Maximum: A scanning photoelectron microscope at Aladdin

Nuclear Instruments and Methods in Physics Research A266 (1988) 303-307 North-Holland, Amsterdam

303

MAXIMUM: A SCANNING PHOTOELECTRON MICROSCOPE AT ALADDIN F. C E R R I N A 1), G. M A R G A R I T O N D O 2), J.H. U N D E R W O O D 3), M. H E T T R I C K 4) M.A. G R E E N 2), L.J. B R I L L S O N 5), A. F R A N C I O S 1 6), H. H O C H S T 2), P.M. D E L U C A Jr. 7) a n d M.N. G O U L D 7) 1) Department of Electrical and Computer Engineering, University of Wisconsin-Madison, WI 53706, USA 2) Synchrotron Radiation Center, 3731 Schneider Drive, Stoughton, W1 53589, USA 3j Center for X-ray Optics, Lawrence Berkeley Laboratory, Berkeley, CA 94707, USA 4) Hettrick Scientific, P.O. Box 8046, Berkeley, CA 94707, USA 5) Xerox Corporation, Webster Research Center, Webster, N Y 14580, USA 6) Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA 7) Department of Medical Physics, University of Wisconsin-Madison, WI 53706, USA

The recent successful installation of the 30-period undulator on Aladdin, the 1 GeV electron storage ring at the Synchrotron Radiation Center of the University of Wisconsin, opens new possibilities for photoelectron spectroscopy. In particular, the high brightness of the machine, together with innovative optics, make possible the application of photoelectron spectroscopy to high-resolution soft X-ray microscopy. We call this system MAXIMUM (for Multiple Application X-ray IMaging Undulator Microscope). The proposed optical system will have a lateral resolution of better than 1000 ~, and a resolving power of better than 200 at 100 eV. After monochromatization, the radiation will be focused on a pinhole that can range in diameter from 1 to 100 ~m, and will be prepared by lithographic techniques on a thin nickel film. The image of the pinhole, suitably demagnified, will be relayed to the sample. The image resolution and magnification can be adjusted by changing the pinhole size and the scanning step. A Schwartzschild objective can produce a demagnified image of the pinhole which is diffraction limited even at a wavelength of 40 A. At 100 ,A and at a numerical aperture of 0.2, the objective can produce a 250 ,~ diameter spot. High flux will be achieved with a Mo-Si multilayer coating, for which preliminary experiments have demonstrated reflectivities near 40% at normal incidence. Other focusing elements (Fresnel zone plates and Kirkpatrick-Baez objectives) will also be implemented.

1. Introduction The advent of high-brightness sources opens the possibility of X-ray photoelectron microscopy. Traditionally, microscopy is performed by using either imaging or scanning systems. Hybrid techniques, such as photoelectron microscopy, require both high spatial and high energy resolution at the same time. Scanning microscopy allows the separation of the two functions, and can satisfy both requirements. The spatial resolution is provided by a sharply focused X-ray beam, while the energy resolution is provided by post-analyzing the photoemitted electrons with an electrostatic energy analyzer. In this way parameters and setup conditions can be modified independently to suit the experimenter's needs. Undulators are important for X-ray microscopy because their radiation can be focused to very small dimensions without excessive loss of flux, as discussed in section 3. The interest in the study of two- and three-dimensional non-uniform systems, as found, for example, in the early stages of semiconductor interface formation, has prompted us to develop the design of an undulatorbased X-ray microscope with very high spatial and energy resolution. The performance of the undulator is 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

presented elsewhere in these Proceedings [1]; here we present a description of the beamline optics studied so far. The optical system is complex and is comprised of several subsystems. Critical to the overall performance is the efficiency of the optical elements, such as the reflectivity of the multilayer coatings to be employed in some of the optics. We report on this aspect elsewhere in these Proceedings [2], while the performance of the focusing optics has already been discussed [3,4]. First results show that the microscope should indeed be feasible and, although some refinement of the optics will be necessary, a simple optical system can deliver a high photon flux to a spot of 1000 ,A or less in diameter. Besides presenting the conceptual design of the microscope, we also present an estimation of the photon flux delivered to the sample.

