Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
Monochromator manual Titan V1.2
Author
Title
Date
: Peter Tiemeijer
: Monochromator manual Titan V1.2 and higher
: March 16, 2011
77 K
85 K
94 K
105 K
Transition to superconductivity probed at the nanometer scale: the local density of states around
4eV changes below Tc = 90K in Bi2Sr2CaCu2O8-δ (data acquired on a monochromated Titan
with liquid Helium cooling).
Contents
0
1
Introduction ........................................................................................................................... 3
Illumination and probe forming........................................................................................... 3
1.1 Titan condenser column without monochromator ............................................................ 3
1.1.1 Gun lens...................................................................................................................... 3
1.1.2 First condenser lens.................................................................................................... 4
1.1.3 Second and third condenser lens ................................................................................ 5
1.1.4 Minicondenser lens .................................................................................................... 6
1.1.5 Objective lens............................................................................................................. 6
1.2 FEG column with monochromator not used..................................................................... 6
1.2.1 Gun lens...................................................................................................................... 7
1.2.2 Other lenses ................................................................................................................ 8
1.3 FEG column with monochromator used ........................................................................... 8
1.3.1 Gun lens and monochromator .................................................................................... 8
1.3.2 Decelerating and accelerating gun lens...................................................................... 9
1.3.3 First condenser lens.................................................................................................. 11
1.3.4 Second and third condenser lens .............................................................................. 11
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1.3.5 C2 lens for extra spot demagnification .................................................................... 11
Alignment ............................................................................................................................. 13
2.1 Conventional FEG column ............................................................................................. 13
2.2 FEG column with monochromator ................................................................................. 13
2.2.1 Gun tilt and monochromator deflectors ................................................................... 13
2.2.2 Gun shift................................................................................................................... 15
2.2.3 Spot-size dependent gun shift .................................................................................. 16
3 Monochromator Optics....................................................................................................... 17
3.1 Excitation ........................................................................................................................ 17
3.2 Double focusing Wien filter............................................................................................ 18
3.3 Monochromator stigmators............................................................................................. 19
3.4 Monochromator slit......................................................................................................... 20
4 User interface ....................................................................................................................... 23
4.1 Monochromator panel..................................................................................................... 23
4.1.1 Settings flap-out ....................................................................................................... 24
4.1.2 Gun lens flap-out...................................................................................................... 25
4.1.3 Find Beam flap-out................................................................................................... 26
4.2 Monochromator Tune panel............................................................................................ 28
4.2.1 Offsets flap-out......................................................................................................... 29
4.2.2 Outputs flap-out........................................................................................................ 30
4.3 FEG Registers ................................................................................................................. 31
4.3.1 Options flap-out........................................................................................................ 33
5 Monochromator alignment procedures............................................................................. 35
5.1 Gun Alignment................................................................................................................ 35
5.1.1 Preparation ............................................................................................................... 35
5.1.2 Find Beam ................................................................................................................ 35
5.1.3 Gun shift pivot point ................................................................................................ 36
5.1.4 Gun tilt and monochromator shift ............................................................................ 36
5.1.5 Gun shift................................................................................................................... 38
5.1.6 Spot-size dependent gun shifts................................................................................. 38
5.2 Monochromator Table alignments.................................................................................. 39
5.2.1 Excitation alignment for decelerating mode ............................................................ 39
5.2.2 Accelerating gun alignment ..................................................................................... 39
5.2.3 Excitation alignment for accelerating mode............................................................. 40
5.3 Direct Alignments for the monochromator..................................................................... 40
5.3.1 Focus Slit.................................................................................................................. 40
5.3.2 Fine Mono Shift and Reset Fine Mono Shift ........................................................... 41
6 Practical guidelines.............................................................................................................. 42
6.1 General way of adjusting monochromator...................................................................... 42
6.2 Recommended settings ................................................................................................... 43
6.3 How to find the beam...................................................................................................... 47
6.4 How to switch on the monochromator............................................................................ 48
6.5 How to switch to accelerating mode............................................................................... 50
6.6 How to find the beam on the GIF ................................................................................... 51
6.7 How to tune the GIF for the highest resolution .............................................................. 51
6.8 How to make small monochromized probes and STEM ................................................ 54
2
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Monochromator manual Titan V1.2
0
Version 1.2 and higher
March 16, 2011
Introduction
This is the user manual for the monochromator. It is assumed that the reader is familiar with the
basic principles of the Titan operation. This manual does not discuss other parts of the
microscope, except when these differ from the conventional instrument (without
monochromator) or when these are essential for a good understanding of the operation of the
monochromator. This manual does also not discuss the use of the GIF or Tridiem or Quantum.
