Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy

Ultramicroscopy North-Holland

36 (1991) 1-28

Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy An Introduction

to the Proceedings

and Workshop

on Emission

of the Second

Microscopy

International

and Related

Symposium

Techniques

0. Hayes Griffith Institute of Molecular Biology and Department

of Chemistry,

University of Oregon, Eugene, OR 97403. USA

and Wilfried Engel Department of Electron Microscopy, W-1000 Berlin 33, Germany Received

3 October

Fritz-Haber-Institut

1990; at Editorial

der Max-Plan&Gesellschafi,

Office 7 December

Faradayweg

4-6,

1990

Emission microscopes and related instruments comprise a specialized class of electron microscopes that have in common an acceleration field in combination with the first stage of imaging (i.e.. an immersion objective lens, also called a cathode lens or emission lens). These imaging techniques include photoelectron emission microscopy (PEEM or PEM), electron emission induced by heat, ions, or neutral particles, mirror electron microscopy (MEM), and low-energy electron microscopy (LEEM), among others. In these instruments the specimen is placed on a flat cathode or is the cathode itself. The low-energy electrons that are emitted, reflected, or backscattered from the specimen are first accelerated and then imaged by means of an electron lens system resembling that of a transmission electron microscope. The image is formed in a parallel mode in all of the above instruments, in contrast to the image in scanning electron microscopes, where the information is collected sequentially by scanning the specimen. A brief history and introduction to emission microscopy, MEM, and LEEM is presented as a background for the Proceedings of the Second International Symposium and Workshop on this subject, held in Seattle, Washington, August 16-17, 1990. Current trends in this field gleaned from the presentations at that meeting are discussed.

1. Introduction The Second International Symposium and Workshop on Emission Microscopy and Related Techniques was part of the XII International Congress for Electron Microscopy, held in Seattle, Washington, USA, August 12-17, 1990. The Symposium consisted of the two half-day sessions titled “Emission Microscopy A and B”. The theme was the development and applications of a family of surface imaging techniques that share common technologies. One common technology shared by emission microscopy and related techniques is the 0304-3991/91/$03.50

0 1991 - Elsevier Science Publishers

accelerating and imaging of low-energy electrons or other charged particles emitted, reflected, or backscattered from a planar specimen surface, as opposed to a pointed specimen surface (not included in this discussion are tip microscopes such as field emission microscopes or scanning tunneling microscopes). This electron-optical arrangement, called an immersion lens or cathode lens, is used in emission microscopy, low-energy electron microscopy (LEEM or low-energy reflection microscopy) and its older cousin, mirror electron microscopy (MEM). All of these methods can be thought of as belonging to a broader family of

B.V. (North-Holland)

“low-energy electron microscopy” techniques. In LEEM and MEM an electron gun supplies a beam which is directed toward the specimen, decelerated. and backscattered from the specimen or reflected just before reaching the specimen. In photoelectron emission microscopy (PEEM or PEM) the same specimen geometry and immersion lens are used, but the electron gun is omitted. UV light is most often selected to eject the electrons in PEEM, but other wavelengths (e.g., Xrays) are preferred where analytical information is required. Therefore, a second theme is the broader area of photoelectron imaging, which includes PEEM and all other methods of imaging surfaces by means of the photoelectric effect. Here the common technology is the photon source (X-rays. synchrotron radiation, UV arc lamps, and lasers). The aim of all of these approaches is the development of non-destructive (or minimum damage) methods for imaging surfaces. 2. Concepts in

emission

croscopy

and definitions microscopy,

and low-energy

of terms encountered mirror electron mi-

electron

microscopy

Photoelectron imaging. Photoelectron imaging includes any form of imaging in which the source of information is the distribution of points from which electrons are ejected from the specimen by the action of photons (i.e., the photoelectric effect). The technique with the highest resolution photoelectron imaging is presently photoelectron emission microscopy using UV light. Electron emission microscopy. This is a type of electron microscopy in which the information-carrying beam of electrons originates from the specimen. The source of energy causing the electron emission can be heat (thermionic emission), light (photoelectron emission), ions, or neutral particles, but normally excludes field emission and other methods involving a point source or tip microscope. Emission microscopy differs from the more familiar transmission electron microscopy (TEM) and scanning electron microscopy (SEM) approaches in that an electric accelerating field is present at the specimen surface. This specimen is, in effect, part of the electron-optical system. Typically. the specimen is held at a negative potential

of from - 10 to -40 kV on the cathode, and the electrons are accelerated by the electric field between the cathode and the grounded anode. In some instruments the specimen is grounded and the anode is at positive potential. Photoelectron emission microscop.v (PEEM, also called photoelectron microscopy, PEM). It is the most widely used type of emission microscopy. The excitation is usually produced by UV light, but synchrotron radiation or X-ray sources can be used. In physics, this technique is referred to as PEEM. which goes together naturally with low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy, which fits logically with photoelectron spectroscopy (PES), TEM, and SEM. These two abbreviations, PEM and PEEM, are so similar that there is little chance of confusion in the literature. We will use PEEM here because most of the papers presented at this symposium are from surface physics laboratories, but PEM is used in some of the following articles. Immersion lens und cathode lens. The term immersion lens comes from the analog in light optics. When the space between the specimen and the objective lens in a light-optical microscope is filled with liquid (e.g., oil), the refractive index in object and image space are different and the lens designed for this use is called an oil immersion objective lens. In electron optics this corresponds to different potentials in object and image space. Whereas the refractive index n between the specimen and objective lens in light optics is constant (e.g., that of the oil), in the electron immersion lens the refractive index is in general not constant but increases as the electron moves from the cathode to the anode (the index of refraction relative to the emission point is ( V/ VC)‘/‘, where eV is the electron’s kinetic energy as a function of position between the cathode and the anode. and eV, is the emission energy). The objective lenses used in emission microscopy, mirror electron microscopy, and in LEEM are a special kind of immersion lens. In emission, mirror, and LEEM microscopes designed for high resolution, the electric field at the sample surface is maximized. The most common way to do this is to divide the function of the immersion lens into two parts. the accelerating

0. H. Griffith, W. Engel / Historical perspective and current trends

field and a conventional objective (magnetic or electrostatic) lens. The combination of these two parts is the immersion lens, also called an immersion objective lens, a cathode lens, or emission lens. None of these terms is ideal. The term “immersion lens” is somewhat ambiguous since it can refer to several different configurations. The term “cathode lens” is not sufficiently broad to cover imaging with positrons or positive ions, where the surface is not the cathode. Likewise, the term “emission lens” is fine for emission microscopes but does not describe the imaging in mirror microscopy and LEEM, where the electrons are reflected or backscattered rather than emitted. However, until there is a better description, the words immersion lens, immersion objective lens, cathode lens, and emission lens will be used interchangeably. Mirror electron microscopy (MEM). An electron mirror is a conducting surface biased slightly more negative than the electron source. High-velocity electrons are decelerated in the electrostatic mirror field and are turned back just before reaching the negative surface. Mirror electron microscopy is unique in that the specimen is neither bombarded by electrons nor emits electrons. The specimen is the mirror surface, and the specimen stage resembles that of a photoelectron emission microscope. Some mirror microscopes were built as simple devices without beam-separating systems or electron lenses, but both have been improved in the evolution of MEM technology. The beam-separating system results in a branched (“V” or “Y” shaped) electron-optical system whereas the conventional PEEM is a linear electron-optics system similar to that of a TEM. Mirror electron micrographs are difficult to interpret without complementary information. Thus, mirror electron microscopes are often combined with other surface techniques, such as low-energy electron diffraction (LEED). MEM-LEED is LEED performed in a mirror electron microscope rather than in a conventional LEED apparatus. A MEM-LEED can be either a “straight” (i.e., no beam-separating system) or branched electron-optical system. With improvements in beam-separating technology and imaging systems it has become possible to design microscopes that function in several modes, in-

3

cluding MEM, LEED, LEEM, and PEEM (i.e. the instrument below). Low-energy electron microscopy (LEEM). LEEM has also been called low-energy electron reflection microscopy (LEERM), which emphasizes the nature of the process. (Note this is very different from the reflection electron microscopy, REM, performed with high-energy electrons at small angles to the surface in a TEM.) LEEM is a relatively recent development that utilizes an immersion lens similar to that of mirror electron microscopy and electron emission microscopy. Fast electrons produced by a high-brightness electron gun are directed towards the surface of the specimen, which is held at a slightly less negative potential than the electron source. The incident beam is focused to a small spot in the back focal plane of the immersion objective lens. The parallel beam of electrons then decelerates and strikes the surface of the specimen at near normal incidence with the desired low energy, typically in the range 0.2 to 100 eV. In the most common mode of operation, the elastically backscattered electrons are then re-accelerated and used to image the surface. On clean crystal surfaces low-energy electron diffraction takes place and a diffraction pattern is formed in the back focal plane of the objective lens. An aperture rejecting the diffracted intensity gives rise to the so-called diffraction contrast in the final image. A second contrast mechanism is electron interference (phase contrast), and this helps make possible the mapping of certain geometric shapes such as atomic steps. Regardless of the contrast mechanism, the images can be optimized by varying the kinetic energy of the input electrons. This is accomplished simply by changing the bias on the cathode (specimen). A LEEM can also observe LEED patterns by imaging the back focal plane of the objective lens, just as transmission electron diffraction patterns can be observed in a TEM. In LEEM, a magnetic deflecting field is needed to separate the incoming electron beam from the outgoing electron beam, resulting in a branched electron-optical system similar to that of one type of MEM. A LEEM can function in several modes, including LEEM, MEM, LEED and, with an appropriate light source, PEEM.

SPECIMEN

ANODE

EMITTING POINT

TO 06JECTIVE LENS

U

Fig. 1. (a) Trajectories of electrons emitted from a point on the axis showing the curved paths in the acceleratmg region and the diverging action of the aperture lens (radial distances have been exaggerated. the beam is much narrower than shown here). (b) Detail of the accelerating region showing a trajectory and a tangent ray defining the position of the virtual specimen at a distance I* from the anode. The components of the emission velocity ~1~are ;, and t, and the components of the velocity L>‘~ after acceleration are ;, and r,. The initial and final tangents make angles a, and a,. respectively, with the axis. (c) Electron optical equivalent for the case of a uniform accelerating field combined with the diverging aperture lens. The focal length of the aperture lens for low emission energies 15 fA = - 41. The virtual specimen is at a distance of 21 from the aperture lens. The aperture lens forms a virtual image of the virtual M ,A= 2/3. The ray angle after divergence specimen at a distance of t/ and magnification hy the aperture lens is a, = :a,,. From ref. [9].

