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Multipole WIEN-filter for a high-resolution X-PEEM
Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
Multipole WIEN-filter for a high-resolution X-PEEM G.K.L. Marx*, V. Gerheim, G. Scho¨nhense Institut fu¨r Physik, Universita¨t Mainz, Staudinger Weg 7, D-55099 Mainz Germany Accepted 20 January 1997
Abstract The development and construction of an X-PEEM with integral multipole WIEN-filter for spectromicroscopy with high lateral resolution is presented. All relevant electron-optical properties of the filter have been examined by field and trajectory calculations. Electric and magnetic multipole components (dipole, quadrupole and hexapole) can be chosen independently from each other, thus allowing electron-optical corrections. Theoretical values for the lateral resolution are in the range of 10 nm, experimental values of # 25 nm have been achieved using the column without filter. The instrument is designed in particular for the use of Sychrotron radiation as well as new, high-brilliance laboratory X-ray sources. q 1997 Elsevier Science B.V. Keywords: WIEN filter; Spectromicroscopy; Photoemission microscopy
1. Introduction The motivation for the development of an imaging WIEN-filter was based on theoretical work of Bauer [1] for element specific imaging of photoelectrons using core level photoemission. Parallel to the development of the LEEM (low energy electron microscope) at the University of Clausthal by Telieps and Bauer [2] the imaging of surfaces via electrons excited by high-energy photons has been explored. Electron storage rings as dedicated sources for high-brilliance synchrotron radiation make it possible to combine electron spectroscopy such as XPS or UPS with high spatial resolution. The latter can be achieved either by scanning techniques or by parallel image acquisition. Most important for this kind of microscopy of both categories is
* Corresponding author.
the signal to noise ratio and the instrument function for the formation of an image. These two quantities determine at which lateral magnification and resolution information can be extracted from the image. For practical reasons the signal to noise ratio should not be smaller than 5. This problem is strongly related to the kind of sample. Estimations carried out for ‘ideal’ samples are often not significant for an evaluation of the method for a ‘general’ sample. For such an energy discriminating microscopy with parallel image acquisition, the electron optical properties of the energy filter will be most important. The ultimate resolution can, however, only be achieved if the stability and precision of the electron optical column and the sample mount is sufficient also. It is the purpose of this paper to present a new development of an imaging energy filter based on an electric and magnetic multipole arrangement used as correcting WIEN-filter.
0368-2048/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 7 )0 0 02 9 -7
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2. Construction of the X-PEEM 2.1. Basic specifications of the microscope When designing the X-PEEM the following specifications where aimed at: 1. Field of view variable from 5 to 150 mm diameter. 2. Variation of the starting energy of the electrons from 0 # E s # 1000 eV. 3. Energy resolution of the WIEN-filter 1 # DE # 5 eV. 4. Sample stage with vibrational amplitudes # 4 nm at frequencies of 0.1 # u # 200 Hz. 5. Point resolution 1000 × 1000 pixel. (ad 1) The large range of the field of view has proven to be very important for emission electron microscopy. Magnetic domain patterns are often visible on a large scale of 100 mm and more, whereas chemical defect structures often lie in the submm range. For practical applications it must be taken into account that the current density on the fluorescent screen shows an 1/M 2L dependence with M L being the lateral magnification. The practical resolution limit is about 10 nm. This is for our X-PEEM the practical limit with respect to all parameters such as maximum field strength between sample and extractor, starting energy of the electrons, the chromatic, spherical and diffraction aberrations of the electrostatic objective lens and all combination image aberrations of the round lenses and of the WIEN-filter. The theoretical resolution limit lies below 5 nm. (ad 2) The starting energy of the electrons is determined by the type of excitation source. These cover the region from threshold excitation by Mercury highpressure lamps and various discharge sources in the UV and VUV-wavelength range up to X-ray synchrotron radiation which reaches the inner core levels of the elements. Alternatively, the excitation can be performed using primary electrons and imaging of the secondary electrons produced in the sample surface. It can be shown [3] that parallel image acquisition using electron-induced Auger-electrons is principally impossible with the exception of a few ideal cases. This is because the electric power load on the sample will destroy the sample surface in most cases. For parallel imaging this power amounts to a few hundred
watts per square millimetre, i.e., comparable current densities like in an Auger-Microprobe (typically E p = 3 keV, I p = 1 nA, d s = 50 nm). These are typical conditions for electron bombardment heating. (ad 3) The development of the WIEN-filter with imaging properties has first been discussed by Rose [4] in the sharp cut-off approximation [5,6]. In this work, practical values for electric and magnetic field strengths have been used. The resulting dispersion is 10 mm eV −1 at a pass energy of 4 keV. This value has been chosen with respect to reasonable sizes of energy discriminating, contrast, and image apertures. (ad 4) In the immersion lens, the sample surface is part of the objective. Hence, it’s mechanical stability is of high importance. For a good sample handling, the samples stage can be shifted in the xy- and z- direction and has two rotating angles, Q and F. In addition, insitu sample cooling and sample transfer in UHV is possible. (ad 5) The point resolution of 1000 × 1000 pixels is ideally matched to the CCD-chips of modern integrating slow scan cameras for image acquisition. 2.2. Mechanical and electron optical realisation A cross section of the microscope and imaging ray paths is shown in Fig. 1. The sample as part of the objective lens is mounted on a ‘heliocentric’ sample manipulator, where it can be tilted and rotated about the centre of the sample. This is most important at high magnifications, because due to the small depth of field of a few hundred nm, a small sample tilt shows up as aberration in the image. The sample can be heated by electron bombardment up to 2400 K and can be cooled down to liquid helium temperatures by means of a cryostat. The sample is near ground potential but electrically isolated in order to facilitate energy sweeps by sweeping the sample potential. The electrostatic tetrode lens has been discussed and compared with magnetic lenses with extractor electrode in [7]. We have chosen a purely electrostatic solution, because such a lens does not show any hysteresis effects and there is no image rotation when changing the excitation of the lens. The lateral resolution of the objective lens is determined by several parameters. These are the field strength between sample and extractor, the width of the energy distribution DE/E, and the starting energy
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
Fig. 1. Cross section of the microscope with multipole WIEN-filter and fundamental rays (right).
