Surface electron microscopy: the first thirty years

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Surface Science North-Holland

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Surface electron microscopy: the first thirty years E. Bauer Physikalisches Institut, Technische Unir~ersitiitClausthal, D-3392 Clausthal-Zellerfeld, Germany Received

2 March

1993; accepted

for publication

4 May 1993

After a brief account of the early years of surface electron microscopy the evolution of ultrahigh vacuum surface electron microscopy from the early sixties to the early nineties is described. Low energy electron microscopy is selected as a case study to illustrate the difficulties encountered in the development of a new method, but all other successful imaging methods are discussed

1. The prehistory More than 30 years before the first issue of Surface Science appeared, the first images of surfaces obtained with electrons using electron lenses were published [l-6]. These images were of low magnification and resolution but demonstrated that surfaces could be imaged with thermionically emitted electrons [1,21, photoelectrons [3], secondary electrons produced by electron [4] or ion bombardment [5] and with reflected electrons [6]. Soon afterwards scanning electron microscopy (SEMI [7] and mirror electron microscopy (MEM) [8] were added to the list of direct surface imaging methods and in 1940 high energy electron reflection microscopy (REM) [9] in the mode used in todays instruments. Resolutions beyond that of the light optical microscope were, however, not obtained in direct electron optical surface imaging until the early forties [lO,ll], ten years after the invention of electron microscopy. In the meantime transmission electron microscopy (TEM) had already become “ubermikroskopie” (“super microscopy”), that is microscopy with a resolution beyond that of the light microscope, and the introduction of the replica technique in 1940 [12] had produced the first “super microscopic” images of metal sur0039-6028/94/$07.00 0 1994 - Elsevier SSDI 0039-6028(93)E0279-4

Science

faces using TEM. Indirect surface electron microscopy by surface replication and TEM soon received a further boost by the shadow casting method [13] in which a thin heavy metal layer was deposited obliquely onto the (rough) surface before the thin electron-transparent replication layer was applied. This easy way of imaging surfaces indirectly by TEM made direct imaging techniques such as emission, mirror or reflection microscopy much less attractive. Also, SEM had already reached a high level of sophistication in 1942 1141 and world war II soon terminated further developments. After the war widespread efforts were made to develop emission and mirror electron microscopy into viable imaging methods, in spite the rapid progress of TEM and of the replica method. The motivation was the desire to study phenomena which are inaccessible to the replica method such as electron emission, oxidation or recrystallization in situ. These developments which are very well described in several reviews (see, e.g., Refs. [15-171) actually lead to some commercial instruments, the most sophisticated being the Balzers Metioskop KE3, an instrument which allowed several emission modes [18]. Although a large amount of interesting work was done with these instruments it soon become evident that surface contamination was a serious handicap which lim-

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E. Bauer / Surface electron microscopy

ited their usefulness. Furthermore, in the mid-sixties the first commercial SEMs became available and soon displaced emission, mirror and reflection microscopes although having similar contamination problems. In view of these limitations, indirect surface electron microscopy continued to be an attractive alternative. The carbon replica technique, introduced 1954 [19] and its extension, the Pt-C shadowing technique allowed replication in vacua with a resolution not achievable by direct imaging at that time. In favorable cases, modifications Ff this technique gave a resolution of about 25 A [201. Another indirect surface imaging mode discovered in these years, however, became much more important in surface science. In my thesis [211 I had used Pd shadow casting to study the growth of thin fluoride films. On one occasion the Pd layer had become much too thin and the Pd crystals had decorated the growth steps. This was obvious to me and I did not pay much attention to it. It was Bassett who recognized the potential of the decoration technique [22] and Bethge who made it popular in surface science by his and his coworkers’ beautiful studies of monatomic steps on alkali halide surfaces [23]. The decoration replica technique is still the most important technique for the study of the surface microstructure of electron-sensitive materials such as ionic crystals and of surface processes on them. In the sixties several attempts were made to overcome the contamination problem in conventional electron microscopes. One route was to use differential pumping with well-trapped diffusion pumps augmented by sorption pumps such as Koch’s photoemission electron microscope (PEEM) in Tiibingen which achieved a base pressure in the lop8 Torr range [24]. Another, still used method was to prepare the surface in a UHV system and to transfer it to a conventional vacuum electron microscope, hoping that the exposure to atmosphere and the subsequent contamination in the microscope would not influence the surface features of interest. None of these attempts, of course, can satisfy the pure surface scientist who wants to study surface phenomena under well-defined conditions in the system in which he prepared and characterized his surface.