O.H. Griffith,

Engel

/ Hutmeal

How emission microscopes, mirror microscopes, and low-energy electron microscopes form an image of a surface. In all of these instruments the images are formed in a parallel mode, in contrast to scanning electron microscopes where the information is gathered sequentially by scanning a finely focused beam. In parallel imaging, the specimen is illuminated by a broad beam (e.g., of light, electrons, or ions) and information is collected from all points in the field of view simultaneously. The electron trajectories are illustrated for an emission microscope in fig. 1. MEM and LEEM have the same specimen geometry as in emission microscopy, and the electrons leaving the specimen are accelerated and imaged in the same way. The electrons usually have low emission energies and are emitted in a wide range of directions, so they are first accelerated in order to collect the electrons into a small enough angle for imaging by an electron-optics system similar to that of a transmission electron microscope. (Problems with stray magnetic and electric fields are also greatly reduced with fast electrons.) If we ignore the effect of the small hole (aperture) in the anode for a moment, the field between the cathode and anode would be uniform. It has long been known that the electron trajectory of an electron in a uniform field is the arc of a parabola [l-3]. If the axial component of the initial velocity of the electron is zero, the vertex of the parabola coincides with the emitting point (for the trajectories of an electron in an accelerating field written in the standard form of a parabola see ref. [2]). The electrons appear to be coming from a point where the tangent intersects the axis of the parabola, and this defines the virtual specimen. It is a property of a parabola that the point of intersection of a tangent with the axis occurs at a distance behind the vertex equal to the distance of the point of tangency in front of it. Hence in fig. lb, for a uniform field, I* = 21 (if ;, = 0. The quantities ;, and z’, are the axial components of the initial emission velocity and the final velocity after acceleration, respectively. A more general expression for the location of the virtual object is /* = 21/(1

+ ;,/;,),

(1)

perspective und current trends

5

which approaches 21 as the ratio t&z’, approaches 0. Thus, the distance of the virtual object from the anode is approximately 21 since the initial kinetic energy of the electrons ejected by UV light in the threshold region is very small compared with the kinetic energy after acceleration. The dependence of I* on L’, in eq. (1) shows why the image formed by the accelerating field has chromatic and spherical aberrations. If all of the electrons were emitted with the same kinetic energy and at the same angle, the tangents to the parabolas would intersect at the same point. However, the tangents to the parabolas corresponding to electrons emitted with different kinetic energies do not intersect at exactly the same point on the axis, and neither do electrons emitted in different directions. The effect of the aperture at the terminus of the acceleration field is illustrated in fig. lc. It acts as a weak diverging lens. Although not as important as the accelerating field, the effect of the anode aperture is significant, for example, in magnification and resolution calculations. Literature on the physical 1 arumeters of emission microscopy and related techniques. The principal factors that determine the applicability and limitations of emission microscopy, and some useful references on these subjects, are: (1) lateral resolution, which depends on the kinetic energy distribution and angular distrbution of the emitted electrons, the magnitude of the accelerating field, the chromatic and spherical aberrations of the accelerating field, and the aberrations of the objective lens. For resolution in emission microscopy in general see refs. [1,3-91. These references are also relevant to MEM and LEEM calculations, but do not discuss these cases specifically. For MEM see refs. [lO,ll] and for LEEM see ref. [12]. (2) Topographic contrast is an important factor that is greatly enhanced in emission microscopy because the specimen is in a high electric field [2,13]. (3) Depth of field, the range of depth in the specimen that appears to be in focus, is treated for emission microscopy in refs. [14,15]. Two other important factors, (4) muterial contrast and (5) depth of information (i.e., depth resolution) are more difficult to generalize because they are dependent on both the specimen and the type of excitation. For reviews that discuss all of the above factors in

emission microscopy see refs. [16-181. For reviews of MEM see refs. [lO,ll] and for reviews that focus on LEEM see refs. [19,20].

standing emission

of current microscopy

developments and uses and related techniques.

of

3.1. The pioneer.s in Germunp 3. A brief history related techniques,

of emission microscopy MEM and LEEM

and the

References to early w0i.k on emission microscopy can be found in older books on electron microscopy [21l24], and a 1963 review by Mollenstedt and Lenz [25]. A bibliography at the back of the Proceedings of the First International Conference on Emission Electron Microscopy [26] gives an extensive list of work in this field, from the early work through 1979, including Balzers Literature Searches by Wegmann and the papers presented at that meeting. For a history of mirror electron microscopy see Bok, Le Poole, Roos and de Lang [ll] or Bok [27]. and the reviews by Oman [28] and Luk’yanov, Spivak and Gvozdover [lo]. Other lists of references on mirror electron microscopy can be found in refs. [29-311. The present brief tracing of the development of instruments utilizing the cathode lens (i.e.. immersion lens) is by no means a complete historical accounting. It is selective rather than comprehensive. In electron optics, where much of the information was gained in the 1930’s and 1940’s. it is not uncommon for one or more groups to rediscover designs that, if one looked carefully back through the literature. had been developed previously. Our goal is simply to provide a perspective for the developments in this field. especially the cathode lens and electron-beam-separating systems. and to present some information we know first-hand that is not available elsewhere. We encourage others to do the same. The following article by Mundschau [32] provides a current overview with a different purpose. Instead of stressing the development of instrumentation as we do here, Mundschau focuses on all of the possible combinations of excitation and emission, and their applications. In this introduction. we found it necessary to have some overlap with Mundschau’s excellent article in order to trace the instrument development. Our hope is that these two articles together will provide a better under-

The observation of electron emission from metal surfaces has an old and rich history. In 1878 Eugen Goldstein presented to the Berlin Academy his observations of images formed by “cathode rays” in an evacuated glass tube. This paper. published in 1880, described many experiments including forming an unfocused image of the surface features of a coin mounted as the cathode. The image was recorded on “light-sensitive materials” attached to the vessel wall [33]. It is interesting to note that this early observation predates, by almost twenty years. the discovery of the electron by J.J. Thompson in 1897. Later. Otto Wolff noted in his recollections that he observed the image of an emitting cathode in 1929 while adjusting the two coils of a cathode ray oscillograph [34]. There probably have been other occasional observations of emission images in Geissler’s or Braun’s tubes or cathode-ray oscilloscopes. However, it was not until the birth of electron optics and electron microscopy in the early 1930’s in Berlin that emission microscopy became possible as a systematic science. Many detailed descriptions of the early history of electron lenses and electron microscopy have been published [35-371. The focus of these reviews is generally on transmission electron microscopy. The best known of the early pioneers was Ernst Ruska, who shared the Nobel Prize in Physics in 1986 “for his fundamental work in electron optics and for the design of the first electron microscope” [38]. During these investigations of magnetic and electrostatic lenses very often images of the electron source, i.e. emission images. were also produced, reminiscent of Goldstein’s and Wolff’s early observations. Soon thereafter, in 1932, appeared the first reports of instruments built with the specific aim of producing magnified emission images of hot planar cathodes and investigating the emission and activation processes. One report was given by M. Knoll. F.G. Houtermans and W. Schulze [39], another by E. Briiche and H. Johannson [40.41].

0. H. Griffiith, W. Engel / Hisroricul perspective and current irends

The instrument of Knoll, Houtermans and Schulze was a two-stage microscope constructed with two magnetic lenses. A magnification of 150 times was achieved. It was the first time that iron-clad coils were used as lenses. They had been designed by E. Ruska [35] who worked in the group of Max Knoll in the high-voltage research laboratory at the Berlin Technische Hochschule. A detailed study of thermal cathodes had been performed with this instrument in the Physical Institute (directed by G. Hertz) at the Berlin Technische Hochschule. Brtiche and Johannson [41] developed their instrument in the AEG (Allgemeine ElektrizitatsGesellschaft) Research Institute in Berlin in a parallel effort. They used one electrostatic immersion lens with which a magnification of about 100 times could be obtained. This type of lens [42] is still in use in some of the present instruments. Briiche and Johannson also used their microscope for an investigation of hot cathodes [43], and they were able to produce excellent films of the activation process [44]. By 1933 and 1934 investigations in the field of metallurgy had been performed with this microscope [45,46]. These and other important contributions of the AEG group to the field of emission microscopy are reviewed in the book by Ramsauer [21]. In 1933, Briiche reported images of cathodes illuminated by UV light [47]. This work was extended by two of his colleagues, H. Mahl and J. Pohl, at AEG [48,49]. Brtiche made a sketch of his photoelectron emission microscope in his 1933 paper, and it is reproduced in fig. 2. This is evidently the first successful photoelectron emission microscope (PEEM). Fig. 3 shows some representative micrographs taken with an instrument of this type. We include fig. 2 here not just for its historical interest, but because it illustrates the basic design of a PEEM perhaps better than modern instruments with all of their refinements. Similarly, fig. 3 illustrates many of the factors that are important in emission microscopy today. Fig. 3a illustrates the sensitivity of PEEM to topographic contrast (the scratches) and surface contamination (the fingerprint). Fig. 3b is an optical micrograph of the same area as in fig. 3a. The bottom two figures illustrate the observation of crystal grains

t

Q

Fig. 2. An early photoelectron emission microscope of E. Briiche at AEG, Berlin, reproduced from his 1933 paper [47]. Light from a quartz mercury lamp (Q) was focused by a quartz lens (L) onto the cathode (Z, for zinc, the metal used in Briiche’s initial study). The emitted electrons were accelerated through a potential difference of from 10 to 30 kV between the cathode and the anode (R, for Rohr or tube) and focused onto the phosphor screen (S).