Manuals for the standard microscope and the GIF, Tridiem, or Quantum are available on-line.
This manual is also available on-line.
Chapters 1, 2, and 3 discuss in a general fashion the optics of the illumination system with
monochromator. It is advised to read and understand these chapters before reading the remaining
chapters and before working at the microscope.
Chapter 4 describes all controls in the monochromator software. This chapter is intended as a
reference, but it is advised to read the sections about the two main control panels before working
with the monochromator.
Chapter 5 lists the alignment procedures. This chapter is intended as a reference. When the
microscope is properly aligned, the user will not need this section.
Chapter 6 gives some guidelines, checklists, and recipes for the beginning user on how to do
basic things such as finding the beam, making HR-EELS spectra and making small probes.
1
Illumination and probe forming
1.1 Titan condenser column without monochromator
The operation of the Titan condenser column without monochromator is extensively described
in the 'Titan condenser manual'. This section partly copies the description of the optical schemes
in that manual.
The conventional Titan column has six lenses in the illuminating system, namely the gun lens,
the first condenser lens (C1 lens), the second condenser lens (C2 lens), the third condenser lens
(C3), the minicondenser lens (MC lens) and objective lens (Obj). The main function of these
lenses is summarized in the following table.
Function
Beam current/Probe size
Beam width/Beam convergence
Parallel beam/focused beam
Compensate upper objective lens
Probe forming
Name
gun lens
spot number
Aera/Semi-angle
TEM/Probe
Microprobe/Nanoprobe
Lens
gun lens
C1-C2 zoom
C2-C3 zoom
C3
minicondenser
upper objective lens
1.1.1 Gun lens
The first lens is the electrostatic gun lens positioned directly behind the field emitter and
extractor. The user can choose eight settings for this lens (called gun lens 1 to 8). The figure
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below gives three sketches for the rays in these different settings for the gun lens. Increasing gun
lens leads to increased demagnification of the source and decreased current in the beam.
Increasing the gun lens by 1 leads to a reduction of the beam current by approximately 35% .
gun lens
C1
C2
C2 aperture
C3
Obj
Obj
‘gunlens 1’
‘gunlens 5’
specimen
‘gunlens 8’
1.1.2 First condenser lens
The next lens is the first condenser lens (C1). The user can choose from eleven settings for this
lens (called spot number 1 to 11). The next figure gives three sketches for the rays in these
different settings of the first condenser lens. Increasing ‘spot number’ leads to increased
demagnification of the source and decreased current in the beam. Increasing the spot number by
1 leads to a reduction of the beam current by roughly 50%.
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gun lens
C1
C2
C2 aperture
C3
Obj
Obj
‘spot number 8’
‘spot number 4’
specimen
‘spot number 1’
1.1.3 Second and third condenser lens
The second condenser lens (C2) and third condenser lens (C3) together form a zoom system for
continuously varying the size of the parallel illuminated area (in TEM) or the convergence angle
of the probe (in STEM).
An alternative way of looking at this is to say that the C3 lens (de)magnifies the size of the C2
aperture.
gun lens
C1
C2
C2 aperture
C3
Obj
Obj
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specimen
Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
Changing the position of the intermediate image between C2 and C3 changes the illuminated
area in TEM mode.