103

This lead early, shortly after metal UHV technology had became commercially available, to the development of UHV surface electron microscopy, the main subject of this review.

2. The childhood

years of UHV surface electron

microscopy

As far as I can determine from the literature, my group at the Michelson Laboratory, China Lake, California, seems to have been the first one who dared to attack the task of building an UHV surface electron microscope. According to Ref. [17], this effort was soon followed by another one, also in a military laboratory, the Night Vision Laboratory, Fort Belvoir, Virginia, in which Burroughs built about 1965 the electrostatic flange-on PEEM and TEM designed by Gertrude F. Rempfer in 1963. We were not aware of this work and nothing was apparently published about it so that I can recount only the Michelson Lab Story. Our effort was a true child of surface science. Shortly after Germer et al. had revived LEED by developing the display type LEED system they reported streaked LEED patterns [25] which I subsequently attributed to linear adsorbate structures caused by preferred adsorption at surface steps 1261. It was obvious that an unambiguous interpretation of the LEED pattern required independent information on the microstructure of the surface. At this point I remembered what I had learned in my thesis: the Boersch ray path, introduced by Boersch in 1936 in TEM [27]. It allowed to take the diffraction pattern of the specimen in a TEM by imaging the back focal plane of the objective lens, in which the Fraunhofer diffraction pattern is located, onto the fluorescent screen, using an auxiliary lens. Why could not the same be done in reflection with the slow electrons used in LEED? This idea was the birth of a new surface imaging technique, low energy electron (reflection) microscopy (LEEM) which uses elastically backscattered electrons and is based on diffraction contrast. The experience gained in writing a book on electron diffraction [28] and building an electron optical bench as well as the basic re-

search-friendly atmosphere and the excellent workshop facilities at the Michelson Laboratory at that time gave me the necessary confidence for this endeavour. The project started in a somewhat funny way. Although I wanted to build a metal system right away our glass blowing contractor succeeded to convince management that he could build a glass system with the necessary tolerances and conductive coating (to avoid charging) much faster and much cheaper than a metal system could be built. Of course, it took much longer and probably was also much more expensive than he had promised, but nevertheless it was finished in time for presentation at a conference in 1962 [29]. Fig. 1 shows the schematic (a) and the physical appearance (b) of this curiosity. The double 90” deflection (3,4) was dictated by the limitations of precision glass blowing. The electrostatic lenses (6,11,12) were pre-aligned with precision-ground glass cylinders. Electrons from a pointed filament cathode (1) could be accelerated up to 25 keV and decelerated in the cathode lens to the desired low energy before being reflected in or before the specimen (8) for LEEM and MEM, respectively. Although I got a good direct and a distorted reflected beam on the auxiliary fluorescent screen (14) and with deflector 4 some strangely looking features on the final image screen 13, repeated accidents such as arcovers in the gun - which destroyed the emitter 1 - or in the cathode lens - which made repolishing necessary -, burnout of the filaments of the ionization getter pump 10 and of the electron bombardment heater 9 for specimen cleaning soon made the system irrepairable. This convinced management that glass was not quite the right way to go and 1 was allowed to build a metal system. Similar to the glass system it relied on proven lens designs: an electrostatic triode as cathode lens [301, magnetic intermediate and projective lenses 1311 and a filter lens [32]. The precision possible in an all-metal construction allowed to deviate from the 90” beam deflection, but first a simple straight beam set-up was chosen in order to test the various components. One of my coworkers, George Turner, a very skilled designer and experimentalist, took over

(4

TO PUYP

Fig. I. First model of a LEEM.

