A new ESCA instrument with improved surface sensitivity, fast imaging properties and excellent energy resolution

Journul of E&ctron Spectroscopy and Reluted Ph~nmnena, 52 (1990) 741-785 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

141

A NEW ESCA INSTRUMENT WITH IMPROVED SURFACE SENSITIVITY, IMAGING

FAST

PROPERTIES AND EXCELLENT ENERGY RESOLUTION

U. GELIUS’, B. WANNBERG’, P. BALlZER’, H. FELLNER-FELDEGG*, C.-G. JOHANSSON*, J. LARSSOti, P. MONGER* and G. VEGERFORS*.

G. CARLSSOti,

~Department of Physics, University of Uppsala. Box 530, S-751 21 Uppsala, Sweden. 2)Scienta

Instrument AB, Seminatiegatan 33H, S-752 28 Uppsala, Sweden.

SUMMARY

Results from the first experiments with a new ESCA instrument with monochromatic X-ray excitation are presented. The measurements were selected to assess the performance of the instrument in terms of energy and spatial resolution, information rate and surface sensitivity, and to probe the new dimension added to XPS analysis with the introduction of an imaging system. Novel design features described in the paper include a high-power monochromatic X-ray source and an electrostatic lens system permitting either large luminosity or high spatial resolution. A new geometry allows excellent access to the sample area and gives maximum surface sensitivity at glancing angles. The best energy resolution achieved so far with the instrument has been evaluated from the Fermi edge of silver to 0.27 eV (FWHM). The FWHM of the Ag 3ds, line at this resolution is 0.44 eV. At an instrument energy resolution of 0.37 eV (FWHM) a peak intensity of the Ag 3d5j2 line corresponding to a single-channel detector count rate of 1.9*106 cps has been measured. At this resolution, high quality spectra from surface elements of 30 urn x 30 urn can be recorded in a few minutes. The spatial resolution (20% - 80%) has been measured to 23 um. The enhanced surface signal at glancing angles is demonstrated in a series of measurements of the angular dependence of the XPS spectrum from a silicon surface. Preliminary experiments using an image integrating system show the potential of the combination of high spatial resolution in XPS with such techniques to give new information on the distribution of chemical properties or electrostatic potential variations over a surface. Future developments are discussed, including parallel recording of spectra from a large number of small spots, advanced handling of information from an imaging XPS system and further improvements of the spatial resolution.

1.

INTRODUCTION X-ray crystalmonochromatization was introduced in X-ray photoelectron spectroscopy more

than twenty years ago (1.2). The first function of such an X-ray monochromator was to filter out the energetically well separated Kat line from the Ka, lime and the X-ray satellite limes in the CuKa and MoKa spectra, and to reduce the Bremsstrahlung background (1). It was also realized that the inherent line width of the Kat line contributes to the observed line width in XPS, and that this contribution can be eliminated by counter-coupling the energy dispersions of the X-ray monochromator and the electron spectrometer (2).

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The dispersion compensation technique was for the first time implemented in the HewlettPackard 595OA ESCA instrument, in which a double-focusing X-ray monochromator was integrated together with an AlKa X-ray anode (3). The X-ray monochromator incorporated three spherically bent a-quartz crystals placed in a symmetric Johann arrangement (4) on three Rowland circles intersecting each other along a line through the X-ray source and X-ray focus. The width of the total spectrometer function, including the X-ray contribution, could in this way be reduced to below 0.5 eV. In order to compensate for the reduced X-ray intensity caused by the monochromatization. this instrument introduced an electron multidetection system based upon micro-channel plates and a phosphor screen (3). The advantage of the dispersion compensation method is primarily that it allows a higher X-ray power to be used, since the electron beam does not have to be focused to a narrow line on the anode. Thus, it is today possible to continuously use 1200 W X-ray power with a stationary aluminum anode in the HP ESCA instrument (5). The dispersion compensation method, in conjunction with a wide X-ray source, does not create a true monochromatization of the X-rays, but merely a dispersion of the photon energies over the sample surface. Therefore, it works best for samples with a flat surface at a particular angle. When the polar angle is changed, or when the sample is not flat, a broadening of the spectrometer function is obtained. True monochromatization of the AlKa line in XPS was used for the first time in an instrument built for studying gases (6). This instrument was equipped with a water cooled rotating anode and a high power. fine focusing, two-stage Pierce type (7) electrostatic electron gun in order to allow a higher power density on the aluminum anode and, consequently, to achieve a higher intensity of monochromatic

AlKa radiation at the sample position. In addition, a multidetection system was

included, similar to that in the HP ESCA instrument. A further improvement in the intensity of the monochromatized X-ray source was obtained by developing a multi-crystal monochromator which incorporated two side-rows of crystals in addition to the earlier used central row. The crystals were placed in a close-packed arrangement on both sides of the central row. In this way, wide angle crystal monochromators with 19 and 25 double-focusing circular crystals, each with a 35 mm diameter active surface were built, spanning a solid angle of 0.12 and 0.16 steradians. respectively (8,9). One of the crucial points in the development of water cooled rotating anodes for high power has been the sealing technology. In the early seventies, the best technique was still based on mechanical seals between a soft and a hard material, such as a radial lip seal of a rubber material in contact with a chromium plated shaft, or an axial seal between a graphite disc and a Tic plated hard metal plate. The lifetime of the vacuum and water seals, however, was limited to at best a few months of continuous operation at 3000 r-pm. Moreover, the vacuum achieved was generally limited to the lOA mbar range. Therefore, this technology was still not sufficiently developed to be used in commercial ESCA instruments.

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A further step in the development of more powerful ESCA instruments having monochromatic X-ray excitation was taken with the introduction of a new type of rotating anode equipped with a differentially pumped contact free vacuum seal and capable of achieving oil-free UHV (8). Also the water seal was built in a contact free way. With this design the previous problems with the limited lifetimes of the vacuum and water seals were eliminated. Moreover, the rotational speed of the anode could be increased, which in turn allowed a continuous operation at a higher X-my power. Finally, the UHV performance of this type of rotating anode makes it suitable for incorporation in ESCA instruments for surface analysis. As the field of X-ray photoelectron spectroscopy has matured over the last decade, the merit of having an X-ray source with a sharp and well defined photon energy has become more generally accepted. Thus, today all the leading manufacturers of XPS instruments have developed such sources. The advantages of monochromatic X-ray excitation in XPS over non-monochromatized Xray excitation are listed below: * higher energy resolution * improved signal-to-background ratio giving higher sensitivity * elimination of undesirable satellites limes * distinct valence band structures (all materials) and Fermi edges (metals) can be recorded * considerably reduced X-ray damage to sensitive materials * negligible IR radiation and destructive electron bombardment at the sample The disadvantages of monochromatic X-ray excitation in XPS have generally been considered to be the following: * reduced intensity and sometimes reduced sensitivity * reduced surface sensitivity due to decreasing intensity at grazing electron emission angle * variable energy resolution with polar angle (HP ESCA instrument) * more expensive instruments With the development

of powerful monochromatic

X-ray sources based on the above

described new types of wide-angle, multi-crystal monochromators and fast UHV rotating anodes, the implementation

of monochromatic X-ray excitation in XPS can now be made without the

previous sacrifices in intensity and sensitivity. Moreover, in the instrument described hem a new geometry of the monochromator relative to the electron lens and the analyzer is introduced, which results in an enhanced surface sensitivity, thereby eliinating

the second disadvantage listed above.

The third disadvantage is totally eliminated by the use of true monochromatization

instead of the

dispersion compensation method. The last disadvantage, however, obviously will remain because of the inevitable costs for incorporating these more sophisticated components into ESCA instruments.

An additional important advantage with such an advanced monochromatic high-power X-ray source in an ESCA instrument, is connected with the recent trend for ESCA to develop into a microscopic tool. The minimum surface element size observable with XI’S will in practice be limited by the demand for sufficient photoelectron intensity. It turns out to be possible to achieve a higher photon density in the focus of a monochromatic high-power X-ray source than can be practically achieved with the divergent X-ray radiation from conventional non-monochromatized X-ray sources. This will be discussed in section 2.4 below. Thus, to the above list of advantages of monochromatic X-ray excitation in XPS the following important point should be added: * increased photon density enabling higher spatial resolution in ESCA microscopy. Realizing all the above stated merits of monochromatic high-power X-ray sources in ESCA, we have developed the ESCA300 instrument incorporating such a source. The ESCA300 was from the very beginning developed with this as its main excitation source, and therefore its position in the instrument has been optimized such as to create an easy-touse instrument with excellent access to the sample analysis position and the sample preparation region. In addition, the new geometry will give a better surface sensitivity at glancing emission angles. The instrument

also incorporates a novel

electrostatic lens with two modes of operation, one high transmission mode for conventional large area analysis, and one mode with high spatial resolution for ESCA microscopy. The following section will describe the most important components of the ESCA-300 instrument. Recent results obtained with the first ESCA-300 instrument will be presented in section 3, selected in order to clearly illustrate its performance in terms of energy resolution, information rate, surface sensitivity, spatial resolution etc. The last section discusses some future developments of the ESCA-300 instrument, in particular concerning improvements in ESCA microscopy performance.

2.

THE

ESCA-300

INSTRUMENT

2.1 A Versatile and User Friendlv Confieuration The first and foremost criterion in the design of the ESCASOO instrument has been to create a versatile and user friendly instrument. The layout of the entire instrument has as its priority the sample itself, and the requirements to have easy access to it during analysis and various preparation treatments in the preparation chamber. The first ESCA300 instrument is shown in Fig. 1. The sample analysis position is in the center of a spherical analysis chamber having a diameter of 300 mm. A large number of vacuum ports point towards the analysis position, permitting the flexibility of appending various accessories to the analysis chamber, such as a UV lamp, ion gun, flood guns. a conventional X-ray source, electron gun, residual gas analyzer, gas inlet system, etc.

