and also having the advantage that the complementary information obtained by the techniques may be compared under identical conditions of resolution and sensitivity. Finally, and most important, one knows that one is studying the same surface condition of the sample by each of the techniques if one has sequential operation inside one UHV system. II. INSTRUMENTATION (a)
The vacuum system
A block diagram of the complete vacuum system is shown in Figure 1. It may be sub-divided into four constituents; the analyzer/target chamber; the pumping arrangements for the X-ray and UV sources; the preparation chamber, and the gas handling facilities. Stainless steel and glass are used throughout. Most of the vacuum seals are metal gaskets though the three gate valves have viton O-ring seals. The analyzer/target chamber is a cylinder of 20 cm diameter and 30 cm height. The 4” oil (polyphenyl ether) diffusion pump is used in conjunction with a specially designed (Vacuum Generators) liquid nitrogen cold trap which maintains two cold walls for about 22 hours. Pressures are measured by VIG 20 ion gauges mounted near the cold trap and in the chamber. Bake-out to 250°C is possible with the viton-sealed valves open, but in practice bake-out at 180°C for 8 hours is sufficient to give a base pressure
243
I
I
I I
I
-__---__-
I
Figure 1. Schematic block diagram of complete vacuum system_ T/AC = target/analyzer chamber, SC = sample chamber, X = X-ray source, U = UV source, TB = target bridge piece, SP = sample probe, A = hemispherical analyzer, T = liquid nitrogen trap, D = diffusion pump, S = molecular sieve trap, R = rotary pump, 1 = ion gauge, P = Pirani gauge, M = metal valve, V = Viton sealed valve, L = metal leak valve, G = gate valve (Viton sealed), B = gas bulb. The dotted circle, U, in the target/analyzer chamber denotes the mating flange for the UV source.
reading of ca. 2 x lo- lo torr in the chamber. The chamber carries on ports around it the X-ray source, the UV source, the preparation chamber, and a gas inlet line. The X-ray source (described in section II(b)) is separately pumped by an 8 litre/sec ion pump which maintains a pressure of better than 1 x lo-’ torr during operation. The base pressure in the chamber rises to about 5 x lo- lo torr during X-ray operation, owing to heating of the aluminium window separating the source from the chamber (see Figure 2). A metal bypass valve allows the source and chamber to be pumped together when necessary to avoid a large pressure differential across the window. The windowless UV source (described in section II(b)) has two-stage differential pumping allowing the maintenance of a pressure of less than 1 x lo-’ torr of helium in the analyzer/target chamber during operation in the He I mode (less than 1 x lo-’ torr for He II mode). There is a viewing port in line of sight with this source to which a quadrupole mass analyzer may be connected for checking the residual gas constitution, The gas inlet line from the gas handling system enters the chamber through a bakeable leak valve. The preparation chamber (described in section Ilc) may be isolated from the analyzer/target chamber by an O-ring sealed gate valve. The 2” pumping stack is
244
WATER WATER
WATER ANODE
COOLED ASSEMBLY
/INSULATOR
CERAME
COPPER GASKET SEAL BETWEEN
X-RAY
WATER
COOLING
SOURCE
FILAMENT
COIL
Figure 2. Cross-section through X-ray tube assembly.
capable of producing a base pressure of 3 x lo- ‘a torr after bakeout. A V.G. Q7 quadrupole mass analyzer mounted on one of the preparation chamber access ports allows residual gas analysis. At a pressure of - 1 x lo-’ torr the only constituents observable are CO,, CO and H,O, and in particular there is no evidence of any high mass contamination from the pump fluid or viton O-rings, though opening or closing the gate valve separation preparation chamber from target/analyzer chamber produces a momentary pressure burst (3 x IO-’ torr) of hydrocarbons. The gas handling system consists of two interconnectable parts. One is a nonbakeable trapped rotary pumped manifold used for admitting permanent gases from high pressure (1 atm) storage bulbs. The other is a small bakeable fine pumped by a
245 1” oil diffusion pump stack capable of reaching a base pressure of 5 x lo-’ torr. This is used for the preparation and purification of liquids (e.g. H,O) prior to their use as sources of vapour in adsorption studies and for situations where extreme purity of gaseous samples is required. (b)
X-ray, WV, and electron
sources
X-ray Source A cross-section through the X-ray tube is shown in Figure 2. It is a modified Henke design’ 5 with no direct line of sight from filament to anode. The aluminium anode is water cooled with a maximum usable power of 1000 Watts (10 kV at 100 mA). Higher powers than this are likely to melt the anode or the aluminium window, and in practice we (at Bradford) never operate for any length of time above 503 W. If the filament is new or if the apparatus has just been baked or exposed to atmospheric pressure it is necessary to outgas the filament and then to operate first at low wattage before increasing to 500 W. The power supply for the X-ray source consists of a thyristor type emission control circuit stabilising the filament emission to 0. I 0/Owith switched control ranges of 25, 50, 75 and 100 mA. The anode voltage is stabilised to 0.3 % using a transductor arrangement. As is usual in such X-ray sources a small percentage of Al Ka,,, radiation at 1496.2 eV is emitted along with the main Al Ka, ,2 component at 1486.6 eV. Its presence and relative intensity may be observed in Figure 10 from the satellite Au 4f peaks 9.6 eV below the main 4f lines. The aluminium anode may be replaced by a magnesium one (Mg Ka, ,2 at 1256 eV) if required. UV source The UV source (Figure 3) is a gas discharge lamp in which the discharge is confined by a quartz capillary tube. External fan cooling is employed as necessary on to a copper cooling disc attached to the anode feedthrough. Gas is admitted via a bakeable leak valve. Two stage differential pumping is employed from a roughing pump and to the sample preparation diffusion pump so that the analyzer can operate in the lo-’ torr range whilst the lamp is operating. Provision is made for fitting a window on the end of the lamp. The lamp is attached to a 22” flange for matin$ with the spectrometer by a bellows arrangement so that correct alignment of the cabillary with the sample probe can be made by means of external positioning screws .on the body of the bellows. A liquid nitrogen cooled sorption trap containing Linde 5A molecular sieve and a bakeout jacket for regenerating the sieve at 300 “C are provided for helium purification before admission through the leak valve. The UV lamp power supply provides 5 kV for starting the discharge and a 1 kV current-stabilised supply variable from 20 to 50 mA to run the lamp. The lamp voltage is determined by the discharge characteristics of the lamp itself. When operated with helium, a discharge
246
COOLING
FIN
ANODE J CAPlLLlARY
INLET
DIFFERENTIAL ALIGNMENT SCREW\
Figure 3. Cross-section
through UV source.
FILAMEN
ANODE \
\ Figure 4. Cross-section
through electron gun.
INSULATOR
247 lamp of this type always consists of a mixture of He I (21.2 eV) and He II (40.8 eV) radiation, plus small percentages of other He lines and impurity lines16* I’. Only at very low helium pressures however does the percentage of He II radiation become significant. The maximum He II/He I ratio (about 1:3) is therefore obtained by starting the lamp at a high helium pressure (giving about 1 x lo- ’ torr He in the analyzer chamber) and then gradually reducing the pressure. When the maximum He lI/He I ratio has been reached the colour of the discharge has changed from a characteristic “peach”, to a pinkish blue. The analyzer pressure is then in the region of 5 to 10 x lo-lo torr. Any attempt to reduce the pressure further causes the lamp to go out and necessitates re-starting at the higher pressure.
Electron gun source The electron gun (Figure 4) is of the triode flood type, comprising a directly heated filament, Wehnelt cylinder for beam current control, and an anode which is connected to the target bridge piece. The gun may be operated from a few volts to 2.5 kV with current at the specimen of l-500 mA.
Overall configurution The position of the X-ray source relative to the target analyzer chamber can be seen from Figure 1. The relative positions of the other two sources may be judged
X-RAY
SOURCE X-RAY
ELECTRON
APERTURE
GUN
COLLIMATING
CAPILLARY
COLLIMATING TUBE FOR VIEWING
ANALYSER PIECE
I
BRIDGE
Figure 5. Overall configuration of the three electron inducing sources.
248 from the schematic diagram in Figure 5. The electron gun is actually mounted inside the analyzer chamber directly on to the target bridge piece (section He). The X-ray source and the UV source are mounted on flanged ports on the target/analyzer chamber.