For explanation

SW text

E. Bauer / Surface electron microscopy

design and assembly of the instrument in early 1963. According to his somewhat sketchy notebook, assembly started in October 1964 and on Christmas we had the first beam through the system. The first mirror and secondary electron images, however, were not obtained until 8 months later after the filter lens had been removed again and an earth magnetic field compensation had been installed. They were from a polycrystalline surface on which a (BaGlO layer was deposited through a 1000 mesh grid. We then switched to single crystals in order to be able to obtain also LEED patterns. By that time the publication pressure had become so strong that we decided to interrupt instrument development and to study with the operation modes available at that time a specific surface science problem, the microstructural aspects of electron emission from alkaline earth oxide films on tungsten. This had become possible because (i) due to many bake-outs we had reached a base pressure in the low lo-” Torr range, (ii) we could clean the W(110) crystal and deposit SrO in situ using the procedures developed in a separate LEED system and (iii> we had photoelectric and retarding field work function measurements available in addition to MEM and PEEM. The instrumental set-up used in this work is shown in Fig. 2. Some of the results were reported briefly at a conference in 1966 [33]. The instrument development work was accompanied from the very beginning by theoretical work on the optics of the cathode lens together with Cruise and on low energy electron scattering and diffraction with Browne. The major hurdle to LEEM was from the very beginning the Recknagel formula for the resolution 6 of a cathode lens, 6 N V,,/E, where V, is the start voltage of the electrons and E the electric field strength at the cathode [34]. Applied indiscriminately to LEEM where I’, is much larger than in PEEM or microscopy thermionic electron emission (THEEM), a much poorer resolution would be expected in LEEM than in the emission modes. I had addressed this problem early in the game by looking at the aberrations of the homogeneous field in front of the cathode but did not report the results until Cruise had confirmed my predic-

Fig. 2. Temporary LEEM development

105

straight beam set-up during the initial phase with the then still young developfZK5.

tions of a better resolution of LEEM than PEEM by his calculations for the Bartz lens 1301 used in our instrument. Cruise’s main job was computational chemistry of propellants so that he could do the calculations for me only on the side. He first developed a new computational procedure for electron optics [35], the charge density method, and applied it then to our specific problem [36] confirming that the homogeneous field calculations [37] were realistic and that the LEEM instrument design based on them [38] was promising. The homogeneous field calculations were so simple that I did not publish them until much later [391 together with some of Cruise’s data in order to counter the persistent misunderstood Recknagel resolution argument from the electron microscope community. In writing my book on electron diffraction [28] I had become not only aware that an adequate description of the diffraction of slow electrons required a dynamical theory but also that the

106

E. Bauer / Surface electron microscopy

scattering of slow electrons by the individual atoms in the crystal could not be calculated within the Born approximation. In order to understand the diffraction contrast in LEEM we embarked, therefore, early in an effort aimed at a managable multiple scattering theory for slow electrons. We started by developing effective scattering potentials V,, for free atoms taking into account exchange and correlation and applied them successfully to free atoms for which measurements and more precise calculations were available [40]. For atoms in solids we used superposition of atom potentials truncated at the nearest-neighbor distances plus an energy-dependent V,, from the free-electron approximation. The Z(I/) curves obtained with these potentials from a multiple scattering calculation in a column approximation which we presented at the first LEED theory seminar in Brooklyn 1967 had much broader maxima than experiment, due to a trivial error: in the amplitude attenuation due to inelastic scattering I had used the inelastic mean free path characteristic for the intensity attenuation. This lead us to give up our theoretical efforts, in particular as in the meantime high power professional theoreticions such as Charlie Duke had entered the field. Some results of our early work have been reported much later [41,42]. Returning to the LEEM instrument development, further progress was very slow due to the many problems encountered. For example, in a period of changes on the instrument we moved it into a then low AC field region further away from the power lines which ran past our room. When we resumed microscopy again with the supposedly improved conditions, we got very poor images. There were many possible causes such as instability of the specimen holder, of power supplies, charging, etc. which we eliminated until we finally discovered that the public works department had improperly re-wired in the meantime the AC power distribution grid in the basement below us, ca,using AC fields not existing before. Other problems were increasing high voltage arcovers with increasing complexity of the system which frequently damaged power supplies causing considerable delays until we could reduce them with inductive damping and other tricks.