in a polycrystalline metal sample imaged by thermionic emission (fig. 3c) and by photoelectron emission (fig. 3d). In other micrographs of this series, Pohl illustrated how layers of gases on the surface could obscure the detail seen in (c) and (d). Hence it was clear from these early experiments that PEEM is a sensitive method of studying events occurring at surfaces. The origins of mirror electron microscopy (MEM) can also be traced to the early days of electron-optics in Germany. The acceleration stage and electron lens combination (i.e., cathode lens) is the same as in emission microscopy, so that development naturally came first. In MEM, the cathode does not emit electrons. A separate electron gun is supplied for this purpose. The accelerated electron beam approaches the cathode and is decelerated by the retarding field and then turned back before striking the surface. Calculations suggesting the feasibility of operating a cathode lens in a mirror mode were reported by Henneberg and Recknagel at the AEG Research Institute in Berlin in 1935 [50], and a MEM made of glass with a single magnetic lens and a magnetic deflector was reported by Hottenroth also at AEG in 1937 [51] (see also work performed by R. Orthuber at the AEG Research Institute in 1939, but published in 1948 [52]). However, MEM has been even slower to develop than emission microscopy. The additional complications of separat-

Fig 3. Early emiwon Top par:

Identification.

reported ix J. Pohl at AEG.

emission micrograph

(h) An optical micrograph

the micrographa 1040°C‘.

micrographs

(a) Photoelectron are at 7 x

(d) A photoelectron

magnification.

of an aluminum

Berlln. in 1934 using a microxwpz surface with tvx

of the same area of the aluminum Bottom

emission micrograph

pair:

(c) Thermlomc

of the same platinum

vertical xratchcs

vrr> Gmllar to that of fig. 2 [4X]. and one horizontal

specimen. Note the fingerprint t‘mislon

micrograph

wratch

for

visible in (a). In ;I and h

from ;I plntlnum

specimen after It had hecn cooled to 20°c‘.

surface heated

to

Both (c) and (d)

arc at 25 X magnification.

ing the incoming from the reflected electron beam and forming a focused image of the mirror surface in MEM and in LEEM (in which the electrons are allowed to strike the surface) are challenging. and they are still an important area of investigation today.

There was considerable activity in North America in the 1930’s, stimulated by the interest

in electron-optics for oscilloscopes, television. and by the electron microscopy developments in Berlin. V.K. Zworykin at the RCA Laboratories in Princeton, New Jersey. built his own version of a thermionic emission microscope consisting of ;I simple electrostatic two-cylinder lens [53]. At the Bell Telephone Laboratories Davisson and Calbick derived the equation that bears their names and is still used to calculate the diverging effects of the anode aperture in emission microscopy and other techniques that utilize the cathode lens. The results were published in two short notes [54.55].

0. H. Griffith, W. Engel / Historical perspectrve and current trend.7

Zworykin also demonstrated the possibility of utilizing low-energy electrons to form images. For this he added a crude electron gun to his emission microscope. The gun generated 450 V electrons at an angle of about 45 o to the specimen. The specimen was biased 20 V positive with respect to the electron source. Thus, the specimen was bombarded with 20 V electrons, and this experiment was the forerunner of modern LEEM. (One difference is that, in a modern LEEM, the electrons form a parallel beam which is normal to the surface.) The specimen was a cold specimen cathode. In one experiment it was a piece of carbonized nickel which had been scratched to remove the carbon, forming a letter “A”. The backscattered electrons, and perhaps some secondary electrons, returning from the specimen were accelerated and imaged by the electrostatic optics, and formed a bright letter “A” on the phosphor screen (Zworykin called all electrons returning from the specimen “secondary” but his meaning is somewhat different from today’s common usage). Zworykin described a second experiment in which the specimen was a sheet of mica with a coating of silver. In this case the low-energy electron microscope images of the scratches were poorly defined, and he concluded that this was due to charging on the insulating mica surface [53]. Zworykin presented these results along with a discussion of electron-optics, at an Optical Society of America Meeting in February of 1933 and published them the same year [53], later summarizing the work in the book “Electron Optics and the Electron Microscope” by the RCA group

WI. In Toronto, in 1936, Cecil E. Hall imaged heated cathodes in an experimental verification of the Briiche and Johannson experiments (p. 196 of ref. [23]). (At about this time, as a footnote of personal interest, our colleague Gertrude F. Rempfer began her work on electron emission as a graduate student in Seattle in 1935. Although not involved in imaging at this point. her thesis was a study of energy loss accompanying thermionic and field emission [56].) In 1937 McMillen and Scott constructed a thermionic emission microscope at Washington University, St. Louis [57], using a single magnetic lens patterned after one described

9

by Knoll and Ruska in 1932 [58]. It was used to determine the calcium and magnesium distributions (i.e., the sum of Ca and Mg, but not the individual ions) in biological tissues. This was done by freezing, dehydrating, and incinerating the specimens, heating the remaining mineral deposits, and observing the emission patterns. The specimens were treated with barium and strontium carbonates to enhance the emission of the Ca and Mg salts. The barium and strontium carbonate treatment and the final imaging were similar to techniques used in the early thermionic emission experiments on metals. Intact biological specimens were not viewed directly. Nevertheless, this is an interesting early example of analytical electron microscopy, and the effort was directed at a goal that is still an active area of investigation today [59,60]. In 1937, D.B. Langmuir at the RCA Laboratories, New Jersey, published a useful theoretical study of the resolution of the acceleration stage of emission microscopes in addition to a broader study of cathode-ray tubes [3]. At about the same time (1936) in Japan, E. Sugata constructed an emission microscope, according to the historical account of K. Kanaya [61]. 3.3. The 1940’S through the early 1960’s 3.3.1. Emission microscopes in Europe und the USSR (I 940 - ear!)> 1960’s) In the 1940’s and 1950’s very few emission microscopes were built compared to the number of other types of electron microscopes put into service during this period. Many of the advances were taken from successive generations of transmission electron microscopes, since the available resources and effort were largely devoted to TEM. The first attempt at a commercial emission microscope was evidently by Mecklenburg in the early 1940’s, based on an instrument he developed in 1942, with a claimed resolution of 50 nm [62] (see also p. 1 of ref. [26]). The timing of this project could not have been worse, as the development was stopped by World War II. After the war, a commercial two-lens thermionic emission microscope was built at Philips, Eindhoven, in 1947 by H.J. De Heer in collaboration with J.B. Le Poole at the Technical University of Delft. This instru-

mt‘nt had an operating resolution of 100 nm and Jr-eu freeI>, upon the design of ;I commercial f’hilips TI3M. the “EM 75 kV” (refs. [h3.64] and f_e Poole. personal communication). A cross-sect~onal drakving of the Philips instrument is reproduced on p. 324 of ref. [25]. The emission microscopes built in the 1950’s and early 1960’s are revielved in refs. [24.25]. We will only briefly mention some of them here. In Paris. a thermionic emission microscope with a r.chc,lution of 100 nm was conceived b> .I. Vastel and P. Grivet. and improved by A. Septier and M. (;auzit. The early results from this laboratory wrc reported in 1950 at the First International (‘onI‘erenct’ on Electron Microscopy in Paris [65]. Sqtier madt: a detailed study of the optical propertic> of the electrostatic immersion objective. and examined the influence of heating treatments up to 7500°C on pure refractory metals (Nb. Ta. W) and on Ba-acti\,ated metal surfaces at much lower temperatures. Descriptions of the apparatux and the results are given in Septier’s thesis [66]. The resolution of this instrument \vas essentialI> limited h> vibrations. to between 100 and 200 nm ( 4. Sttptier. personal communication). A photograph and ;I cross-sectional \iev of the immersion IN\ of this microscope appear in the book ” Elrctron Optic,” h! Grivt’t [24]. This microscope utilized an electrostatic objective lens and u+as deigned *‘to be conwnient to use. and that changing of the specimen should he rapid” [24]. Some secondary electron images produced by ion bombardment of the cathode were obtained mith this instrument in 1953 [66.67]. A PEEM was dccelopzd in Septier’s group by EL. Huguenin in 19461955 [6X.69]. Ref. [24] contains. in chapter 13 entitled “The Emission Microscope”. a brief description of the original instrument. including a low-energ! ion gun used for cleaning the sample xurf’ace .just before switching on the UV light. Images v.ere given by a low-magnification immerhion ohjectiw followed by a large-bore three-electrode projection lens. The final resolution \vas in the micrometer range. Pictures obtained with copper and silver bulk polycrystalline samples are reported in ref. 1241. showing a high crystalline contrast.

The

Institut

fiir Angewandte

Physik.

under

G.

Mtillenstedt’s direction. at the Llniversity of Tiibingen \vas the birthplace of several emission microscopes. Miillenstedt and Diiker designed an emission niicr~wwpe in 1953 [70]. and improvements arc described in subsequent papers 171.721. .A unique feature \~;Is the introductic)n of an ion ~LII~. thus adding another option in emission microscopy in addition to thermionic emission. (Some earlier instruments used ions from a gas discharge to strike the specimen. but the image quality was limited by collisions between the emitted electrons and gas molecules: the separate ion ~LIII eliminated this problem by improving the c’acu~~nl in the microscope column.) In the MKllenstedt and Diiker microscope the inn beam was emitted through an aperture. striking the cathode surface at an oblique angle of incidence. and producing electron emission (sometimes referred to LIP secondary electron emission). Direct magnifications 01’ 500 X . 1200 x and 2000 X wwe obtained on the photographic plate. With a 35 pm aperture a resolution of 50 nm was achieved [71]. This instrument has not equipped for photoemission studies. f 1. Bethgc and co\vorkers at Hallc built an emission microscope L\ith a UV light source and an ion gun in parallel Cth the Ttibingen effort and described their v,ork at the Fourth International Conference on Electron Microscopy in 195X [73]. An improved commercial version of the Mollcnhtedt and Diiker emission microscope with an electrostatic objective lens and a double magnetic projector Icns was marketed in the early 1960s under the name “Metioskop” bv L. Wcgmann at Triib, Tiiuber in Ziirich. and then briefly b> Balzerx Ltd. Liechtcnstcin after B&ers purchased Triib. TBuber. A cross-sectional diagram of the Metioskop is given in ref. [25]. The Bal/ers specification sheet lists ;I limit of rcwlution of 50-60 nm for the ” Metioskop KE2”. It was equipped for both thermionic emission and ion-induced emission. but not photoelectron emission. Another type of emission microscope huilt at I‘iibingen was an X-ray image converter microscope (i.e.. an X-ray shadow image made \.isihk hy converting the X-rays to electrons). This instrument was described by Lan Yu Huang in 1957 1741. X-rays were generated hy an electron heam

0.