Fig. 1. Front view photograph of the first ESCA-300 instrument. The spherical sample analysis chamber is seen in the center of the photograph. A stereo microscope is mounted on the top of this chamber. The sample can also be observed directly through the large viewport at the front. The preparation chamber is mounted to the right of the sample analysis chamber. The sample manipulator extends to the left of the analysis chamber. The large chamber at the top contains the hemispherical electron analyzer and the electron multidetector system The electron lens is situated in the chamber partially seen behind the stereo microscope. The chamber containing the wide angle X-ray crystal monochromator extends to the left of the central analysis chamber, directed 45’ upwards and backwards. A part of the rotating anode structure (black) extends below and to the left of the analysis chamber, whereas the anode disc and the high power electron gun are placed behind this chamber. A conventional X-ray anode is also mounted on the analysis chamber. Five diffusion pumps with UHV cold traps, specified by the first customer, are mounted on both sides of the instrument. The standard ESCA-300 uses ion pumps and turbomolecular pumps, and has provisions for additional pumping with Ti-sublimation pumps or cryopumps.

The electron lens is directed 45” upwards and backwards, that is away from the operator. The large hemispherical electrostatic analyzer. with 300 mm mean radius, is directed further backwards, away from the user region. The enttance slit to the analyzer is hotizontal. The electron lens has been made extra long, 800 mm, in order to remove the analyzer sufficiently from the analysis chamber, thereby avoiding interference with possible future accessories on the analysis chamber. Moreover, the large length is advantageous to reach higher spatial resolution over extended samples. The sample

can be observed directly through a 100 mm diameter viewport, and through a stereo microscope or, alternatively, by means of a CCD camera and a monitor. The whole region to the right of the analysis chamber is reserved for the user. A versatile preparation chamber is available as a standard (somewhat larger than the one shown in Fig. l), but the configuration also allows special chambers of practically any shape and function to be attached to the analysis chamber on this side. Sample transfer in the standard chamber is made by means of a rack and pinion manipulator. Samples are first introduced into a separate load-lock chamber, which is evacuated by a turbomolecular pump, for quick introduction into the preparation chamber. Single samples as well as carrousels carrying many samples can be handled in this way. The region straight to the left of the analysis chamber is used for mounting a universal high precision sample manipulator. The standard manipulator has a horizontal main axis. Rotating the sample around this axis has the effect to vary the angle of emission for the analyzed photoelectrons relative to the sample surface (the polar angle), i. e. to vary the depth of analysis and the surface sensitivity. The standard universal manipulator has a 50 mm translational movement in the axial direction. However, a special long travel high precision manipulator, with its own sample inlet chamber and a separating gate valve. can also be connected here. In this way, both the right and the left sides of the analysis chamber can be used for sample preparation arrangements, one side perhaps reserved for the surface scientist’s time demanding research with single crystal surfaces, whereas the other side may be used for applied surface analysis of many different kinds of samples. 2.2 A Mom Surface Sensitive Geometrv In all kinds of surface analysis, from basic research on single crystal surfaces to characterixation of less well defined surfaces in applied surface science, it is frequently essential to get information on the depth distribution of the compounds present within the range of the escape depth of the photoelectrons. In particular it is important to be able to identify the very outermost atomic or molecular layers. The standard way of doing this in XPS is to tilt the sample and register the electron spectra at several polar angles (10.11). At grazing emission angle photoelectrons escaping from the surface without inelastic scattering originate primarily from the outermost few atomic or molecular layers. Therefore, a highly desirable property of an ESCA instrument is to be able to take spectra even at very small grazing angles with good efficiency. In instruments working with conventional X-ray anodes there are normally no problems in recording electron spectra with good intensity even at grazing angles, whereas the intensity has decreased more rapidly towards grazing angles in instruments with an X-ray monochromator. Up till now all manufacturers of such ESCA instruments have placed the monochromator, relative to the analyzer, principally in the same way as was first done in the Hewlett-Packard instrument, and that instrument was designed before the advantages of varying the emission angle had been generally realized.

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In the ESCA-300 instrument a new monochromator position has been chosen, which has proved to enhance the intensity from the outermost surface layer so that spectra can be recorded at high intensity even at 0’ grazing angle between the sample surface and the electron lens axis. For the first time, a commercial ESCA instrument has been built with a geometry that allows the photons to impinge at grazing angle onto the studied surface simultaneously as the photoelectrons are analyzed at grazing angle. The enhancement in the observed intensity from the top surface layer under this condition originates almost entirely from the fact that more and more surface atoms ate brought into the X-ray focus, and that the photoelectrons from these irradiated surface atoms are to a large extent still within the acceptance area of the electron lens and the entrance slit of the analyzer. This new geometry is shown in Fig. 2.

Fig. 2. Schematic drawing of a new arrangement of the X-ray monochromator in an ESCA instrument leading to enhanced surface sensitivity. The center of the monochromator lies in the plane defined by the electron lens axis and the entrance slit to the electron analyzer.

Let us use the name analysis plane for the plane defined by the electron lens axis and the straight entrance slit to the electron analyzer. Another plane, here called the incident X-ray plane, is defined by the central X-ray beam from the monochromator

and the line focus of the X-ray

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monochromator. In earlier instruments the incident X-ray plane (or just the central X-ray beam in instruments with a point focus) form an angle &of at least 30’ with the analysis plane; the planes intersecting each other along the sample surface. In the ESCA300 instmment these two planes have been brought to coincide. As is seen in Fig. 2 the main rotational axis of the universal manipulator also lies in this common plane and is perpendicular to the electron lens axis. Let a denote the grazing angle between the sample surface and the analysis plane. The angle between the sample surface and the incident X-ray plane (central beam) then becomes a+ 8.TO a first approximation the number of irradiated surface atoms is proportional to l/sin(a+ 6). In the new coplanar geometry the surface intensity at grazing angle a is increased by a factor F compared to the old geometry, where F is given by : F = sin(a+e)/sin(a) For example, compared to the Hewlett-Packard ESCA instrument, which has a 72’ angle between the analyzer plane and the incident X-ray plane, the enhancement in intensity at 5Ograzing angle is above a factor of 10. The angle tl = 35” represents probably the minimum angle found in modem instruments, and at the same grazing angle the factor F is still as large as 7.4. The grazing angle 5’ represents a suitable angle for studies of the topmost surface composition in the ESCA-300 instrument. The analysis of the enhancement factor above was intentionally made in an oversimplified way in order to stress the most important factor governing the intensity at grazing angles with monochromatic X-ray excitation. It is likely that the given improvement factors will be somewhat reduced (30 50 %) when all relevant geometrical factors are considered -

the lengthening of the elliptic focus at

grazing angles, the gradual reduction of X-ray intensity when some regions of the monochromator descend behind the horizon of the sample, the convergence of the X-ray beam, low-angle X-ray reflection-refraction

effects at very grazing angles (12), the reduction of the solid angle into the

electron lens at small grazing angles, and the effect of surface roughness. A computer simulation of all these effects is under development (13). The enhancement of the surface intensity at grazing angles is not only applicable for large flat samples. At 5” grazing angle the FWHM of the X-ray focus on the sample is only about 6 mm, which is a quite normal sample size in ESCA analysis, and in particular most of the surface science research is performed on such surfaces. Moreover, it is not necessary that the surface is flat. For example, the intensity from the surface layer of fibers can be enhanced similarly by orienting the fibers so that their length directions are parallel and form a small angle with the electron lens axis. Single fibers can be studied in this way as well. Even spherical particles, spread out over a flat surface, will exhibit a corresponding enhancement in the surface layer intensity at grazing angles. The coplanar geometry will, however, not enhance the surface intensity at grazing angles compared to the older geometries for small structures on a surface, such as a small corrosion spot with a diameter below 1 mm. Such analysis problems, on the other hand, often concern undesired surface defects in applied surface science, and in these cases it is normally more interesting to investigate the spots at a normal take-off angle in order to reach deeper below the topmost surface

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layer. For analysis at normal angles the new coplanar geometry works equally well as the older geometry. If it is desirable to look at small spots under a graxing angle this can still be done with the ESCA-300 instrument in its high spatial resolution mode with a spatial resolution normal to the lens axis of about 20 urn. Thus, spots with diameters down to about 200 pm can be studied under grazing angles with relatively low spectral contributions from the surroundings. The new coplanar geometry has one additional advantage. In the conventional geometry, where the incident X-ray plane intersects the analysis plane at an angle 8. it is important to position the sample surface along the intersection line. In particular for instruments having an intersecting angle close to 90” the adjustment becomes critical. Thus, changes of the height position of the sample as small as 0.1 mm can have a noticeable effect on the intensity. This also makes accurate angular dependent measurements more difficult since it requires the axis of rotation to pass through the surface within such small tolerances. The reason for the sensitivity in the height position of the sample is because of the fact that in the conventional geometry the X-ray focus on the sample moves perpendicularly across the analysis plane as the height position of the sample surface is changed. When the sample surface is normal to the analysis plane, the displacement equals Ah*tg(e), and since the tangent term is as large as 0.7 at 35” and 3.1 at the Hewlett-Packard geometry the height displacement Ah must be small. In the coplanar geometry this problem is much reduced, because the X-ray focus always lies within the analysis plane, and the only effect of changing the sample height is to move the X-ray line focus along the entrance slit direction on the sample. Since the X-ray line focus has about a 1O:l length/width ratio the height positioning problem is reduced by about one order of magnitude. In the spatially resolving mode, the depth of field at highest resolution is about 0.2 mm, so this will be the most important factor for the positioning in many cases. The requirement of having the axis of rotation passing through the analysed surface is relaxed to the same amount. By the same reason the coplanar geometry is also of advantage in studying curved samples. Thus, it can be concluded that the new coplanar geometry introduced in the ESCA-300 has many important advantages over the traditional geometry for ESCA instruments with X-ray monochromators. 2.3 The UHV Rota-

Anode and the Hieh-Power Electron Gun

The design of the rotating anode for the ESCA-300 instrument is based on the long experience of developing and testing such anodes in Uppsala (2,6,9.14,15). The latest generations of these anodes are differentially pumped and have contact free vacuum and water seals. The last version of this type of anode (15) has performed well for several years in a UHV ESCA instrument for surface science research (16). The rotating anode in the ESCA-300 instrument is a further development along this line. The limiting factors for the rotational speed in our previous anodes have been the vibration and high sound level in the instruments. Therefore, a main concern developing the anode for the ESCA300 was to eliminate this problem This has been done by taking a series of new measures. First, the