The C3 lens transports the beam to the minicondenser lens and the objective lens. In STEM, this
is the lens that is used to focus the probe on the specimen. Therefore, the C3 lens is connected to
the focus knob in STEM.
1.1.4 Minicondenser lens
The user can choose two settings for the minicondenser lens. With these settings he can choose
between little demagnification of the probe (called microprobe) and much demagnification of
the probe (called nanoprobe). The ratio between the probe size in these two modes is 5:1 for a
FEG column. Typically, microprobe is used for TEM and nanoprobe for STEM.
Different settings for the minicondenser lens
gun lens
C1
C2
C2 aperture
C3
MC
Obj
Obj
‘nanoprobe’
specimen
‘microprobe’
1.1.5 Objective lens
The specimen is located approximately in the middle of the magnetic field of the objective lens.
This field gives lens action in front of the specimen (the condenser-objective lens) and lens
action after the specimen (the imaging-objective lens). The focal distance of both lenses is 2.0
mm for a SuperTwin objective lens. In nanoprobe, the demagnification of the source is very
sensitive to the exact height of the specimen. A height change of 10 μm typically leads to a
change of demagnification of more than 20%. It is therefore important to use the eucentric
height.
1.2 FEG column with monochromator not used
The monochromator introduces three new optical elements above the C1 lens:
•
The 100 μm monochromator entrance aperture, located directly behind the gun lens.
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The monochromator itself, located directly behind its entrance aperture.
The (removable) monochromizing slit, which is placed in the C1 aperture holder, which is
76 mm in front of the C1 lens.
These components affect the optics considerably.
•
•
The monochromator has two modes: monochromator not used & monochromator used. These
modes are also called “the unfiltered mode” & “the filtered mode”. The optics of these two
modes is described in section 1.2 and 1.3, respectively.
1.2.1 Gun lens
Below is a series of ray diagrams in the condenser column with (from left to right) decreasing
strength of the gun lens. The monochromator is off so no deflection fields are present.
(Extractor=3.8kV, Monochromator potential=3kV, C2 aperture=100μm)
gunlens=740V
focus=-35V
gunlens=760V
focus=-15V
gunlens=775V
focus=0V
gunlens=795V
focus=20V
gunlens=840V
focus=65V
In the first two diagrams, the first cross-over is in the monochromator. About 20 mm behind the
monochromator starts the accelerator. The accelerator also acts as a lens, and a second crossover is made in the accelerator. In the middle diagram, the first cross-over is located almost
precisely at the end of the monochromator. The accelerator has been designed such that, in this
case, it focuses the second cross-over precisely at the C1 entrance aperture. In the last two
diagrams, there is no cross-over in the monochromator, and the only cross-over is made by the
accelerator lens.
In contrast to a conventional system, the gun lens of the monochromized system is continuously
variable. The user can enter the potential for the gun lens electrode (in the FEG control panel)
which can be anything between 300 and 5500V. However, it is simpler to use the default option
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‘gun lens controlled by monochromator’ (in the gun lens flap-out of the monochromator panel),
and to use the monochromator Focus control to set the gun lens strength. When the
monochromator is properly aligned, Focus=0V corresponds to the middle diagram in the figure
above. Negative Focus gives larger beam current than positive Focus (because the
monochromator aperture accepts larger angles from the FEG), but positive Focus gives larger
brightness than negative Focus (because the Coulomb interactions are less in the
monochromator).
These diagrams illustrate one important aspect of the column with the monochromator:
depending on the strength of the gun lens, the beam limiting aperture is either the C2 aperture
(first and last diagrams) or the monochromator aperture (middle three diagrams).
In the latter case, the spot can be relatively imperfect for the following two reasons.
•
The monochromator aperture is at low kV and therefore its edges are less sharp.