H. Griffrrh, W.

Engel / H~storrcul per.ywcr~oe und currenr

striking a slanted metal target located above the specimen. The specimen was mounted on a thin structureless silver foil and the X-rays passing through the foil generated secondary electrons. These electrons were accelerated and imaged in the emission microscope. High-contrast images of diatoms (the mineral remains of a type of algae), including a pair of stereoscopic images, were produced in this way [74]. The resolution attained was about 250 nm. A stereo pair of images obtained by tilting the specimen by 3” in an emission microscope is also shown on p. 310 of the review [25]. At Ttibingen in the late 1950’s and early 1960’s. W. Koch built two photoelectron emission microscopes (at the time referred to within the Institute as “PEM 1” and “PEM 2”) [75,76]. Koch shaped the electrodes of the electrostatic immersion lens in an unusual cone-like arrangement in order to achieve direct illumination of the specimen by UV light at large angles of incidence (i.e., 60” for the center ray). With this geometry Koch was able to employ an unusually wide range of UV excitation sources, including a very-short-wavelength hydrogen lamp used in combination with a thin LiF window on the microscope. As in earlier emission microscopes. the electrostatic cathode lens contained a Wehnelt or control electrode, so the electric field at the specimen was not maximized. Nevertheless. electron-optical magnifications of from 200 x to 800 x and a resolution of 100 nm was achieved. The second instrument designed by Koch incorporated the same electron-optics and UV-excitation system of the first instrument. In addition, it had a completely redesigned vacuum pumping system with liquid air traps that made possible an impressive lo-’ Torr in the specimen area [76]. This instrument was used for several including one by Hofmeister and studies, Schwarzer on the effects of polarized UV light [ 17,771. Recently, this instrument was improved further by the addition of a liquid-helium-cooled stage (the only one we are aware of) [78]. It was used for measurements of mean free paths of slow electrons in thin metal films. with a technique called metal-insulator-metal (MIM) cathodes [78] (see also p. 101 of ref. [26]). One key to improving resolution in emission microscopy and other techniques that employ the

trendc

11

cathode lens is to maximize the electric field at the specimen surface. The improvement is achieved mainly by separating the accelerating field and imaging field. This requires removing any intermediate electrodes (e.g., Wehnelt) between the cathode and anode. The electron-optics then becomes the acceleration region (the cathode-anode gap) followed by a magnetic or electrostatic objective lens. Septier [67] discussed the use of this arrangement with an electrostatic objective lens in 1954, although he experienced difficulties in translating this idea into a working instrument. This same year M. Keller presented a design for a cathode-anode gap followed by a magnetic objective lens in his Diplomarbeit [79]. Keller’s thesis presents some results using an instrument incorporating this magnetic immersion objective lens, although this work was not published elsewhere. C. Fert and R. Simon. at the Laboratoire d’Optique Electronique in Toulouse, built an emission microscope in the late 1950’s that omitted the Wehnelt electrode and thereby increased the electric field at the cathode surface. This instrument utilized a magnetic objective lens and a magnetic projector lens and was equipped with an ion gun. It achieved a resolution on the order of 25 to 30 nm. A cross-sectional diagram of the upper part of this instrument is reproduced in refs. [25,80]. Fert. Simon, Fagot, Pradal and others used this emission microscope to investigate the origin of contrast in ion-induced secondary electron emission from metallic specimens (including oriented crystals), as described in a series of papers from 1956 to 1966. [80-841. Using the same concept to maximize the electric field, Dtiker and Illenberger designed a cathode lens consisting of an accelerating field followed by an electrostatic (einzel) lens [85]. These developments are of historical interest because they are early examples of one of the principles used in modern high-resolution instrument designs (see discussion of the Engel and the Rempfer et al. photoelectron emission microscopes below). In the USSR, G.V. Spivak and his colleagues in Moscow and others built relatively simple emission microscopes for the study of metals (including surfaces coated with Sb-Cs) during this same period, and obtained images by photoemission

and by thermionic emission [86&89]. Additional references on these and other research efforts are given in the bibliography of ref. [26].

A simple thermionic emission microscope similar to that of McMillen and Scott [57] was built at the University of Oregon in 1946 and used to demonstrate thermionic emission images of heated oxide-coated cathodes [90]. In the 1950’s R.D. Heidenreich built a thermionic emission microscope at the Bell Telephone Laboratories [91]. This instrument, patterned after that of Mecklenburg [62], utilized an electrostatic immersion objective lens and a double unipotential projection lens. It was used in the study of transformations in carbon steels [92]. Another emission microscope was built at the North American Philips (Norelco) laboratories in Mt. Vernon, New York. and used from about 1954 to 1958. This development was inapired by the earlier emission microscopes built at Philips in the Netherlands (J.B. Le Poole. personal communication; see also refs. [63.64]). In the early to mid 1960’s. some of the Balzerh Metioskop KE2 thermionic emission microscopes were brought to government and industrial laboratories in the USA. Metioskop KE2 instruments were purchased or shared by NASA -Lewis in Cleveland. Ohio: Monsanto Research Corporation. Miamisburgh, Ohio; Battelle Memorial lnstitute. Columbus. Ohio: North American Rockwell. Lo:, Angeles: General Motors Corporation, Indianapolis: and NASA-Goddard. Greenbelt. Marvland.

When work in this field resumed following World War II. MEM designs were of two types. as illustrated in fig. 4. One type was a linear optics system (fig. 4a). This design avoided some of the difficulties caused by the astigmatism of the magnetic deflection field of the earliest mirror microscopes. which were more like fig. 4b. (Hottenroth’s 1937 microscope. for example. was similar to fig. 4b in that it had a magnetic beam separating system.) In its simplest form the linear design consisted of a single straight tube in which the

H

4

82

EO

M

-b

illuminating beam \\as directed toward the mirror through a small hole in the viewing screen. The anode aperture of the mirror acted as a beak lens. diverging the electron beam both on entering and on leaving the mirror unless followed by an electron Icns. The reflected beam spread out over the viewing screen. The electrons were reflected just in front of the specimen (mirror) surface. Specimen topography. or variations in potential or magnetic fields on the surface, gave rise to contrast in the image by deflecting electrons away from the direction they would have taken from a perfect plane surface (i.e.. the specular reflection direction). A perfectly plane and uniform surface produces no contrast. The image was thus a type of Schlieren-

0. H. Griffirh, W. Engel / Hisforicul perspeciioe and current trends

shadow representation of the specimen surface. In some instruments the addition of an electron lens in front of the mirror (at EO in fig. 4a) made it possible to focus the beam incident on the specimen so that it was parallel to the axis. This arrangement is appropriate for viewing low-energy electron diffraction patterns, as noted more recently by Delong and DrahoS [93], with the specimen biased positively rather than negatively with respect to the electron source. However, without a separating system a focused image of the specimen surface was not viewable, as the reflected beam returned through the opening in the center of the viewing screen. Although the straight design was attractive in its symmetry and simplicity, the beam could not be optimized simultaneously for the incident and returning beams. Eventually, interest in the straight design waned for most applications in favor of improving the second type (fig. 4b). In this second type of MEM, a magnetic field was used to separate the path of the reflected beam from that of the incident beam. At this stage of MEM development there were inadequate optics to form a focused (Gaussian) image of the mirror, so the imaging yielded only shadow images, as mentioned above (e.g., shadow projection mirror images or Schlieren patterns). L. Mayer, at the Wright Air Development Center, Wright-Patterson Air Force Base in Ohio, and later at the General Mills Corporation, Minneapolis, Minnesota, constructed a series of instruments of two general types he illustrated in fig. 4, and wrote a series of papers on MEM from 1952 to about 1962 that examined many types of specimens and provided insights into the origins of contrast [94-991. In addition to metallic specimens, Mayer, for example, was able to observe the 2.4 nm steps in successive layers of barium stearate, which showed the sensitivity of MEM to topography and provided the first evidence that MEM might be used to observe organic and biological specimens [99] (for observations on biological specimens see also ref. [28]). In his 1961 review, Mayer writes of observing real-time events such as “wave-like motion” of “electric charge movements” on a selenium surface. He recorded the contrast waves as a 16 mm motion picture and predicted that MEM would be particularly useful

13

for time-dependent surface effects. He also pointed out the importance of improving the vacuum system which was at best 10eh Torr in this generation of instruments [99]. In Moscow, G.V. Spivak built a MEM and published the first MEM observations of surface magnetic fields (e.g., magnetic contrast) in 1955 [loo]. Shortly thereafter Mayer observed periodic patterns associated with recordings on magnetic tapes [97]. In 1956 D. Wiskott at Ernst Leitz, Wetzlar, contributed two articles on the theory of MEM [101,102], and in 1957 Bartz and Weissenberg, also at Ernst Leitz, reported in a short note a MEM study of p-n junctions on a silicon semiconductor surface as a function of voltage [103]. The visual observations of contact potentials, surface charges, electrical conductivities, magnetic domain patterns, and surface relief structures by this small group working with relatively simple instruments rekindled interest in MEM, in spite of the modest resolution (e.g., 100 nm at best). The continuation of these developments in the 1960’s is discussed in section 3.4.4 below. Additional references can be found in the reviews by Oman [28] and Luk’yanov. Spivak, and Gvozdover [lo]. By adjusting the voltage on the mirror, a fraction of the faster-moving electrons was sometimes allowed to strike the specimen in order to enhance contrast in films of limited conductivity [99]. However, without a good way to form an image with the backscattered electrons, if the beam as a whole was allowed to strike the surface, an effect aptly named “washout” was produced, so LEEM was not technically feasible in these simple mirror electron microscopes. 3.4. Emission microscopy from 1960 to 1980

and related

techniques

3.4.1. Emission microscopes in Europe and the USSR (1960-1980) In the 1960’s there were interesting developments in many nations. There were several instituteor research-laboratory-built instruments developed during this period (i.e., one-of-a-kind instruments built in academic or industrial laboratories, as distinguished from commercial instruments produced in quantity). Dr. E. Bas and col-