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vacuum chamber for the rotating anode is mechanically decoupled from the rest of the instrument by a metal bellows connecting to the monochromator chamber. The monocbmmator chamber is likewise coupled to the analysis chamber via another bellows. Second, the rotating anode itself is decoupled from its vacuum chamber by letting it float on Viton O-rings instead of using a metal gasket. The UHV requirement is still met by using differential pumping between the O-rings. Third, the whole rotating anode unit and its vacuum chamber are mounted on an adjustable x-y table which is fixed to a very rigid and sound absorbing base structure bolted to the main framework of the ESCA-300 instrument. The new mounting of the rotating anode has turned out to work very satisfactorily. A well balanced anode running at 10 OCOrpm can hardly be heard in a normal laboratory environment. ‘Ihe vibrations transmitted to the instrument and the sample manipulator are also low. Thus, future improvements in the spatial resolution can be made without requiring the standard manipulator to be modified. The rotating anode and its adjustable mounting on the rigid base structure are shown in Fig. 3. The oblique mounting of the anode is necessary in the coplanar geometry discussed above. The main part of the anode disc is made of a high strength titanium alloy. It is provided with an outer rim containing the water-cooling channels. The water to the anode is supplied via the shaft of the anode from the outside. The cooled section of the rim is made of OFHC copper which has been

Fig. 3. The UHV rotating anode mounted on a large x-y table which allows the anode to be adjusted precisely to its proper position behind the analysis chamber during operation, and to be retracted from this position during service, cf. Fig. 1. The water-cooled anode disc, 300 mm in diameter, is seen up to the right in the figure. The main metal sealed flange for the anode vacuum chamber is seen next to the anode disc. The anode normally spins at 10 000 rpm, and is then capable of working continuously with 8 kW electron beam power (16kV. 500 mA).

plated with aluminum. It is also possible to plate half the width of the copper rim with chromium in order to work with monochromatic Cr Kg excitation. This radiation has almost exactly four times the photon energy of Al Ku and therefore allows the same wide angle crystal monochromator to be used in fourth order diffraction. The main use for this higher photon energy is to increase the attenuation length for the photoelectrons by almost a factor of three, thus making it possible to reach deeper down below the surface for a nondestructive analysis, Such measurements, however, will be much more time consuming than working with Al Ka due to reduced X-ray reflectivity, decreased photoelectron cross sections and higher kinetic energies of the photoelectrons, requiring the use of larger retardation factors in the electron lens. The rotating anode is driven by an air turbine motor connected to a speed regulating system Normally it operates at 10 000 rpm, then allowing the use of an electron beam power of 8 kW focused into an elliptic spot about 3.5 x 1.7 mm in size (FWHM). The UHV vacuum performance of the contact free vacuum seal is achieved by using two stages of differential pumping in addition to using three internal stages of narrow cylindrical gaps engraved with multi-grooves like in the early molecular drag pumps by Holweck (17). and more recently in the hybrid turbomolecular pump by Maurice (18). The first differential pumping stage is evacuated by a trapped standard rotary vane pump, and the second stage is pumped with a small turbomolecular pump. The base pressure in the X-ray compartment lies in the low 10-8 to high 10-g mbar region at 10000 rpm. This is sufficient to allow a trouble free operation of the anode without contamination being formed on the target rings. The water flow through the anode is typically 6 liters per minute, provided by a closed loop cooling system. The speed of the anode is measured with a magnetic sensor. A second sensor supervises the condition of the ball bearings. A special computer system continuously monitors a number of functions in order to guarantee the safe operation of the rotating anode. Functions being controlled are; the flow of cooling water, the temperature of the cooling water, the vacuum in the Xray compartment and in the first differential pumping stage, the condition of the water seal, the X-ray power, the speed of the anode and the condition of the ball bearings. The same computer system monitors a number of other instrument parameters as well. The two-stage high power Pierce type electron gun has an electrode structure of the same kind as described earlier (9). The two-stage design has the advantage of having a very stable electron focus and a high perveance. It is capable of giving a high emission current in the second stage with a stable emission and long lifetime. A long lifetime of the first stage, which is the heating stage of the second stage, is also important and is achieved by the use of a thick filament. The gun operates normally at 8 kW and an acceleration voltage of about 15 kV. The ESCA intensity has been found to scale linearly with the power on the anode and to be insensitive within several kV to the acceleration voltage. The gun is mounted on a hinged CF flange on a bellows so that it can be slightly tilted by means of a screw on a handle. By turning the handle, it is possible during operation, to redirect the electron beam from the aluminum to the chromium target ring, and vice versa. This adjustability is also of advantage during the starting up operation of the X-ray source to adjust the electron beam to the center of the target rings.

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2.4 The wide angle. double focusing X-rav monochromator The X-rays generated in the 3.5 x 1.7 mm focus on the anode are emitted towards the monochromator at an average angle of about 15’. The apparent shape of the focus as seen by the monochromator is therefore about 3.5 x 0.45 mm. First the X-rays pass a thin beryllium window which prevents secondary high energy electrons from bombarding the monochromator crystals. Such irradiation would probably have a deteriorating effect on the crystal structure after long exposures. The energy spread of the photons due to the apparent width of the X-ray source is given by the size of the Rowland circle and the crystal dispersion. The ESCA-300 instrument uses a Rowland circle diameter of 650 mm and a-quartz crystals. This choice of size was to a large extent governed by the demand to create enough space around the sample position. The energy dispersion from reflection of the Al Ka radiation in the (1010) crystal planes using this Rowland circle diameter is 2.1 mm/eV. Thus, the finite width of the electron beam on the anode results in an energy uncertainty of about 0.21 eV. In addition to this energy uncertainty there is the diffraction spread in the a-quartz crystal. This has been measured to be 0.16 eV for Al Ka radiation (3). Therefore, the expected total energy spread of the monochromatized Al Ka radiation becomes about 0.26 eV. The wide angle monochromator consists of seven crystals mounted in a hexagonal close packed pattern as is seen in Fig. 4. The total effective area of the seven quartz crystals is 319 cm*. This is probably the largest double focusing X-ray monochromator used so far in XPS. Despite its large shape the spherical aberrations are still negligible. The crystals are mounted on the top flange of the moncchromator chamber which has been made from extra thick stainless steel to be sufficiently

Fig. 4. Photograph of the seven double focusing X-ray crystals in the ESCA-300 monochromator. The quartz crystals are toroidally bent and soldered onto thick quartz crystal mandrels. Each crystal is individually adjustable.

stable. The crystals are individually adjustable, and it has proven to be easy to align all the crystals to have a common focus at the same photon energy. Each crystal consists of a circular thin wafer which has a diameter of 76 mm. The thin crystal wafers are soldered onto thick substrates which have been machined to a nearly perfect tomidal surface in order to be double focusing. The advantage of using a toroidal bending of the crystals over the spherical bending introduced by Hewlett-Packard is that this eliminates the lengthening of the line focus produced by a spherical bending, an effect which at 650 mm Rowland circle diameter no longer is negligible. The use of the soldering technique has the definite advantage over using glues that the entire monochromator is bakeable together with the rest of the instrument. Also, it would be more difficult to achieve a perfect shape of the bent crystals using the gluing technique. Moreover, soldering has proven to be a reliable technique, guaranteeing that the crystals do not change their curvature over the years. This technology was used already in the HewlettPackard ESCA instrument seventeen years ago. and these monochromators are still working well. It is far more likely that the stresses of bending a wafer in two dimensions will slowly lead to deformations due to creeping in a polymer film than in a solder metal film. The monochromator crystals are heated and temperature stabilized by quartz lamp radiation heaters mounted at the bottom of the monochromator chamber. This ensures a constant photon energy, since otherwise temperature variations would give rise to differences in the quartz crystal lattice spacing, allowing other photon energies to be Bragg diffracted in a monochromator with fixed Bragg angle. The vacuum in the monochromator chamber is separated from the vacuum in the analysis chamber by a thin Al foil. This is necessary in order to achieve a good UHV in the analysis chamber since the hot materials in the rotating anode chamber are more outgassing than the cold surfaces in the analysis chamber. The foil also prevents electrons from the X-ray source from entering the analysis region where they could give rise to an enhanced background. The foil can be inspected through a large viewport on the side of the monochromator. A broken foil can easily be replaced by removing this flange after having first closed the two gate valves separating this chamber from the analysis chamber and the rotating anode chamber. During evacuation of the chamber, opening a small bypass valve prevents breaking of the foil. The number of monochromatized AlKa photons per unit area on the sample is proportional to the product of the X-ray power, the monochromator solid angle, and the reflectivity of AlKa photons in the crystals, and is inversely proportional to the area of the X-ray focal spot. The maximum X-ray power is at least 8 000 W. the solid angle spanned by the crystals is 0.076 steradians, and about 10% of the AlKa photons incident on the monochromator are reflected back onto the sample. The X-ray focal spot projected onto the sample is about 0.5 x 6 mm in size, i e 3 mmz. Thus, the photon density is proportional to about 20 W sr/mm2. A conventional X-ray source in XPS operates generally with about 600 W. The anode surface inside the X-ray tube can normally not be closer to the sample surface than 15 mm, and the mean incident angle of the X-rays onto the surface can then be about 45 degrees, Each mm2 of the sample corresponds to a solid angle at the

anode of about l/ (fi * 1Zi2)= 0.003 steradians. Thus, the photon density on the sample in this case is proportional

to about 600*0.003 = 1.8 W sr/mm 2. This is considerably

smaller than the

corresponding monochromatized AlKa value given above (20 W sr/mm2). Even if we would have overestimated the reflectivity of the crystals by a factor of two, and if we include the effect of the absorption of the X-rays in the extra beryllium window in front of the rotating anode (80% transmission) , the conclusion is still that a high power X-ray source in combination with a wide angle monochromator can give an increased photon density at the sample surface compared to what is obtained with a conventional non-monochromatic X-ray source. 2.5