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E s. The resulting aberrations in connection with the energy distribution and the starting angle a 0 of the electrons at the sample surface determine the total resolution. Fig. 2(a) and (b) show the total resolution as a function of the starting angle of the electrons for different extractor voltages and an assumed starting energy of 100 eV (Fig. 2(a)) and 500 eV (Fig. 2(b)) as well as an energy width of 1 eV. It is clearly to be seen, that the total resolution increases as a function of electric field strength between sample and extractor. The extractor voltage, U ex, is the voltage between sample and first electrode of the objective lens. The gap between sample surface and lens is d = 2 mm. Since with increasing starting energy E s the electron beam becomes more ‘stiff’, the allowed starting angle for high resolution work decreases. As a consequence, the usable solid angle interval also decreases strongly, because it depends quadratically on the starting angle. This results in a decrease of the image intensity with increasing starting energy. The selection of the starting angle interval is facilitated by an exchangeable contrast aperture in a diffraction plane. Since the phase space volume depends on starting energy, different energies correspond to different angular information in the image. In addition, the objective lens needs to be refocussed if the starting energy changes, hence its focal length changes and consequently the optimum position of the contrast aperture will shift along the electron optical axis. Close to the backfocal plane of the objective lens a magnetic stigmator is located correcting astigmatism of the objective lens due to mechanical tolerances. Alignment lens I forms a telescopic beam which is important for adjustment of the electron beam with respect to the WIEN-filter. We selected a lateral magnification of the objective lens of M L # 10. Hence, the 5 contrast apertures in the backfocal plane of alignment lens II have diameters of D min . 10 mm. In the first real intermediate image plane between alignment lens I and alignment lens II an electrostatic image stigmator is located. The contrast aperture can be shifted in xyz-direction. The imaging subsystem of
Fig. 2. Total resolution versus starting angle of the photoelectrons for different extractor potentials, for starting energies of 100 (a) and 500 eV (b). In both cases the energy width is 1 eV.
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
alignment lens I and II retains the telescopic beam when varying the starting energy. The WIEN-filter consists of a multipole arrangement, entrance and exit lens and adjustment and correcting elements. The beam in the WIEN-filter is telescopic which allows a simple adjustment. A problem is that the first dispersive plane is not in a field free space. Therefore this plane has to be imaged in a field free region. The first dispersive plane is imaged with a lateral magnification of M L = 1 by means of exit lens II. Between exit lens I and II an image stigmator is located which allows to correct small aberrations of the WIEN-filter. The multipole arrangement has been selected, because this allows a wide variation of all electric and magnetic field components and allows their optimization on maximum image quality. Curved or tilted surfaces for a pure magnetic or electrostatic
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arrangement are technically possible. They allow, however, no corrections of the image quality inside of the filter. Behind the WIEN-filter unit a two stage projective lens system serves for final image magnification. Projective I and II allow a variation of the total magnification in a wide range and in addition the energy spectrum of the dispersion plane can be imaged. The retardation lens produces a telescopic beam which increases the sensitivity of the multichannel plates (MCP) and eliminates the dependence of the efficiency of the plates as a function of impact angle on the plate surface [8]. The lateral resolution of the double MCP with phosphor screen is about 100 mm (contrast transfer function assumed to be one). Hence, a lateral magnification of at least 10 4 is necessary in order to reach a lateral resolution of 10 nm on the sample surface.
Fig. 3. Paraxial fundamental trajectories of the WIEN-filter in SCOFF-approximation.