(c) Preparation chamber The preparation chamber (Figure 6) consists of a cylinder of 10 cm radius and,16 cm length. At one end it is attached to a port from the analyzer chamber by a 23” flange (A). The opposite end:plate, (B), is demountable and carries a translational drive mechanism, (C), which utilises an edge-welded bellows. The sample probe is
mounted into the drive mechanism at a mini-conflat copper gasket sealed flange (1.44 inch diameter), (D). A ceramic section, (E) in the drive mechanism electrically isolates the probe from the preparation chamber. Two rings of ports are set into the chamber. One ring, (F), contains five 2%” flanged ports. The other, (G), contains one 2r port and four mini-conflat ports. Three of the mini-conflats are used for connec-
tion to the gas handling system via bakeable leak valves or miniature metal valves, the fourth carries a pirani gauge. The 2%” port in this ring usually carries the 47 quadrupole. To one of the ports in ring F is attached the bellows mechanism which operates the gate valve used for isolating the preparation chamber from the analyzer/ target chamber. This valve operates by moving two metal discs across the face of the sealing O-ring. When in place the final sealing pressure is provided by the discs being forced apart by stainless steel ball bearings. The remaining flanges in ring F carry a viewing port, an argon ion gun for sputter cleaning, an ion gauge and a filament
Figure 6. Scale drawing of the preparation
chamber
and sample probe.
X-RAY
SPECIMEN
GUIDE
ELECTROSTATK:
SOURCE
UV SOURCE FLANGE (FC
BUSH
MOUNTING 36)
SHIELD
VP PUMPING
LINE
ADJUSTABLE PLdTE PlVO
FC 150
FEEDTHROUGH (aI
X-RAY
SOURCE
ANALYSER HEMISPHERES\
ANALYSER CHAMBER S’ECYHEN PREMRATION CHAMBER MOUNTlNG FLANGE (FC 38) ADJUSTABLE SLIT PLATE ‘SPECIMEN ANALYSER
ENTRANCE
MOUNT
SLIT
MULTIPLIER
-EXIT
\ SINGLE -WAY ELECTRlCAl.
SLIT
FEEDTHROUGHS
( bl Figure 7. (a) Cross-section through analyzer chamber viewed from preparati on chamber (b) Cross-section through analyzer chamber (front view).
position;
250
BRIDGE
PIECE
CHAMELTRDN
HIGH
YDLTAGE
SUPPLY X-Y
R-R
Figure 8. Schematic of energy analysis, detection, and data acquisition system.
evaporation source. When fully withdrawn the probe face is in line with ring F. When the gate valve is open and the probe drives fully home into the analyzer/target chamber a seal is made again between the chambers by the flange, H, mounted on the probe shaft. (d)
Sample probes We have four types of probe in operation, two of which are now supplied as standards by VG. The simplest probe is non-rotatable but may be fastened into the drive mechanism in the required orientation to face the desired source when translated into the analyzer chamber. The geometry of the sources (section IIb) is such that the probe face can be pre-set to accept radiation from the X-ray source and the U-V source in one position and the X-ray source and the electron source in another. These are the most useful combinations. The disadvantages of a non-rotatabIe probe are that when set in the compromise X-ray/UV position one does not obtain full UV sensitivity, and that this orientation is not suitable for viewing the probe face. The probe may be heated to about 300°C or cooled to temperatures approaching - 195°C by circulating heated nitrogen gas or liquid nitrogen through a tube which passes down the inside of the probe to the end block, I, (constructed of nickel for good thermal contact). A Cr/Al thermocouple is buried in the end block. The rotating motion of the rotary probe is provided by a bellows mechanism on the probe itself, allowing a rotating motion of about f 45 degrees, enabling the sample to face all three sources. This probe is also temperature controlled by flowing nitrogen but because of the extra
251 mass of metal it is difficult to exceed 200°C. The third probe is similar to that shown in Figure 7, except that the circulating nitrogen tubes are replaced by an electrical heater embedded in the end block. This allows the face of the probe to be raised to about 6OO”C, but of course removes the possibility of cooling. A selection of sample plates may be attached to the probe face. These are useful for retaining powdered samples by gluing, evaporating from solution, or pressing into a fine wire mesh. The fourth probe is a prototype ribbon or single crystal holder system which allows a typical tungsten ribbon (2 cm x 0.5 cm x 0.003 cm) to be resistively heated to about 2000 “C.
(e) Analyzerlfarget
chamber
The important features of the analyzer/target chamber are shown in Figures 7 and 8. The analyzer’ a (detail in Figu re 7) is made of gold plated aluminium and is of the concentric spherical type’ ‘, utilizing a 150” sector of the full spherical configuration (10 cm radius). The field is terminated by Herzog correction plates. A rotatable slit plate carries three pairs of diametrically opposite slits of 1 cm length and width 0.25, 1 and 2 mm. The plate is connected by a crank to a linear motion drive so that any pair may be selected as entrance and exit slits to the analyzer without breaking vacuum. The equation relating the potential applied between the analyzer plates, V, and the energy of the electrons passed, E, is given by”
(1) where R, and R, are the radii of the inner and outer hemispheres here) which reduces to
V = EfH
(9 cm and 11 cm
(2)
where H, the hemispherical constant, is theoretically equal to 2.48. _ The voltage supply to the analyzer may be preset to pass electrons of energy, E, equal to 5, 10, 20, 50 or 100 eV. By varying the specimen potential, I?, ejected electrons are accelerated or retarded to match the analyzer pass energy being used. Thus if KE is the kinetic energy of the ejected electrons KE = E’ + E
(3)
The voltage, E’, is derived from a O-1500 V variable power supply (to select a kinetic energy scan starting position) in series with a ramp generator which can be switched to give kinetic energy scan ranges of 10, 30, 100, 300, or 1000 eV, in times variable from 30 to 3000 seconds.
252 The resolution aberrations. dE E
-=-
W
2R
of the analyzer is given by the following
equation,
neglecting
(4)
where dE is the full signal width at half-height, w is the total slit-width and R the mean radius. Thus by varying E and w the theoretical instrumental resolution can be changed from 12.5 meV to 2.0 eV. The transmission of electrons at any given energy is proportionalzl to l/E so that high resolution is only obtained at the expense of sensitivity (as usual). By operating the spectrometer as described the resolution is constant and independent of the initial kinetic energy. However since the spectrometer entrance angle is fixed and electrons leaving the specimen are diverged by a retarding field and converged by an accelerating field, sensitivity will decrease with increasing initial kinetic energy, and vice versa. To a first approximation, and when initial KE is high compared to E (i.e. E’ N ICE) relative sensitivities are inversely proportional to kinetic energy. Signal detection is by a channel electron multiplier mounted over the exit slit; the output pulses are amplified and counted by a ratemeter and readout is direct to an X-Y recorder (Figure 8). The analyzer chamber is surrounded by a p-metal shield during operation, to minimise the effect of external magnetic fields. III. PERFORMANCE (a)
Sample preparation The illustrative examples of the spectrometer performance presented below are all of polycrystalline films. These were prepared by evaporation from filaments, either by melting small strips of the metal concerned (low melting point) on to a tungsten filament, or from a filament of the metal concerned (high melting point). No special filament cleaning procedures in vacua were adopted, but during and after bakeout the filament was thoroughly outgassed so that during evaporation pressures of 1o-9 torr were maintained. This results in films largely clean of surface contaminants”, 23. (b)
Vacuum ultraviokt spectra The He I and He II photoelectron spectra of a clean gold film are shown in Figure 9. The slit-width was set at 1 mm and the analyzing energy at 10 eV, thus giving an instrumental resolution of 0.1 eV. The steeply rising back-ground at low electron energies in the He I spectrum (trace (b)) is due to the build-up of multiply inelastically scattered electrons. Electrons of less than 10 eV energy are accelerated into the analyzer and as already mentioned the acceleration converges electrons through the
253
(b)
BE.