The ESCA300 Imagine Electron Outical Svm One of the objectives in the development of the new instrument was to make a significant

improvement in the use of ESCA for small samples and to combine the detailed chemical information with imaging at a spatial resolution of a few tens of microns. Thus, the photoelectron intensity should be measured as a function of three variables, i.e. two spatial coordinates (x.y) and the energy (E). Using a two-dimensional

multidetector one then has the choice of either producing a direct

image of the object for each electron energy of interest, or to produce at each instant an “image” of the energy distribution along one line on the sample, and obtaining the complete image by successively moving the sample across the imaged line. The first approach is that taken in the “ESCASCOPE” by VG (19). while we in the present instrument have adopted the second technique, as schematically shown in Fig. 5. The double-focusing property of the hemispherical analyzer implies that for each electron energy it produces a real, inverted image with unit magnification of its entrance plane on the detector plane. This property was experimentally demonstrated for a double focusing magnetic spectmmeter by Siegbahn and Svartholm already 1946 (20). Its utilization for ESCA imaging was first proposed by Gurker. Ebel and Ebel(21). who considered the case where the sample is situated in the object plane of the hemispherical analyzer. The introduction of a magnifying lens between the sample and the analyzer obviously improves the possibility to achieve a high spatial resolution. Since the electrons reaching the entrance plane have a more or less continuous distribution in energy, only the direction perpendicular to the energy dispersive direction can be used for imaging, i.e. each point on the entrance slit gives rise to its own energy spectrum. In the resulting image on the detector, we thus have one direction representing the energy (E) and the perpendicular direction representing the position (x) in a direction parallel to the slit. We will in the following use the name E-x (ESCA) image for this type of data representation. The detection system, in our case consisting of a micro channel plate (MCP) electron multiplier observed by a CCD camera, gives an image of the electron intensity distribution with a spatial resolution (defined at the 20-80 % intensity levels) of about 0.10 mm, while the abenations of the hemispherical analyzer can, with the limitations in solid angle brought about by the lens, be made negligible compared to the other parts (22). Thus, the electrostatic lens system has to image the sample onto the entrance slit of the analyzer with such a magnification and acuity that the total resoht-

761

electron

h-resolution detector

magnifying, high-transmission, acromatic. Imaging electron lens

ESCA IMAGING

Fig. 5. Schematic view of the E-x ESCA Imaging system. The lens gives a magnified image of the sample in the entrance slit plane of the analyzer. The slit selects electrons from a narrow rectangular area on the sample, while it also defines the object size for the hemispherical energy analyzer. The analyzer gives a succession of inverted images of the entrance slit on the detector. dispersed radially with respect to the electron energy.

tion at the sample fulfills the above requirement of a few tens of microns over an extended sample. The complete system of lens, analyzer and detector then produces, for each sample position, a parallel set of energy spectra, each corresponding to the composition of one small spot on the line across the sample imaged on the entrance slit of the analyzer. Besides these requirements dimensional

of the spectrometer itself, collection of data from a two-

detector and the subsequent handling of the information from a number of such

“images” to give the complete three-dimensional picture of the intensity distribution, puts quite high demands on the data processing system. This point will be shortly discussed in the last section.

The lens system between the sample and the analyzer serves several purposes and has to be optimized accordingly. In an instrument with multi channel detection, it is most convenient to operate the analyzer at a constant dispersion and hence a constant pass energy during the acquisition of a spectrum. Thus, the lens system should retard (or accelerate) the electrons from their original energy at the sample to the pass energy of the analyzer, while maintaining as well as possible a constant transmission. The lens should also match the electron-emitting area on the sample to the acceptance of the analyzer to provide optimum utilization of the photoelectrons. It should also provide a convenient working distance between the sample and the analyzer, while allowing a sufficiently large free space around the sample for handling and manipulation of large samples. In earlier designs, where the emphasis has been on obtaining maximum intensity at high energy resolution from an extended sample (9) or where the spatial resolution has been achieved by concentrating the exciting radiation to a small spot , the requirements of image acuity have been very moderate. In the present instrument, the above-mentioned aim at high spatial resolution puts more stringent demands on the lens, while the possibility to record spectra with high sensitivity from an extended sample area should not be impeded. This implies that the lens has to be able to operate in two distinct modes, and shifting between these two modes should be rapid. In the high transmission mode, the objective is to fill the entrance slit area and the allowable solid angle of the analyzer as completely as possible with photoelectrons. In the energy dispersive direction the slit width and the acceptable beam divergence are limited by the required energy resolution. The product of slit width and beam divergence is normally much smaller than the corresponding product at the sample. Thus, as long as the magnification is large enough that no regions outside the width of the X-ray spot are imaged on the analyzer slit, this direction poses no difficulties. In the perpendicular direction, the useful length of the analyzer entrance slit is limited primarily by the detector size, due to the unit magnification of the analyzer. In this direction, the analyzer can accept a large angular spread without degradation of the resolution. At the sample, the angular spread is limited by the entrance angle into the lens, while the useful sample length is defined by the X-ray spot. Therefore, since at a given retardation ratio the product of lateral and angular magnification is constant, and the acceptance angle of the analyzer is larger than the corresponding angle at the sample, the magnification in this mode should not be larger than the ratio between the detector height and the length of the X-ray spot. On the other hand, it is advantageous from the point of view of detector saturation to make use of the entire detector area. Thus, with a 6 mm long X-ray spot and 30 mm useful detector height, a magnification of 5x was chosen for the high transmission mode. In the spatially resolving mode, one also has to take the finite spatial resolution of the detector into account. In addition, one has to realize that the off-axial aberrations of the lens system would make it very difficult to achieve high spatial resolution over the entire length of the X-ray spot. These factors make the use of a larger magnification advantageous. On the other hand, the distance between the sample and the tirst lens element has to decrease with increasing magnification. magnifications between 10x and 25x are used in this lens mode.

Hence,

763

Maintaining

a constant magnification and constant object and image positions when the

retardation ratio is changed requires at least two variable focusing voltages besides the retardation voltage, which is applied between the sample (normally on ground potential) and the center of the spheres. The introduction of one extra variable voltage gives an additional degree of freedom, which can be used to fulfill other requirements, such as keeping all electrode potentials within convenient boundaries for insulation and power supplies. It is also desirable that the electron energy nowhere in the lens is very low, since this will make the trajectories more sensitive to perturbations by charging or residual magnetic fields, and also increases the chromatic aberrations of the lens. The lens system for the high transmission mode thus consists of five elements with cylindrical symmetry, i.e. a first element on the sample potential, three focusing elements and a final element on the sphere center potential. The distance between the sample and the entrance slit of the analyzer is 800 mm and the free space between the sample and the first lens element is 55 mm. The center element is much longer than the others, so the lens system acts essentially as two decoupled threeelement lenses. In order to increase the magnification in the spatially resolving mode, an additional optical element can be inserted as a prelens by use of a manipulator. The prelens is an Einzel lens with its outer elements on the sample potential and connects directly to the first element of the main lens. Thus, one has in this mode four adjustable voltages. The angular acceptance is limited by an aperture stop inside the prelens. This aperture can be easily replaced. although it requires breaking of vacuum. The distance from the sample to the front of the prelens is 16 mm. In the high transmission mode, the lens gives a direct image of the sample on the entrance slit of the analyzer, and this is also the case in the spatially resolving mode when the magnification factor 10x is used. For pass energies below 300 eV, it is more advantageous to use stronger excitation of the lens, giving rise to an intermediate image in the long central element. The lens system then works principally as an electron microscope, with the prelens and the first three elements in the main lens taking the role of the objective, and the last three-electrode part working as a projective lens. The main advantage in using the higher magnification is that the detector contribution to the resolution is reduced. In this imaging mode, the optimum resolution is obtained at a magnification of 15x. It is in this imaging mode possible to obtain a spatial resolution ( defined as the distance between 20-80 % of full intensity at an edge) of better than 30 urn everywhere on a 3 mm long object. The optimum resolution measured at the center is 23 pm using 15x magnification. This figure includes the contribution from the finite resolution of the detector. This contribution is about 16 pm. implying that the resolution of the electron-optical system is also about 16 pm. The depth of field is at this spatial resolution about 0.2 mm. Besides the rotationally symmetric electrodes, the lens also contains a pair of deflection plates, situated near the center of the lens, and oriented to deflect the beam perpendicular to the direction of the slits. By applying a voltage to these plates, the position of the area giving an image on the slit can

764

be moved, thus making it possible to compensate a slight misalignment between the lens axis and the X-ray spot on the sample. Experiments have shown that in the imaging mode good spatial resolution can be maintained for deflections of the beam corresponding to a displacement of fl mm at the sample. Thus, one could in principle use this deflection instead of moving the sample in order to scan over an area, but the observable area would in this case be limited by the width of the X-ray spot. This feature has turned out to be very convenient to use in the adjustment of the X-ray monochmmator insofar that it effectively decouples the intensity variations of the monochromator from the transmission of the lens. The analyzer is of the hemispherical type with a main radius of 300 mm and an inter electrode gap of 100 mm. ‘Ihe field is terminated by a Herzog plate on the potential corresponding to the main radius. In order to obtain an energy dispersed image of an entrance slit in the plane of the Herzog plate on a detector in the same plane, the geometry deviates somewhat from the simple hemispherical one, Thus, the electrodes extend somewhat beyond the full hemisphere, and the Herzog plate is situated yet some millimeters from the end planes of the electrodes. The lens axis, defining the direction of the central beam of electrons, makes an angle inwards from the normal to the Herzog plate. The position of the radial image plane is a sensitive function of this angle (23), and we have hence provided for its further adjustment by introducing a pair of deflection electrodes immediately after the entrance to the analyzer. So far, however, the optimum setting of these electrodes corresponds to zero deflection, indicating that the chosen geometry is adequate for its purpose. The acceptance of the analyzer is defined by a pair of slits situated at its entrance. The last slit, which is in the plane of the Herzog plate, defines the object size for the analyzer, while the aperture angle in the high transmission mode is defined by the combination of the two slits. Eight different slit pairs, each matched for optimum intensity at a given resolution, are mounted on a carrousel and am easily interchanged via a vacuum feedthrough. The object-defining slits vary in width from 0.2 to 4.0 mm, while all slits are 30 mm long. The analyzer can be operated at a number of predefiied pass energies, ranging from 20 eV to 1000 eV. During sweeping of a spectrum, the focusing voltages in the lens have to be varied accordiig to calculated curves. The electron detector is based on the technique introduced already on the HP ESCA instrument, i.e. an electron multiplier consisting of two MCPs in a chevron mount gives rise to light flashes on a phosphor screen, which is subsequently viewed by a video camera. In the present instrument, the MCP detector is a circular 40 mm diameter standard detector with 12 urn channel diameter, and a specified resolution of 16 lines/mm . The video camera is a standard CCD camera The signal from the CCD camera is fed into a separate micro computer interface, which translates the position on the CCD array into energy, correcting for the nonlinearity in dispersion, and the counts ate then accumulated in the proper energy channel in the main computer.