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3. Design of the multipole-WIEN-filter The very flexible but technically elaborate design consists of a mechanical multipole (12 poles) with independent electric and magnetic excitation of each individual pole. Thus, all relevant multipole components are individually adjustable. 3.1. Theory of the multipole-WIEN-filter According to the WIEN-condition the following equations of the dipole-fields have to be fulfilled in order to obtain a linear electron optical axis: − eE = − ev × B, E = − F1c ex , B = − W1s ey , Ex F1c = =n By W1s
(1)
In addition, quadrupole fields have to be excited in order to operate the imaging energy filter as a round lens: F2c =
F21c 3 W , W2s = n 1s 2F 8 F
(2)
Furthermore, hexapole fields can be superimposed in order to compensate second order aberrations: F3c − nW3s =
3 F31c 32 F2
(3)
Then, the paraxial trajectories and the dispersion are determined by the following differential equation: q0 +
1 F21c 1 1 F21c F2c k F1c q9 + − q= − 2 F 8F 4 2 F2 4 F
(4)
Fig. 4. Axial multipole components of the WIEN-filter. Note that the fringing fields of the various components differ from each other.
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
Paraxial multipole components and coordinates: F F 1c W 1s F 2c W 2s F 3c W 3s q = x + iy DF k= F
Electric potential. Electric dipole component. Magnetic dipole component. Electric quadrupole component. Magnetic quadrupole component. Electric hexapole component. Magnetic hexapole component. Complex coordinates. Relative energy deviation.
In the frame work of SCOFF-approximation (short cut-off fringing field), the filter is geometrically corrected in the second order. The calculated paraxial basic trajectories of the filter in SCOFF-approximation are shown in Fig. 3. The fundamental rays in the xy-plane are denoted by X a, Y b, X g, Yd, where a (b) are the aperture coordinates and g(d) the object coordinates.
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In the next step the fringing fields of the various multipole components have to be taken into account. Therefore, a three dimensional analytical field calculation of the electric and magnetic scalar potentials was necessary. We assumed a rotationally symmetric arrangement of the electrodes, with a linear variation of the potential between the electrodes (linear potential field). The resulting electric multipole coefficients are shown in Fig. 4. The fringing fields of the different components deviate from each other. Therefore, the conditions (eqns (2) and (3)) of the filter are not reached simultaneously in every plane z. The paraxial fundamental rays in Fig. 3 have been calculated using these components. As a consequence of the fringing fields, the trajectories in x- and ydirection are splitted and hence the system is no longer rotationally symmetric. The calculated energy dependent dispersion ray is
Fig. 5. Energy dependent dispersion ray and corresponding coefficients.
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shown in Fig. 5. In order to reach a dispersion of more than 10 mm eV −1 in the energy selection plane, the coefficient Cg needs to be maximized. In addition, Ca has to vanish in the Gaussian image plane behind the filter (achromatic plane). These conditions are automatically fulfilled via the mirror symmetry of the filter with respect to the central plane. 3.2. Technical realization of the multipole filter Figs 6 and 7 show the mechanical construction of the WIEN-filter. All pole shoes have separate coils for magnetic excitation and are electrically isolated from each other. In order to minimize the magnetic losses and mutual influences, the pole shoes become wider towards the outer part of the filter. The magnetic pole shoes and the magnetic yoke and mirrors consist of Hyperm 766 (WIDIA), a nickel– iron-alloy (Ni 80Fe 20). The material has been thermally
treated after mechanical finishing. The resulting size of crystallites is of the order of 10 mm. The surface has been plated by gold, in order to reach a homogeneous distribution of the work function inside of the filter. The necessary electrical insulation of the pole shoes causes gaps for the magnetic flux in various places. These are located between the outer end of the pole shoes and the magnetic yoke. These gaps have been minimized such that the magnetic losses became as small as possible. The ‘natural’ gaps lie between the inner ends of the pole shoes facing each other. The inner gap is determined by the maximum field of view to be transferred through the filter. For the calculation of the required ampere-turns, the multipole arrangement has been quantitatively treated in the framework of a resistor network. The magnetic losses are about 60%, In order to minimize hysteresis effects, only the magnetic dipole and quadrupole fields are excited and the correction of the second order aberrations is performed via the electrostatic field alone. Mechanical adjustment of the pole shoes is possible by means of a special device. The filter and the electron optical column is kept at a potential of 4 kV. All electrical supplies are referenced to this potential. Power supplies for the multipole exhibit a relative ripple and a stability of , 10 −5. Adjustment is possible either manually or by means of a PC. The digital data are transferred via optical fibres to the 16-bit DAC-converters on the reference potential. All electrostatic potentials of the various current and voltage power-supplies are electrically separated. The whole supply is contained in a Faraday cage.
4. Summary and first results without energy filter
Fig. 6. Longitudinal section and cross section of the multipole WIEN-filter.