•
The monochromator aperture can not be aligned with respect to the tip and gun lens. A
misalignment will give coma in the spot, resulting in spots like shown below for different
defocus.
Therefore, when the monochromator is not used, it is better not to use the monochromator
aperture as beam limiting aperture.
1.2.2 Other lenses
The behavior of the other lenses is similar to that in the conventional column.
1.3
FEG column with monochromator used
1.3.1 Gun lens and monochromator
The monochromator acts as a lens with a strength approximately proportional to its excitation.
The gun lens is tuned such that the gun lens and the monochromator together make a focus of
the dispersion in the plane of the energy selection slit. This is illustrated in the figure below.
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March 16, 2011
gun lens
slit
C1
C2
C3
Obj
Excitation=0.0
Excitation=0.8
Excitation=1.6
Excitation=2.2
This figure illustrates the following two important points:
•
when the monochromator is off, the gun lens is strongest (most left figure);
•
when the monochromator is strongest, the gun lens is off (most right figure).
These two points imply that
•
when the monochromator is off, the aberrations of the gun lens are strongest;
•
when the monochromator is strongest, the gun lens aberrations are absent.
The most important aberration of the gun lens is coma (introduced by the misalignment of the
tip with respect to the gun lens and monochromator aperture). Thus strong excitation of the
monochromator improves the energy resolution not only because the dispersion is increased, but
also because the aberrations of the gun lens are reduced.
1.3.2 Decelerating and accelerating gun lens
The gun lens is an electrostatic three-electrode lens. The three electrodes are the extractor, the
gun lens electrode and the monochromator. The strength of the gun lens is proportional to the
difference of potentials of these three electrodes. The standard way of operating the gun lens is
to lower the potential of the gun lens electrode compared to the extractor and the
monochromator. This decelerates the electrons in the gun lens. An alternative way of operation
is to raise the potential of the gun lens electrode compared to the extractor and the
monochromator (because of the limited range of the gun lens supply, this only gives sufficient
gun lens strength when the monochromator is at a potential ≤ 800V). This accelerates the
electrons in the gun lens. As will be explained at the end of this section, this accelerating mode
is only provided if the monochromator is combined with the standard Schottky FEG (SFEG), not
if it is combined with the high brightness FEG (XFEG).
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(b) accelerating gun lens
potential V
(a) decelerating gun lens
potential V
March 16, 2011
extractor gun lens monochromator
extractor gun lens monochromator
The figure sketches that the lens action is concentrated at the gun lens electrode in the
decelerating mode, whereas the lens action is concentrated at the entrance of the monochromator
in the accelerating mode. In accelerating mode, the lens action is therefore further away from the
tip. In accelerating mode, the monochromator captures therefore a smaller part of the cone
emitted at the tip. This has the following consequences
1. In accelerating mode, the beam current in the monochromator is about ten times lower
than in accelerating mode.
2. In accelerating mode, the aberrations of the gun lens are negligible, and so very small
excitation (e.g. 0.2) of the monochromator already gives useable energy resolution.
Other differences are
3. In accelerating mode, the electrons travel faster in the gun lens, so the Coulomb
interactions are smaller in the gun lens, so the energy resolution is a little better than in
decelerating mode.
4. In accelerating mode, the electrons travel slower in the monochromator, so the beam is
more sensitive to changes of the deflections fields in the monochromator, and therefore
the beam position re-produces less well in accelerating mode.
The decelerating mode is more easy to use because of points 1 and 4. However, the ultimate
energy resolution is obtained in accelerating mode because of point 3, and the ultimate
monochromized HR-STEM and HR-TEM resolution is obtained in accelerating mode because
of point 2.
The current density in the cone emitted at the tip is about 5 times higher for the high-brightness
FEG (XFEG) then for the standard Schottky FEG (SFEG). Consequently, for the same total
current, the beam diameter in the monochromator can be about √5 times smaller for the XFEG.
