Hawkes P.W., Spence J.C.H. (Eds.) Science of Microscopy. V.1 and 2

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Chapter 8 LEEM and SPLEEM 615

a magnetic stigmator (6). The primary image in the center of the beam

separator is imaged with a magnetic intermediate lens (9) and a pro-

jective lens (10) onto the ﬁ nal screen, and the diffraction pattern by

adjusting the focal length of the intermediate lens. The purpose of the

electrostatic ﬁ lter lens (11) was to ﬁ lter out secondary and inelastically

scattered electrons but was later removed because it was found that the

dispersive properties of the magnetic beam separator (1) were sufﬁ cient

to eliminate them from the image. A pair of multichannel plates (12)

enhances the image intensity on the ﬂ uorescent screen (8), allowing

observation and image recording with a video camera (13) at very low

beam currents. Both illumination and imaging columns are equipped

with deﬂ ectors and stigmators, some of which are indicated (16). Emis-

sion microscopy is possible with thermionic emission by heating the

specimen, with photoelectrons generated by a 100-W high-pressure Hg

arc lamp (14) and with secondary electrons using an auxiliary electron

gun (15).

While Figure 8–9 shows the principle of the LEEM system, more

recent instruments differ considerably in detail. For example, a transfer

lens, which transfers the diffraction pattern from the back focal plane

of the objective lens toward the front of the intermediate lens, is now

inserted just behind the beam separator, so that the illuminating beam

does not have to pass through the contrast aperture as in the original

design. The electrostatic triode lens is now replaced by lenses with

better resolution such as the electrostatic tetrode lens, the magnetic

diode lens, or the magnetic triode lens, which will be brieﬂ y discussed

below. The beam separator has been improved in a variety of ways,

such as by close-packed or separated multiple magnetic prisms, con-

centric square or round pole pieces, or a Wien ﬁ lter, resulting in deﬂ ec-

tion angles ranging from 16° to 90° compared to the original 60°. In

addition, energy ﬁ lters have been added to instruments so that they

can also be used for AEEM, low electron energy loss microscopy

(LEELM), energy-ﬁ ltered SEEM, and spectroscopic PEEM.

Before discussing these components of an LEEM instrument, a short

account of the various designs is appropriate. The ﬁ rst major develop-

ment after the original instrument and a similar one

also used a beam

separator with 60° deﬂ ection but with close-packed multiple magnetic

prisms and only magnetic lenses, including the objective lens, a LaB

cathode, and a transfer lens so that the contrast aperture could be

placed behind the beam separator.

The magnetic prism on the

illumination side of the instrument can be excited differently from that

on the exit side so that a higher beam energy is possible than on the

imaging side, which allows AEEM when an energy ﬁ lter is added to

the instrument. The dense packing of the magnetic lenses together

with the shielding of the beam separator and the specimen region

makes the magnetic shielding used in the more open earlier instru-

ments unnecessary. The addition of an energy ﬁ lter allows not only

AEEM but in combination with synchrotron radiation also spectro-

scopic X-ray PEEM.

13,45,46

From this instrument the presently most wide-

spread commercial instrument shown in Figure 8–10 developed later.

The version with energy ﬁ lter, the SPELEEM, which is presently the

616 E. Bauer

most versatile instrument, has been described repeatedly

in connec-

tion with PEEM. Therefore, a cross section of the presently best home-

built pure LEEM instrument (Figure 8–11)

is shown here. Similar to

the previous instrument (Figure 8–10) it has densely packed magnetic

lenses both in the illumination (top) and in the imaging (bottom)

section. However, it uses a beam separator with 90° deﬂ ection that is

incorporated in the vacuum system (center). The objective lens (right

Figure 8–10. Commercial LEEM instrument. The illumination column is on

the right side and the imaging column on the left side. The 60° beam separator

is hidden behind the specimen chamber in the center, which also contains the

objective lens. (Courtesy of ELMITEC GmbH.)

Figure 8–11. Cross

section of a LEEM

instrument with a 90°

separator. The illumina-

tion column with the

ﬁ eld emission gun is on

top and the imaging

column is at the bottom.

The right side shows the

specimen chamber, air-

lock, and part of the

pumping system.

Chapter 8 LEEM and SPLEEM 617

side) is a magnetic diode and consists of two sections with opposing

image rotation. A commercial cold ﬁ eld emission electron gun (top)

with an energy spread of about 0.25 eV allows a theoretical resolution

of 0.4 nm at 10 eV. The right side is the specimen chamber and a prepa-

ration chamber plus airlock. This instrument presently holds the

resolution record (0.5 nm) among the instruments without aberration

correction.