The electron optical design and properties of an XPEEM with imaging multipole WIEN-filter has been described. A principal advantage of this concept is its linear electron optical axis, which results in an easy adjustment procedure. An additional in-situ RHEEDsystem allows to obtain simultaneously information about the crystalline structure of the sample. As an example for the determination of lateral resolution, Fig. 8 shows the oxidation of a carbon overlayer on a Mo(110) single crystal. At a temperature of 1500 K,
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
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Fig. 7. Photo of the multipole WIEN-filter.
the carbon layer is partially removed by heating in oxygen. In the initial stage, bright spots become visible representing nucleation of carbon-free domains with sharp fringes. A line scan reveals a lateral resolution of 25 nm. The theoretical lateral resolution is better than 15 nm at a field strength between sample
Fig. 8. Carbon covered molybdenum (110) crystal during oxygen treatment at 1500 K.
and extractor of 10 kV mm −1 and an energy width of 1 eV. Fig. 8 has been obtained by using a mercury high-pressure lamp with hu = 4.9 eV. Fig. 9 illustrates the importance of an energy filter by means of comparing the same sample area imaged using different excitation mechanisms. The sample is polycrystalline titanium. Fig. 9 (top) shows a threshold image taken with a mercury high-pressure lamp. Differences in work functions of the different crystallises result in a good contrast. The grain boundaries exhibit a good lateral resolution, because threshold electron emission leads to a small energy width. The excitation at the photo-threshold results in a width of the electron energy distribution of only a few hundred meV. Fig. 9 (middle) shows the same area being excited with the H-Lyman a line from a discharge lamp (hu = 10.2 eV). In this image, the work function contrast is strongly reduced, because the excitation energy lies far above the photo-threshold. Hence, lateral resolution is clearly diminished due to the increased chromatic aberrations. The width of the photoelectron energy distribution is approximately 5 eV. The enhanced contrast at the grain boundaries is resulting from the sample topography owing to the grazing-incidence illumination from one side. Fig. 9 (bottom) shows the same area imaged by
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Fig. 10. Thermal emission of electrons from a molybdenum (110) single crystal sample showing monatomic steps (Smoluchowskicontrast).
Fig. 9. Image of a polycrystalline titanium sample. (a) Excited by UV-light (hu # 5.2 eV). (b) Excitation by means of a H-Ly a discharge source, hu = 10.2 eV. (c) Excitation of secondary electrons by means of argon ions (5 keV).
using secondary electrons which have been released by an ion beam (Ar, 5 keV, 10 mA). Clearly, the lateral resolution became even worse due to the increased energy width of the secondary electrons. In contrast to the Lyman, a image a contrast of the different crystallites occurs most likely due to differences in the secondary electron yield of different crystallographic orientations. These three examples clearly demonstrate the importance of energy filtering for high resolution emission microscopy. Secondary electron distributions are typically several 10 eV wide. The resulting chromatic aberrations can reduce the lateral resolution to several 100 nm, if no filter is employed. Fig. 10 shows thermal electron emission from a clean Mo(110) single crystal surface. The temperature of the crystal was T = 1500 K. The local reduction of the work function at the atomic steps (or groups of steps) leads to an increased emission. Thus, the steps appear as bright lines in the image. This phenomenon can be explained by the Smoluchowski-effect [9], describing a variation of the work function due to the local electron density distribution with respect to the position of the ion cores. Smearing out the electron charge distribution results in dipole field at the steps and hence in a reduction of the local work function.
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
This example demonstrates the high sensitivity of the imaging device. The Peltier-cooled slow-scan CCD camera used has a 14 bit converter. A weak contrast like that of Fig. 10 is invisible for standard CCD cameras. In conclusion, we have described the design and development of an X-PEEM with integral multipole WIEN-filter for microspectroscopy. The filter has been characterized by trajectory calculations. The independent choice of electric and magnetic dipole, quadrupole and hexapole field components allows a very effective correction of electron-optical aberrations. The column without filter recently went into operation. First images revealed a lateral resolution of # 25 nm, which lies about a factor of 2 above the theoretical value. The full performance of the instrument with multipole filter will be examined at Synchrotron-radiation sources in the near future. In addition, a set-up exploiting a high-brilliance laboratory X-ray source (laser-based plasma in combination with multilayer optics) is under construction.
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Acknowledgements The authors would like to thank M. Dunin v. Przychowski for the good cooperation. Financial support by Deutsche Forschungsgemeinschaft through Sonderfors-chungsbereich 262 as well as Materialwissenschaftliches Forschungszentrum der Universita¨t Mainz is gratefully acknowledged. References [1] E. Bauer, Ultramicroscopy 12 (1985) 51. [2] W. Telieps, E. Bauer, Ultramicroscopy 17 (1985) 57. [3] E. Bauer, T. Franz, C. Koziol, G. Lilienkamp, T. Schmidt, in: R. Rosei (Eds.), Chemical, Structural and Electronic Analysis of Heterogeneous Surface on Nanometer Scale, Kluwer, Dordrecht, 1995. [4] H. Rose, Optik 77 (1987) 26. [5] M. Scheinfein, Optik 82 (1989) 99. [6] K. Tsuno, Optik 89 (1991) 31. [7] J. Chmelik, L. Veneklasen, G.K.L. Marx, Optik 83 (5) (1989) 155. [8] W. Engel., M.E. Kordesch, H.H. Rotermund, S. Kubala, A. von Oertzen, Ultramicroscopy 36 (1991) 148. [9] R. Smoluchowski, Phys. Rev. 60 (1941) 661.