CCW-
Figure. 9. Ultraviolet photoelectron spectrum of a clean gold surface. (a) He I (21.2 eV) spectrum, analyzer scan mode; (b) He I spectrum, retarding potential scan, 10 eV analyzing energy; (c) He II (40.8 eV) spectrum, retarding potential scan, 10 eV analyzing energy.
slit, thus increasing sensitivity, hence accentuating the scattered background. A more common method of recording band structure by photoemission is to analyze the electrons at their ejection energies by sweeping the analyzer voltages. An He I spectrum taken in this manner is shown in trace (a) (cf. Eastman”). The “slow” peak is depressed since transmission is proportional to E, and the spectrum is therefore an E x n(E) versus E plot. In fact at very low energies (-c 1 eV) small inhomogeneities of field can cause sufficient deflection for the signal to fall even more rapidly. The He II spectrum may be compared
to previously
published
data24.
be cases where high resolution is required to look for possible vibrational fine structure. To test the high-resolution capabilities of the analyzer, the He I spectra of gaseous argon were taken (using the 0.25 mm slit-width) and a There
may
254 special gas probe which admits sample gas at moderately high pressure (- 0.01 torr in the collision region) directly into the target chamber. A best resolution of 30 meV for 5 eV electron energy was recorded, more than adequate to discern any solid state fine structure resolution requirements. The theoretical resolution of 12.4 meV is presumably not obtained owing to residual magnetic fields and the presence of contact potentials.
(c) X-ray spectra Figure 10 shows the full ESCA spectrum of Au taken with the aluminium source, plus expanded regions for the 4f core levels and the band structure. All recording parameters are given in the figure caption. The major portion of the experimental half-width is of course due to the line width of the X-rays3. Figure 11 shows the 3d peaks of silver taken with a magnesium anode at 10 eV analyzing energy. This is the best absolute resoultion we have achieved in the X-ray mode. Figure 12 shows a spectrum of freshly evaporated MO, and the same sample after several days exposure to a residual background pressure (mainly CO) of ca. 5 x lo-‘* torr at room temperature. The O(ls) peak height has just reached a maximum at this point, which we know from chemisorption studiesz5 corresponds to ca. 60% surface coverage. Since the sticking probability of CO on MO is close to unity up to this coverage, and therefore MO is an example of one of the most reactive surfaces, the above result emphasises the need for UHV conditions to study such
4 I El:
5
10 1 200
I 4MI . . (cV) BE
I MO _
Figure 10. X-ray photoelectron spectrum of clean gold surface (Al Ku source, 1486.6 eV). Main spectrum: 50 eV analyzing energy, 250 watts X-ray power, scan time 2ooO sec. Band structure inset: 20 eV analyzing energy, 500 watts X-ray power, scan time 1000 sec. 4f peaks inset: 20 eV analyzing energy, 500 watts X-ray power, scan time 100 sec.
255
1
366
I
I
I
366
I
I
466
I
462
I
I
404
,
I
a8
RE. W’,
Figure 11. X-ray photoelectron spectrum (Mg Ka source, 1253.6 ev) of 3d peaks of silver. Analyzing energy 10 eV, scan time 330 sec.
surfaces adequately. The spectrum of Au on the other hand remains unchanged in the spectrometer over a period of weeks since it is unreactive tb the residual gases present (CO, H,O, CO,). Hydrocarbon residues, however, which are usually present in non-UHV systems, would quickly build up on the Au surface to give a carbon signal.
(d) Auger electron spectra Auger electrons are of course generated by X-ray impact as well as electron impact, and these may be recorded together with the photoelectron spectra (for example see Sch6nZ6). Auger electron energies often fall in the 300-O eV range, as is the case for example in MO (Figure 13). In this range one gets a rapid build up of multiply scattered electrons giving rise to a steeply sloping background on which sit the Auger peaks (trace a). This may be counteracted (with some loss of resolution towards high electron energies) by analyzing at the ejected electron energy (of He I spectra) as in trace (b). This is the manner in which commercially available Cylindrical Mirror Analyzers are usually operated for electron impact Auger spectroscopy”, but the spectrum is then conventionally recorded in the first differential mode. Such a recording2’ for MO has been re-integrated in Figure 13, trace (c), and so may be compared to the X-ray induced spectra of (a) and (b). We have not yet been able to use the electron gun arrangement on the present spectrometer as a source for producing Auger spectra. This is because with the present design a large number of electrons escape from the rear of the gun and find their way through the analyzer, creating SI very high background. A modified, capped version
256
c cw
‘WITION
I
;‘- :
Figure 12. X-ray photoelectron spectrum of a “clean” molybdenum surface. 50 eV analyzing energy, 500 watts X-ray power, scan time 500 sec. Insets: Expanded C(ls) and O(ls) regions of “clean” surface and after 2 days’ exposure to residual background pressure. 100 eV analyzing energy, 500 watts X-ray power, scan time C(ls) 300 sec. 0( 1s) 500 sec.
of the gun has been used successfully with an otherwise very similar spectrometer. A typical spectrum, recorded in the dN/dE mode is shown in Figure 14. (c)
Calibration of absolute energy scale for the spectrometer Though the normal practice in ESCA is to calibrate spectra against some “standard” line, direct spectrometer calibrations are feasible and useful in some cases. For example the ‘Labsolute” binding energy of Au 4f,,2, which is commonly used as a calibrant, has been quoted at values between 82.0 and 84.0 eV3* 26* 2g. We have measured the value by several calibration procedures within this spectrometer and obtain an internally consistent value of 83.7 & 0.1 eV, which is in agreement with the most carefully determined literature value 2g_ The procedure used was to apply equation 2 to any of the sharply defined peaks in the He I spectrum recorded in the analyzer scanning mode (Figure 9c). The experimental hemisphere voltage, V,, for the peak concerned was recorded on the digital voltmeter, and an accelerating voltage of 4.000 V was then applied to the sample, the experimental spectrum recorded again,
257
Figure 13. Auger spectrum of molybdenum. (a) Al Ka induced, retarding potential scan mode, analyzing energy 50 eV; (b) Al Ka induced, anaIyzer scan mode; (c) Electron impact induced, spectrum re-integrated28.
and the new hemisphere H(V,
-
voltage
V,) = El -
E,
V, measured. = 4.000 eV
Since (5)
one has a unique determination of 27, the hemisphere constant. The value obtained was H = 2.62. c_&, the spectrometer work function, was then found by locating the Fermi edge’accurately in the He I spectrum and noting the experimental hemisphere voltage, V, at that position. Since HV,
= EF = 21.22
-
&,
I ,
I 500
400
I 700
I 600
t IO
5 K.E. ICVl
Figure 14. Differential Auger spectrum (electron impact induced) of an argon-ion bombarded sample of stainless steel. Gun energy 2.4 kV, current 350 ,vA. Analyzer energy 50 eV, scan time 1000 sec.
&, can therefore be determined. A value of &, = 4.02 eV was obtained. H can also be found directly from X-ray spectra, taken in the retarding potential mode but with lower accuracy_ The kinetic energy of an ejected electron using AI Kcc radiation is given by KE
=
1486.6 -
(E, + &,)
(7)
where Eb is the binding energy of the electron with respect to the Fermi level. After retardation to the analysis energy, E, E = 1486.6
-
E’ -
(E, + I&,) = HP’,.
(8)
Recording at two different hemisphere voltages, v1 and Vz and measuring the retarding potentials required El and E; to focus a particular binding energy peak, E,,, for analyzing energies E, and E,, gives (E, -
E2) = E’, -
EL = H(r/, -
V.)
Applying this equation to the 4f lines of Au resulted in an identical value of 2.62 for H. Substituting for N and c&, in eqn. (8) provided the value of 83.7 + 0.1 eV for Eb
259
4f,,2 Au.