The instruments described above are free-standing instruments.

There has long been the wish to add a LEEM instrument to existing

UHV systems. To be practical, such LEEM systems must be much

smaller and have small deﬂ ection angles. Two solutions were chosen:

one uses a simple beam separator with a small deﬂ ection angle (10°)

and the other one uses three deﬂ ections by 45°.

In both cases the

instrument is mounted on an 8-in.-diameter UHV ﬂ ange and can be

attached to a UHV system via a 6-in.-diameter UHV ﬂ ange. All lenses

are electrostatic, including the electrostatic tetrode lens. In contrast to

magnetic lenses, electrostatic lenses can be easily ﬂ oated at high voltage.

Therefore the complete electron optics can be at high voltage so that

the specimen can be near ground potential, whereas in the magnetic

lens systems the specimen is at high voltage. Because of the compact

design, high-voltage insulation requirements allow ﬁ nal energies of

only 5 keV, in contrast to the 15–20 keV used in the larger systems. Only

the extraction electrode of the tetrode lens can be increased to 15 kV.

External ﬁ elds are screened by internal µ-metal screening. Figure 8–12

shows the mechanical conﬁ guration of one of these ﬂ ange-on LEEM

instruments.

Its overall length is 60 cm and its weight is about 20 kg.

Several other LEEM instrument designs have been proposed and in

part realized. One design

included some interesting features such as

a combined magnetic-electrostatic beam separator that allows different

energies to be used in the illumination and imaging beams, an electro-

static tetrode combined with a Schwarzschild-type optical mirror objec-

tive, which focuses UV radiation onto the specimen for PEEM, and a 70°

spherical condenser for electron energy ﬁ ltering. Unfortunately it never

came into operation. In another design that is used in a commercial

instrument

the beam separation is achieved by a Wien ﬁ lter into which

the illumination beam enters at an angle of 36° while the imaging beam

runs along its optical axis. A second Wien ﬁ lter is used as an energy

ﬁ lter. This instrument has been used mainly for PEEM, MEM, and

6˝ FLANGE

VACUUM

CHAMBER

APERTURE

MANIPULATOR

FLANGES

BASIS

FLANGE 8˝

ELECTRON

GUN UNIT

IMAGE

INTENSIFIER

UNIT

MAGNETIC

SCREENING

OBJECTIVE

LENS

BEAM

SEPARATOF

SPECIMEN

Figure 8–12. Schematic of the mechanical conﬁ guration of a ﬂ ange-on LEEM

system.

618 E. Bauer

metastable impact electron emission microscopy (MIEEM). Apparently

the difﬁ culty of aligning the incident beam normal to the surface and

keeping the imaging beam on axis makes systematic LEEM studies with

acceptable resolution difﬁ cult. A similar problem has to be overcome in

an instrument in which the beam separator is replaced by a W single

crystal.

This crystal is tilted 45° against the optical axis and reﬂ ects a

low-energy electron beam from a side-mounted gun along the optical

axis. Exact normal incidence on the sample requires that the reﬂ ector is

on the optical axis, which would obstruct the imaging beam. This

instrument, called a double reﬂ ection electron emission microscope

(DREEM), is also commercially available. Other LEEM instruments

have been built as well but their design has not been published.

All instruments discussed up to now suffer from the large chromatic

and spherical aberration of the objective lens. That these aberrations

can be corrected with an electron mirror has been known for some time

but only during the past decade have efforts been made to develop

aberration-corrected instruments. Simultaneously, however, the aber-

rations of the beam separator have to be corrected. This has led to the

design of a complex system

54,55

that has already been realized in the

so-called SMART (Spectromicroscope for All Relevant Techniques)

instrument,

17,56,57

schematically shown in Figure 8–13. Similar to the

SPELEEM system, the instrument is designed for a wide range of opera-

tion modes,

one of which is LEEM. The centerpiece is the beam separa-

objective beam separator transfer optics energy filter projector / detector

X-ray optics

X-ray

illumination

X-ray

mirror

mirror corrector

axial ray

optic axis

field ray

dispersion ray

0 100 200 300 400 500 600 mm

specimen

objective

lens

electric-magnetic

deflectors

field emission gun

deflector D1

condensor lens

transfer lens T1 field aperture energy selection slit projector lens

dipole P1

quadrupole

dodecapole

L5 T2L3 L4 D4D3D2

beam

separator

aperture

hexapole H1

electric-magnetic

deflectors/dodecapoles

outer

electrodes

tetrode

mirror

monochromator exit slit

inner

electrode

field

lens L1

channel plate

slow scan CCD

Figure 8–13. Schematic of the electron-optical conﬁ guration of the SMART system.