Multipole WIEN-filter for a high-resolution X-PEEM G.K.L. Marx*, V. Gerheim, G. Scho¨nhense Institut fu¨r Physik, Universita¨t Mainz, Staudinger Weg 7, D-55099 Mainz Germany Accepted 20 January 1997
Abstract The development and construction of an X-PEEM with integral multipole WIEN-filter for spectromicroscopy with high lateral resolution is presented. All relevant electron-optical properties of the filter have been examined by field and trajectory calculations. Electric and magnetic multipole components (dipole, quadrupole and hexapole) can be chosen independently from each other, thus allowing electron-optical corrections. Theoretical values for the lateral resolution are in the range of 10 nm, experimental values of # 25 nm have been achieved using the column without filter. The instrument is designed in particular for the use of Sychrotron radiation as well as new, high-brilliance laboratory X-ray sources. q 1997 Elsevier Science B.V. Keywords: WIEN filter; Spectromicroscopy; Photoemission microscopy
1. Introduction The motivation for the development of an imaging WIEN-filter was based on theoretical work of Bauer [1] for element specific imaging of photoelectrons using core level photoemission. Parallel to the development of the LEEM (low energy electron microscope) at the University of Clausthal by Telieps and Bauer [2] the imaging of surfaces via electrons excited by high-energy photons has been explored. Electron storage rings as dedicated sources for high-brilliance synchrotron radiation make it possible to combine electron spectroscopy such as XPS or UPS with high spatial resolution. The latter can be achieved either by scanning techniques or by parallel image acquisition. Most important for this kind of microscopy of both categories is
* Corresponding author.
the signal to noise ratio and the instrument function for the formation of an image. These two quantities determine at which lateral magnification and resolution information can be extracted from the image. For practical reasons the signal to noise ratio should not be smaller than 5. This problem is strongly related to the kind of sample. Estimations carried out for ‘ideal’ samples are often not significant for an evaluation of the method for a ‘general’ sample. For such an energy discriminating microscopy with parallel image acquisition, the electron optical properties of the energy filter will be most important. The ultimate resolution can, however, only be achieved if the stability and precision of the electron optical column and the sample mount is sufficient also. It is the purpose of this paper to present a new development of an imaging energy filter based on an electric and magnetic multipole arrangement used as correcting WIEN-filter.
0368-2048/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 7 )0 0 02 9 -7
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G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
2. Construction of the X-PEEM 2.1. Basic specifications of the microscope When designing the X-PEEM the following specifications where aimed at: 1. Field of view variable from 5 to 150 mm diameter. 2. Variation of the starting energy of the electrons from 0 # E s # 1000 eV. 3. Energy resolution of the WIEN-filter 1 # DE # 5 eV. 4. Sample stage with vibrational amplitudes # 4 nm at frequencies of 0.1 # u # 200 Hz. 5. Point resolution 1000 × 1000 pixel. (ad 1) The large range of the field of view has proven to be very important for emission electron microscopy. Magnetic domain patterns are often visible on a large scale of 100 mm and more, whereas chemical defect structures often lie in the submm range. For practical applications it must be taken into account that the current density on the fluorescent screen shows an 1/M 2L dependence with M L being the lateral magnification. The practical resolution limit is about 10 nm. This is for our X-PEEM the practical limit with respect to all parameters such as maximum field strength between sample and extractor, starting energy of the electrons, the chromatic, spherical and diffraction aberrations of the electrostatic objective lens and all combination image aberrations of the round lenses and of the WIEN-filter. The theoretical resolution limit lies below 5 nm. (ad 2) The starting energy of the electrons is determined by the type of excitation source. These cover the region from threshold excitation by Mercury highpressure lamps and various discharge sources in the UV and VUV-wavelength range up to X-ray synchrotron radiation which reaches the inner core levels of the elements. Alternatively, the excitation can be performed using primary electrons and imaging of the secondary electrons produced in the sample surface. It can be shown [3] that parallel image acquisition using electron-induced Auger-electrons is principally impossible with the exception of a few ideal cases. This is because the electric power load on the sample will destroy the sample surface in most cases. For parallel imaging this power amounts to a few hundred
watts per square millimetre, i.e., comparable current densities like in an Auger-Microprobe (typically E p = 3 keV, I p = 1 nA, d s = 50 nm). These are typical conditions for electron bombardment heating. (ad 3) The development of the WIEN-filter with imaging properties has first been discussed by Rose [4] in the sharp cut-off approximation [5,6]. In this work, practical values for electric and magnetic field strengths have been used. The resulting dispersion is 10 mm eV −1 at a pass energy of 4 keV. This value has been chosen with respect to reasonable sizes of energy discriminating, contrast, and image apertures. (ad 4) In the immersion lens, the sample surface is part of the objective. Hence, it’s mechanical stability is of high importance. For a good sample handling, the samples stage can be shifted in the xy- and z- direction and has two rotating angles, Q and F. In addition, insitu sample cooling and sample transfer in UHV is possible. (ad 5) The point resolution of 1000 × 1000 pixels is ideally matched to the CCD-chips of modern integrating slow scan cameras for image acquisition. 2.2. Mechanical and electron optical realisation A cross section of the microscope and imaging ray paths is shown in Fig. 1. The sample as part of the objective lens is mounted on a ‘heliocentric’ sample manipulator, where it can be tilted and rotated about the centre of the sample. This is most important at high magnifications, because due to the small depth of field of a few hundred nm, a small sample tilt shows up as aberration in the image. The sample can be heated by electron bombardment up to 2400 K and can be cooled down to liquid helium temperatures by means of a cryostat. The sample is near ground potential but electrically isolated in order to facilitate energy sweeps by sweeping the sample potential. The electrostatic tetrode lens has been discussed and compared with magnetic lenses with extractor electrode in [7]. We have chosen a purely electrostatic solution, because such a lens does not show any hysteresis effects and there is no image rotation when changing the excitation of the lens. The lateral resolution of the objective lens is determined by several parameters. These are the field strength between sample and extractor, the width of the energy distribution DE/E, and the starting energy
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
Fig. 1. Cross section of the microscope with multipole WIEN-filter and fundamental rays (right).