Fig. 3. Final configuration

of the first LEEM.

Good THEEM was delayed by these problems until summer 1967, when we could take images of BaO layers on W(110) at magnifications up to 8000 X with about 0.3 pm resolution, while PEEM was still limited by intensity problems to magnifications below 3000 x . Still no acceptable LEED patterns could be obtained. This lead to the unsuccessful1 attempt to build a stable Zr0-W(lOO> field emission gun. Although activation of a small spot source was no problem, frequent reactivation by in situ Zr deposition made operation at high voltages impractical. We, therefore, decided to use a cored oxide cathode when we assembled the system in January 1968 in the final set-up (Fig. 3) with the 60” deflector block which separates incident and reflected beam and allows LEEM in addition to LEED. MEM and the emission imaging modes. In mid-February we had the beam to the final screen and to the specimen but it took until the middle of March before we had (poor) MEM, THEEM and what we believed to be LEEM images which later, however, turned out to be secondary electron emission (SEEM) images. Proper handling of the remanence problems of the Armco iron deflector and compensation of its astigmatism made for slow progress, but in May good THEEM images could be obtained as well as reflected or secondary electron images over an energy range from O-40 eV. By that time management had changed and was understandably loosing patience. Nevertheless, work continued.

107

E. Bauer / Surface electron microscopy

In autumn 1968 we finally received the channel plate image intensifier which had long been classified for military reasons and from spring to summer 1969 we made a last attempt at the Zr-O-W(100) field emission gun. Then Turner left my group. Shortly afterward I left the Michelson Lab with the permission to take all electron optical components with me which was the basis on which the work in the seventies in Clausthal was built. I have described the first decade of LEEM in so much detail for two reasons. First of all to give recognition to George Turner who has done all the early pioneering LEEM instrument development work without receiving recognition for his achievements by the scientific community. Secondly, this example is a good illustration of how tedious and time-consuming the development of a new method is, in particular if a lot of new physics and technology is needed. Less ambitious endeavours, extending or combining known methods, obviously bring faster success. For example, by improving the vacuum using UHV technology to a large extent, Eichen et al. [43] built a MEM capable of working in the lop8 Torr range. Delong and Drahos [44] used differential pumping with an Orbion pump in an emission microscope and could, thus, reach about lOwE Torr which enabled them to publish the first LEED patterns in a surface electron microscope [45] with the basic set-up shown in Fig. 2. Although this mode of operation is the one which stimulated the invention of LEEM [29] we had not used it in our early work [33] which concentrated on the connection between microstructure and emission properties. By the end of the first decade of surface electron microscopy two true, bakeable emission microscopes were in operation. One, which had been built by Griffith et al. [46], partially from the components of the Fort Belvoir instrument, however, was not used in surface science, the other one by Recknagel et al. [47] apparently did not produce any additional results beyond those reported in Ref. [47]. This instrument, which incorporated a LEED and a RHEED system, was the most versatile operating surface electron microscope at that time. The impact of all these instruments on surface science, however,

was small not only in the first decade but also in the one to follow.

3. The second decade: adolescence formative

or the

years

In the late sixties Auger electron spectroscopy (AES) had been developed as a tool for chemical surface characterization. In view of the large practical importance of the distribution of the chemical species on the surface, the seventies became the decade of scanning Auger electron microscopy (SAM). MC Donald and Waldrop demonstrated the feasibility of SAM already in 1971 by incorporating a cylindrical mirror analyzer (CMA) into a conventional scanning electron microscope (SEMI [48]. This approach was also followed later in commercial UHV-SEMs, the JEOL-JAMP [49] and the VG HB50 which was equipped with a field emission gun [50]. There were also numerous other developments (for a review see Ref. [511). The most successful incorporated the illumination system coaxially into the CMA and found widespread distribution over the whole world [52]. Soon most users of these instruments found out, however, that even with the best-designed illumination system the signal decreased very rapidly with decreasing spot size making it practically impossible to use the smaller spot sizes for high resolution imaging. This is an inherent problem of AES caused by the low Auger electron yield and the high background on which the Auger electron signal rides. As a result, SAM never developed into a high resolution surface imaging technique but is more or less used on the 0.1-l pm level. Chemical analysis of smaller areas is usually accomplished by spot or line profile analysis combined with SEM imaging. Another problem is quantification in SAM. The Auger signal depends generally upon angles of incidence and emission and is, thus, very sensitive to surface roughness. Although this problem has been reduced later significantly by various correction algorithms, SAM has found no widespread application in pure surface science, as important as it has become for practical surface analysis.