This smaller beam diameter reduces the aberrations of the gun lens and monochromator
significantly. This makes it superfluous to switch to accelerating mode to reduce gun lens
aberrations (point 2 above). Therefore, and in order to simplify the operation of the
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monochromator, the accelearting mode is not made available when the monochromator is
combined when an XFEG.
1.3.3 First condenser lens
The behavior of the C1 lens is similar to that in the conventional column. However, in a
conventional FEG, increasing the spot size leads to decreasing beam current; the beam is spread
out more by C1, and more current is stopped by the C2 aperture. When the monochromator is
used, this is not always the case. This is illustrated in the figures below which sketch the
trajectories when the beam is increased from spot 3 to spot 8.
spot number 3
spot number 5
spot number 8
As can be seen, the 150μm C2 aperture starts to limit the beam only at spot number 8. The
current in the probe remains constant between spot 1 and spot 7.
1.3.4 Second and third condenser lens
Normally, the C2 and C3 lens together form a zoom system for continuously varying the size of
the parallel illuminated area (in TEM) or the convergence angle of the probe (in STEM), similar
to that of the conventional column.
However, this zoom system is not available in the special mode described in the next section.
1.3.5 C2 lens for extra spot demagnification
When the monochromator is used to reduce the energy spread, an intermediate image of the
source is focused at the monochromizing slit. The size of this image is about 1μm. In probe
mode, this image is demagnified and focused on the specimen. Normally, only the C1 lens is
used to demagnify the source and this gives the standard range of spot numbers 1-11. Since this
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range ends at about ∼1000x demagnification at spot number 11, it is not sufficient for subnanometer monochromized probes.
More demagnification can be obtained in the 'Monochromator spot number' mode. Then the C1
lens is set a maximum strength (corresponding to spot 11) and C2 lens is set such that it helps
the C1 lens to further demagnify the source image. The C2 – C3 zoom system is no longer
operational, and the C3 aperture must be used in stead of the C2 aperture as the beam limiting
aperture. The figure below sketches the optics.
gun lens
monochromator
slit
C1
C2
C3
Obj
Obj
‘spot number 13’
C3 aperture
specimen
‘spot number 16’
Increasing strength of C2 gives increasing demagnification. The strength of the C2 lens is
denoted by a spot number ranging from 12 to 17 (Note: since the C1 lens is at a strength
corresponding to spot 11, some parts of the software keep indicating spot 12 to 17 as spot 11).
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Monochromator manual Titan V1.2
2
Version 1.2 and higher
March 16, 2011
Alignment
conventional FEG
monochromator
gun tilt pivot point
gun tilt
gun shift pivot point
gun shift
spot-size dependent
gun shift
at FEG
select central part of
FEG for maximum
intensity
at the end of the
accelerator
align accelerator to
condenser
correct beam shifts at
specimen
monochromator not
used
align FEG to
monochromator
at monochromator
align monochromator
to accelerator
monochromator
used
align FEG to
monochromator
at monochromator
align monochromator
to accelerator
at monochromizing
slit
align accelerator to
condenser
correct beam shifts at
specimen
at monochromizing
slit
align accelerator to
condenser
correct beam shifts at
C2 aperture
The gun shift/tilt deflectors (which are located between the accelerator and monochromizing
slit) and the monochromator deflectors align the FEG and monochromator with respect to the
condenser column. The table summarizes the use of these deflectors for a conventional FEG, for
the monochromized system in the state monochromator-not-used, and for the monochromized
system in the state monochromator-used. The following sections discuss their use in detail.
2.1 Conventional FEG column
In a standard FEG column, the gun alignment has two main steps. In the first step, one selects
the central part of the illuminating cone of electrons coming out of the FEG. This is done by
adjusting the gun tilt and optimizing the intensity.
In the second step of the gun alignment, the beam coming from the gun is aligned with the
optical axis which runs through C1 and C2. Due to image rotations, this optical axis depends a
little on the strength of C1 and C2. Therefore, the gun shift alignment is split in two. First, a
main alignment is made which shifts the gun to the average optical axis. Next, small offsets are
added for each C1 setting separately.