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E s. The resulting aberrations in connection with the energy distribution and the starting angle a 0 of the electrons at the sample surface determine the total resolution. Fig. 2(a) and (b) show the total resolution as a function of the starting angle of the electrons for different extractor voltages and an assumed starting energy of 100 eV (Fig. 2(a)) and 500 eV (Fig. 2(b)) as well as an energy width of 1 eV. It is clearly to be seen, that the total resolution increases as a function of electric field strength between sample and extractor. The extractor voltage, U ex, is the voltage between sample and first electrode of the objective lens. The gap between sample surface and lens is d = 2 mm. Since with increasing starting energy E s the electron beam becomes more ‘stiff’, the allowed starting angle for high resolution work decreases. As a consequence, the usable solid angle interval also decreases strongly, because it depends quadratically on the starting angle. This results in a decrease of the image intensity with increasing starting energy. The selection of the starting angle interval is facilitated by an exchangeable contrast aperture in a diffraction plane. Since the phase space volume depends on starting energy, different energies correspond to different angular information in the image. In addition, the objective lens needs to be refocussed if the starting energy changes, hence its focal length changes and consequently the optimum position of the contrast aperture will shift along the electron optical axis. Close to the backfocal plane of the objective lens a magnetic stigmator is located correcting astigmatism of the objective lens due to mechanical tolerances. Alignment lens I forms a telescopic beam which is important for adjustment of the electron beam with respect to the WIEN-filter. We selected a lateral magnification of the objective lens of M L # 10. Hence, the 5 contrast apertures in the backfocal plane of alignment lens II have diameters of D min . 10 mm. In the first real intermediate image plane between alignment lens I and alignment lens II an electrostatic image stigmator is located. The contrast aperture can be shifted in xyz-direction. The imaging subsystem of
Fig. 2. Total resolution versus starting angle of the photoelectrons for different extractor potentials, for starting energies of 100 (a) and 500 eV (b). In both cases the energy width is 1 eV.
G.K.L. Marx et al./Journal of Electron Spectroscopy and Related Phenomena 84 (1997) 251–261
alignment lens I and II retains the telescopic beam when varying the starting energy. The WIEN-filter consists of a multipole arrangement, entrance and exit lens and adjustment and correcting elements. The beam in the WIEN-filter is telescopic which allows a simple adjustment. A problem is that the first dispersive plane is not in a field free space. Therefore this plane has to be imaged in a field free region. The first dispersive plane is imaged with a lateral magnification of M L = 1 by means of exit lens II. Between exit lens I and II an image stigmator is located which allows to correct small aberrations of the WIEN-filter. The multipole arrangement has been selected, because this allows a wide variation of all electric and magnetic field components and allows their optimization on maximum image quality. Curved or tilted surfaces for a pure magnetic or electrostatic
255
arrangement are technically possible. They allow, however, no corrections of the image quality inside of the filter. Behind the WIEN-filter unit a two stage projective lens system serves for final image magnification. Projective I and II allow a variation of the total magnification in a wide range and in addition the energy spectrum of the dispersion plane can be imaged. The retardation lens produces a telescopic beam which increases the sensitivity of the multichannel plates (MCP) and eliminates the dependence of the efficiency of the plates as a function of impact angle on the plate surface [8]. The lateral resolution of the double MCP with phosphor screen is about 100 mm (contrast transfer function assumed to be one). Hence, a lateral magnification of at least 10 4 is necessary in order to reach a lateral resolution of 10 nm on the sample surface.
Fig. 3. Paraxial fundamental trajectories of the WIEN-filter in SCOFF-approximation.