765

3.

RESULTS 3.1 High Previously, the main disadvantage of monochromatic X-ray excitation in X-ray photoelectron

spectroscopy has undoubtedly been the considerable reduction in intensity. Because of this, and despite the many advantages inherent in using this refined photon source, most ESCA users still mainly use conventional X-ray excitation. The ESCA300 instrument changes this situation dramatically. As demonstrated below, the intensity obtained with the ESCASOO instrument is higher than with any other ESCA instrument. while simultaneously exhibiting all the advantages of monochromatic excitation listed above in the Introduction. In fact, the intensity of the ESCA-300 is so high that under certain conditions problems arise in detecting strong electron lines without saturating the twodimensional detector system. The reason for this is that the CCD camera is limited to recording 30 images per second. When using high pass energies and analyzing strong and narrow electron lines the electron intensity in the most intense regions on the detector can yield more than one flash per CCD pixel and image. Saturation effects are seen well before the intensity reaches this theoretical maximum limit. Therefore, the strongest photoelectron lines should be recorded at a low pass energy where a line is spread over a wider detector area. Well aware of the above disadvantage of the present phosphor and CCD camera read-out system from the MCP detector we have still chos& to use this system because of its superior high spatial resolution, the absence of image distortions, and its inherent long time stability. These properties are so important for a two-dimensional detector system which is also used for imaging that they outweigh the inconvenience of sometimes havig to record strong electron lines with a reduced intensity. Moreover, the technical development in increasing the resolution of both the MCPs and the CCD cameras steadily continues and hopefully CCD cameras with a higher image frequency will also become available. All together. this means an increased detection rate per unit area of the twodimensional detector system. The Ag 3dsur core electron line is one of the strongest and most narrow lines observed in XPS. For this reason this line has become the most generally accepted standard line for demonstrating the intensity performance of new ESCA instruments. We have chosen to demonstrate the intensity of the ESCA300 not on this strong core electron line, but on the Ag 4d valence electron band. This band has in its most intense region about 30 times lower intensity per unit detector area than the peak of the Ag 3d5,, line. Simultaneously, the silver valence band possesses interesting fine structures in its density of states which require monochromatic X-ray excitation to be observed, and which appear increasingly well resolved as the instrument resolution is enhanced. Moreover, the analysis of the Fermi edge region at the top of the weak s-p band with sufficiently good stathtics to allow an evaluation

of the instrument resolution from the shape of this edge represents a difficult

challenge and is simultaneously a sensitive measure of the detector background. Hence, presentations of Ag valence band spectra are much more revealing for comparing the performance of different instruments than merely showing Ag 3d lines.

The ESCA-300 instrument is probably more flexible than other instruments in allowing seven pass energies, from 20 eV up to 1000 eV, and eight different entrance slits, from 0.2 mm up to 4 mm widths, to be used in any combination. The electron lens can be operated in two modes, the transmission mode and the spatial mode, and the effective region of the two-dimensional detector can easily be specified in a menu in the software. Thus, the count rates obtained by using different combinations of these parameters span over many orders of magnitudes. Fig. 6 represents an example of how to achieve a good balance between high intensity and high resolution without going to extremes in either of these. The spectrum was recorded in the swept mode over a 15 eV wide energy region. The step size was chosen to 0.1 eV which means that the detected photoelectrons are sorted into 151 different energy channels. The X-ray power on the rotating anode was 8 kW, the slit width into the analyzer 0.8 mm, the pass energy 150 eV, and the full size of the detector was active. The spectrum was recorded during one hour, not in order to get sufficiently good statistics, but because a 1 hour run on the Ag valence band spectrum has been chosen as the performance test for proving the high intensity at a high energy resolution for the ESCA-300. The original specifications, given at the early development stage of this project, were. 150 000 counts in the channel at the valence band maximum recorded at a resolution of s 0.55 eV. The spectrum shown in Fig. 6 shows that the insuument performed much better than our original conservative specifications.

10

5 Binding Energy IeVl

1

Binding

EnJgy

[eV]

Fig. 6. Valence band of a sputtered silver foil recorded at a normal energy resolution with the ESCA300 instrument. The 15 eV wide spectrum was recorded in the swept mode with 0.1 eV step size during 58 minutes. The number of counts at the Ag 4d band maximum is above 850 000. The spectrum is printed without any smoothing and the statistical spread is seen to be negligible. The Fermi edge is seen magnified to the right. The instrument energy resolution was determined to 0.37 eV from the shape of this distribution. This spectrum is used as the standard intensity test for the ESCA-300 instrument.

767

From a separate study the intensity ratio between the Ag 3d peak intensity and the intensity at the Ad 4d band maximum in the ESCA-300 instrument was determined. This ratio was found to be about 31 at this resolution (it should be somewhat less at a lower resolution). Thus. the ESCA-300 would in the same time take a 15 eV wide Ag 3d spectrum with 26.4 million counts in the peak channel. Since the detector width at 150 eV pass energy corresponds to an energy width of 10 eV, sweeping a 15 eV final spectrum with the multidetector system means that 251 steps have been taken. The total time used was 58 minutes, i.e. 3480 seconds, and the dwell time per step is therefore 3480/251 = 13.86 seconds. Therefore, the equivalent single detection intensity can be given as 2.64 x 107/13.86 = 1.9 x 106 cps. This way of calculating the intensity of an instrument with a multidetector system seems to have become the standard procedure among all instrument makers. Thus, the ESCA-300 has demonstrated a Ag 3Qn intensity of 1.9 x 106 cps at 0.37 eV instrument resolution. This is about 20 - 100 times higher intensity than obtained with other ESCA instruments using monochromatic Al Ka excitation, and it is even far more than achieved with ESCA instruments using standard non-monochromatized X-ray sources at 1 eV instrument resolution. It should be noted that the count rates given in the spectrum are corrected for the multiple counting that occurs on the CCD sensor. The multiple counting on the CCD occurs because the image of a light flash on the phosphor screen behind the MCP detector normally covers several CCD pixels. The software of the ESCA-300 contains a calibration routine that makes an analysis of the statistical spread of the counts in a region of a spectrum which is a straight background, and determines the average multiple counting factor from this. The intensity scales of all spectra are automatically corrected for this multiple counting by division with this factor, which typically lies between 3 and I. The the multiple counting factor is dependent on the size and brightness of the light spots on the phosphor screen, the lens aperture on the detector camera, the correct focusing of the camera, and the setting of a discriminator level. In the old Hewlett-Packard ESCA instruments, which used detector cameras with vidicon tubes, the proper adjustment of all these parameters was difficult to perform and was not stable in time. The CCD sensors, however. are much more light sensitive than the old vidicon cameras were, and are much less temperature sensitive. Moreover, the temperature dependent vidicon lag, which gave rise to multiple counting on subsequent vidicon images, is totally eliminated by the CCD technology. Therefore, it is easier to find a suitable combination of the parameters which influence the multiple counting, and, most important, this combination is much more stable than for the Hewlett-Packard instrument. 3.2 J-Iiah Enerav Resolution The performance of the ESCA-300 in the vicinity of the energy resolution limit was examined by again studying valence band spectra. First a highly resolved silver band spectrum is shown recorded recently with the first ESCA-300 instrument. The silver sample was a thin plate, cleaned in the preparation chamber by ion bombardment with Ar. The slit size was in this experiment 0.2 mm, the pass energy was 75 eV. and the effective detector height was reduced to 30% of the full width.

The electron analyzer has in this mode an energy resolution corresponding to 0.030 eV (FWHM) which is almost ten times smaller than the contribution from the X-ray monochromator, estimated above to be 0.26 eV. Therefore, the analyzer contribution to the total resolution is practically negligible in this experiment. The spectrum shown in Fig.7 was recorded automatically over a night.

L

0.

\

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I

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I

8

6

4

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Binding

0

Energy [eV]

Fig. 7. The valence band spectrum of silver recorded at high energy resolution with a step size of 0.03 eV. The individual energy channels are plotted in this figure in order to more clearly show the details in the spectrum The sharpness of the Fermi edge indicates that the energy resolution has been 0.27 eV or slightly better. At this resolution and statistics fine structures appear which have not been observed earlier.

An analysis of the Fermi edge gives an energy resolution of about 0.27 eV. However, there appears to be an asymmetry in its shape with a somewhat more extended tail above the Fermi edge. The origin of this slight asymmetry is not clear. It might be a statistical artifact, but it may also be a real phenomenon caused by the sputtering of the silver without annealing. Changes of the band structure due to sputtering can be expected in general, but for silver no obvious broadening of the core electron lines is observed after sputtering, and there is no Ar 2p electron line seen in the total spectrum. However, this may not be sufficient evidence to rule out structure modifications which might show up in the band structure, and which could influence the sharpness of the Fermi edge. The establishment of such an effect for silver obviously requires more detailed studies.