Alternatively one may calibrate without reference to &, found from the He I spectrum by locating the Fermi edge in the X-ray spectrum itself, though this is less accurate. Agreement with the above figure was obtained by this method. IV. POSSIBLE
INSTRUMENTAL
IMPROVEMENTS
The instrument described here is a prototype and can certainly be improved upon. Some improvements have already been incorporated in more recent versions of ESCA-3. Base pressures of 2-3 x 10-l’ torr are normally adequate, but for extended studies of clean surfaces it would be very useful to be able to reduce this by half an order of magnitude. One of the inherent disadvantages of the XPS technique is the long recording times that are required for good statistics on the small surface feature signals at high resolution. An increase in total signal strength by about a factor of five is thought to be attainable by changing the X-ray source geometry, the shape of the aluminium window, and increasing the analyzer slit widths (increase of w and the solid angle of acceptance) while maintaining resolution by analyzing at a lower kinetic energy. An enhanced surface/bulk signal ratio is to be expected if the angle of exit of the detected electrons with respect to the sample surface is smal13’. A more advanced data collecting system would be an advantage. A fully integrated computer controlled system capable of handling data simultaneously from up to eight regions of the electron spectrum is being developed by VG. Particularly useful features will be averaging, deconvoluting, and base-line correction facilities, storage of standard spectra for matching purposes, and considerable programmable control for the study of time-dependent surface processes. The preparation chamber is not ideal. It is too small for completely successful evaporation of refractory metals, since the chamber walls can reach quite high temperatures by radiation from the filament. Cooling externally with dry ice helps but does not completely alleviate the problem. A liquid nitrogen shrouded evaporating filament would probably be successful. The small size of the chamber also restricts the number of line of sight ports to the sample. The presence of the preparation chamber enables one to perform reactions with reactants and at pressures which one would be reluctant to use in the UHV analyzer chamber. However, much of the work with which we are concerned involves vacuum worthy reactants at low pressures, and as such is carried out in the analyzer chamber. For such situations the preparation chamber is largely superfluous, and the mechanics of moving the sample probe from one to the other restricts flexibility. V. POTENTIAL USES OF COMBINED SURFACE STUDIES
All the electron
spectroscopic
ELECTRON
techniques
SPECTROSCOPY
can be surface
TECHNIQUES
sensitive,
FOR
but as
mentioned earlier the controlling factor as far as the escape or probing depth is concerned is the kinetic energies of the ejected electrons. Since in most cases the energies of the photoelectrons measured in XPS are greater than those in UPS or Auger the latter two techniques will generally probe the region closer to the surface. However this is not all that should be considered3’ . For instance the recording of Auger data is generally much quicker than XPS by virtue of the high electron beam currents available, but this advantage is often negated because of strong interaction between the electron beam and the surface being studied (desorption, dissociation of adsorbed species, physical disruption of the substrate). XPS though slow, is generally “milder” in causing the above effects and more readily offers chemical shift data. The electron energies of UPS are in the range that should make them the most surface sensitive in terms of escape depth. However the fact that one is experimentally dealing with a region of overlapping band structures, the interpretation of which may be important but extremely complex, and which provide no atomic identification (cf. XPS or Auger), could in general reduce its worth as an individual surface probe. The combination of the various favourable aspects of the three techniques makes the effective use of each of them far greater than any of the techniques individually. We should in principle have available a fast, semi-quantitative, surface element analysis (Auger), detailed information on the different binding environments of these elements (Chemical Shifts and other effects in XPS), plus high-resolution data on the band structures (UPS). Our aims for the spectrometer described here are to bring this combination of techniques to bear mainly on problems of adsorption on to clean metal surfaces with the object of moving on to problems in catalysis once it is clearer from the basic studies what the subject has to offer in such a complex area. REFERENCES 1
2 3
10 11 12 13 14
D. W, Turner, A. D. Baker, C. Baker and C. R. Brundle, Molecular Photockctron Spectroscopy, Wiley-Interscience, London, 1970. W. E. Spicer, in F. Abeles (editor), Optical Properties of Solids, North-Holland Publ. Co., Amsterdam, 1970. K. Siegbahn et al., ESCA-Atomic, Molecular, and Solid State Structure Studied by Means of EZectron Spectroscopy, Nova Acta Regiae Sot. Sci. Upsaliensis Ser. IV, Volume 20, 1967. For example, G. K. Wertheim and S. Hiifner, Phys. Rev. Lett., 28 (1972) 1028. K. Siegbahn et al., ESCA AppIied to Free Molecules, North-Holland Publ, Co., Amsterdam, 1969. D. E. Eastman and J. K. Cashion, Phys. Rev. L&t., 24 (1970) 310. W. N. Delgass, T. R. Hughes and C. S. Fadley, Catal. Rev., 4 (1971) 179. T. A. Carlson and G. E. McGuire, J. Electron Spectrosc., 1 (1972/73) 161. M. Klasson, J. Hedman, A. Bemdtsson, R. Nilsson, C. Nordling and P. Melknik, Phys. Ser., 5 (1972) 93. C. R. Brundle and M. W. Roberts, Chem. Phys. Left., 18 (1973) 380. D. E. Eastman and J. K. Cashion, Phys. Rev. L&t., 27 (1971) 1520. C. R. Brundle, in M. W. Roberts and J. M. Thomas (editors), Surface and Defect Properties of Solids, Vol. I, Specialist Periodical Report, Chemical Society, London, 1972. S. Trajmar, J. K. Rice and A. Kupperman, Advan. Chem. Phys., 18 (1970) 15. H. Ibach, J. Pac. Sci. Tech&., 9 (1972) 713.
261 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
B. L. Henke, in I-I. H. Pattee, V. E. Cosslett ?nd A. Engstrbm (editors), X-Ray Optics andX_Ray Microanalysis, Academic Press, New York, 1963. C. R. Brundle, Chem. Whys. Lett., 5 (1970) 410. J. A. R. Samson, Techniques of Vacuum Ulfraviolet Spectroscopy, Wiley, New York, 1967. The analyzer was designed in collaboration with Dr. K. J. Ross, Department of Physics, University of Southampton. J. A. Simpson, Rev. Sci. Instrum., 35 (1964) 1698. C. E. Kuyatt and J. A. Simpson, Rev. Sci. Instrum., 38 (1967) 103. J. C. Helmer and N. H. Weichert, A@. Phys. Lert., 13 (1968) 266. C. R. Brundle and M. W. Roberts, Proc. Roy. Sot. (London), A 331 (1972) 383. S. J. Atkinson, C. R. Brundle and M. W. Roberts, J. Electron Spectrosc., 2 (1973) 105. D. E. Eastman, Phys. Rev. Lett., 26 (1971) 1108. R. R. Ford, Advun. Catal., 21 (1970) 51, and references quoted therein. G. Sch6n, J. EIecfron Spectrosc., 1 (1972/73) 377. H. E. Bishop, J. P. Coad and J. C. RiviBre, J. Electron Spectrosc., 1 (1972/73) 389. P. W, Palmberg, G. E. Riach, R. E. Weber and N. C. MacDonald, Handbook of Auger Electron Spectroscopy, Physical Electronic Industries, Minnesota, 1972. G. Johansson, J. Hedman, A. Berndtsson, M. Klasson and R. Nilson, J. Electron Spectrosc., 2 (1973) 295.
30 31
W. A. Frazer, J. V. Floris, W. D. Robertson and W.-N. Delgass, Surface Science, 36 (1973) 661. C. R. Brundle, J. Vuc. Sci. Technol., Jan/Feb 1974 issue.