The fixation in the seventies on chemical imaging had as a consequence that hardly any UHVSEM surface imaging with true secondary electrons was done although these are much more suitable for imaging because of their usually high yield. This method is particularly well suited for fundamental in situ film growth studies at elevated temperatures at which sufficiently large three-dimensional crystals form [531. The desire to study nucleation and growth in situ with high resolution under UHV conditions lead to the incorporation of UHV specimen chambers in conventional transmission microscopies (TEM). The most successful approach, the “cryocage” system was first used by Poppa [541 and perfected by Honjo’s group [55]. A small cage surrounding the specimen and cooled with liquid nitrogen or helium was inserted in a UHV chamber between objective and condenser of a commercial TEM. High pumping speeds of Vacion, Orbion, Ti sublimation or cryopumps, combined with small apertures between the specimen chamber and the rest of the microscope allowed to maintain a pressure difference of several orders of magnitude between the conventional vacuum and UHV parts of the instrument. Thus, experiments in the 10~y-lO-‘O Torr range at the specimen became possible and were done with great success in the late seventies and in the eighties. A parallel and equally important development was the introduction of UHV into REM in a very similar manner, also by Honjo’s group [56]. While SAM and UHV-TEM attracted increasing attention in the seventies, interest in nonscanning emission microscopy decreased, largely because the limitations of non-UHV instruments became increasingly evident. A conference in 1979 dedicated solely to emission microscopy [57] summarized the results of the late years of non-UHV emission microscopy (see also the review [58]). Only in a few laboratories UHV instruments were developed. Griffith, Rempfer et al. started in 1975 with a second generation PEEM instrument (see Ref. [17]). Bethge et al. combined a UHVPEEM with LEED and AES [59] and my group in Clausthal continued working on LEEM. Progress was slow because I was busy building up a surface science institute and fighting student

unrest while Koch, the experienced PEEM microscopist, was so absorbed by administrative duties that the master’s students working on the instrument were more or less left to themselves. Although the instrument was rebuilt by 1972 [60] it was not operating until the late seventies when a particularly good student, W. Telieps, took it over. He did a very thorough study of the electron optics of the system and by summer 1978 he obtained THEEM and PEEM images of a quality comparable to that obtained ten years before in China Lake. It took, however, two more years and many instrument improvements, in particular in the field emission illumination system, before he succeeded with MEM and LEEM, initially with a resolution of about 200 nm. By the time he finished his thesis [61], LEEM and PEEM resolution was improved to 50 and 60 nm, respectively. This leads us to the third decade of surface electron microscopy.

4. The third decade: maturity In the early eighties the instrument development efforts of the seventies began to bear fruits and in the second half of this decade UHV surface electron microscopy was flourishing. Initially the TEM and REM studies of the Tokyo Institute of Technology group (Takayanagi, Yagi and Honjo) produced the most impressive results on the structure of metal and Si(ll1) surfaces and its modification by metal condensation (see the review in Ref. [62]). The scanning counterpart (SREM) of conventional REM (CREM), proposed in the seventies by Cowley (see Ref. [63] and realized in a non-UHV system [64], was adapted by Petroff’s group [65] to UHV in a manner similar to that of Honjo’s group. It was used successfully in scanning TEM and REM studies of reconstructed Si(ll1) and GaAs(100) surfaces [66]. The poor accessibility of the specimen was eliminated in a later modification of another commercial STEM which allowed AES and in situ MBE growth [671. A quite different approach to SREM (“p-RHEED”) was taken by Ichikawa et al. [68] who started from a UHV system equipped with a CMA and other surface