2.2 FEG column with monochromator
The basic principle is that the gun shift/tilt strengths and pivot points do not depend on the
monochromator settings (except for the spot-size dependent gun shift which depends on the
monochromator mode unfiltered/filtered). Details are explained in this section.
2.2.1 Gun tilt and monochromator deflectors
When the monochromator is present, gun tilt is no longer needed to select a specific part of the
illuminating cone of the FEG, since this is already done by the entrance aperture of the
monochromator. This aperture is mechanically pre-aligned and fixed. Instead, the gun tilt is
used to help aligning the FEG with respect to the monochromator and accelerator.
Misalignment of the FEG with respect to the monochromator entrance aperture results in a beam
which is not parallel to the axis of the accelerator. This can be corrected by the deflectors in the
monochromator but it can also be corrected by the gun tilt. It is best to use the monochromator
deflectors. The three figures below show trajectories for a misaligned tip (all other components
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perfectly aligned) where the gun shift/tilt deflectors are set such that all monochromator
deflectors are off at zero excitation. As can be seen from the figures, the beam goes far off-axis
in the monochromator at all excitations. Large off-axis trajectories are undesirable since they
give less energy resolution due to third and fourth order aberrations of the monochromator.
Excitation=0.0
Excitation=1.6
Excitation=2.3
In the next three figures below, the monochromator deflectors are set such that the beam crosses
the axis at the monochromator exit and thus also at the monochromizing slit. Since the gun shift
pivot point is at the monochromizing slit, only a gun shift is needed to align the beam to the axis
of the condenser. So this setting does not require a gun tilt correction when the rest of the
column is mechanically perfect. As can be seen from the figures, this is an optimal setting since
the beam stays fairly close to the axis in the monochromator at all excitations.
It should be noted that one can not simply set the current for the gun tilt to zero in order to get
this optimal alignment, since an unknown amount of gun tilt is still needed to correct mechanical
imperfections between monochromator, accelerator and condenser. Instead, one finds this
optimal alignment by deflecting the beam in the monochromator in four directions all the way
up to the edges of the large 1mm exit hole of the monochromator; and taking the center of these
four edges as the center of the monochromator. This is done in the “gun tilt and monochromator
shift” alignment procedure (see section 5.1.4).
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Excitation=2.3
2.2.2 Gun shift
The gun shift is used to make the beam as parallel as possible with the axis which runs through
C1 and C2 and the objective lens. The pivot point of the gun shift is taken at the
monochromizing slit. The left figure below shows the trajectories for a well-aligned gun shift.
The three figures on the right below show the trajectories for a misaligned gun shift. As
illustrated in these figures, a misaligned gun shift can be recognized from the fact that the beam
moves with respect to the C2 aperture when C1 is changed.
Gun shift good
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Gun shift wrong, increasing spot size from 5 to spot 9
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2.2.3 Spot-size dependent gun shift
The optical axis through C1 and C2 varies slightly with the setting of C1 (or “spot number” or
“spot size”) due to the image rotations in these lenses. When C1 is changed, the beam moves
slightly with respect to the second condenser aperture and with respect to the specimen. In a
conventional microscope, the beam is very broad at the second condenser aperture and
movements of the beam with respect to this aperture are of no importance. However, in the
monochromized microscope, the beam can be very narrow due to the monochromator entrance
aperture and slit.
The spot-size dependent corrections to the gun shift can compensate for this slight movement of
the beam. In a standard FEG microscope, these corrections are chosen such that focused probe
does not change position on the specimen. In the FEG column with monochromator, the spotsize dependent corrections depend on whether the monochromator is used or not.
•
When the monochromator is not used, the beam is usually fairly broad. Therefore the spotsize dependent gun shift is taken such that the focused probe changes not position on the
specimen (as in a conventional microscope).