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3. Design of the multipole-WIEN-filter The very flexible but technically elaborate design consists of a mechanical multipole (12 poles) with independent electric and magnetic excitation of each individual pole. Thus, all relevant multipole components are individually adjustable. 3.1. Theory of the multipole-WIEN-filter According to the WIEN-condition the following equations of the dipole-fields have to be fulfilled in order to obtain a linear electron optical axis: − eE = − ev × B, E = − F1c ex , B = − W1s ey , Ex F1c = =n By W1s
(1)
In addition, quadrupole fields have to be excited in order to operate the imaging energy filter as a round lens: F2c =
F21c 3 W , W2s = n 1s 2F 8 F
(2)
Furthermore, hexapole fields can be superimposed in order to compensate second order aberrations: F3c − nW3s =
3 F31c 32 F2
(3)
Then, the paraxial trajectories and the dispersion are determined by the following differential equation: q0 +
1 F21c 1 1 F21c F2c k F1c q9 + − q= − 2 F 8F 4 2 F2 4 F
(4)
Fig. 4. Axial multipole components of the WIEN-filter. Note that the fringing fields of the various components differ from each other.
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Paraxial multipole components and coordinates: F F 1c W 1s F 2c W 2s F 3c W 3s q = x + iy DF k= F
Electric potential. Electric dipole component. Magnetic dipole component. Electric quadrupole component. Magnetic quadrupole component. Electric hexapole component. Magnetic hexapole component. Complex coordinates. Relative energy deviation.
In the frame work of SCOFF-approximation (short cut-off fringing field), the filter is geometrically corrected in the second order. The calculated paraxial basic trajectories of the filter in SCOFF-approximation are shown in Fig. 3. The fundamental rays in the xy-plane are denoted by X a, Y b, X g, Yd, where a (b) are the aperture coordinates and g(d) the object coordinates.
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In the next step the fringing fields of the various multipole components have to be taken into account. Therefore, a three dimensional analytical field calculation of the electric and magnetic scalar potentials was necessary. We assumed a rotationally symmetric arrangement of the electrodes, with a linear variation of the potential between the electrodes (linear potential field). The resulting electric multipole coefficients are shown in Fig. 4. The fringing fields of the different components deviate from each other. Therefore, the conditions (eqns (2) and (3)) of the filter are not reached simultaneously in every plane z. The paraxial fundamental rays in Fig. 3 have been calculated using these components. As a consequence of the fringing fields, the trajectories in x- and ydirection are splitted and hence the system is no longer rotationally symmetric. The calculated energy dependent dispersion ray is
Fig. 5. Energy dependent dispersion ray and corresponding coefficients.
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shown in Fig. 5. In order to reach a dispersion of more than 10 mm eV −1 in the energy selection plane, the coefficient Cg needs to be maximized. In addition, Ca has to vanish in the Gaussian image plane behind the filter (achromatic plane). These conditions are automatically fulfilled via the mirror symmetry of the filter with respect to the central plane. 3.2. Technical realization of the multipole filter Figs 6 and 7 show the mechanical construction of the WIEN-filter. All pole shoes have separate coils for magnetic excitation and are electrically isolated from each other. In order to minimize the magnetic losses and mutual influences, the pole shoes become wider towards the outer part of the filter. The magnetic pole shoes and the magnetic yoke and mirrors consist of Hyperm 766 (WIDIA), a nickel– iron-alloy (Ni 80Fe 20). The material has been thermally
treated after mechanical finishing. The resulting size of crystallites is of the order of 10 mm. The surface has been plated by gold, in order to reach a homogeneous distribution of the work function inside of the filter. The necessary electrical insulation of the pole shoes causes gaps for the magnetic flux in various places. These are located between the outer end of the pole shoes and the magnetic yoke. These gaps have been minimized such that the magnetic losses became as small as possible. The ‘natural’ gaps lie between the inner ends of the pole shoes facing each other. The inner gap is determined by the maximum field of view to be transferred through the filter. For the calculation of the required ampere-turns, the multipole arrangement has been quantitatively treated in the framework of a resistor network. The magnetic losses are about 60%, In order to minimize hysteresis effects, only the magnetic dipole and quadrupole fields are excited and the correction of the second order aberrations is performed via the electrostatic field alone. Mechanical adjustment of the pole shoes is possible by means of a special device. The filter and the electron optical column is kept at a potential of 4 kV. All electrical supplies are referenced to this potential. Power supplies for the multipole exhibit a relative ripple and a stability of , 10 −5. Adjustment is possible either manually or by means of a PC. The digital data are transferred via optical fibres to the 16-bit DAC-converters on the reference potential. All electrostatic potentials of the various current and voltage power-supplies are electrically separated. The whole supply is contained in a Faraday cage.
4. Summary and first results without energy filter
Fig. 6. Longitudinal section and cross section of the multipole WIEN-filter.