Chapter 8 LEEM and SPLEEM 619

tor to which the illumination system, a ﬁ eld emission gun, the objective

lens, and the mirror corrector are attached via ﬁ eld lenses (L). Beyond

the beam separator ﬁ ve electrostatic lenses transfer either the diffrac-

tion pattern in the plane of the contrast aperture between L

and L

the primary image at various magniﬁ cations into the entrance of the

Ω-type energy ﬁ lter where the ﬁ eld limiting aperture is located. With

the energy selection slit inserted, the subsequent projective lens system

produces an energy ﬁ ltered image or diffraction pattern on the Channel

plate-ﬂ uorescent screen unit, which is ﬁ beroptically coupled to the CCD

camera. Numerous deﬂ ectors (D

) allow precise alignment and several

n-pole elements (n = 2, 6, 12) are used for the correction of residual

aberrations. The instrument operates at 15 kV and is expected to have

a resolution in LEEM of 1 nm at 10 eV.

Unless aberrations of the beam separator, the energy ﬁ lter, other optical

components or vibrations, electromagnetic ﬁ elds, charging, or other dis-

turbances are resolution limiting the chromatic and spherical aberra-

tions of the objective lens determine the resolution. The aberrations of the

accelerating ﬁ eld of the cathode lens cause much higher limiting values

of the resolution than in transmission microscopy. These values can be

calculated analytically by assuming that the lens may be separated into

a homogeneous ﬁ eld in front of the specimen, which produces a virtual

image behind the specimen, and an einzel lens, which produces a real

image of the virtual image. Realistic calculations have to consider the

cathode lens as a unit and have been made by many authors. A compari-

son of the resolution obtainable with an electrostatic triode, electrostatic

tetrode, and magnetic triode lens (Figure 8–14) shows that the electro-

static tetrode and the magnetic triode lenses are much better than the

original triode lens because in the latter the ﬁ eld strength at the sample

is low under focusing conditions (1.18–0.52 kV/mm vs. 10 kV/mm in the

other two lens types) (Figure 8–15).

The data are for the optimum aper-

ture, which is determined by minimizing the contributions of the aber-

ration discs due to diffraction at the angle-limiting aperture, chromatic

and spherical aberrations. These contributions can be seen in Figure 8–16

together with the radius of the optimum aperture. From these ﬁ gures it

is clear that the magnetic triode is superior to the electrostatic tetrode

both in resolution and transmission. Transmission does not play an

important role in LEEM but is important in XPEEM with photoelectrons

and secondary electrons, which have a wide angular distribution. Today

the magnetic diode, which differs from the triode only in that both pole

shoes are at the same potential, is the standard in the best LEEM instru-

ments without aberration correction while the electrostatic tetrode is the

domain of the smaller, purely electrostatic LEEM instruments. In the

multimethod instruments

15–17,44,56,57

combined electrostatic–magnetic

cathode lenses, such as the magnetic triode, are useful because they

allow the ﬁ eld strength at the specimen to be varied. Such a lens with a

design somewhat different from that shown in Figure 8–14 is used in the

4 Electron Optics

620 E. Bauer

SMART instrument. Because the chromatic and spherical aberrations of

the objective lens are corrected in this instrument by the mirror corrector,

the largest aberrations are now those of the energy ﬁ lter. The resolution

improvement obtained by correction is shown in Figure 8–17 as a func-

tion of angular acceptance for a start energy of 10 eV, an energy width of

b. ELECTROSTATIC TRIODE

c. ELECTROSTATIC TETRODE

d. MAGNETIC TRIODE

TADIAL POSITION (mm)

24681012

AXIAL POSITION (mm)

E(Z)

r(Z)

(Z)

Figure 8–14. Schematic conﬁ gurations of LEEM cathode lenses. One-half of

the electrodes/pole pieces, the electron energy E(z), and the electron ray path

r(z) is shown.

1.18

1.15

1.12

1.08

.98

FOCUSSED E.S. TRIODE

.87

.52

E.S. TETRODE

MAGNETIC TRIODE

HOMOGENEOUS

(NON-FOCUSSING) FIELD

INITIAL ENERGY (eV)

RESOLUTION δ (nm)

10 100

Figure 8–15. Comparison of the resolution of the lenses shown in Figure 8–14

for a ﬁ nal energy of 20 keV, an energy spread of 0.5 eV, and optimized

aperture.

Chapter 8 LEEM and SPLEEM 621

2 eV, and a ﬁ nal energy of 15 keV.