2. Microscope layout MAXIMUM can be divided into five subsystems: the undulator, the monochromator/condenser stage, the reduction optics, the scanning system and the detect i o n / d a t a acquisition system. Fig. 1 shows a schematic layout of the entire beamline. Of particular concern is lI(e). MICROSCOPY BEAMLINES

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F. Cerrina et al. / M A X I M U M SAMPLE

UERT l CAL CONDENSER

CONDENSER

ENTRANCE PINHOLE UNDULATOR

HOR IZONTAL CONDENSER SCHWARTZSCHILD OBJECTIUE SPHERICAL GRATING

Fig. 1. Schematic of the proposed beamline. The overall length is of the order of 6-8 m.

an assessment of the number of photons reaching the sample under typical experimental conditions. Here we concentrate on the monochromator and the reduction optics. We discuss in particular the problem of the most efficient matching of the optical elements forming the system. Fig. 2 shows a schematic view of the reduction optics, based on the use of a Schwartzschild objective. The Schwartzschild objective was selected as the primary reduction system because of its, (a) high lateral resolution, (b) simple optical surfaces (spheres), and (c) large numerical aperture [3,4]. In a two-stage construction process, a simple system is constructed first, in order to begin experiments as soon as possible. In parallel to the first operation of M A X I M U M , more sophisticated optics will be designed, tested, and finally implemented as part of the second stage of construction.

2.1. M o n o c h r o m a t o r

Due to the rather modest requirements of the microscope in terms of photon energy resolution, we have selected a design based on a simple spherical grating monochromator (SGM) in an asymmetric mount, optimized for a resolution of 0.5% at the center of the band 120-240 #,. A condenser system in a modified Kirkpatrick-Baez configuration is used to relay the photon beam to the monochromator: one spherical mirror focuses vertically onto the entrance slit and a second focuses horizontally onto the exit slit. Ray tracing analysis shows that, using evenly-spaced straight-groove gratings, the monochromator performs as expected, with spherical aberration being the main aberration left. In the second stage of implementation, a varied line spacing (VLS) grating [5] will be employed to remove this aberration in order to achieve higher resolution and better transmission. A condenser system is used after the monochromator to relay the beam to the pinhole that defines the object to be demagnified. This optical element is necessary in order to match the monochromator output to the acceptance of the objective. The condenser will be a simple elliptical mirror working in a 10 : 1 reduction scheme. 2.2. Pinhole

Fig. 2. Schematic view of Schwartzschild objective. The source point is located to the far left and is demagnified by the two mirrors to a small focus.

An essential part of the instrument is the image defining aperture, i.e., the pinhole. It is worthwhile briefly discussing its role in the system, since this will elucidate some of the possible operational modes of the microscope. The Schwartzschild objective has a large field of view, so that it is possible to use large pinholes in order to work at reduced image magnifications. Of course, this will also improve the signal level, allowing a faster setup and alignment. The pinholes themselves

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may be of two types: windowless and supported. In the former, a pinhole will be defined by plating nickel on a photoresist-coated silicon wafer. The silicon will then be removed by back-etching, leaving a self-supporting film carrying the pinhole. Due to the large absorption coefficient of nickel, a thickness of a few microns will be enough to define the pinhole. Tungsten or gold can also be used as overlayers to improve the optical contrast. The supported pinholes will instead be defined on a thin Si3N 4 film ( = 1000-2000 ,~ thick) and processed similarly to an X-ray lithography mask [6]. The silicon nitride film may be left in place to act as a vacuum contamination barrier. The magnification of a scanning microscope can be changed, without modifying the optical setup, by switching pinholes and, at the same time, changing the step size of the scanning stage. In MAXIMUM, pinholes of several sizes that are defined on a single wafer can be sequentially brought into the beam. Due to the large field of view of the Schwartzschild objective, the exact location of the pinholes is not critical and will not affect the resolution. 2.3. Reduction optics

As mentioned in the introduction, the principal demagnifying optics will be based on the Schwartzschild objective, as illustrated in fig. 2. In this arrangement, two spherical mirrors of opposite curvature are used in a Cassegrain-type mount. The radiation transmitted by the pinhole is imaged in a virtual image by the first, convex, mirror. This virtual image becomes the object of the second mirror and is focused at the final image point. This geometry is advantageous because: (a) the optics are centered so that aberrations are reduced, and (b) the large numerical aperture gives a very small diffraction limit. The optical performance of these objectives has been discussed in detail [3,4]. Here we note that the excellent performance is due to the opposite curvatures of the mirrors, which give rise to aberrations of opposite signs that cancel out. Furthermore, the optical magnification of the objective itself can be adjusted simply by changing the distance between the two mirrors. The two mirrors will be ground and polished from Zerodur to a figure accuracy of ~,/50, coated with molybdenum-silicon multilayers and mounted on a suitable frame. Piezoelectric drivers will be used to accurately set the gap between the two mirrors and for lateral adjustment in order to achieve the best resolution. Several objectives will be installed on a revolving carrier, similar to those in conventional microscopes. This is necessary because the narrow-band reflectivity of multilayer films means that different wavelength ranges require matching coatings. The final resolution will probably be limited by the surface errors on the mirrors, and modelling of this is in progress.