leagues at the Institut fur Technische Physik, ETH Zurich, built a versatile electrostatic emission microscope. equipped with a high-temperature oilcooled goniometric stage. a 60 kV ion source to produce electron emission by ion impact, a quartz window for photoelectron emission. and a 6 kV electron gun to produce secondary electron emission [104.105]. The instrument was not designed for ultrahigh vacuum (UHV) applications. but was used as a complementary tool for surface analytical studies with Auger electron microscopy. LEED. thermal denorption spectrometry. ion scattering spectrometry and work function analysis (see p. 153 in ref. [26] and also ref. [106]). At Carl Zeiss in Jena in the early 1960’s. Ernst-Adolf Soa designed and built an emission microscope called the “EF/6”. EF being a designation for a series of electron-optics instruments. This instrument was designed around a magnetic objective lens and was equipped for thermionic emission and ion-induced electron emission. This instrument also featured an electromagnetic stigmator with eight coils. Soa stated that it was possible to resolve lines with a separation of 10 nm in the thermionic emission mode and 15 nm in ion-induced emission mode with this instrument. an impressive achievement at that time [107.108]. The Technical University of Dresden became a center for the development of emission microscopy. A. Recknagel, who had earlier made significant contributions to the theory of resolution of emission microscopy and mirror microscopy [1,50] while at the laboratory of Briiche at AEG, Berlin (193441945) and had developed electron microscopes at C. Zeiss, Jena (19455 1948) became a professor at Dresden (194881975). In Dresden, Recknagel and his colleagues built a versatile microscope that combined emission microscopy at moderate resolution with low- and high-energy electron diffraction. and Auger electron spectroscopy. This was accomplished by incorporating a standard LEED system and a separate electron source for the high-energy electron diffraction (as in TEM), together with an immersion lens and projector lens for emission microscopy, into the same vacuum chamber (i.e.. the LEED was a separate unit, and not a selected-area LEED using the immersion lens). This microscope

is described in a 1972 paper [109], and represents a step forward in combining emission microscopy with other surface analytical techniques. as vvell as in situ specimen preparation in a clean vacuum environment. F. Hasselbach developed at Tiibingen a combination of a scanning electron microscope and an electron emission microscope in the early 1970s. adding to the growing list of contributions to this field by the Institut fur Angewandte Physik [110]. Two different geometries were included in the design, one for the measurement of secondary electron distributions induced by a transmitted electron beam. and the other for the measurement of reflected (backscattered) electron distributions. This type of approach illustrates an important difference between image formation in emission microscopy and in SEM. In SEM a spectrum of secondary electrons is collected for each point of the scanning beam. The detector may average together electrons from a sharply defined area under the electron probe and electrons from a wider bloom area produced by electron scattering. In emission microscopy. instead of an average current being detected, a Gaussian image of the emitting points is formed. Thus, emission microscopy can be used to provide a better understanding of the lateral widening of electron beams due to scattering in thin films of metals. and to study other processes that are important in determining contrast and resolution in SEM [ill]. Yet another application of the immersion lens and emission microscopy. that of ion emission microscopy, is reviewed by Grivet and Septier [ 1121 and by Castaing and Slodzian [113] (see also ref. [321).

In the Institute of Electron Microscopy at the Fritz-Haber-Institut in Berlin (directed by E. Ruska) one of us (W. Engel), as a graduate student, developed a photoelectron emission microscope with which a point-to-point resolution of 12 nm could be obtained [7,114]. This microscope also allowed thermal emission and secondary electron emission induced by a beam of fast neutral particles. The two keys for the improvement of photoelectron emission microscopy were ( 1) an optimized magnetic cathode lens with stigmator and (2) a considerably improved UV-illumination

O.H. Griffith.

W. Engel

/ Hlstorrcal perspective

system giving sufficient intensity for high magnification. The first results were reported in 1964 at a Joint Swiss-German Conference on Electron Microscopy in Zurich. At that conference one of us (W. Engel) met L. Wegmann for the first time. Wegmann was very interested in the results and in the microscope design. Somewhat later Wegmann arranged with E. Ruska to build a commercial version at Balzers, the Metioskop KE3.

3.4.2. The Balzers Metioskop KE3 emission microscope Approximately eleven Balzers Metioskop KE3 emission microscopes were sold in the early 1970’s before production was discontinued about 1975. A photograph and cross-sectional diagram of the Metioskop is published in the review by Wegmann [16]. As a historical note, there were four KE3s located in Germany (Tubingen, Karlsruhe, Aathen, and Mtinster), three in Switzerland (Neuchatel, Zurich, and Wegmann’s instrument at Triibbach), and one each in France (St. Martin d’Heres), the UK (Leeds), Brazil (Campinas), and the USSR (Moscow). None were sold in North America or Asia. The Balzers KE3 was aimed primarily at the metallurgical market, hence the name Metioskop. For the first time it brought emission microscopy to a larger community of metallurgists and other physicists, and a flurry of papers resulted from the introduction of this new instrument. Unfortunately for the Balzers KE3, its popularity was short-lived, although the Tiibingen microscope was modified for UHV in the specimen area, as indicated in a 1985 abstract [115], and the Leeds instrument is still in service [116]. About the time the Balzers KE3 was introduced, the first successful SEM became available from Cambridge Instruments. The Metioskop KE3 was a technical achievement, but the market rapidly changed soon after it was introduced. Metallurgists are more concerned with analytical capabilities and a broader range of specimen geometries, as provided by SEM, than they are with surface imaging. The other potential market, the community of surface scientists. requires ultrahigh vacuum, and the pumping system of the KE3 resembled those of

und current trends

15

commercial TEMs of the 1970’s, and was not a UHV design. Nevertheless, the Balzers KE3 had a substantial impact. This can be seen by looking over publications in emission microscopy during the period of 1968, when papers using the Metioskop KE3 first appeared, through 1979, the year the First International Conference on Emission Microscopy was held. During these twelve years there were a total of about 326 papers on emission microscopy. and of these studies, 190 or 60% were performed using the Balzers KE3 (the percentage is even higher when theoretical papers and reviews are deleted from the total). It is not possible to review all of these applications here. We refer the interested reader to the bibliography of the First International Conference on Emission Electron Microscopy [26]. Some of the highlights are covered in review articles by Wegmann [16], Schwarzer [17] and Griffith and Rempfer [18]. We also note that L. Wegmann (Lieni Wegmann, 191881986) and Balzers. in addition to making available the KE2 and KE3 instruments, also performed an important service by encouraging the exchange of information on emission microscopy through personal communications and through publications of the Balzers Literature Search Service. 3.4.3. Emission microscopes in North America, Japan, and China (1960- 1980) Gertrude F. Rempfer, who had been working in electron optics and TEM, turned her attention to emission microscopy in the 1960’s. She designed the electron optics for an early ultrahigh-vacuum (UHV) PEEM in 1963. Rempfer was involved in designing two instruments at the same time, one PEEM and one TEM. Both were built by Mr. Gil Burroughs at the Night Vision Laboratory, Fort Belvoir, Virginia, about 1965. The PEEM and TEM were made with bakeable electrostatic lenses (e.g., stainless steel electrodes and ceramic insulators) and metal-sealed valves. These instruments were designed as modular vertical attachments for UHV work chambers, evacuated by ion pumps, and used in research on cesium-based semiconductor photocathodes for image intensifiers (night vision devices). In the PEEM, as well as the TEM, the specimens were grounded and could be trans-

ferred in the UHV environment to several positions for photocathode formation, processing. and observation. These electron microscopes were used for only a brief period of time, but the components live on. The electron lenses and voltage divider of the PEEM were sent to Oregon around 1970 and were incorporated into one version of a PEEM for biological studies in Eugene. The TEM was also shipped to Oregon. Later it was modified by Gail Massey into a PEEM, a clean but modest-resolution instrument that is still in service in his laboratory at San Diego State University. In 1968 work on the photoelectric behavior of biological specimens began at the University of Oregon, at Eugene. We (Griffith and colleagues) were not aware of photoelectron emission microscopy at the time. We arrived at it by asking: What is the electron-optical analog of fluorescence microscopy (i.e.. microscopy? Fluorescence photo-induced light emission) was, and continues to be. the most useful form of optical microscopy in molecular and cell biology. In collaboration with Gertrude F. Rempfer. we constructed a twolens low-magnification UHV PEEM in 1970 (we informally call it “PEM 1” and the second-genera“PEM 2”. as were Koch’s emistion instrument, sion microscopes referred to in Tubingen, unknown to us at the time). The first photoelectron micrographs of organic and biological specimens were obtained in 1971 and published in 1972 [117]. Those of us involved recall the excitement when the UV lamp was first switched on and an image of the organic test pattern showed up faintly on the phosphor screen at 35 x We were just as relieved when the image disappeared instantly as the UV light was blocked. And, we were lucky to have seen an image at all. It was discovered later that day that one of the UV lenses had flipped over during evacuation of the microscope, and was lying on its side, so that only stray UV light was reaching the specimen! The initial biological specimen, sectioned rat epididymis, also produced an image on the first attempt. Whereas we had worried that most organic and biological materials might not emit sufficient electrons to be imaged, almost all organic and biological specimens produced an image of some kind. This PEEM for biological studies in Oregon

was, in many ways, a rather sophisticated instrument for 1970. This instrument was entirely UHV-compatible, with bakeable electron lenses (stainless steel electrodes and ceramic insulators. as in the Night Vision Laboratory PEEM). metalsealed UHV valves. copper-sealed Conflat flanges. an ion pump, and molecular sieve sorb pumps. It was equipped with a Varian microchannel plate image intensifier and a liquid-nitrogen cold stage. The UV source was a hydrogen lamp. with magnesium fluoride optics to permit a wide range of UV excitation. The PEEM was entirely enclosed in an oven for bake-out. Many 400°C bake-outs were performed, including one runaway bake-out caused by a heater thermostat malfunction that brought part of the PEEM (“PEM 2”) to an alarming dull red glow (a truly photo-emitting electron microscope!). Soon thereafter it was decided that high-temperature bake-outs were unfor organic and necessary, and even inadvisable biological specimens. The other features (far-UV light source and cold stage) were useful for the model studies that were carried out with this instrument between 1971 and 1981, when it was disassembled. Meanwhile work had begun on a second-generation PEEM about 1975. this time for maximum resolution and higher magnifications. This second instrument, after several successive design changes, is probably the highest resolution PEEM in operation as of this writing, achieving a resolution within a factor of two of the design limit of 5 nm. It is described elsewhere in this volume [118]. Another effort involving emission microscopy took place at the General Motors Research Laboratories in Warren, Michigan. Here, one of the Philips thermionic emission microscopes built in Eindhoven [63] was purchased and operated in the thermionic mode by S.R. Rouze and W.L. Grube to study phase transformations in steels in the late 1950’s and 1960’s (see note by Baxter and Rouze in ref. [26]). In the early 1970’s. Baxter and Rouse modified this instrument for UV photoelectron emission microscopy [119]. They used it for a series of studies of the surfaces of stressed metals (e.g., deformation-induced rupture of thin surface oxide films on metals) throughout the 1970’s [120,121].