The dashed lines show the contribu-

tions of the aberrations of the uncorrected lens and the thin solid lines

those of the energy ﬁ lter. For LEEM instruments with LaB

or ﬁ eld emis-

sion guns that have lower energy widths, the resolution is still improved

by about a factor of 5. The main advantage of aberration correction, the

large increase in transmission, however, comes to bear only in emission

microscopy, in particular in AEEM and XPEEM.

b. ELECTROSTATIC TRIODE

c. ELECTROSTATIC TETRODE

d. MAGNETIC TRIODE

FIELD STRENGTH 1.18→ .58 kV/mm

FOR FOCUS AT INFINITY

FIELD STRENGTH 10

kV/mm

FIELD STRENGTH 10 kV/mm

a – NET RESOLUTION

b – DIFFRACTION

c – CHROMATIC

d – SPHERICAL

ϒ – APERTURE RADIUS

NET RESOLUTION δ

DIFFRACTION .6 λ

/α

opt

CHROMATIC C

opt

∆E/E

SPHERICAL C

opt

INITIAL ENERGY (eV)

110100

RESOLUTION COMPONENT (nm) OR RADIUS (µm)

Figure 8–16. Energy

dependence of the contri-

butions of the spherical,

chromatic, and diffrac-

tion aberrations (dashed,

dotted, and dash–dotted

lines) to the resolution

(solid line) at the optimum

aperture with radius r

for the lenses shown in

Figure 8–14. In the triode

the ﬁ eld strength has to

be changed for forming a

real image; in the other

two lenses 10 kV/mm has

been choosen. Energy

spread 0.5 eV.

Figure 8–17. Resolution limit d as a

function of the acceptance angle α

without and with correction of the

spherical and chromatic aberra-

tions. Data for the system shown in

Figure 8–13 for 10 eV start energy

and 2 eV energy spread. In the

uncorrected system d is limited by

the chromatic and spherical aberra-

tions (dashed lines) and in the cor-

rected system by higher order

aberrations (solid lines).

622 E. Bauer

The next critical component of a LEEM instrument is the beam sepa-

rator. Early beam separators

3,43

aimed only at the reduction of the uni-

directional beam dispersion in the deﬂ ection plane. This was achieved

with a D-shaped cutout in the round pole pieces.

The focusing action

of the fringing ﬁ eld of the magnet was compensated by magnetic

quadrupoles. In contrast to these ﬁ rst separators, which tried to elimi-

nate its focusing action, later separators made use of it in order to

obtain optimum image and diffraction pattern transfer in close-packed

magnetic prism arrays. They consist of an array of inner pole pieces

surrounded by a single outer pole piece with different relative excita-

tions. Such arrays act almost like round lenses. They transfer image

and diffraction planes stigmatically and distortion free to correspond-

ing planes behind the separator but at different settings.

61,62

A 90°

deﬂ ector with four inner prisms was suggested for the addition of a

mirror corrector

and a 60° deﬂ ector with three inner prisms was real-

ized in the ﬁ rst fully magnetic LEEM instrument.

If illumination and

imaging beam have the same energy as in LEEM—in contrast to the

more versatile instruments—then a single inner pole surrounded by

an outer pole ring are sufﬁ cient. Both square

48,54,63–65

and round 90°

64,66

separators have been proposed and built. With proper geometry and

excitation ratio, astigmatic and distortion-free imaging can be achieved

for image and diffraction plane with the same settings. For an aberra-

tion-corrected microscope the aberrations of these separators would

limit the resolution and have to be corrected. This is achieved in the

highly symmetric beam separator seen in Figure 8–13.

67–69

In small

ﬂ ange-on LEEM instruments the design of the beam separator is deter-

mined less by minimizing aberrations than by geometry and space

considerations.

49,50

For example, to achieve parallel illumination and

imaging beams within a minimum of space, the beams were translated

achromatically

by magnets with ﬁ eld boundaries perpendicular to

the beam and electrostatic cylinder lenses in order to achieve

stigmatic focusing.

Finally a Wien ﬁ lter may also be used as a beam

separator

though with some alignment difﬁ culties.

Although the use of electron mirrors for the correction of lens aber-

rations was proposed many years ago

72,73

and suggested for the correc-

tion of the cathode lens aberrations in LEEM and PEEM instruments

in the early 1990s

54,61,74

serious efforts have been made only in the past

10 years

75–77

and only one instrument, the SMART, is presently equipped

with a mirror corrector and is in its testing stage.

An energy ﬁ lter is not needed in simple LEEM instruments with suf-

ﬁ ciently large separator deﬂ ection angles because at low energies, at

which the energy of the secondary electrons differs little from that of the

elastically reﬂ ected electrons used for the LEEM image, the secondary

electron intensity is small compared to the diffracted beam intensity.