•
When the monochromator is used, the beam is usually rather narrow (because of the inserted
slit). Therefore the spot-size dependent gun shift is taken such that the beam stays centered
on the second condenser aperture.
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Monochromator Optics
This section discusses the optics and working principle of the monochromator in some detail.
3.1 Excitation
In its most simple form, a Wien filter monochromator has only an electric deflection field E1 and
a magnetic deflection field B1 which are perpendicular to each other. Both fields are constant
over the length L of the filter. The deflecting forces -eE1 of the electric field and –evB1 of the
magnetic field cancel for electrons with average velocity v0 = E1/B1.
The above figure sketches some electron trajectories in the Wien filter. Electrons with average
velocity which enter at the center of the filter experience no deflection force and follow the
optical axis. If an electron enters with a velocity v0 + dv slightly above the average velocity, the
magnetic force dominates and the electron is deflected towards the electrode at negative
potential. However, when the electron approaches the negative electrode, its kinetic energy
decreases; the electric deflecting force starts to dominate and the electron is redirected to the
central axis. Analogously, an electron with a velocity v0 - dv just below the average velocity, is
deflected by the electric force initially, but wins energy upon approaching the positive electrode
and returns to the central axis.
Another example is given in this figure. Electrons with average velocity v0 enter the
monochromator with off-axial positions. The upper electron enters close to the positive electrode
and is accelerated by it. Due to its increased speed, the magnetic force dominates and the
electron is deflected towards the negative electrode. However, when the electron approaches the
negative electrode, its kinetic energy decreases; the electric deflecting force starts to dominate
and the electron is redirected to the positive electrode.
In both figures, all trajectories can be decomposed as the sum of a straight line movement and a
circle movement:
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In these examples, the Wien filter was tuned such that the electrons completed precisely one
circle movement. This is setting is called a 2π excitation. When the Wien filter is shorter, or
equivalently, when the deflection fields are smaller, different excitations are obtained. This is
illustrated in the following figure:
Excitation = π/4
Excitation = π/2
Excitation = 3π/4
This figure nicely illustrates that the Wien filter acts as a thick lens, of which the strength
depends on the excitation. In order to have a focus at the exit of the monochromator, the
convergence of the beam in front of the monochromator must be properly matched to its
excitation. The tuning of the convergence is done with the gun lens:
Excitation = 0
Excitation =π/4 =0.78
Excitation =π/2=1.57 Excitation=3π/4=2.36
The dispersion increases with the excitation.
3.2 Double focusing Wien filter
In section 3.1 we discussed a simple Wien filter which only has a homogeneous magnetic and a
homogeneous electric deflection fields, perpendicular to each other and perpendicular to the
beam. This simple filter only focuses in the xz-plane (the dispersive plane) and has no effect on
the beam in the yz-plane (the non-dispersive plane). Thus the monochromator makes a line focus
at the monochromizing slit. This is illustrated in the left figure below.
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For reasons of alignment it is easier to have a point focus at the monochromizing slit. Therefore
an electric quadrupole field E2 is added over the whole length of the monochromator. This field
focuses in the yz-plane and defocuses in the xz-plane (see right figure above). The strength of the
quadrupole field is chosen as E2 = -E12/V (E1 is the electric deflection field, V is the
monochromator beam potential); at this strength the total focus in both planes is equal.
3.3 Monochromator stigmators
The monochromator has two stigmators for correcting astigmatism of the gun lens and
astigmatism of the monochromator. In a conventional column, astigmatism is corrected such that
the probe becomes as small and as round as possible. This criterion can not fully be applied
when the monochromator is on, since the monochromator disperses the spot to a line.
The following figure of nine images explains this. When the monochromator is off, and the
beam is in focus, the spot is simply point-like, as in the middle image of the left column.