AN ULTRAHIGH STUDIES
VACUUM
ELECTRON
C. R. BRUNDLE, M. W. ROBERTS School of Chemistry, University of Bradford, Bradford,
SPECTROMETER
Yorkshire 807
FOR
SURFACE
IDP (England)
D. LATHAM and K. YATES Vacuum Generators Ltd., East Grinstead, Sussex (England)
(Received 10 September 1973)
ABSTRACT A high-resolution electron spectrometer has been constructed for the study of clean metal surfaces and their interactions with gases. In its present form it is most suitable for the study of evaporated films but may also be used for ribbons, single crystals, or powders. Electron spectra induced by three sources, X-ray, vacuum ultraviolet, or electron impact, may be recorded sequentially. Electron energy analysis is normally performed by setting the analyzer (10 cm radius, hemispherical) to pass electrons of a fked energy and applying a scanning retarding potential to the sample. The limiting instrument absolute resolution is about 25 meV FWHM at 5 eV analyzing energy, as judged by an He I gaseous argon spectrum. Resolution can be traded for sensitivity by analyzing at higher electron energies (up to 100 eV), or by means of an external slit-width adjustment mechanism. The spectrometer carries a preparation chamber which has a variety of demountable sample preparation, cleaning, and reaction monitoring devices. Spectra from gold, silver, and molybdenum films. are presented by way of illustration. I. INTRODUCTION Photoelectron spectroscopy, as developed over the past five years, has become one of the most useful techniques for studying electronic energy levels. The study of valence electron energy distributions with vacuum UV radiation (5-50 eV, but generally Q 21.2 eV) was termed Molecular Photoelectron Spectroscopy (MPS or UPS) where gases were studied’ and Photoemission for solids’. The study of core level energies (usually solids) with X-radiation (up to 20 000 eV, but generally < 1500 eV) acquired the name Electron Spectroscopy for Chemical Analysis (ESCA)3. Within the last three years improvements in instrumental sensitivity have facilitated
242 the study of valence ievels4 and gaseous studies5 by ESCA. It has been the exception rather than the rule that these complementary studies (X-ray and UV) were performed in the same spectrometer. It had been recognised initially that both techniques were surface sensitive23 3 but the extent of this sensitivity was perhaps not fully appreciated for some time (for example, see Eastman 6 and Delgass et al.‘). There is now available a considerable to indicate that the depths probed are similar (within a factor of body of data’-l’ 2 or 3) to those generally accepted for electron impact Auger spectroscopy, and that the major controlling factor in determining this depth in all three types of spectroscopy is the kinetic energy of the escaping electron being analyzedi2. Energy Loss Spectroscopy, the inelastic scattering of an incident electron beami3, makes a fourth electron energy analysis technique which has been shown to be surface sensitive14. Energy loss spectra are obtained at low resolution in conventional electron impact Auger spectra, and at high resolution by using monoenergetic incident electron beams14. We believe that the instrument described here was the first serious attempt to combine the above techniques. The advantages of such a system for clean surface work are considerable and obvious. A reactive clean surface may become completely contaminated in a few seconds at lo- 6 torr pressure, necessitating base pressures of less than lo-’ torr for their study over any extended period of time. Such systems are expensive and the incorporation of four techniques into one UHV chamber affords a considerable cost saving. Since all the techniques involve the analysis of electron energies, one electron energy analyzer suffices, again reducing cost and also having the advantage that the complementary information obtained by the techniques may be compared under identical conditions of resolution and sensitivity. Finally, and most important, one knows that one is studying the same surface condition of the sample by each of the techniques if one has sequential operation inside one UHV system. II. INSTRUMENTATION (a)
The vacuum system
A block diagram of the complete vacuum system is shown in Figure 1. It may be sub-divided into four constituents; the analyzer/target chamber; the pumping arrangements for the X-ray and UV sources; the preparation chamber, and the gas handling facilities. Stainless steel and glass are used throughout. Most of the vacuum seals are metal gaskets though the three gate valves have viton O-ring seals. The analyzer/target chamber is a cylinder of 20 cm diameter and 30 cm height. The 4” oil (polyphenyl ether) diffusion pump is used in conjunction with a specially designed (Vacuum Generators) liquid nitrogen cold trap which maintains two cold walls for about 22 hours. Pressures are measured by VIG 20 ion gauges mounted near the cold trap and in the chamber. Bake-out to 250°C is possible with the viton-sealed valves open, but in practice bake-out at 180°C for 8 hours is sufficient to give a base pressure
243
I
I
I I
I
-__---__-
I
Figure 1. Schematic block diagram of complete vacuum system_ T/AC = target/analyzer chamber, SC = sample chamber, X = X-ray source, U = UV source, TB = target bridge piece, SP = sample probe, A = hemispherical analyzer, T = liquid nitrogen trap, D = diffusion pump, S = molecular sieve trap, R = rotary pump, 1 = ion gauge, P = Pirani gauge, M = metal valve, V = Viton sealed valve, L = metal leak valve, G = gate valve (Viton sealed), B = gas bulb. The dotted circle, U, in the target/analyzer chamber denotes the mating flange for the UV source.
reading of ca. 2 x lo- lo torr in the chamber. The chamber carries on ports around it the X-ray source, the UV source, the preparation chamber, and a gas inlet line. The X-ray source (described in section II(b)) is separately pumped by an 8 litre/sec ion pump which maintains a pressure of better than 1 x lo-’ torr during operation. The base pressure in the chamber rises to about 5 x lo- lo torr during X-ray operation, owing to heating of the aluminium window separating the source from the chamber (see Figure 2). A metal bypass valve allows the source and chamber to be pumped together when necessary to avoid a large pressure differential across the window. The windowless UV source (described in section II(b)) has two-stage differential pumping allowing the maintenance of a pressure of less than 1 x lo-’ torr of helium in the analyzer/target chamber during operation in the He I mode (less than 1 x lo-’ torr for He II mode). There is a viewing port in line of sight with this source to which a quadrupole mass analyzer may be connected for checking the residual gas constitution, The gas inlet line from the gas handling system enters the chamber through a bakeable leak valve. The preparation chamber (described in section Ilc) may be isolated from the analyzer/target chamber by an O-ring sealed gate valve. The 2” pumping stack is
244
WATER WATER
WATER ANODE
COOLED ASSEMBLY
/INSULATOR
CERAME
COPPER GASKET SEAL BETWEEN
X-RAY
WATER
COOLING
SOURCE
FILAMENT
COIL
Figure 2. Cross-section through X-ray tube assembly.
capable of producing a base pressure of 3 x lo- ‘a torr after bakeout. A V.G. Q7 quadrupole mass analyzer mounted on one of the preparation chamber access ports allows residual gas analysis. At a pressure of - 1 x lo-’ torr the only constituents observable are CO,, CO and H,O, and in particular there is no evidence of any high mass contamination from the pump fluid or viton O-rings, though opening or closing the gate valve separation preparation chamber from target/analyzer chamber produces a momentary pressure burst (3 x IO-’ torr) of hydrocarbons. The gas handling system consists of two interconnectable parts. One is a nonbakeable trapped rotary pumped manifold used for admitting permanent gases from high pressure (1 atm) storage bulbs. The other is a small bakeable fine pumped by a
245 1” oil diffusion pump stack capable of reaching a base pressure of 5 x lo-’ torr. This is used for the preparation and purification of liquids (e.g. H,O) prior to their use as sources of vapour in adsorption studies and for situations where extreme purity of gaseous samples is required. (b)
X-ray, WV, and electron
sources
X-ray Source A cross-section through the X-ray tube is shown in Figure 2. It is a modified Henke design’ 5 with no direct line of sight from filament to anode. The aluminium anode is water cooled with a maximum usable power of 1000 Watts (10 kV at 100 mA). Higher powers than this are likely to melt the anode or the aluminium window, and in practice we (at Bradford) never operate for any length of time above 503 W. If the filament is new or if the apparatus has just been baked or exposed to atmospheric pressure it is necessary to outgas the filament and then to operate first at low wattage before increasing to 500 W. The power supply for the X-ray source consists of a thyristor type emission control circuit stabilising the filament emission to 0. I 0/Owith switched control ranges of 25, 50, 75 and 100 mA. The anode voltage is stabilised to 0.3 % using a transductor arrangement. As is usual in such X-ray sources a small percentage of Al Ka,,, radiation at 1496.2 eV is emitted along with the main Al Ka, ,2 component at 1486.6 eV. Its presence and relative intensity may be observed in Figure 10 from the satellite Au 4f peaks 9.6 eV below the main 4f lines. The aluminium anode may be replaced by a magnesium one (Mg Ka, ,2 at 1256 eV) if required. UV source The UV source (Figure 3) is a gas discharge lamp in which the discharge is confined by a quartz capillary tube. External fan cooling is employed as necessary on to a copper cooling disc attached to the anode feedthrough. Gas is admitted via a bakeable leak valve. Two stage differential pumping is employed from a roughing pump and to the sample preparation diffusion pump so that the analyzer can operate in the lo-’ torr range whilst the lamp is operating. Provision is made for fitting a window on the end of the lamp. The lamp is attached to a 22” flange for matin$ with the spectrometer by a bellows arrangement so that correct alignment of the cabillary with the sample probe can be made by means of external positioning screws .on the body of the bellows. A liquid nitrogen cooled sorption trap containing Linde 5A molecular sieve and a bakeout jacket for regenerating the sieve at 300 “C are provided for helium purification before admission through the leak valve. The UV lamp power supply provides 5 kV for starting the discharge and a 1 kV current-stabilised supply variable from 20 to 50 mA to run the lamp. The lamp voltage is determined by the discharge characteristics of the lamp itself. When operated with helium, a discharge
246
COOLING
FIN
ANODE J CAPlLLlARY
INLET
DIFFERENTIAL ALIGNMENT SCREW\
Figure 3. Cross-section
through UV source.
FILAMEN
ANODE \
\ Figure 4. Cross-section
through electron gun.