The evolution of the energy resolution in XPS is clearly reflected in the observation of more and more fine structures in the silver valence band. The first spectrum recorded with monochromatized Al Ka excitation, recorded with the Hewlett-Packard instrument, shows only the two strongest structures (3). In 1975 Banie recorded a silver band structure (24). which clearly showed two mote fine structures at 4.2 eV and 6.8 eV, and perhaps also the existence of a weak structure at 5.5 eV. as predicted in the band structure calculations by Christensen (25). The structure at 5.5 eV was well resolved with one of our previous instruments (9). The accurate positions of the earlier observed five structures are from the data shown in Fig. 7 determined to 4.23 eV, 4.87 eV. 5.55 eV. 6.30 eV and 6.72 eV relative to the Fermi edge. The minima in the band structures occur at 4.50 eV, 5.43 eV and 5.96 eV. The accuracy of all these values are within 0.03 eV. except for the broad structum at 6.72 eV where the large width makes it less meaningful to define a precise energy value. The spectrum in Fig. 7 shows the existence of an additional sixth structure situated on the left side of the strong peak at 4.87 eV. The position of this structure is at 5.07 k 0.03 eV.The 1972 band structure calculation by Christensen actually predict three sharp structures in the density-of-states around 4.9 eV. It is therefore likely that the somewhat broad peak at 4.87 eV actually gets intensity contribution from one additional and somewhat broader structure at a slightly lower biding

energy

around 4.60 eV. It should be noted that the binding energy scale given in the figure unfortunately is shifted by about 0.1 eV. The energy determinations above have been made by counting individual channels relative to the Fermi position. Platinum has its Fermi edge at the top of the 5d band where the intensity is much higher than at the Fermi edge of the silver 5s-p band. A shiny platinum foil was cleaned by simply wiping it off a few times with tissues wetted with iso-propanol before insertion in the spectrometer. The few monolayers of hydrocarbon present at the surface after such a treatment will contribute less than 1% of the intensity of the platinum band because of the difference in cross sections and, moreover, will not give rise to any sharp structures. This justifies the simple cleaning procedure. The Pt band spectrum was run at 40 eV pass energy and a slit width of 0.5 mm with an X-ray power of 8 kW. The spectrum was recorded over night with an energy step size of 0.05 eV. The spectrum is shown in Fig. 8. The three most distinct peaks in the band structure appear at 0.27 eV. 1.83 eV and 4.20 eV. Two additional structures, which to our knowledge have not been observed before, appear as shoulders, one more distinct at about 2.60 eV and a second very broad one at 5.8 eV. The peaks and shoulders correspond well to peaks in relativistic density-of-states calculations for platinum by Christensen (26, 27). Thus, the peaks at 0.27 eV, 1.83 eV and 2.60 eV originate from peaks with predominantly Pt 5dS,, symmetry at 0.15 eV, 1.94 eV and 2.70 eV. The peak at 4.20 eV corresponds to two closely spaced structures with predominantly 5d3h symmetry at 4.04eV and 4.45 eV having their centroid at about 4.2 eV. The broad shoulder at 5.8 eV corresponds in the density-of-states

distributions to a broad distribution at 6.05 eV which has only slightly more

contributions from 5d3&symmetry states than 5dsjz states (26).

710

10

0

6 4 BindingEnergy [eV]

2

0

Fig. 8. High energy resolution valence electron spectrum of platinum from a Pt plate. The shoulders observed at 2.6 eV and 5.8 eV have not been observed in XPS spectra before. All structures correspond well with structures appearing in relativistic density-of-states calculations on Pt (26).

A first approximation to the spectrometer.function was obtained by assuming that the densityof-states distribution resembles a step function convoluted by a Fermi-Dirac function for T=3OOK. This resulted in a spectrometer function of 0.265 eV. The convolution of a Gaussian of this halfwidth with a model density of states distribution, which resembles the Christensen’s theoretical density-of-states distribution in the vicinity of the edge, showed that this value is somewhat too low, and that the spectrometer function in this case probably is around 0.29 eV.

3.3 HH$

n

Operating the ESCA-300 instrument in the high transmission mode with a large entrance slit and a high pass energy increases the intensity considerably, and the instrument is then operated in a high sensitivity mode. Consider for example the Ag 3d5n line which at 150 eV and 0.8 mm slit width had an intensity of 1.9 x lo6 cps at an instrument energy resolution of 0.37 eV. Keeping the pass energy at 150 eV and opening the slit to 4 mm increases the intensity to 23 x 106 cps and increases the energy resolution to 1.1 eV. This is a convenient energy resolution for running survey spectra on a sample to search for unknown surface constituents. Weak structures situated in the

771

background of strong core electron lines, which in the high resolution mode could have been overlooked, will appear much more clearly in this high sensitivity mode. The core levels of the substrate are likely to be saturated in such a survey spectrum, but their true relative intensities can easily be obtained in a quick separate measurement, by deliberately defocusing the electron lens and recording the peak-to-background ratio for the strongest peaks in this mode. Fig. 9 illustrates the high sensitivity of the ESCA-300 by studying a silicon sample in detail. The surface of the silicon sample originally contained some large Si@ structures which in the present experiment were first removed by etching the wafer in HF until all the Si@ structures were removed. The sample was then rinsed six times in distilled water, dried, mounted on a stub with Aquadag. a colloidal suspension of graphite in iso-propanol, and inserted in the spectrometer. First a series of angular dependent measurements were made of the type described in the next section. At the end of this series the sample was oriented perpendicularly to the lens axis, the spectrometer was set

I

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4500- w 4000-% 3500- 8, 151 _u m2500 2 3000-

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BindingEnem

BiO.

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6;O ’ 4;)o BindingEnergy[eVl

2oo

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Fig. 9. Survey spectrum demonstrating the high sensitivity for low abundant elements achievable with the ESCA-300 instrument. The inserted spectrum in the upper left comer shows an enlargement of the energy region around 400 eV binding energy. The observed weak Nls double line has an intensity corresponding to approximately 0.5 % of a monolayer. This structure has a S/N ratio of 4. indicating the possibility to detect even lower abundances of foreign substances on a surface. This spectrum was recorded at a normal emission angle. Further enhancement of the S/B ratio at grazing angks will make it possible to detect surface constituents with concentrations corresponding to less than 0.1% of a monolayer.

in a high sensitivity mode, and an survey spectrum was set up to be run automatically over night. The slit width and pass energies were 4 mm and 150 eV, respectively, and the energy step was 0.5 eV. The acquisition time was 8 hours. The spectrum shows the presence of several elements of low concentration in addition to the signals from Si, SiOx and iso-propanol. Thus Cu is seen at about 933 eV (Cu 2~3~~)and 76 eV (Cu 3~). and 570 eV (Cu LMM), fluorine is seen at 688 eV (Fls), 833 eV and 860 eV (F KLL) and nitrogen is seen around 400 eV (N 1s). The presence of fluorine was expected, since we have observed this every time we study a silicon wafer, including an as received wafer from a supplier. The Cu is likely to originate from the distilled water that was produced within our university. The nitrogen signal originates from two Nls lines, about 2.8 eV shifted from each other as shown in detail in the inserted spectrum. Their binding energies are about 400.8 eV and 403.6 eV. It is likely that the nitrogen originates from the atmospheric exposure of the wafer after the etching and rinsing in water. We will not tty to give an explanation to their presence on the Si wafer, but will just stress the fact that nitrogen is there and can be detected by XPS. An evaluation of the abundance of the nitrogen leads to an amount of about 0.6 % of a monolayer for the Nls peak at 403.6 eV and slightly more for the other nitrogen. The signal-to-background ratio is seen to be as small as 0.01 but nevertheless the signal-to-noise

ratio is as large as 4. As discussed above, the new coplanar

geometry of the ESCA-300 will enhance both the signal and, even more, the signal-to-background ratio by changing the electron angle from normal to grazing angle. It is reasonable to assume that the lower limit for detection of low abundant elements and compounds on surfaces has now been decreased to below 0.1 % of a monolayer.

3.4 High Surface Sensitivitv In order to examine the angular dependence of the intensity for the ESCA300 instrument a study on the polished side of a silicon wafer was made. Special precautions were taken to avoid that the silicon surface was covered by particles which would affect the results at the most grazing angles. A standard 3” silicon wafer in its original dust free package was opened in a clean air cabinet and immediately dipped in a liquid polymer used as a strip-coat in optical industry (28). The polymer was allowed to dry and form a protective skin on both sides of the wafer. The wafer was subsequently sliced with a diamond saw on the back side of the wafer into rectangular pieces, 10 x 2.5 mm in size, without cutting through the wafer. The many tiny silicon particles formed during this operation were then embedded in the polymer by painting a second thin layer of the polymer onto the back side. After drying, the wafer could easily be broken into rectangular pieces. The strip-coat on the backside of the wafer was removed, and the sample was mounted on a stub by a silver glue. The stub was placed in the receptacle of the load-lock chamber, and immediately before closing this for pumpdown the strip-coat on the polished top side of the wafer was pulled off. In this way the silicon surface entered the vacuum system with a minimum of particles on its surface.

Fist

1150 CV widesurvey

spectra were recorded at normal emission and at 5’ glancing

emission. Besides the expected lines from the silicon bulk and oxide, two Cls lines from -CHa- and -COH carbon atoms were seen and also tmces of Sn, Na and F. The Cls lines mainly originate from a residual overlayer from the strip-coat polymer but some hydrocarbon contamination contribution was also present. A crude analysis of the thicknesses of the films involved in this system, based on the observed intensities, Scofield cross sections (29). attenuation lengths of 26

A and

30

A for Si

and Si&, respectively (30,31) and an assumed attenuation length 40 A for the polymer film, gives a SiOx thickness of about 10 A and a polymer layer thickness of about

sA.

Fig. 10 shows the angular dependent relative intensities I,(a)/I,(W)

for the five most

prominent peaks in the spectrum The intensity of the Cls limes from the outermost polymer layer is seen to increase steadily towards grazing angles until about 7.Y. If the layer had not been 5 A

thick,

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Fig. 10. Angular distribution of relative intensities Ix(a)&(W) for the 01s. Cls and Si 2p lines from a silicon wafer with an oxide layer about 10 A thick and with a polymer coating about 5 A thick. The Si 2p bulk intensity and the 01s intensity show effects of electron diffraction in their angular distribution functions.

but only 1 A, the maximum intensity would have occurred at an even smaller grazing angle at about 5”. This can be concluded by calculating the attenuation within a 5

A film and a 1 A film, and

correcting the intensities for the difference in attenuation. The geometrical cut off of photon intensity and electron transmission starts gradually somewhat below 10’. and it dominates over the expected intensity increase for the outermost atomic layer at angles below 9, resulting in a steep fall of intensity for this top layer at the smallest angles. Fadley has measured the angular dependence of the intensity from a very thin carbon contamination overlayer with the Hewlett-Packard instrument (32). He found the maximum intensity for this instrument to occur at 30’. At this angle the surface sensitivity is only very little enhanced. This is evident if the relative intensities in Fig. 10 are normalized to the relative intensities of the Si 2p bulk intensity by forming the ratio between these relative intensities for each angle. These normalized intensities are shown in Fig. 11. The enhancement of the relative intensity for the outermost polymer

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I

0

EMISSION ANGLE RELATIVE TO THE SURFACE Fig. 11. Angular distribution of the relative intensities shown in Fig.10 after normalization to the Si 2p bulk relative intensities. The normalization removes most of the artificial effects at very small grazing angles due to onset of cut off of X-ray intensity and electron lens transmission as well as surface roughness effects. Note the very high enhancement in surface sensitivity obtainable with the new geomeay introduced in the ESCA300 instrument,