O.H. Gnffith,

W. Engel

/ Historical perspectwe

In Japan, in the early 1960’s, an emission electron microscope effort was organized in the Faculty of Engineering, Department of Electronics, Nagoya University. Y. Uchikawa, M. Kojima, M. Ichihashi and S. Maruse described an ionbombardment-type emission microscope built by this group and used for surface science applications [122]. It featured an ion source based on a high-frequency gas discharge which produced a stable, finely focused ion beam. Y. Uchikawa and S. Maruse also performed calculations on the resolving power of the cathode lens [123]. See also p. 155 of ref. [26] for brief comments by Dr. Yoshiki Uchikawa and a list of 10 references of the Nagoya group spanning the years 1963 to 1970. In China, an emission electron microscope was developed by Lanyou Huang in 1963. This instrument apparently still exists and is located in the Sixth Research Laboratory, Institute of Electronics, Academica Sinica, Beijing (Junen Yao, personal communication). 3.4.4.

Mirror

electron

microscopes

and

LEEM

(1960-1980)

Several different developments took place during this period. First, there was a continuing interest in the design and applications of classical mirror electron microscopes of the two simple types diagrammed in fig. 4. Second, work began on the development of MEMs with the capability of forming a focused image of the specimen. Third, MEM-LEED (selected-area LEED using a conventional emission microscope or MEM) was achieved. Fourth was the beginnings of LEEM. We will discuss the developments in this order. However, much of the work was done in parallel, and oftentimes independently of the other efforts, so the order and classification of these advances is somewhat arbitrary. L. Mayer at General Mills, Inc., in Minneapolis continued his work on mirror electron microscopy into the early 1960’s [99] and designed a MEM with the cathode-specimen grounded and the column at high positive potential [124]. G.F. Rempfer designed and studied the properties of mirror electron microscopes in the 1960’s. One of her mirror microscopes, built about 1962 at Tektronix, Inc., in Beaverton, Oregon, was of the straight

and current trends

17

type, with no beam separator. At Tektronix she also designed a cathode ray oscilloscope with an electron mirror to fold back the beam. At Portland State University she supervised student theses on the properties of a plane electron mirror [125] and a hyperbolic mirror [126]. In Delft in 1964, Bok and Le Poole designed a scanning MEM with magnetic quadrupoles [ll]. In 1960, H. Bethge, J. Hellgardt and J. Heydenreich reported a MEM without an objective lens (i.e., of the type as in fig. 4a) constructed as early as in the late 1950’s and used for the imaging of electrical surface inhomogeneities [127]. During the 1960’s, H. Bethge and J. Heydenreich built a MEM equipped with a magnetic prism [128] which was used primarily for investigating conductivity inhomogeneities in insulating layers [128]. From about 1960 to 1970 several mirror microscopes were built by C. Guittard and his colleagues at the Universite de Lyon, and some are still in use in French laboratories [29,129,130]. These were relatively simple microscopes of the general type diagrammed in fig. 4b. They utilized a cathode lens, described as an electrostatic Johannson lens, after the early German microscopes designed at the AEG in Berlin. There were no intermediate or projector lenses, and usually no focusing lens following the mirror. Although the magnification is limited, the advantage of a simple MEM is that it can be combined with other surface-analysis techniques in a work station. An interesting example is the integration of a MEM of this general design into a multipurpose UHV apparatus by R. Pantel, M. Bujor and J. Bardolle at Orleans [131,132]. Besides MEM, this instrument had a conventional LEED, an Auger spectrometer, an ion gun, and a manipulator in the center that permitted the rotation of the specimen from one analytical station to the next. G.V. Spivak, A.E. Luk’yanov and their colleagues at Moscow State University continued their work in mirror electron microscopy in the 1960’s. Several straight-type mirror electron microscopes were designed [133,134] and a study of the geometric optics of an electron mirror microscope based on a two-electrode immersion objective lens was published in 1968 [135]. One of the trends in the field during the 1960’s was the utili-

zation of advances made in emission microscopy to improve the straight MEM design. Several groups designed instruments that could function as either a MEM or an emission microscope. Barnett and Nixon built such a microscope at the University of Cambridge in the mid 1960’s, although it was evidently operated primarily in the MEM mode [136-1381. Barnett and Nixon omitted the Wehnelt electrode, treating the cathode lens as an accelerating field followed by a focusing field of an objective lens. Partly because it was built so that it could be easily converted into an emission microscope. this MEM had an objective lens. as well as two projector lenses, all magnetic. At the bottom of the column was added the electron gun and condenser lenses. As is characteristic of the straight design MEM, the electrons from the gun passed through a small hole in the center of the viewing screen. up through the projector lenses and objective lens to the electron mirror (specimen). At the mirror, the electrons reversed their direction and passed back through the objective. projectors. and either struck the phosphor viewing screen or were lost through the hole in the viewing screen. Barnett and Nixon noted that to form a truly focused (point-to-point) image would result in an almost zero field of view. That is, when the image is really in focus, the electrons would reach the screen nearly on axis and would pass right through the hole in the screen. This is a fundamental limitation of a straight MEM design, since without a beam-separating system the incident and reflected electrons must traverse the same optical system. The answer in this microscope was to defocus the image somewhat, resulting in a shadow image. Nevertheless, this type of instrument optimized the straight design of fig. 4a, and it was a step toward a versatile instrument with MEM and emission microscope capabilities. A similar instrument, but with somewhat different optics, was produced as a commercial MEM (the JEM-Ml) by Japan Electron Optics Laboratory Co. in Tokyo as early as 1968 [139.140]. Another MEM with the electron-optics of an emission microscope was built at the Moscow State University [lo]. Interestingly, a similar MEM-emission microscope design was used by A. Delong, V. DrahoS. V. Kolaiik and others at the Czechoslovak

Academy of Sciences in Brno to obtain the first MEM-LEED, as discussed below. Delong and DrahoS also combined MEM with several emission microscope capabilities. including thermionic emission and PEEM [10.141]. The extensive review by Luk’yanov. Spivak. and Gvozdover [lo] discusses additional designs. modes of operation. theory, and applications of mirror electron microscopes. and includes many diagrams. To summarize a few highlights. E. lgraa and T. Warmihski at the Polish Academy of Sciences designed a straight-type MEM for aemiconductor studies that featured a specimen temperature range of 77 to 1300 K [142]. Igras and Warminski also designed a MEM with a beam separator that provided a magnification of LIP to 2000 X and a resolution of about 200 nm [lo]. This instrument was evidently designed for mass production and was available from the Polish Optical Plant PZO, Warsaw. In the USSR, a multipurposc MEM-remission microscope was built based on the design of their EM-7 transmission electron microscope. The authors claim a recordsetting resolution of 80 nm for this MEM using a gold film specimen [10,143]. The second development in the field of MEM was the increased emphasis of forming a focused image. rather than a shadow image. For example. in the 1971 review by the Philips and Delft groups [ll] it is mentioned that Le Poole noted in 1964. in the proceedings of a conference in Cambridge. how a MEM could be improved considerably by forming a focused image of the mirror surface onto the fluorescent screen (see also related comments by others in refs. [ 10.136]). It also became increasingly evident to several investigators in this field that mirror electron microscopes require. in addition to an objective lens. a better means of separating the illuminating and reflected beams in order to obtain a focused image with a suitable field of view. Of particular interest in this informative review [ll], and in other papers by Bok [27], is the description of an advanced MEM designed for focused images. Bok’s microscope, built in the 1960’s, used a magnetic prism with a minimum deflection angle to separate the incident and reflected beams [144]. Three additional “bridge-deflectors” made the beam coincide again

O.H. Griffith, W. Engel / Historrcal perspective and current wends

with the main axis of the vertical microscope. The imaging system consisted of the objective lens, intermediate lens and projector lens, allowing a continuously variable magnification of 250 X to 4000 x Bok was a student of J.B. Le Poole, a pioneer in electron diffraction in TEM, at the Technical University of Delft. Bok discusses the possibility of “obtaining secondary emission images by bombarding the specimen with low-energy electrons (in the order of tens of electron volts)” and performing LEED experiments in his thesis [144] and other publications [11,27]. However, progress was hampered by alignment problems and the moderate vacuum in this otherwise sophisticated instrument. Thus, even though this instrument produced excellent mirror micrographs, no LEEM or LEED results were obtained. A simpler instrument designed to produce focused MEM images was reported by Schwartze at the Friedrich-Schiller-Universitat, Jena in 1967 [145]. Schwartze’s MEM was a two-lens instrument. It had a magnetic objective lens and one magnetic projector lens and a magnetic prism to separate the incoming and reflected beams. Schwartze also succeeded in obtaining shadow images and focused, or partially focused, MEM images [145]. The third development during this period was MEM-LEED. The group of A. Delong, V. DrahoS, V. Kolaiik and others at the Czechoslovak Academy of Sciences in Brno carried out an independent investigation of the use of an emission microscope for selected-area LEED. One goal, shared with E. Bauer and others, was to provide a way of imaging surfaces and observing LEED of the same area of a specimen. A conventional LEED apparatus lacks the ability to form an image of the surface, and averages information over a large area. The Brno group obtained their first results about 1970 [93], and continued to make progress in the 1970’s on the development of selected-area LEED with a cathode lens, which is analogous to selected area diffraction in a transmission electron microscope (see refs. [146] and references contained therein). Their instrument was based on an emission microscope developed by the same group in the 1960’s [147]. This instrument had a three-stage electrostatic lens system, a