INSULATOR
247 lamp of this type always consists of a mixture of He I (21.2 eV) and He II (40.8 eV) radiation, plus small percentages of other He lines and impurity lines16* I’. Only at very low helium pressures however does the percentage of He II radiation become significant. The maximum He II/He I ratio (about 1:3) is therefore obtained by starting the lamp at a high helium pressure (giving about 1 x lo- ’ torr He in the analyzer chamber) and then gradually reducing the pressure. When the maximum He lI/He I ratio has been reached the colour of the discharge has changed from a characteristic “peach”, to a pinkish blue. The analyzer pressure is then in the region of 5 to 10 x lo-lo torr. Any attempt to reduce the pressure further causes the lamp to go out and necessitates re-starting at the higher pressure.
Electron gun source The electron gun (Figure 4) is of the triode flood type, comprising a directly heated filament, Wehnelt cylinder for beam current control, and an anode which is connected to the target bridge piece. The gun may be operated from a few volts to 2.5 kV with current at the specimen of l-500 mA.
Overall configurution The position of the X-ray source relative to the target analyzer chamber can be seen from Figure 1. The relative positions of the other two sources may be judged
X-RAY
SOURCE X-RAY
ELECTRON
APERTURE
GUN
COLLIMATING
CAPILLARY
COLLIMATING TUBE FOR VIEWING
ANALYSER PIECE
I
BRIDGE
Figure 5. Overall configuration of the three electron inducing sources.
248 from the schematic diagram in Figure 5. The electron gun is actually mounted inside the analyzer chamber directly on to the target bridge piece (section He). The X-ray source and the UV source are mounted on flanged ports on the target/analyzer chamber.
(c) Preparation chamber The preparation chamber (Figure 6) consists of a cylinder of 10 cm radius and,16 cm length. At one end it is attached to a port from the analyzer chamber by a 23” flange (A). The opposite end:plate, (B), is demountable and carries a translational drive mechanism, (C), which utilises an edge-welded bellows. The sample probe is
mounted into the drive mechanism at a mini-conflat copper gasket sealed flange (1.44 inch diameter), (D). A ceramic section, (E) in the drive mechanism electrically isolates the probe from the preparation chamber. Two rings of ports are set into the chamber. One ring, (F), contains five 2%” flanged ports. The other, (G), contains one 2r port and four mini-conflat ports. Three of the mini-conflats are used for connec-
tion to the gas handling system via bakeable leak valves or miniature metal valves, the fourth carries a pirani gauge. The 2%” port in this ring usually carries the 47 quadrupole. To one of the ports in ring F is attached the bellows mechanism which operates the gate valve used for isolating the preparation chamber from the analyzer/ target chamber. This valve operates by moving two metal discs across the face of the sealing O-ring. When in place the final sealing pressure is provided by the discs being forced apart by stainless steel ball bearings. The remaining flanges in ring F carry a viewing port, an argon ion gun for sputter cleaning, an ion gauge and a filament
Figure 6. Scale drawing of the preparation
chamber
and sample probe.
X-RAY
SPECIMEN
GUIDE
ELECTROSTATK:
SOURCE
UV SOURCE FLANGE (FC
BUSH
MOUNTING 36)
SHIELD
VP PUMPING
LINE
ADJUSTABLE PLdTE PlVO
FC 150
FEEDTHROUGH (aI
X-RAY
SOURCE
ANALYSER HEMISPHERES\
ANALYSER CHAMBER S’ECYHEN PREMRATION CHAMBER MOUNTlNG FLANGE (FC 38) ADJUSTABLE SLIT PLATE ‘SPECIMEN ANALYSER
ENTRANCE
MOUNT
SLIT
MULTIPLIER
-EXIT
\ SINGLE -WAY ELECTRlCAl.
SLIT
FEEDTHROUGHS
( bl Figure 7. (a) Cross-section through analyzer chamber viewed from preparati on chamber (b) Cross-section through analyzer chamber (front view).
position;
250
BRIDGE
PIECE
CHAMELTRDN
HIGH
YDLTAGE
SUPPLY X-Y
R-R
Figure 8. Schematic of energy analysis, detection, and data acquisition system.
evaporation source. When fully withdrawn the probe face is in line with ring F. When the gate valve is open and the probe drives fully home into the analyzer/target chamber a seal is made again between the chambers by the flange, H, mounted on the probe shaft. (d)
Sample probes We have four types of probe in operation, two of which are now supplied as standards by VG. The simplest probe is non-rotatable but may be fastened into the drive mechanism in the required orientation to face the desired source when translated into the analyzer chamber. The geometry of the sources (section IIb) is such that the probe face can be pre-set to accept radiation from the X-ray source and the U-V source in one position and the X-ray source and the electron source in another. These are the most useful combinations. The disadvantages of a non-rotatabIe probe are that when set in the compromise X-ray/UV position one does not obtain full UV sensitivity, and that this orientation is not suitable for viewing the probe face. The probe may be heated to about 300°C or cooled to temperatures approaching - 195°C by circulating heated nitrogen gas or liquid nitrogen through a tube which passes down the inside of the probe to the end block, I, (constructed of nickel for good thermal contact). A Cr/Al thermocouple is buried in the end block. The rotating motion of the rotary probe is provided by a bellows mechanism on the probe itself, allowing a rotating motion of about f 45 degrees, enabling the sample to face all three sources. This probe is also temperature controlled by flowing nitrogen but because of the extra
251 mass of metal it is difficult to exceed 200°C. The third probe is similar to that shown in Figure 7, except that the circulating nitrogen tubes are replaced by an electrical heater embedded in the end block. This allows the face of the probe to be raised to about 6OO”C, but of course removes the possibility of cooling. A selection of sample plates may be attached to the probe face. These are useful for retaining powdered samples by gluing, evaporating from solution, or pressing into a fine wire mesh. The fourth probe is a prototype ribbon or single crystal holder system which allows a typical tungsten ribbon (2 cm x 0.5 cm x 0.003 cm) to be resistively heated to about 2000 “C.
(e) Analyzerlfarget
chamber
The important features of the analyzer/target chamber are shown in Figures 7 and 8. The analyzer’ a (detail in Figu re 7) is made of gold plated aluminium and is of the concentric spherical type’ ‘, utilizing a 150” sector of the full spherical configuration (10 cm radius). The field is terminated by Herzog correction plates. A rotatable slit plate carries three pairs of diametrically opposite slits of 1 cm length and width 0.25, 1 and 2 mm. The plate is connected by a crank to a linear motion drive so that any pair may be selected as entrance and exit slits to the analyzer without breaking vacuum. The equation relating the potential applied between the analyzer plates, V, and the energy of the electrons passed, E, is given by”
(1) where R, and R, are the radii of the inner and outer hemispheres here) which reduces to
V = EfH
(9 cm and 11 cm
(2)
where H, the hemispherical constant, is theoretically equal to 2.48. _ The voltage supply to the analyzer may be preset to pass electrons of energy, E, equal to 5, 10, 20, 50 or 100 eV. By varying the specimen potential, I?, ejected electrons are accelerated or retarded to match the analyzer pass energy being used. Thus if KE is the kinetic energy of the ejected electrons KE = E’ + E
(3)
The voltage, E’, is derived from a O-1500 V variable power supply (to select a kinetic energy scan starting position) in series with a ramp generator which can be switched to give kinetic energy scan ranges of 10, 30, 100, 300, or 1000 eV, in times variable from 30 to 3000 seconds.