E. Bauer / Surface electron microscopy

science tools and added a field emission (FE)SEM to it. This allowed them to combine SREM not only with p-RHEED but also with -AES which was very useful in the later in situ growth studies on semiconductor surfaces. A similar system, but without CMA, built by Cowley’s group [69], apparently did not find much application, inspite of its better resolution. Ichikawa’s approach was later extended by incorporating a FE-SEM into a large MBE system with a separate surface analysis chamber [70]. In this system compound semiconductor MBE can be studied under production conditions. An even more versatile UHV scanning electron beam system was built by Ichinokawa et al. [71]. It also combined a field emission SEM with a CMA but in addition had also a LEED optics so that imaging could be complemented not only by AES and energy loss spectroscopy with the CMA but also by LEED and work function change measurements with the LEED optics. The system was not used in the usual SREM mode but was originally intended for low energy SEM with primary energies below 2 keV for which a high surface sensitivity is expected 1721.A natural later extension was to use the diffracted LEED beams for scanning low energy REM [73], the scanning counterpart to LEEM. By the mid-eighties UHV-REM, both in the conventional (non-scanning) and in the scanning mode, had made important contributions to the understanding of the microstructure of clean surfaces. They are summarized in the proceedings of a 1987 NATO conference [74] and several reviews [75,76] which include also the important contributions from non-UHV-REM. UHV-TEM also was further improved [77] and continued to give information on surface microstructure and on surface processes. Its major impact, however, was in the determination of the atomic structure of surfaces, either indirectly via TED or directly, mainly via profile imaging. Thus, Takayanagi et al. [78] succeeded to analyze the TED pattern of the Si(lll)-(7 x 7) structure which lead them to the famous dimer-adatom stacking-fault (DAS) model, ending a long controversy about this surface reconstruction. Atomic resolution in profile imaging had already been achieved in 1983 by

109

Marks and Smith [79] but this was done in conventional vacuum with the accompanying residual gas interactions. The importance of these interactions became evident in the first in situ UHV profile imaging studies [BO]which showed atomic relaxations different from those found in conventional vacuum, and is further discussed in Ref. [Sl]. The contributions of profile imaging to surface science up to 1987 have been summarized by Smith [82]. It should be mentioned that ordinary “plan-view” TEM also continued to find important applications. One of many examples is the study of the growth of Au films on Ag surfaces in the monolayer range, using carbon films for stabilization of the Au layers [83]. Before turning to the evolution of PEEM LEEM and other low energy microscopies in the eighties one scanning imaging mode using fast primary electrons, biased secondary electron microscopy, should be mentioned [84,85]. The most adsorbate-sensitive surface imaging method with wide application range, however, remained PEEM due to the strong work function dependence of the photo yield. Bethge’s instrument became productive in the early eighties [86-881. One interesting result was the strong doping sensitivity of PEEM ranging from 10” to 1016 Zn atoms/cm* in GaAs, and its enhancement by CH, adsorption [BB]. By 1985 our LEEM instrument also produced good PEEM images [89] but LEEM was so exciting that there was little time for PEEM studies. Only later, after Telieps’ untimely death in 1987, was the instrument used more frequently in the PEEM mode, one example being the study of the decoration of monatomic steps on Mo(ll0) by Cu and the step flow growth of the Cu monolayer on this surface [90]. Another example is the PEEM imaging of three-dimensional Cu silicide crystals on Si(ll1) [91]. Until the late eighties Bethge’s, our and an instrument similar to ours at the Fritz Haber Institut (FHI) in Berlin were the only UHV-PEEMs in surface science and the scientific output was correspondingly low. The development of scanning-PEEM in 1989 [92,93] and its application to the study of chemical reactions on surfaces greatly stimulated the further development of PEEM in surface science. The next step was the construction of