2.4. Scanning stage and data acquisition

A piezoelectric-driven scanning system will move the sample across the X-ray focus, and photoemitted electrons will be energy analyzed by a standard cylindrical mirror electrostatic energy analyzer. The stage is based on the use of flexure hinges to avoid sliding motions in vacuo. The position will have both coarse and fine adjustments, through the use of stepping motors and piezoelectric actuators, for a total travel of +0.5 cm with a resolution well below 0.1 /~m. The piezoelectric actuators may also be located outside the vacuum system and coupled to the stage through welded bellows. We expect the sample to be moved across a field of 512 × 512 elements, with the size of each pixel determined by the projected pinhole image. For each pixel, the CMA will analyze the photoelectrons, and the total number of counts in a specified energy interval will be presented on a graphics display. For example, with the analyzer tuned to an atomic core level, a map of the distribution of the selected element(s) on the sample surface will be displayed, with color depicting the signal intensity. The high energy resolution will also allow the distribution of chemically-shifted species to be shown. Due to the limited space available around the sample in the microscope, a separate sample preparation chamber with the standard surface physics implements will be necessary.

3. Estimate of photon flux In a photoelectron microscope the photon throughput on-target is of primary importance because of the relatively low photoemission cross section. It is therefore necessary to establish the performance of the microscope as a whole system. We have based our analysis on simple optical arguments and then verified the viability of the design by using our ray-tracing code SHADOW [7]. The performance of an optical system can be studied in terms of the emittance of the photon beam and of the acceptance of the optical elements. We recall that the emittance of a beam of particles is the phase-space volume occupied by a given fraction of the beam, while the acceptance (or &endue) of an optical system is the phase-space volume which it transmits. The emittance is conserved for dispersionless and lossless systems [8]. If the acceptance is larger than the beam emittance, then the whole beam will be transmitted; if not, only the fraction of the beam failing within the acceptance "window" will be transmitted, resulting in a loss of flux. This framework is quite useful in assessing the performance of an optical system, at least as a first step. Table 1 shows the values of the beam size and divergence at three different positions of the microII(e). MICROSCOPY BEAMLINES

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Table 1 Photon phase-space parameters along the beamline. (a) Optical acceptance Position

Size Divergence I~tendue zax (/~m) Ax' (rad) Ax Ax' Otm rad)

Sample 0.1 Pinhole 2 Exit slit 20 Monochromator Vertical + 20 Horizontal _+250

+ 0.2 + 0.01 + 0.001

0.04 0.04 0.04

_+0.001 _+0.001

0.08 1.0

o

o'

40 emittance

200 1400

60 × 10 6 0.3 × 10 3

0.048 1.68

(b) Source emittance Undulator Vertical Horizontal

scope, i.e., at the sample, the pinhole (the object of the Schwartzschild objective), and the exit slit of the monochromator (the object of the condenser mirror). The monochromator acceptance and the undulator source emittance values are also shown. Since the Schwartzschild objective has cylindrical symmetry, its acceptance is the same in both horizontal and vertical directions. However, the monochromator and the source are not axisymmetric, so that we have to consider their phase-space properties separately in both planes. The photon beam emittance of the undulator needs to be discussed in some detail before proceeding. The radiation emitted by an undulator is characterized by a very narrow cone. Because of the incoherent nature of the source, the overall angular divergence can be obtained (in first approximation) by adding in quadrature the electron and photon beam angular standard deviations. Furthermore, since the source is far from being diffraction limited, the source "size" is the electron beam cross section. The emittance of the source is then approximated by the product of the electron beam cross section and the overall angular divergence of the photon beam. The table is built by starting from the goal to be achieved, i.e., with the smallest image size, 0.1 ~m. The resolution requirement at the operating wavelength (100 ,~) dictates the objective numerical aperture (the sine of the half-angle of the converging beam), since the diffraction limit is given by t / ( 2 N A ) . This shows that N A = 0.2 is more than adequate. We notice from table 1 that the source and the image are quite well matched vertically but rather poorly matched horizontally. A loss by, at most, a factor of two is expected in the vertical direction. In order to match the Schwartzschild objective and the monochromator a condenser mirror working at a reduction of 10 is located between the mono-