252 The resolution aberrations. dE E
-=-
W
2R
of the analyzer is given by the following
equation,
neglecting
(4)
where dE is the full signal width at half-height, w is the total slit-width and R the mean radius. Thus by varying E and w the theoretical instrumental resolution can be changed from 12.5 meV to 2.0 eV. The transmission of electrons at any given energy is proportionalzl to l/E so that high resolution is only obtained at the expense of sensitivity (as usual). By operating the spectrometer as described the resolution is constant and independent of the initial kinetic energy. However since the spectrometer entrance angle is fixed and electrons leaving the specimen are diverged by a retarding field and converged by an accelerating field, sensitivity will decrease with increasing initial kinetic energy, and vice versa. To a first approximation, and when initial KE is high compared to E (i.e. E’ N ICE) relative sensitivities are inversely proportional to kinetic energy. Signal detection is by a channel electron multiplier mounted over the exit slit; the output pulses are amplified and counted by a ratemeter and readout is direct to an X-Y recorder (Figure 8). The analyzer chamber is surrounded by a p-metal shield during operation, to minimise the effect of external magnetic fields. III. PERFORMANCE (a)
Sample preparation The illustrative examples of the spectrometer performance presented below are all of polycrystalline films. These were prepared by evaporation from filaments, either by melting small strips of the metal concerned (low melting point) on to a tungsten filament, or from a filament of the metal concerned (high melting point). No special filament cleaning procedures in vacua were adopted, but during and after bakeout the filament was thoroughly outgassed so that during evaporation pressures of 1o-9 torr were maintained. This results in films largely clean of surface contaminants”, 23. (b)
Vacuum ultraviokt spectra The He I and He II photoelectron spectra of a clean gold film are shown in Figure 9. The slit-width was set at 1 mm and the analyzing energy at 10 eV, thus giving an instrumental resolution of 0.1 eV. The steeply rising back-ground at low electron energies in the He I spectrum (trace (b)) is due to the build-up of multiply inelastically scattered electrons. Electrons of less than 10 eV energy are accelerated into the analyzer and as already mentioned the acceleration converges electrons through the
253
(b)
BE.
CCW-
Figure. 9. Ultraviolet photoelectron spectrum of a clean gold surface. (a) He I (21.2 eV) spectrum, analyzer scan mode; (b) He I spectrum, retarding potential scan, 10 eV analyzing energy; (c) He II (40.8 eV) spectrum, retarding potential scan, 10 eV analyzing energy.
slit, thus increasing sensitivity, hence accentuating the scattered background. A more common method of recording band structure by photoemission is to analyze the electrons at their ejection energies by sweeping the analyzer voltages. An He I spectrum taken in this manner is shown in trace (a) (cf. Eastman”). The “slow” peak is depressed since transmission is proportional to E, and the spectrum is therefore an E x n(E) versus E plot. In fact at very low energies (-c 1 eV) small inhomogeneities of field can cause sufficient deflection for the signal to fall even more rapidly. The He II spectrum may be compared
to previously
published
data24.
be cases where high resolution is required to look for possible vibrational fine structure. To test the high-resolution capabilities of the analyzer, the He I spectra of gaseous argon were taken (using the 0.25 mm slit-width) and a There
may
254 special gas probe which admits sample gas at moderately high pressure (- 0.01 torr in the collision region) directly into the target chamber. A best resolution of 30 meV for 5 eV electron energy was recorded, more than adequate to discern any solid state fine structure resolution requirements. The theoretical resolution of 12.4 meV is presumably not obtained owing to residual magnetic fields and the presence of contact potentials.
(c) X-ray spectra Figure 10 shows the full ESCA spectrum of Au taken with the aluminium source, plus expanded regions for the 4f core levels and the band structure. All recording parameters are given in the figure caption. The major portion of the experimental half-width is of course due to the line width of the X-rays3. Figure 11 shows the 3d peaks of silver taken with a magnesium anode at 10 eV analyzing energy. This is the best absolute resoultion we have achieved in the X-ray mode. Figure 12 shows a spectrum of freshly evaporated MO, and the same sample after several days exposure to a residual background pressure (mainly CO) of ca. 5 x lo-‘* torr at room temperature. The O(ls) peak height has just reached a maximum at this point, which we know from chemisorption studiesz5 corresponds to ca. 60% surface coverage. Since the sticking probability of CO on MO is close to unity up to this coverage, and therefore MO is an example of one of the most reactive surfaces, the above result emphasises the need for UHV conditions to study such
4 I El:
5
10 1 200
I 4MI . . (cV) BE
I MO _
Figure 10. X-ray photoelectron spectrum of clean gold surface (Al Ku source, 1486.6 eV). Main spectrum: 50 eV analyzing energy, 250 watts X-ray power, scan time 2ooO sec. Band structure inset: 20 eV analyzing energy, 500 watts X-ray power, scan time 1000 sec. 4f peaks inset: 20 eV analyzing energy, 500 watts X-ray power, scan time 100 sec.
255
1
366
I
I
I
366
I
I
466
I
462
I
I
404
,
I
a8
RE. W’,
Figure 11. X-ray photoelectron spectrum (Mg Ka source, 1253.6 ev) of 3d peaks of silver. Analyzing energy 10 eV, scan time 330 sec.
surfaces adequately. The spectrum of Au on the other hand remains unchanged in the spectrometer over a period of weeks since it is unreactive tb the residual gases present (CO, H,O, CO,). Hydrocarbon residues, however, which are usually present in non-UHV systems, would quickly build up on the Au surface to give a carbon signal.
(d) Auger electron spectra Auger electrons are of course generated by X-ray impact as well as electron impact, and these may be recorded together with the photoelectron spectra (for example see Sch6nZ6). Auger electron energies often fall in the 300-O eV range, as is the case for example in MO (Figure 13). In this range one gets a rapid build up of multiply scattered electrons giving rise to a steeply sloping background on which sit the Auger peaks (trace a). This may be counteracted (with some loss of resolution towards high electron energies) by analyzing at the ejected electron energy (of He I spectra) as in trace (b). This is the manner in which commercially available Cylindrical Mirror Analyzers are usually operated for electron impact Auger spectroscopy”, but the spectrum is then conventionally recorded in the first differential mode. Such a recording2’ for MO has been re-integrated in Figure 13, trace (c), and so may be compared to the X-ray induced spectra of (a) and (b). We have not yet been able to use the electron gun arrangement on the present spectrometer as a source for producing Auger spectra. This is because with the present design a large number of electrons escape from the rear of the gun and find their way through the analyzer, creating SI very high background. A modified, capped version
256
c cw
‘WITION
I
;‘- :
Figure 12. X-ray photoelectron spectrum of a “clean” molybdenum surface. 50 eV analyzing energy, 500 watts X-ray power, scan time 500 sec. Insets: Expanded C(ls) and O(ls) regions of “clean” surface and after 2 days’ exposure to residual background pressure. 100 eV analyzing energy, 500 watts X-ray power, scan time C(ls) 300 sec. 0( 1s) 500 sec.
of the gun has been used successfully with an otherwise very similar spectrometer. A typical spectrum, recorded in the dN/dE mode is shown in Figure 14. (c)
Calibration of absolute energy scale for the spectrometer Though the normal practice in ESCA is to calibrate spectra against some “standard” line, direct spectrometer calibrations are feasible and useful in some cases. For example the ‘Labsolute” binding energy of Au 4f,,2, which is commonly used as a calibrant, has been quoted at values between 82.0 and 84.0 eV3* 26* 2g. We have measured the value by several calibration procedures within this spectrometer and obtain an internally consistent value of 83.7 & 0.1 eV, which is in agreement with the most carefully determined literature value 2g_ The procedure used was to apply equation 2 to any of the sharply defined peaks in the He I spectrum recorded in the analyzer scanning mode (Figure 9c). The experimental hemisphere voltage, V,, for the peak concerned was recorded on the digital voltmeter, and an accelerating voltage of 4.000 V was then applied to the sample, the experimental spectrum recorded again,
257
Figure 13. Auger spectrum of molybdenum. (a) Al Ka induced, retarding potential scan mode, analyzing energy 50 eV; (b) Al Ka induced, anaIyzer scan mode; (c) Electron impact induced, spectrum re-integrated28.
and the new hemisphere H(V,
-
voltage
V,) = El -
E,
V, measured. = 4.000 eV
Since (5)
one has a unique determination of 27, the hemisphere constant. The value obtained was H = 2.62. c_&, the spectrometer work function, was then found by locating the Fermi edge’accurately in the He I spectrum and noting the experimental hemisphere voltage, V, at that position. Since HV,
= EF = 21.22
-
&,
I ,
I 500
400
I 700
I 600
t IO
5 K.E. ICVl
Figure 14. Differential Auger spectrum (electron impact induced) of an argon-ion bombarded sample of stainless steel. Gun energy 2.4 kV, current 350 ,vA. Analyzer energy 50 eV, scan time 1000 sec.