110

E. Bauer / Surface electron microscopy

flange-on PEEMs with the specimen at ground potential which was started in Clausthal [94] and shortly thereafter in Berlin and lead to an instrument [95] which proved immediately its superiority over the scanning PEEM [96]. Ertl’s group at the FHI Berlin has since obtained stunning videos of oscillatory and chaotic surface reactions. The photoelectrons in the PEEMs discussed up to now originate from valence levels or from the conduction band. The contrast is largely determined by the work function and contains only indirect information on the chemical nature of the surface. Direct information can be obtained by imaging with photoelectrons originating from core levels which requires excitation with synchrotron radiation. The possibilities of this type of imaging have been discussed already in 1984 by Cazaux [97] but applications in surface science appeared only in the late eighties. Both modes of operation, scanning (“microspectroscopy”) and non-scanning (“spectromicroscopy”) were developed at the various synchrotron radiation facilities. The Wisconsin [98], Brookhaven [99] and DESY Hamburg groups [loo] choose the first path, all with different photon focussing systems, Tonner’s group, also at Wisconsin [loll, and the Stanford group [102,1031, the second route. While Tonner et al. used an immersion lens as in ordinary PEEM, the Stanford group used a commercial magnetic projection PEEM [104] based on the concepts of Turner et al. [10.5,106]. Returning to slow electrons, the lowest energies are encountered in MEM, in which the electrons are reflected in front of the surface. MEM is a natural by-product of LEEM but (short-lived) MEM instruments have also been developed independently [107,108]. An interesting non-UHV MEM system which allowed interferometry [109] should also be mentioned. We have used our LEEMs only occasionally in the MEM mode and only sometimes in the zero-impact energy mode for local work-function measurements because LEEM is much more powerful for surface science studies of crystalline materials. The first images of monatomic steps on Mo(ll0) and the measurement of the step height [89] were soon followed by a study of the Si(l1 l)-(7 x 7) ++ (1 x 1) phase transition at 1100 K [llO]. It revealed many more

details than the preceeding REM studies [ll 11 which were hampered by the strong foreshortening caused by the small grazing angle of incidence. In particular the videos of the phase transition made such a strong impression on the surface science community that several groups started to build LEEMs too: at the FHI Berlin, according to our design, at IBM Yorktown Heights, at the Max Planck Institute for Plasma Physics in Munich and in several other places. The Berlin LEEM became operational with limited resolution in 1992 and soon produced interesting results on reactions in adsorbed layers 11121. The Munich and IBM instruments which contained several new design ideas were described in 1991 [113,114]. The IBM group became very rapidly productive but work on the Munich instrument was terminated unfortunately. An instrument built by Delong et al. [ 1151 had suffered the same fate already several years before. En route to the third generation instrument which will allow energy-filtered imaging with Auger and core photoelectrons [116,117], in the meantime we had built a second generation instrument which soon after completion was used in semiconductor surface studies [118,1191, similar to the IBM instrument [120,121]. In spite of the small number of LEEM instruments in operation a large amount of valuable information on surface structure and processes has already been obtained with this method (for recent reviews on instrumentation and applications see Refs. [122,123]). The goal of the microscopies discussed up to now was the determination of the surface topography, work function, crystal structure and orientation or chemical species distribution on the surface. Only charge and energy of the electrons are used in these imaging modes. If the spin of the electron is also used in imaging, information on the magnetic microstructure of the surface can be obtained. One possibility, the spatially resolved measurement of the spin polarization of secondary electrons was already discussed in the late seventies [124]. By the mid-eigthies two groups had demonstrated the power of this technique (spin-SEM or SEMPA = scanning electron microscopy with polarization analysis) [125,126].

E. Bauer / Surface electron microscopy

The success of this initial work and the large interest in the magnetic microstructure of surfaces and thin films soon lead to the development of other SEMPA instruments [127,128] and to impressive results which are reviewed up to 1989 in Ref. [129]. A more recent example of the power of this technique is the demonstration of the oscillatory magnetic coupling between iron films through a Cr film as a function of its thickness [130]. A second magnetization-sensitive surface imaging mode, spin-polarized LEEM (SPLEEM), is based on the spin dependence of the elastic scattering from magnetic materials. This imaging mode is easily realized in a LEEM instrument equipped with a spin-polarized electron source. This was done recently in Clausthal in cooperation with IBM San Jose [131]. After the usual initial difficulties in situ SPLEEM studies have already produced interesting results for ultrathin epitaxial Co layers [132], for example on the evolution of the magnetic microstructure during in situ film growth or annealing [1331, with much shorter data aquisition times than those needed in SEMPA. This brings us to the present state of the art.