19

differential pumping system which could attain pressures on the order of lo-’ Torr, and several other features. This emission microscope was modified for LEED by adding an electron gun at the bottom of the column, below the fluorescent screen. The final instrument was similar in concept to that of Barnett and Nixon [136], although the motivation was different. Another MEM-LEED development occurred in the late 1970’s. Berger, Dupuy, Laydevant and Bernard at the Institut National des Sciences Appliquees de Lyon [148] developed the design of a MEM-LEED. This instrument was based on the earlier developments in MEM at Lyon discussed above. It utilized an electrostatic cathode lens and, like all the early MEMs, did not have a means to focus the reflected (or refracted) electrons. There was no intermediate or projector lens, since the emphasis was on the LEED mode and high-magnification images were not required. The incident electron beam was separated from the returning beam by means of a magnetic deflection field. The 1977 paper by these authors provides a detailed discussion of the use of a cathode lens to obtain electron diffraction data (i.e., the MEM-LEED technique). This instrument was used to demonstrate that the diffraction pattern in MEM-LEED is independent of the energy of the diffracted electrons [148]. The fourth development that occurred during the 1960’s was the beginnings of LEEM. At the Michelson Laboratory, US Naval Ordnance Test Station, China Lake, California, a surface physicist, Ernst Bauer, began designing instruments in 1961 with the specific objective of LEED and LEEM. Bauer conceived his approach based on his previous work in LEED [149]. Thus, he approached this field from an independent perspective, rather than an extension of mirror electron microscopy. Bauer’s first instrument was made of glass, and was used to test a beam-separating system and other experimental aspects, but did not produce LEED or LEEM images. A photograph of this instrument and a discussion of the basic concept of imaging diffracted “beams” of slow electrons was presented by Bauer at the Fifth International Congress for Electron Microscopy in 1962 [150]. In 1964 Cruise and Bauer carried out a

study of a cathode lens. and Turner and Bauer began designing a second instrument of stainless steel (as reported in abstracts [151,152]). This second instrument was an all-metal, bakeable, UHV instrument with an electrostatic objective lens and magnetic intermediate and projector lenses. The primary design goals were LEED and low-energy reflection electron microscopy (i.e., LEEM), but the instrument was also to have emission microscopy and MEM capabilities. A photograph of this instrument, and a preliminary emission micrograph and LEED pattern, were presented at the Sixth International Congress for Electron Microcopy in 1966 [153]. In 1969 Bauer moved to the Technische Universitat Clausthal, Germany, and took the UHV microscope with him. Many years later, in 1985, after a series of improvements in the instrument, his student, and later postdoctoral fellow, W. Telieps. succeeded in obtaining LEEM images. This important development is discussed further below (sections 3.6 and 4). 3.5. The First International Microscy~

Conference

on Emission

The First International Conference on Emission Microscopy was organized by G. Pfefferkorn and held on September 13-14. 1979 in Tiibingen, as part of a European electron microscopy meeting. The main impetus for this 1979 meeting was the results obtained in many laboratories with the Balzers emission microscope (the Metioskop KE3). This session involved a modest-size group (about 40 to 50). as one would expect for a specialized field. Those of us who attended the Tiibingen meeting recall it with mixed emotions. The excitement of new data and the stimulation of meeting others in the field, many for the first time, was clearly evident. On the more sobering side was the feeling that many of the participants were in the process of saying “Auf Wiedersehen” to emission microscopy and the Metioskop KE3. This was especially true of the surface physicists, who were learning the importance of having instruments of UHV design. The Proceedings of the First International Conference on Emission Electron Microscopy was edited by Pfefferkorn and Schur and published as a special edition of Beitrage zur

elektronenmikroskopischen Direktabbildung von Oberflachen [26]. This volume. although hard to locate outside of European libraries. is of lasting importance. The importance of U HV-designed instruments can hardly be overemphasized. A clean vacuum is needed to prevent the buildup of amorphous surface layers which obscure detail in PEEM. MEM, and LEEM, and prevent the observation of LEED. An equally important reason for a high vacuum is that a poor vacuum results in a gas discharge and an uncontrolled positive ion bombardment of the specimen-cathode in the immersion lens. This situation is reminiscent of some of the early work in electron microscopy where the electron beam was produced by positive ions striking a cathode in a gas discharge (e.g.. the early observations of E. Goldstein, 0. Wolff. and in the cold-cathode TEM gun designs of Knoll and Ruska). A similar effect can occur in a modern emission microscope or related instrument if there is a problem with the high-vacuum system, or if a gas is deliberately introduced. This can, in extreme cases, result in an instability in the electron beam and an erosion of the specimen by the internally generated ion beam. 3.6. Emission microscop~~~, MEM. lured techniques (I 980 1990)

LEEM,

cd

w-

A number of new instruments were introduced in the 1980’s. David W. Turner’s group at Oxford introduced a technique which he called photoelectron spectromicroscopy (PESM) [154-l 571. PESM is a projection-type instrument. without lenses, in which the specimen is placed in a high magnetic field and the photoejected electrons follow helical trajectories along field lines diverging in the direction of decreasing magnetic strength. Although the magnification is low (e.g. 100 x ), PESM provides analytical information and is a useful addition to photoelectron imaging techniques. Bethge and coworkers in Halle built a modular horizontal PEEM and incorporated it into a versatile UHV work station that also had ports for an Auger electron spectrometer, LEED, an ion gun. and evaporation sources [158]. Cazaux carried out a series of scanning X-ray photoelectron

O.H. Griffith, W. Engel / Historical perspectwe und current trends

microscopy experiments in Reims using a SEM and a thin sample, which is another approach to combining analytical and imaging information [159,160]. Other types of photoelectron imaging described as selected-area XPS are under development in several laboratories (see, for example, ref.

11611). At the Institut National des Sciences Appliquees de Lyon, developments and uses of MEM continued into the 1980’s [29,1622165]. Meanwhile, H. Lichte at the Institut fur Angewandte Physik der Universitat Tiibingen built a MEM that utilizes a magnetic prism (Castaing-type) to separate the incoming beam from the reflected beam [166,167]. The magnetic prism deflects the 20 kV electrons from a field emission gun by 90 o onto the mirror, and again by 90” when leaving the mirror. By introducing electron biprisms in the illuminating and imaging part of the microscope, H. Lichte obtained a surface interference microscope which, in principle, can be regarded as the electron-optical analogue of the Tolansky interference light microscope. H. Lichte was able to measure topographical surface structures of the order of a few Angstrom units as well as potential steps at the surface of less than 1 mV. Paralleling this work, a pratical prism-mirror-prism was developed by Ottensmeyer in Toronto as an electron energy-loss spectrometer in TEM, and it is now available commercially (the Zeiss EM 902) [168]. This is not a MEM but it illustrates the versatility of the electron mirror in combination with a beam-separating system. (The development of electron mirrors for energy analysis is outside the scope of this review. For a list of references on this subject, see ref. [31]). A MEM-LEED was designed and constructed at the University of Cambridge by M.S. Foster, J.C. Campuzano, R.F. Willis, in collaboration with J.C. Dupuy of Lyon [169] (there is no connection between this effort and the earlier instrument built at the University of Cambridge by Barnett and Nixon). The design goal was simplicity rather than high resolution. The instrument utilized the same electron optics as that of the Lyon group, except that the incident and diffracted beams are not separated. This MEM-LEED has recently been used to study order-disorder phase transitions

21

[170,171]. C.S. Shern and W.N. Unertl also built a straight MEM-LEED [172] at the University of Maine. It shares some design aspects with the Cambridge MEM-LEED and the Lyon MEM. Like the Cambridge MEM-LEED, the Shern and Unertl MEM-LEED can be viewed as simplified versions of the general type of instrument introduced in 1968 by Delong and DrahoS [147]. Shern and Unertl point out that even a simple and inexpensive MEM-LEED can outperform a conventional retarding-field-display LEED apparatus [172]. The advantages of MEM-LEED over conventional LEED include: (1) lower sensitivity to magnetic fields since the electrons travel at high speed except in the immediate vicinity of the mirror, (2) the diffraction pattern does not depend on the energy of the incident electrons, (3) the viewing screen is flat and more readily equipped with image intensifiers, and (4) there is the possibility of imaging the diffracted electrons, and hence to “see” the portion of the surface contributing to the LEED pattern. The LEED pattern in a MEM-LEED should, in principle, be sharper since it is a focused image. In 1985 Telieps and Bauer at the Technische Universitat Clausthal reported the first successful demonstration of a modern LEEM [173]. This is a significant achievement and opens up new opportunities in surface science. The Telieps and Bauer instrument has the objective lens and the beamseparation system required for forming a focused (Gaussian) image of the mirror surface, unlike the earlier MEMs. A diagram of this instrument is shown in fig. 5. This instrument combines several functions. It is capable of obtaining LEEM, LEED, MEM, and PEEM. The history of this development was marred only by the tragic death of Dr. W. Telieps in a car accident on May 31, 1987. The development and applications of LEEM, and of LEED and PEEM obtained with this instrument, continue in the laboratory of Bauer [19,20,174,175]. This development has inspired several other laboratories to design LEEM microscopes (see section 4 below). Also in 1985, Delong and Kolafik, pioneers in the field of MEM-LEED, proposed an instrument based on a cathode lens and a magnetic prism for selected-area LEED and imaging of the diffracted

Fig 5. Schematic diagram of the LEEM of l‘eliep\ anti Bauer [ 1731. I‘he elrctron beam produced h> ;I field smiasion gun (2) I\ focuwd hy tuo quadrupole\ (3). deflected h! the magnetic field (1). pa\eh through a collimator lenb (4) and unto a region resembling the fIr\t wct~on of an rmiaion microscope (5). Here the electron heam passes through a magnetic stlgmator (6). and IS focused into thr hack focal plane of the cathode lens. The electrons form B parallel bcum as they approach the \ptwmen (7). and arc decelerated htxauw the speumen IS at high nrgatice potential. Low cnergq electron\ \trihe the specimen. the hackxattered hum 1’1rr-accelcrntcd and again passes through the deflecting field (1 ). which eparate the outgoing bcum from the incoming hum. The image i\ tran\ferrctl and magnified hy a magnetic intermediate Ien\ (9). and magnrtlc prqrctcx Iena (IO) before bcin, 0 Intensified h\ the multichannel plates (12). displayed on the phosphor wreen (X) and rccordcd by a TV system or camera (13). The dcllectlon coils (16) flanking the hending magnet serve for heam alignment. (11 ) is an optional filter Iens. Thl\ mv.xoscope can he operated in two additional mode\: ;15 ;I PEEM. nith the addition of a UV lamp (14). and 1t1 a scondary rmision mode. with the addition of an auxilinrv clcctron gun (15) From ref. 11731. The L.EEM ha\ Gnce heen modlfled wmewhat. a\ drscrihed in ref. [ 1741.