The electron optical design and properties of an XPEEM with imaging multipole WIEN-filter has been described. A principal advantage of this concept is its linear electron optical axis, which results in an easy adjustment procedure. An additional in-situ RHEEDsystem allows to obtain simultaneously information about the crystalline structure of the sample. As an example for the determination of lateral resolution, Fig. 8 shows the oxidation of a carbon overlayer on a Mo(110) single crystal. At a temperature of 1500 K,
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Fig. 7. Photo of the multipole WIEN-filter.
the carbon layer is partially removed by heating in oxygen. In the initial stage, bright spots become visible representing nucleation of carbon-free domains with sharp fringes. A line scan reveals a lateral resolution of 25 nm. The theoretical lateral resolution is better than 15 nm at a field strength between sample
Fig. 8. Carbon covered molybdenum (110) crystal during oxygen treatment at 1500 K.
and extractor of 10 kV mm −1 and an energy width of 1 eV. Fig. 8 has been obtained by using a mercury high-pressure lamp with hu = 4.9 eV. Fig. 9 illustrates the importance of an energy filter by means of comparing the same sample area imaged using different excitation mechanisms. The sample is polycrystalline titanium. Fig. 9 (top) shows a threshold image taken with a mercury high-pressure lamp. Differences in work functions of the different crystallises result in a good contrast. The grain boundaries exhibit a good lateral resolution, because threshold electron emission leads to a small energy width. The excitation at the photo-threshold results in a width of the electron energy distribution of only a few hundred meV. Fig. 9 (middle) shows the same area being excited with the H-Lyman a line from a discharge lamp (hu = 10.2 eV). In this image, the work function contrast is strongly reduced, because the excitation energy lies far above the photo-threshold. Hence, lateral resolution is clearly diminished due to the increased chromatic aberrations. The width of the photoelectron energy distribution is approximately 5 eV. The enhanced contrast at the grain boundaries is resulting from the sample topography owing to the grazing-incidence illumination from one side. Fig. 9 (bottom) shows the same area imaged by
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Fig. 10. Thermal emission of electrons from a molybdenum (110) single crystal sample showing monatomic steps (Smoluchowskicontrast).
Fig. 9. Image of a polycrystalline titanium sample. (a) Excited by UV-light (hu # 5.2 eV). (b) Excitation by means of a H-Ly a discharge source, hu = 10.2 eV. (c) Excitation of secondary electrons by means of argon ions (5 keV).
using secondary electrons which have been released by an ion beam (Ar, 5 keV, 10 mA). Clearly, the lateral resolution became even worse due to the increased energy width of the secondary electrons. In contrast to the Lyman, a image a contrast of the different crystallites occurs most likely due to differences in the secondary electron yield of different crystallographic orientations. These three examples clearly demonstrate the importance of energy filtering for high resolution emission microscopy. Secondary electron distributions are typically several 10 eV wide. The resulting chromatic aberrations can reduce the lateral resolution to several 100 nm, if no filter is employed. Fig. 10 shows thermal electron emission from a clean Mo(110) single crystal surface. The temperature of the crystal was T = 1500 K. The local reduction of the work function at the atomic steps (or groups of steps) leads to an increased emission. Thus, the steps appear as bright lines in the image. This phenomenon can be explained by the Smoluchowski-effect [9], describing a variation of the work function due to the local electron density distribution with respect to the position of the ion cores. Smearing out the electron charge distribution results in dipole field at the steps and hence in a reduction of the local work function.
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This example demonstrates the high sensitivity of the imaging device. The Peltier-cooled slow-scan CCD camera used has a 14 bit converter. A weak contrast like that of Fig. 10 is invisible for standard CCD cameras. In conclusion, we have described the design and development of an X-PEEM with integral multipole WIEN-filter for microspectroscopy. The filter has been characterized by trajectory calculations. The independent choice of electric and magnetic dipole, quadrupole and hexapole field components allows a very effective correction of electron-optical aberrations. The column without filter recently went into operation. First images revealed a lateral resolution of # 25 nm, which lies about a factor of 2 above the theoretical value. The full performance of the instrument with multipole filter will be examined at Synchrotron-radiation sources in the near future. In addition, a set-up exploiting a high-brilliance laboratory X-ray source (laser-based plasma in combination with multilayer optics) is under construction.
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Acknowledgements The authors would like to thank M. Dunin v. Przychowski for the good cooperation. Financial support by Deutsche Forschungsgemeinschaft through Sonderfors-chungsbereich 262 as well as Materialwissenschaftliches Forschungszentrum der Universita¨t Mainz is gratefully acknowledged. References [1] E. Bauer, Ultramicroscopy 12 (1985) 51. [2] W. Telieps, E. Bauer, Ultramicroscopy 17 (1985) 57. [3] E. Bauer, T. Franz, C. Koziol, G. Lilienkamp, T. Schmidt, in: R. Rosei (Eds.), Chemical, Structural and Electronic Analysis of Heterogeneous Surface on Nanometer Scale, Kluwer, Dordrecht, 1995. [4] H. Rose, Optik 77 (1987) 26. [5] M. Scheinfein, Optik 82 (1989) 99. [6] K. Tsuno, Optik 89 (1991) 31. [7] J. Chmelik, L. Veneklasen, G.K.L. Marx, Optik 83 (5) (1989) 155. [8] W. Engel., M.E. Kordesch, H.H. Rotermund, S. Kubala, A. von Oertzen, Ultramicroscopy 36 (1991) 148. [9] R. Smoluchowski, Phys. Rev. 60 (1941) 661.