&, can therefore be determined. A value of &, = 4.02 eV was obtained. H can also be found directly from X-ray spectra, taken in the retarding potential mode but with lower accuracy_ The kinetic energy of an ejected electron using AI Kcc radiation is given by KE
=
1486.6 -
(E, + &,)
(7)
where Eb is the binding energy of the electron with respect to the Fermi level. After retardation to the analysis energy, E, E = 1486.6
-
E’ -
(E, + I&,) = HP’,.
(8)
Recording at two different hemisphere voltages, v1 and Vz and measuring the retarding potentials required El and E; to focus a particular binding energy peak, E,,, for analyzing energies E, and E,, gives (E, -
E2) = E’, -
EL = H(r/, -
V.)
Applying this equation to the 4f lines of Au resulted in an identical value of 2.62 for H. Substituting for N and c&, in eqn. (8) provided the value of 83.7 + 0.1 eV for Eb
259
4f,,2 Au.
Alternatively one may calibrate without reference to &, found from the He I spectrum by locating the Fermi edge in the X-ray spectrum itself, though this is less accurate. Agreement with the above figure was obtained by this method. IV. POSSIBLE
INSTRUMENTAL
IMPROVEMENTS
The instrument described here is a prototype and can certainly be improved upon. Some improvements have already been incorporated in more recent versions of ESCA-3. Base pressures of 2-3 x 10-l’ torr are normally adequate, but for extended studies of clean surfaces it would be very useful to be able to reduce this by half an order of magnitude. One of the inherent disadvantages of the XPS technique is the long recording times that are required for good statistics on the small surface feature signals at high resolution. An increase in total signal strength by about a factor of five is thought to be attainable by changing the X-ray source geometry, the shape of the aluminium window, and increasing the analyzer slit widths (increase of w and the solid angle of acceptance) while maintaining resolution by analyzing at a lower kinetic energy. An enhanced surface/bulk signal ratio is to be expected if the angle of exit of the detected electrons with respect to the sample surface is smal13’. A more advanced data collecting system would be an advantage. A fully integrated computer controlled system capable of handling data simultaneously from up to eight regions of the electron spectrum is being developed by VG. Particularly useful features will be averaging, deconvoluting, and base-line correction facilities, storage of standard spectra for matching purposes, and considerable programmable control for the study of time-dependent surface processes. The preparation chamber is not ideal. It is too small for completely successful evaporation of refractory metals, since the chamber walls can reach quite high temperatures by radiation from the filament. Cooling externally with dry ice helps but does not completely alleviate the problem. A liquid nitrogen shrouded evaporating filament would probably be successful. The small size of the chamber also restricts the number of line of sight ports to the sample. The presence of the preparation chamber enables one to perform reactions with reactants and at pressures which one would be reluctant to use in the UHV analyzer chamber. However, much of the work with which we are concerned involves vacuum worthy reactants at low pressures, and as such is carried out in the analyzer chamber. For such situations the preparation chamber is largely superfluous, and the mechanics of moving the sample probe from one to the other restricts flexibility. V. POTENTIAL USES OF COMBINED SURFACE STUDIES
All the electron
spectroscopic
ELECTRON
techniques
SPECTROSCOPY
can be surface
TECHNIQUES
sensitive,
FOR
but as
mentioned earlier the controlling factor as far as the escape or probing depth is concerned is the kinetic energies of the ejected electrons. Since in most cases the energies of the photoelectrons measured in XPS are greater than those in UPS or Auger the latter two techniques will generally probe the region closer to the surface. However this is not all that should be considered3’ . For instance the recording of Auger data is generally much quicker than XPS by virtue of the high electron beam currents available, but this advantage is often negated because of strong interaction between the electron beam and the surface being studied (desorption, dissociation of adsorbed species, physical disruption of the substrate). XPS though slow, is generally “milder” in causing the above effects and more readily offers chemical shift data. The electron energies of UPS are in the range that should make them the most surface sensitive in terms of escape depth. However the fact that one is experimentally dealing with a region of overlapping band structures, the interpretation of which may be important but extremely complex, and which provide no atomic identification (cf. XPS or Auger), could in general reduce its worth as an individual surface probe. The combination of the various favourable aspects of the three techniques makes the effective use of each of them far greater than any of the techniques individually. We should in principle have available a fast, semi-quantitative, surface element analysis (Auger), detailed information on the different binding environments of these elements (Chemical Shifts and other effects in XPS), plus high-resolution data on the band structures (UPS). Our aims for the spectrometer described here are to bring this combination of techniques to bear mainly on problems of adsorption on to clean metal surfaces with the object of moving on to problems in catalysis once it is clearer from the basic studies what the subject has to offer in such a complex area. REFERENCES 1
2 3
10 11 12 13 14
D. W, Turner, A. D. Baker, C. Baker and C. R. Brundle, Molecular Photockctron Spectroscopy, Wiley-Interscience, London, 1970. W. E. Spicer, in F. Abeles (editor), Optical Properties of Solids, North-Holland Publ. Co., Amsterdam, 1970. K. Siegbahn et al., ESCA-Atomic, Molecular, and Solid State Structure Studied by Means of EZectron Spectroscopy, Nova Acta Regiae Sot. Sci. Upsaliensis Ser. IV, Volume 20, 1967. For example, G. K. Wertheim and S. Hiifner, Phys. Rev. Lett., 28 (1972) 1028. K. Siegbahn et al., ESCA AppIied to Free Molecules, North-Holland Publ, Co., Amsterdam, 1969. D. E. Eastman and J. K. Cashion, Phys. Rev. L&t., 24 (1970) 310. W. N. Delgass, T. R. Hughes and C. S. Fadley, Catal. Rev., 4 (1971) 179. T. A. Carlson and G. E. McGuire, J. Electron Spectrosc., 1 (1972/73) 161. M. Klasson, J. Hedman, A. Bemdtsson, R. Nilsson, C. Nordling and P. Melknik, Phys. Ser., 5 (1972) 93. C. R. Brundle and M. W. Roberts, Chem. Phys. Left., 18 (1973) 380. D. E. Eastman and J. K. Cashion, Phys. Rev. L&t., 27 (1971) 1520. C. R. Brundle, in M. W. Roberts and J. M. Thomas (editors), Surface and Defect Properties of Solids, Vol. I, Specialist Periodical Report, Chemical Society, London, 1972. S. Trajmar, J. K. Rice and A. Kupperman, Advan. Chem. Phys., 18 (1970) 15. H. Ibach, J. Pac. Sci. Tech&., 9 (1972) 713.
261 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
B. L. Henke, in I-I. H. Pattee, V. E. Cosslett ?nd A. Engstrbm (editors), X-Ray Optics andX_Ray Microanalysis, Academic Press, New York, 1963. C. R. Brundle, Chem. Whys. Lett., 5 (1970) 410. J. A. R. Samson, Techniques of Vacuum Ulfraviolet Spectroscopy, Wiley, New York, 1967. The analyzer was designed in collaboration with Dr. K. J. Ross, Department of Physics, University of Southampton. J. A. Simpson, Rev. Sci. Instrum., 35 (1964) 1698. C. E. Kuyatt and J. A. Simpson, Rev. Sci. Instrum., 38 (1967) 103. J. C. Helmer and N. H. Weichert, A@. Phys. Lert., 13 (1968) 266. C. R. Brundle and M. W. Roberts, Proc. Roy. Sot. (London), A 331 (1972) 383. S. J. Atkinson, C. R. Brundle and M. W. Roberts, J. Electron Spectrosc., 2 (1973) 105. D. E. Eastman, Phys. Rev. Lett., 26 (1971) 1108. R. R. Ford, Advun. Catal., 21 (1970) 51, and references quoted therein. G. Sch6n, J. EIecfron Spectrosc., 1 (1972/73) 377. H. E. Bishop, J. P. Coad and J. C. RiviBre, J. Electron Spectrosc., 1 (1972/73) 389. P. W, Palmberg, G. E. Riach, R. E. Weber and N. C. MacDonald, Handbook of Auger Electron Spectroscopy, Physical Electronic Industries, Minnesota, 1972. G. Johansson, J. Hedman, A. Berndtsson, M. Klasson and R. Nilson, J. Electron Spectrosc., 2 (1973) 295.
30 31
W. A. Frazer, J. V. Floris, W. D. Robertson and W.-N. Delgass, Surface Science, 36 (1973) 661. C. R. Brundle, J. Vuc. Sci. Technol., Jan/Feb 1974 issue.