electrons (i.e.. a LEEM) [176]. Although they had not yet achieved a working LEEM. this paper presents a useful discussion of MEM-LEED. This 19X5 paper also presents LEED data and defocused images of the reflections. both obtained using a straight column MEM-LEED developed much earlier by this group [147]. In Oregon, the development of a high-resolution PEEM continued throughout the 19X0’s [1X,1 1X,177.178]. Rempfer published a detailed study of electrostatic lenses appropriate for emission microscopes and related techniques in 1985 [ 1791. Rempfer’s work on electron mirrors evolved into a hyperbolic mirror-based aberration corrector to improve resolution of PEEM and other electron microscopes [180-1X2]. The beam-separating system developed for the mirror aberration corrector can also be used in MEM and LEEM. This is a good illustration of the interrelationship

between developments in PEEM. MEM. and LEEM. Some of these recent advances are discussed in the reviews [1X~20.155~1_57.160,161]. More recent work is covered in the following papers of this special issue of Ultramicroscopy. From the material presented in this review it is clear that the advances in this field have been substantial, but the number of laboratories involved remains quite small compared to those developing or using TEM and SEM. The reason5 that emission microscopy. MEM and LEEM have been slower in developing than TEM and SEM are that the range of specimen geometries is more restricted (e.g., relatively flat specimens, a requirement imposed by the cathode lens). the resolution is less than TEM. and the magnifications achievable are. for PEEM at least, often limited by the emission current densities. Also, LEEM involves

0. H. Griffith. W. Engei / Hlstoriccrl perspecrlue und current trends

development of a beam-separating system not required in TEM. The vacuum requirements for PEEM, MEM and LEEM are stringent because these are surface techniques and the specimen surface must be clean. A review of the past developments provides strong evidence that this family of techniques will continue to develop, as it has in the past, as a specialized field filling an important need in surface research. The growth of surface science and the microelectronics industry has created an increasing demand for surface imaging techniques, especially those that cause minimum damage of the specimen surface. New developments in other fields, for example, ultra-high-vacuum systems, image intensifiers, image processors, energy analyzers, lasers, and synchrotron sources, have substantially increased the potential usefulness of photoelectron imaging (e.g., PEEM, PESM, and LEEM) as methods of imaging metal and semiconductor surfaces. Also, in molecular biology there is a need for improved imaging of cytoskeletal elements, membranes, and DNA, for which PEEM and, in principle, LEEM have some unique advantages.

4. Current trends Eleven years have elapsed between the First International Conference on Emission Electron Microscopy in Ttibingen and the Second International Symposium in Seattle. There have been many exciting changes in the field of emission microscopy and new imaging methods based on the photoelectric effect. We are pleased to note that there is some recognition of these recent developments in the broader community of scientists. For example, Dr. E. Bauer and (posthumously) Dr. W. Telieps were awarded the 1988 Gaede Prize in Germany for their contributions to the development of LEEM. In the USA, Dr. Gertrude F. Rempfer’s work on the theory and development of emission microscopy was a factor in her receiving the Howard Vollum Award in 1987, and the Electron Microscopy Society of America Distinguished Scientist Award in the Physical Sciences in 1990.

23

The major trends in emission microscopy and related techniques, as evidenced in the papers in this issue of Ultramicroscopy, are: (1) the introduction of low-energy electron microscopy (LEEM), (2) UHV instrumentation in both LEEM and PEEM, (3) progress in developing better beam-separating systems needed for forming focused images in mirror microscopy and LEEM, (4) integration of PEEM and LEEM with other surface analysis techniques, (5) less expensive and hence more broadly available PEEM designs (i.e., as an attachment to a UHV workstation, but with the trade-off of lower instrument resolution), (6) new applications of emission microscopy and related techniques to specific problems of broad scientific interest, (7) the introduction of photoelectron spectromicroscopy (PESM, also called magnetic projection photoelectron microscopy, or MicroESCA), and (8) additional progress in combining imaging and analytical information (e.g., core-level excitation by synchrotron and X-radiation, and energy analysis). This merging of technologies is coming from two directions: instruments designed solely for imaging are being upgraded to include energy analyzers (i.e., analytical or spectroscopic microscopy) and the instruments originally designed for spectroscopy are being reconfigured to include imaging capabilities (i.e., microspectroscopy, selected-area analysis, or spatially resolved photoelectron spectroscopy). However, analytical capability is not always the top priority, for example when other instruments such as standard Auger and LEED attachments are available on a UHV work station, or in biological applications where photoemissive labels are used. Also, there appears to be a trade-off between the amount of analytical information obtained at a given resolution and the damage to the specimen surface because of the increased input signal required for the energy analysis.

5. Organization

of this issue

This issue of Ultramicroscopy begins with a review by M. Mundschau that discusses the various possibilities of imaging surfaces by emission microscopy (he discusses no less than 46 varia-

tions!), with references where these combinations have been carried out or proposed in the literature [32]. The subject of the next five articles is the newI\, developed field of LEEM. The paper by E. Bauer discusses the various possibilities of introducing analytical methods in PEEM and LEEM by exciting core-level electrons [1X3]. L. Veneklasen compares parallel versus sequential imaging. ii matter of some importance in making the decision of what type of instrument to select for a specific application [1X4]. In the following paper Vencklasen describes the second-generation LEEM in the Bauer laboratory at Technische Universitiit Clausthal, an instrument which will include an energy analyzer [ 1X5]. Clausthal-Zellerfeld is the only place in which LEEM is in operation at this writing, but the situation is changing rapidly. A similar instrument is being built at the FritzHaber-Institut in Berlin, and two other instruments are described in this issue: H. Liebl and B. Senftinger describe a LEEM of novel design that is being built at Garching/Munich [1X6], and R.M. Tromp and M.C. Reuter describe a LEEM of somewhat different design under construction at IBM. New York [1X7]. Other novel types of photoelectron imaging approaches are discussed in the next four articles. A. von Oertzen. H.H. Rotermund, S. Jakubith and G. Ertl describe a new design of a scanning photoemission microscope (SPM) [1Xx]. The SPM complements PEEM since it has a different set of advantages and limitations. The next two papers deal with synchrotron and X-ray excitation: P.L. King. A. Borg. C. Kim, S.A. Yoshikawa. P. Pianetta and 1. Lindau discuss the method 01 magnetic projection photoelectron microscopy. spectromicroscopy a Iso called photoelectron (PESM) or MicroESCA [1X9]. B.P. Tonner describes photoelectron holography as an approach to high-resolution spectromicroscopy [190]. In a different approach. combining a number of options in one instrument, Y. Kondo, K. Yagi, K. Kohayashi. H. Kobayashi. Y. Yanaka and K. Kise describe progress towards building a modified commercial TEM in collaboration with JEOL Inc. to include PEEM at modest resolution [191]. The resulting UHV instrument will be capable of per-

forming TEM, reflection electron microscopy (REM). and PEEM. on the same specimen. To help solve the problem of the high cost and lack of a\ailabilitg of UHV photoelectron emission microscopes. W. Engel and coworkers describe a small PEEM (“PEEMchen” or “PEEM Jr.“) based on electrostatic electron-optics [ 1921. Several of these modular instruments have been built in Berlin and a commercial version is available. The advantage is that this PEEM is simple. Instead of requiring its own specimen translator or vacuum system. it is designed to bolt onto a standard LEED or other port of a UHV station. The trade-off is that the resolution is modest. The next two articles involve uses of this new instrument. C. Wang and M.E. Kordesch describe PEEM studies of the morphology of carbon films [193]. H.H. Rotermund. W. Engel. S. Jakubith. A. van Oertzen and G. Ertl report on oscillating reactions observed during the catalytic oxidation of carbon monoxide on platinum single-crystal surfaces [ 1941. This is a good example of the need for PEEM. Illumination by an electron beam would probably have interfered with the observation of weak]! adsorbed monolayers of gases. Applications of PEEM in the metallurgical field using an existing Balzers KE3 emission microscope are discussed in the paper of C. Hammond and M.A. Imam [I 161. The article by G. Massey. T.J. Ash and H. Zhou describes laser-induced electron emission from organic thin films 11951. This article also discusses space charge. a problem that should be considered in all PEEM experiments utilizing laser excitation (space charge is minimized in continuous lasers but can be a serious problem in pulsed lasers). The final four papers are from the groups in Oregon. The first paper [ll X] documents a photoelectron emission microscope designed for maximum resolution and biological applications. The second paper [17X] describes the computer-aided control and image-processing system assembled for this instrument. The third paper [196] describes strategies for applying photoelectron emission microscopy in cell biology. and some examples. The last paper [197] outlines a proposed modular microscope that would be the first attempt at designing a photoelectron emission rn-

0. H. Griffith, W. Engel / Historical perspective and current trends

croscope with corrected optics. The goal is to cancel out the aberrations of the accelerating field and objective lens by means of a hyperbolic mirror, and by doing so, to increase the resolution to the diffraction limit. The modular instrument would also have LEEM, MEM, LEED, electron probe and TEM modes of operation. This issue concludes with a comprehensive bibliography of work that was published during the last five years in the fields of emission microscopy, MEM, and LEEM [198].

References [l] W. Henneberg

[2] [3] [4] [5] [6] [7] [8] [9] [lo]

Acknowledgements

We are pleased to acknowledge useful discussions on the history of emission microscopy, MEM, and LEEM with Drs. E. Bauer. W.J. Baxter, A.B. Bok, J.C. Campuzano, A. Delong, R.M. Fisher, F. Hasselbach, J. Heydenreich, W. Koch, V. Kolaiik, J.B. Le Poole, V.S. Letokhov, H. Liebl, M. Mundschau, S. Newberry, W. Nisch, F. Pradal, A. Recknagel, G.F. Rempfer. R. Schwarzer, A. Septier, E.-A. Soa, W.N. Unertl and K. Yagi. We are indebted to Ms. Rachel Di Noto for her photographic skills in preparing the figures for this manuscript. We thank the organizers of the XII International Congress for Electron Microscopy for including the sessions on emission electron microscopy and related techniques in the program of events. We are also indebted to Dr. Elmar Zeitler for his encouragement for publishing the proceedings of the symposium as this issue of Ultramicroscopy. Finally, we thank all of the authors for participating and for delivering their manuscripts on schedule, which made possible the timely publication of this volume. Since there is no textbook on emission microscopy, it is our hope that this special issue of Ultramicroscopy will help fill the void by collecting together examples of many of the successful types of emission microscopy and related techniques in use today. This work was supported in part by grants PHS CA11695 from the National Cancer Institute and DIR-8907619 from the National Science Foundation.

25

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and current trends

27

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