With increasing energy, when the elastically backscattered intensity

decreases and the secondary electron intensity increases, the dispersive

action of the sector ﬁ eld deﬂ ects the secondary electrons sufﬁ ciently so

that only a small fraction passes through the contrast aperture. Like-

wise, inelastic scattering in the high LEEM energy range occurs mainly

in the forward direction so that it can be seen in the backward direction

Chapter 8 LEEM and SPLEEM 623

only through diffraction, either via energy loss before or after diffrac-

tion. Therefore it is a second-order effect. Furthermore, the dispersion of

the beam separator and the momentum transfer in the energy loss

ensure that most inelastically scattered electrons do not pass through

the contrast aperture. In the low LEEM energy range inelastic scattering

is less in the forward direction but in general is weak, so that it does not

contribute noticeably to image formation. The main advantage of having

an energy ﬁ lter in a LEEM instrument is that it eliminates the secondary

and inelastically scattered electrons in the LEED pattern. In many cases

this is not important but in materials with high secondary electron yield

it improves the LEED pattern dramatically. Also, for quantitative analy-

sis of the background in LEED patterns an energy ﬁ lter is useful.

The main motivation for adding an energy ﬁ lter to a LEEM

instrument is its importance in multimethod instruments such as the

SPELEEM or the SMART. It allows LEEM to be combined with low-

electron energy loss spectroscopy and microscopy. Furthermore,

when higher beam energy can be used in the illumination beam

than in the imaging beam, Auger electron emission spectroscopy

(AEES) and AEEM are possible too. These various operation modes of

such an instrument are schemetically shown in Figure 8–18.

15,47

SEEM, whether excited by electrons, X-ray photons, or energetic ion/

neutrals, it allows selection of a narrow energy window from the wide

Figure 8–18. The three fundamental operation modes of a SPELEEM system. The various sections of

the instrument are shown folded into one plane. In imaging and diffraction the energy selection slit

is inserted in the dispersive plane DP and the image/diffraction pattern behind DP is imaged with

the projector. The intermediate lens IL is used to switch between imaging and diffraction, simultane-

ously with the exchange of the contrast aperture in FPI and the ﬁ eld-limiting aperture in IIP. For fast

spectroscopy both apertures are inserted, the energy selection slit is removed, and the dispersive plane

is imaged by the projector.

(See color plate.)

624 E. Bauer

secondary electron energy distribution. This leads to a noticeable reso-

lution improvement, also in the aberration-corrected systems, in which

the correction rapidly deteriorates at low energies with decreasing

energy.

A number of different energy ﬁ lters are used in these multi-

method instruments: one hemispherical analyzer, two hemispherical

analyzers, a Wien ﬁ lter, or an Omega ﬁ lter. As they are not essential

for LEEM they will not be discussed here.

For imaging with LEEM several contrast mechanisms are available,

depending upon the specimen to be imaged. The fundamental contrast

is diffraction contrast in crystalline samples or backscattering contrast

in amorphous or ﬁ ne-grained crystalline materials. The origin of the

backscattering contrast is evident from Figure 8–1. An example of its

consequences is shown in Figure 8–19. Because of the higher backscat-

tering cross section of Co at the selected energy the ﬁ ne-grained poly-

crystalline Co squares appear bright compared to the Si surrounding,

which is covered with native oxide. A slight preferred orientation of the

Co layer enhances the contrast.

In general, however, larger crystals or

single crystalline layers with a well-deﬁ ned orientation are studied. In

this case not only the specular beam [(00) beam] but other diffracted

beams may be used for imaging. An atomically ﬂ at single crystal surface

without steps and other defects produces contrast only when regions

with different crystal structure are present. A standard example is the

Si(111) surface when the unreconstructed (1 × 1) and reconstructed (7 ×

7) structure coexist. Here, both normal and lateral periodicity differ and

produce strong contrast (Figure 8–20a).

The Si(100) surface recon-

structs with the formation of dimer rows whose orientation rotates from

terrace to terrace by 90°, with constant normal periodicity. Here, the two

resulting (2 × 1) domains are equivalent at normal incidence and well-

centered aperture. Contrast is obtained by either tilting the incident

beam or shifting the aperture somewhat in the direction of one of the

rows. Using the ½ order spot of one of the domains gives maximum

contrast (Figure 8–20b).

Similar domain contrast can be obtained on all

Figure 8–19. Backscattering contrast

from 20-nm-thick Co squares on an Si

substrate. The electron energy is 5.1 eV

and the diameter of ﬁ eld of view is

10 µm.

5 Contrast