Fig. 4. Third generation,

111

5. Present state and future prospects Today surface science has a large arsenal of methods for surface imaging with electrons available, with resolutions from the sub-nm to the pm range. They give information on the atomic structure, the microstructure, the chemical and the magnetic structure, all under well-defined vacuum conditions and frequently with in situ manipulation possibilities such as heating, cooling, gas exposure, vapor deposition, sputtering etc. For most surface science problems several methods are suited but one will always choose the simplest and cheapest one which can solve the problem. A good example are the oscillatory and chaotic surface reactions for which the low resolution of a simple flange-on PEEM is sufficient [96]. On the other hand, the direct measurement of atomic relaxations on a surface by profile imaging or chemical identification in the nm range by AES requires sophisticated expensive high resolution UHV instruments with field emission guns. Instruments of this kind have already been built in the second or third generation, for example for TEM [134], REM combined with PEEM

computer-operated

LEEM.

[135] or SEM

combined with SAM [136]. Also second and third generation PEEM [137] and LEEM instruments are now in operation. Fig. 4 shows a professionally built LEEM based on Veneklasens design [116]. All lenses are computer-controlled and a UHV preparation chamber with airlock allows easy sample exchange and preparation, e.g. outgassing or sputter-cleaning. Electron microscopists usually put a lot of emphasis on resolution. In surface science this is frequently less important than image aquisition time and interpretation. Image interpretation problems have lead to the development of multimethod imaging systems, e.g. the MULSAM [13X], and of sophisticated image analysis software. What is ahead for surface electron microscopy in surface science? One major future effort certainly will be in image processing and analysis. The amount of data produced by a well functioning microscope is so large that much information will be lost without efficient image analysis procedures which are also necessary for the extraction of quantitative data. Instrument development certainly will continue too. Examples close to my interest are new versions of LEEM/MEM/ PEEM [139,140], SPLEEM [141] or scanning LEEM [142]. Further developments may be aberration-corrected LEEM systems which have been proposed recently [1431. The efforts to combine several imaging modes in the same system such as LEEM and core electron PEEM have already been mentioned and other combinations such as LEEM and STM are also obvious. In addition to these more sophisticated systems there is a definite need for simpler systems which can be incorporated in many surface science systems. This is the lesson which the experience of the past teaches us: a large number of surface imaging systems has been built at high financial and human effort cost. Many of these systems were never or only briefly used. Only few made major contributions to surface science over a period of time. The reasons for this unfortunate situation are manyfold: the complexity of many systems which causes long development times, resulting in loss of financial support and exhaustion of the instrument builder; the lack of inclination of many instrument-oriented scientists to use their instru-

ments for systematic studies; the complexity of many instruments which makes their use by measurement-oriented scientists difficult and unattractive. As human nature cannot be changed the obvious route which surface electron microscopy must go - a few sophisticated instruments with very experienced users excepted - is towards simpler affordable and dependable workhorse microscopes. Once this happens, surface science will make a major move forward again similar to the one initiated by STM.

6. Closing

remarks

This historical perspective of surface electron microscopy was strongly method-oriented because the various results obtained with it are dealt with in other chapters of this volume. The review is far from comprehensive and many important aspects, in particular those involving specimen transfer from a surface science instrument to a non-UHV microscope, have only been touched upon. I have selected LEEM as a case study because I know its evolution in detail from the very beginning and because this example has many of the ingredients of unsuccessful and successful efforts of development of a new method. In conclusion I would like to dedicate this review to my late friend and coworker Wolfgang Telieps. Without him the development of LEEM may well have ended up in the category “unsuccessful efforts”.

Acknowledgements The author’s work discussed here was initially supported by Navy Independent Research Funding. Later, the various aspects of the work were and still are supported by the Deutsche Forschungsgemeinschaft, the Volkswagen Foundation and by the Bundesminister fiir Forschung und Technologie. I wish to express my thanks also to all my coworkers in the endeavour “UHV surface electron microscopy”, some but not all of whom have been mentioned in the references.

E. Bauer / Surface electron microscopy

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