775

layer continues all the way even down to negative angles, and reaches an impressive enhancement value of about 60. At 30 ’ the enhancement is very small in this scale. It is interesting to see that at the smallest grazing angles, from 9 and below, there develops a significant

difference between the two Cls lines, indicating that the -CHa- carbon is more

concentrated in the outermost atomic layer. It is possible that, even for disordered molecular layers where electron diffraction effects are averaged out, XFS measurements at such extreme grazing angles can determine which side of a molecule is facing upwards and which is bonding to the substrate with a depth resolution of a single atomic layer. A high surface sensitivity does not only mean an enhanced surface/bulk intensity ratio but requires also good signal/background and signal/noise ratios for the corresponding surface electron lines. Fig. 12 shows the relative signal/background ratio for the Cls (CHi) line. The background has been determined at a binding energy 3 eV lower than the peak. The absolute signal/background ratio at normal emission angle was 0.87, and the signal/noise ratio 8.66. The signal/background ratio is seen to increase steadily towards grazing angles and is enhanced by more than a factor 6 at the smallest angles. The signal/noise ratios behave in a similar way, having the maximum at 0’ and

+ + +

+

-10

0

10

TAKE-OFF

20 ANGLE

30

40 RELATIVE

50

60 TO THE

70

60

90

SURFACE

Fig. 12. Angular distribution of the relative signal/background ratio for the Cls (cH2) line from the same sample as studied in Fig.10 and 11. Due to the negligible detector background the S/B ratio increases even at the most grazing angles when the intensity of the Cls line is decreasing. The S/N ratio was in this experiment highest at OO.

falling sharply by l/3 at -2.5’. The enhancement in the signal/noise ratio at 0’ compared to 90 was 3.72. Thus, it can be concluded that the ESCA-300 instrument has its highest surface sensitivity around 0”. both with respect to the signal/bulk intensity ratio and the signal/background ratio.

3.5 &&r&l

resolution i

In order to evaluate the spatial resolution of the imaging system, a gold plated razor’s edge was used as a sample and positioned on the manipulator to be perpendicular to the slit. With the spectrometer set to transmit the Au 4f lines, the edge is then clearly visible on the real-time monitor connected to the CCD camem. In the normal collection of spectra, the line direction of the camera is perpendicular to the energy dispersive direction of the analyzer, so each line corresponds to a particular energy. For this measurement the camera was turned by 90”. so the different lines instead represent different positions along the slit (the x-direction). Thus, the normal fixed mode data collection routine could be used to give the intensity distribution along the slit of the Au 4f intensity instead of the normal spectral information. The magnification, and hence the distance on the sample corresponding to each camera line, could then be measured using the sample manipulator to move the edge along the slit and measuring the corresponding displacement of the image on the detector. The resulting intensity distribution is shown in Fig. 13.

-50

Distance

0

50

(Frnl

Fig. 13. The spatial intensity distribution is the E-x image of a sharp gold edge. The analyzer is set to accept the Au 4f7/~ line. To average out variations in the detector sensitivity, this curve is obtained as a sum of ten recordings with the edge at successively displaced positions. The distance between the points on this curve at 20% and 80% of the full intensity is taken as the spatial resolution of the system. As mentioned above. this width of 23 pm gets about equal contributions from the electron optics and from the detector.

There seems to bc no generally agreed level on which to measure the spatial resolution, but the 20-804 distance has been used earlier in the field. The unambiguous way to define the resolution is to use the Modulation Transfer Function (MTF). In the definition of this function, the resulting intensity modulation of a square wave (e.g. in optics a black and white stripe pattern) after the optical system is given as a function of the spatial frequency of the square wave. We have calculated the MTF curve from the intensity distribution at the edge. with the result shown in Wg.14. In this curve, 23 km corresponds to a modulation of about 30%. In optical systems, the resolution is often given on the 3-546 modulation level, while on the other hand the demand for “clean” spectra would require a modulation close to 100% to be used.

MTF curve

0

10

20

30

40

70

80

LIN!ZwIDTH(ym)

Fig. 14. The Modulation Transfer Function (MTF) corresponding to the intensity distribution shown in Fig. 13. The resolution of 23 l.trn corresponding to the 20-8096 interval gives a modulation of 30%. With the definition of resolution used in optics, the resolution is about 13 pm.

3.6 E-x ESCA irnw The first experiments on the imaging properties of the instrument were performed on a very early stage, before the monochromatic

X-ray source had been taken into operation. For test

purposes, a special sample was prepared by sputter-depositing gold stripes on a silicon backing. The gold stripes were 32 pm and 130 pm wide, with silicon intervals of 168 pm

and 270 km.

respectively. The sample was placed in position with the stripes perpendicular to the direction of the slit, and consequently the instrument sees a narrow rectangular area across a number of the stripes, as indicated in the left part of Fig.15

SPATIALLY RESOLVEDESGA ?S’EGTRA

PHOT-OF Aut&V

si 2p V

DISTANCE(mm)

ku

4f

BtNDtNGENERGY(elf)

Fig. 15. Left part: Photograph of the Au/Si sample with a frame corresponding to the observed area inlaid (slightly displaced downwards). Right part: Photograph of the real-time monitor, 4 sec. expo sure. Non-monochromatized X-rays, 500 eV analyzer pass energy, instrument magnification 13x.

A conventional X-ray source with an Al anode, operated at 500 W, was used in the first experiments. At that stage, the only way to save the complete E-x image was to photograph the real- _ time monitor, using the time-exposure as the means of integration. On the right side of Fig.15 we show the image obtained with an exposure time of 4 sec., using an analyzer pass energy of 500 eV and an 1 mm wide spectrometer slit. The image covers an energy interval of 30 eV, and the magnification is 13x, so the observed area is about 70 urn wide. Even with this short exposure, the Au 4f lines are saturated, and the inelastic background from the gold extends across the figure. The Si 2p bulk line is clearly seen, and also a weak signal from the silicon oxide on the surface. The “fanning” of the lines is an unavoidable effect with this type of analyzer, since all lines converge towards the sphere center. For the later experiments, a special image integrator was used. This integrator can be connected directly to our video camera, and integrates the intensities pixel by pixel. The integration time can be chosen to give the desired signal/noise ratio. The integrated image can be displayed in black/white or color, and the dynamic range or the color coding of the intensities can be chosen to emphasize features of special interest in the image.

T79

Of particular importance for our purposes is the possibility to store in the integrators memory the dynamic range of each individual pixel, i.e. the black and white levels are individually set for each point. In this way, the unavoidable inhomogeneities in the sensitivity of the detector can be corrected at this stage. This feature was not utilized in the images presented here, however. The remaining E-x images in thii section are obtained with thii equipment, by photographing the monitor of the integrator. Using the monochromatic X-ray source, the Au/% sample was studied at higher energy resolution. Fig. 16 shows the Au 4f doublet, recorded with a pass energy of 150 eV. The integration time for this recording was 10 sec. at 4 kW X-ray power. Comparison of this Figure with the preceding one, noting the lower pass energy. also illustrates that the intensities with monochromatic X-rays are of the same order of magnitude as with a non-monochromatixed

source at the closest

possible position.

Fig. 16. E-x image of the sgiped AulSi sample in the energy region around the Au 4f doublet. 10 sec. integration time. Monochromatic X-rays, 150 eV pass energy, magnification 15x. Black/white rendering of color-coded original.

Fig. 17 shows the ener$y region around Si 2p from the same sample. In this case, the integration time was much long& about 1 hour at 4 kW. The Si 2p doublet from the bulk is seen clearly resolved, and the silicon oxide peak is seen to the left.. At this excellent energy resolution the intensity is still rather high. Thus, at the * peak of the Si 2p line each pixel in the CCD image contains more than 1000 counts. and each horizontal pixel row represents a spectrum of a good statistical quality recorded with a very fine energy step. The color-coded intensities reveal also the difference in background level below and above the Si 2p energy.

780

Si AU

35pm

area analyzed tne spectrometer entrance slit

Linear

tnrougn

Fig. 17. Left part: Schematic representation of the Au/Si sample with the observed area marked. Right side: E-x image in the Si 2p energy region. 1 hour integration time, 150 eV pass energy, magnification 15x.

X

1 I

E

Fig. 18. E-x image of a PET sample in a metal mask, taken in the C Is energy region. The curvature of the carbon lines on the PET (lower part) in the vicinity of the metal indicates differential charging of the sample. 500 eV pass energy, magnification 10x. Non-monochromatized X-ray source at low intensity used for chvge neutmlization.

781

In a study of the neutralization

of an insulating sample, a piece of PET (polyethylene-

terephtalate) was mounted in a metal frame, and a conventional X-ray source at a low power density was used simultaneously with the monochromatic source to provide a neutraliing

flow of electrons.

Fig. 18 shows the E-x image of an energy interval around the Cls lines. The image reveals that the surface potential on the PET varies considerably in the vicinity of the metal frame, as manifested in the curvature of the C 1s lines. One also notes that even at the high pass energy used, 500 eV, much detail can be seen in the PET spectrum. This technique can thus be used to find sample areas where differential charging is minii,

and to optimize the charge compensation.

The final example of E-x imaging (Fig. 19) is a sample where a silver foil and a platinum foil are placed side by side, with the silver at the top half of each figure. Four energy regions are shown

Fig. 19. E-x images of a silver-platinum sample, with silver at the upper and platinum at the lower half of each picture. The energy regions are centered on the Ag 3d lines (top left), Pt 4f (top tight), C Is (bottom left), and the valence bands (bottom right). The dark line separating the two metals is due to shadowing of the X-rays. All images are made at 500 eV pass energy, 10x magnification, and cover an energy region of 33 eV with a sample area of 5.0pm x 3 mm. These images were taken at an early stage of development using a detector that had a damaged spot in the center. Black/white renderings of color-coded originals.

782

in the figure, centered around the Ag3d (top left), Pt 4f (top right), C 1s (bottom left), and the valence electron (bottom right) binding energy regions. Of most interest in this figure is that it demonstrates that the intensities are sufficient even in the valence band region to obtain detailed E-x images in a few minutes. Thus, the chemical information contained in the detailed shape of valence bands, which is in many cases more significant than the chemical shifts of core levels, is accessible for spa:ially resolved ESCA investigations. In this case, and also in the many cases, e.g. in the investigation of organic surfaces, where the full chemical information only can be obtained by a detailed investigation of spectral regions with several overlapping structures, the recording of complete spectra at high energy resolution is important, and this is the application where this technique has its greatest advantage.