/ MAXIMUM chromator exit slit and the pinhole. The matching between the monochromator and the undulator source is not a problem vertically since the monochromator 6tendue is larger than the source emittance. The overall matching between undulator source and final image results in a loss by a factor of ---100. The central obstruction of the Schwartzschild objective will cause another factor of 2 loss (unless an Axicon condenser is implemented), bringing the factor to about 200, In the present geometry this is the best that can be achieved because of phase-space conservation. In order to improve the matching, it is necessary to use optics with larger acceptances or sources with smaller emittances. Since the possible gains due to better optics are rather limited (at most a factor of 2-4), the advantages of the new high-brightness sources such as the Advanced Light Source [9] are evident for microscopy applications. At the same time, the inherent limits of small numerical aperture optics, such as Fresnel zone plates, are also evident for high-flux applications. Let us now come to the estimate of the photon flux. As a first approximation, the final flux can be estimated by scaling the initial total flux (at the source) by the ratio of the final image &endue to the source emittance. Under typical conditions the undulator emits about 9 × 1014 p h o t o n s / s (100 mA, 800 MeV) in the region = 100 eV, in a 0.5% bandpass. Estimating the grating efficiency at 10%, the above losses at 200, and maybe an overall optical reflectivity of 5% (because of the two multilayers and all other surfaces), we end up with flux estimated at = 2 × 101° p h o t o n s / s onto the sample in a 0.1 ttm spot. This flux should allow a high counting rate and fast data acquisition.

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We have briefly presented the proposed implementation of M A X I M U M , a scanning photoelectron microscope to be installed at Aladdin. Preliminary analysis clearly indicates the feasibility of the project. The intended applications center on surface-physics problems, such as semiconductor-metal and semiconductor-semiconductor interfaces. As well, M A X I M U M can be operated in transmission mode for biological applications, in which case thin Si3N 4 windows can be used to define a region having higher pressures.

0.4

x (era) Fig. 4. Same as fig. 3, but for horizontal phasc-spacc coordinates).

(x-x' References

In order to verify these calculations, we have performed some ray-tracing and phase-space matching studies. When making phase-space comparisons, it is also necessary to consider the shapes of the volumes involved. It is most efficient to r e v e r s e - t r a c e the optical system from the ideal final image. In this way it is possible to find the phase-space volume of the source that is useful in forming the image. This is easily done by using a ray-tracing code such as S H A D O W [7] and the results are shown in figs. 3 and 4. The figures show the overlap of the Schwartzschild objective focal spot re-imaged back to the source (higher density points) over the source phase-space itself. Notice how the matching is much better vertically (fig. 3) than horizontally (fig. 4), as expected. S H A D O W is being modified to give accurate numerical values for the matching, but a preliminary analysis shows substantial agreement with the simple discussion outlined above.

[1] M.A. Green, M.K. Kelly, B. Lai, R.A. Otte, E.M. Rowe, J.P. Stott, W.S. Trzeciak, D.J. Wallace, W.R. Winter, F. Cerrina, G. Margaritondo and H. Winick, these Proceedings (5th Nat. Conf. on Synchrotron Radiation Instrumentation, Univ. of Wisconsin-Madison, 1987) Nucl. Instr. and Meth. A266 (1988) 91. [2] B. Lai, G.M. Wells, R. Redaelli, F. Cerrina, K. Tan. J.H. Underwood and J. Kortright, ibid., p. 684. [3] F. Cerrina, J. Imaging Sci. 30 (1986) 80 and references therein. [4] B. Lal, F. Cerrina and J.H. Underwood, SPIE 563 (1985) 174. [5] M.C. Hettrick, ref. [1], p. 404. [6] C.C.G. Visser, J.E. Uglow, D.W. Bums, G. Wells, R. Redaelfi, F. Cerrina and H. Guckel, ref. [1], p. 686. [7] B. Lai, K. Chapman and F. Cerrina, ref. [1], p. 544. [8] See for example S. Goldstein, Classical Mechanics, 2nd ed. (Addison-Wesley, Reading, 1980) Section 9-8. [9] Frank B. Selph, ref. [1], p. 44.

II(e). MICROSCOPY BEAMLINES