4.

SOME FUTURE

DEVELOPMENTS

4.1 The Imaeine ESCA Svstem The ESCA-300 instrument. as presented above, with its efficient monochromatic X-ray source and its imaging electron optics, provides the necessary basis for the further development of the ESCA imaging technique. The technique used in the acquisition of the E-x images above, visually pleasing though it is, cle&ly does not represent the final stage in ESCA imaging. The equipment used for the acquisition of these images could, however, be used together with a powerful personal computer (work station) as the basis for a more advanced imaging ESCA system. In such a system, the intensity information from each picture element, with a size appropriate for the resolution in energy and position, would be collected during some time interval for a fixed energy region and a fixed sample position. We do not at present envision the combination of energy scanning with imaging, since a simple analysis shows that this would still be prohibitively costly in terms of computer capacity. During the time when the E-x image from one position is collected, the computer should process the information collected at the previous position, i.e. for each sample point the energy spectrum should be analysed in its components and this information be translated into the chemical composition of the sample point. After the sample area of interest has been scanned over, the distribution of elements in different chemical states over the surface could be displayed. The development of this type of system from the present one is thus essentially a problem of computer software and to some extent also of storage capacity, since each complete E-x-y image will require of the order of 1 Mb for storage. 4.2 Imurovement of Suatial Resolution, In the present system, the spatial resolution is limited by the aberrations of the lens system and by the resolution of the detector system These two contributions are of the same order of magnitude,

783

and hence both have to be improved in order to improve the total spatial resolution significantly. Although the present resolution, of the order of 20 pm , represents a major step forward in the possibility to perform electron spectroscopy with high energy resolution from small samples, there are a number of problems in surface physics and technology which would require significantly better spatial resolution in order to be amenable to electron spectroscopy. It is therefore interesting to consider the factors that limit the resolution. It should be emphasized here, that it is not just the theoretical resolution, but rather the resolution at intensity levels that allow practical spectroscopy that is important. One of the factors that limit the intensity is the photon density at the sample. With the present development of the rotating anode and monochromator. one is probably close to the limits of what can be achieved in this respect, since both the power density on the anode and the focusing of the crystals are limited by the materials. The remaining point where one could hope for significant improvements is in the electron optical system and on the detector, where today’s limitations may be of a somewhat less fundamental nature. The most straightforward way to reduce the contribution from the detection system is to increase the magnification

of the lens system. This also implies that the requirements

of the

tolerances in the hemispherical analyzer would be unchanged. Already the present lens system allows magnifications up to

60x.

Using the present detection system, this would reduce the detector

contribution to about 3 pm. Future developments in the detector technology, or possibly the application of the technique to determine the “mass center” for each electron shower from the MCP instead of using a video camera (33). might improve the detector resolution. In that case, a smaller magnification in the lens could be used, thus increasing the simultaneously studied area. As for the possibilities to significantly improve the resolution of the lens system, one notes that the dominating aberration for objects close to the lens axis is the spherical aberration, which causes electrons ejected at a larger angle to the lens axis to be focused earlier than those ejected at a smaller angle. The magnitude of this aberration is for a given lens proportional to the third power of the acceptance angle, i.e. to the solid angle to the power of 3/2. Although this aberration could be made arbitrarily small by reducing the solid angle, this is not a very attractive option, since the observation of smaller surface elements in itself reduces the available intensity per element in proportion to its area. It is well known (34) that the spherical aberration in a rotationally symmetric lens without charges inside the beam can not be eliminated Magnetic lenses have significantly smaller aberrations than electrostatic lenses, but since the lens system also has the function of changing the electrons’ energy, at least some electrostatic element is needed. Lenses with grids can be made free from spherical aberration, but seem less attractive due to the scattering problems. One then has the possibility to either abandon the rotational symmetry, using multipole lenses (35) or to use lens systems with aberration correcting electrodes on the lens axis. It is not obvious to what extent such techniques would improve the useful solid angles, but it does not seem too unrealistic to hope that one could use at least the same solid angles as we today use at about 20 urn aberration circles at, say, 3 urn.

784

Let us try to estimate the recording times involved in ESCA imaging at improved resolution. In the E-x image we can estimate from the background intensity on the low-energy side of the Au 4f7h and the known peak/background ratio that the peak intensity is 20-30 counts/pixel. Here, each pixel corresponds to an area at the sample of about 7x7 pm, so the collection time of 10 secgives about 0.5 counts/s&m)2.

With image elements of 10x 10 pm we would get 50 counts/s in each element, if

the present solid angle can be retained at this resolution. Using simultaneous recording of 100 such elements along a line. an image of an area of 1 mm2 with 10 000 image elements could be recorded in a few minutes for intense lines. Even if the aberrations of the lens were not improved, but their effect reduced by decreasing the solid angle, the collection times would not increase by more than a factor of 3. Seah and Smith (36) have considered the intensity reductions involved in going to smaller sample sizes, and conclude that the practical limit for XPS will be in tbe order of 10x 10 pm. With the improvements in X-ray intensity achieved in the present instrument, one could perhaps reduce this limit to 5x5 pm. Nevertheless, there may be a point in trying to improve the optical resolution significantly beyond the 5 pm limit, since this would ensure that spectra taken from areas of that size would be free from contributions from the surroundings.

ACKNOWLEDGEMENTS We want to express our sincere gratitude to Drs. G. Beamson, D. Briggs, J. Howard and Prof. D. T. Clark at the ICI Materials Research Center in Wilton for many valuable discussions and ideas throughout the development phase of the ESCA-300 instrument, and for allowing one of us (UG) to use their instrument during a week to record some of the data presented in this report

REFERENCES

A. Fahlman and K. Siegbahn. Arkiv for Fysik. 32 (1966) 111. K. Siegbahn, C. Nordling. A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T. Bergmark, S.-E. Karlsson, I. Lindgren and B. Lindberg. ESCA-Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, Nova Acta Regiae Societatis Scientiarum Upsaliensis, Ser. IV, Vol. 20 (1967). K. Siegbahn, D. Hammond, H. Fellner-Feldegg, E. F. Barnett, Science, 176 (1972) 245. H.H. Johann. Z. Phys., 69 (1931) 185. A modified Al anode with improved cooling has been operated at 1200 Win the HewlettPackard ESCA instrument in Uppsala during approximately one year without showing any signs of degradation. U. Gelius, E. Basilier. S. Svensson. T. Bergmark and K. Siegbahn, J. Electron Spectrosc. 2 (1973) 405. J. R. Pierce. Theory and Design of Electron Beams, D. van Nostrand Co. Inc., New York, 1954.

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8 9

10 11 12 13 14

15 16

17 18 19 20 21 22 23 24 25 26 27 28

29

30 31 32 33 34 35 36

U. Gelius, H. Fellner-Feldegg, B. Wan&erg, A. G. Nilsson, E. Basilier and K. Siegbahn, Uppsala University Institute of Physics report UUIP- 855, April 1974. U. Gelius, L. Asplund, E. Basilier, S. Hedman, K. Helenelund, and K. Siegbahn, Nucl. Instr. Methods, Bl (1984) 85. C. S. Fadley, R. Baird, W. Siekhaus. T. Novakov and S. A. L. Bergstrom. J. Electron Spectrosc. Relat. Phenom.. 4 (1974) 93. C. S. Fadley, Prog. Solid State Chem., 11 (1976) 265. B. L. Her&e. Phys. Rev., A6 (1972) 94. G. Simmons and U. Gelius, in preparation. K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P.F. Hed&t, K. Hamrin, U. Gelius, T. Bergmark, L.-O. Werme, R. Mature and Y. Baer, ESCA Applied to Free Molecules, North-Holland Publ. Co., Amsterdam-London, 1969, pp. 137-143. U. Gelius unpublished results , presented by K. Siegbahn in: I. Lindgren, A. Rosen and S. Svanberg (Eds), Atomic Physics 8, Plenum Publ. Corp., 1983, pp. 277-279. A. Nilsson, Core Level Electron Spectroscopy Studies of Sulfates and Adsorbates, Acta Universitatis Upsaliensis, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science, No. 182 (1989). F. Holweck, C. R. Acad. Science. Paris, 117 (1923) 43. L. Maurice, Jap. J. Appl. Phys., Suppl.2 (1974) 21. P. Coxon. This conference. K. Siegbahn and N. Svartholm, Nature. 157 (1946) 872. N. Gurker, M.F. Ebel and H. Ebel, Surf. Interface Anal., 5 (1983) 13. H. Wollnik, Nucl. Instr. Methods, 52 (1967) 250. P. Baltzer, B. Wannberg and M. Carlsson, Uppsala University Institute of Physics report UUIP-1182, 1989. A. Barrie and N. E. Christensen, Phys. Rev.. B14 (1976) 2442. N. E. Christensen, Phys. Stat. Sol. (b), 54 (1972) 551. N. E. Christensen, J. Phys. F: Metal Phys.. 8 (1978) L51. N. E. Christensen, private communication. The liquid polymer is called OPTI-CLEANPOLYMER 60120, supplied by Bradford Laboratories Inc., 335 Pioneer Way. Mountain View, CA 94041, USA. It is dissolved in denaturated ethanol, and dries to a tough and flexible skin which can be pulled off without leaving any visible residues. Its application is to remove particles from the coated surface by embedding them in the polymer. J. I-I. Scofield. J. Electron Spectrosc. Relat. Phenom., 8 (1976) 129. M. F. Ebel and W. Lieble, J. Electron Spectrosc. Relat. Phenom., 16 (1979) 463. S. Tanuma, C. J. Powell and D. R. Penn, Surf. Interface Anal., 11 (1988) 577. C. S. Fadley, J. Electron Spectrosc. Relat. Phenom., 5 (1974) 725, see Fig. 14. R.W. Wijnaendts van Resandt, H.C. den Harink and J. Los, J. Phys. E: Sci. Instr., 9 (1976) 503. 0. Scherzer, Z. Physik, 101(1936) 23 and 593. A.J. Dragt, Nucl. Instr. Methods in Physics Res. A258 (1987) 339. M.P. Seah and G.C. Smith, Surf. Interface Anal., 11 (1988) 69.