When one goes under or over focus, the point-like spot becomes a round circle, as in the upper
and lower image of the left column. The monochromator focuses different energies at different
positions. The middle image of the middle column illustrates this for five spots which are 0.1 eV
apart. When one goes under or over focus, the five spots become five overlapping circles, as in
the upper and lower image of the middle column. Of course, a real beam consists of a continuum
of energies, and the monochromator focuses these at a continuum of positions, as in the middle
image of the right column. This is the optimal setting for using the monochromator. When one
goes under or over focus, the continuum of spots become a continuum of overlapping circles, as
in the upper and lower image of the right column.
Small note: the images in the right column have been made with a symmetric distribution of
electron energies; however the energy distribution of a FEG is usually asymmetric since the high
energy side is determined by the Fermi-Dirac distribution which is proportional to exp(-E/kBT),
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whereas the low energy side is determined by the probability for tunneling through the potential
barrier between the tip and the vacuum which is more linear in the energy.
When astigmatism is present in the direction 45 degrees to the dispersion, the images become as
in the figure below. This astigmatism can easily be recognized from the fact that the defocused
spot is tilted with respect to the dispersion (as in the upper and lower image of the right column),
and from the fact that it is not possible to focus the image to a small line.
When astigmatism is present in the direction of the dispersion, the images become as in the
figure below. This astigmatism can mislead the user, since it is possible to make a line focus (as
in the lower image in the right column) which suggests that a sharply focused dispersion is
present. However, this astigmatism can be recognized from that the fact it is possible to get a
square probe by defocusing (as in the upper image in the right column). For HR-EELS
experiments it is advised to check whether this astigmatism has been completely corrected by
slightly varying this astigmatism, and looking for the optimal resolution.
3.4 Monochromator slit
The accelerator acts as an electrostatic lens of fixed strength, which images the exit plane of the
monochromator on to the variable C1 aperture. The monochromizing slit is part of the variable
C1 aperture mechanism. It consists of a number of straight slits, wedge-shaped slits and small
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circular holes. The straight slits can be used if a reproducable and specific slit width is needed;
the wedge-shaped slit can be used to continuously change the slit width; the holes can be used
for making small circular monochromized probes (note: the straight slits and small holes are not
present in slits produced before 2010).
2000mu
50mu
slit1
slit2
aperture +x
aperture +y
Sketch of the apertures in the C1 aperture holder. The arrows 'aperture +x' and 'aperture +y'
indicate in which direction the aperture holder moves when its x or y position is increased.
3
3
2
2
2
2
1
1
1
1
μm
1 μ
m diameter
holes
50μm
50μm
holes
100μm
100μm
double wedge slit
μm
7μm
7
100μm
100μm
μm slit
2μm
2
100μm
100μm
μm slit
1μm
1
100μm
100μm
μm slit
0.5
0.5μm
μm
200μm
200
aperture +x
aperture +y
Slit1 with dimensions of the straight slits and small holes which have been introduced in 2010.
The position of the slits is controlled from the Apertures panel of the user interface of the
microscope. The first condensor aperture has four positions corresponding to the 2000μm,
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50μm, 'Slit1', and 'Slit2' apertures. The 'Slits' flap-out of the Apertures control panel offers the
possibility to store three separate positions per slit (in a similar way as the user interface offers
the possibility to store three separate values for the column stigmators or for the image shifts).
This helps to quitckly switch between different slit sizes or slit shapes. The coordinates (in μm)
of the three positions are also shown in the flap-out. If the 'Adjust' knob is activated, only the
position of the currently selected position is adjusted. If the 'Adjust All' knob is activated, the
positions of all three selected positions are adjusted simultaneously.
Panel for controlling the apertures.
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4
User interface
4.1
Monochromator panel
Version 1.2 and higher
March 16, 2011
The monochromator panel contains the main control functions of the monochromator.
State control
With this radio button one can choose between two states for the monochromator
•
Unfiltered: It is assumed that the user does not want to monochromize the beam. The
excitation is set to zero and cannot be changed. The spot-size dependent gun shift is aligned
such that the beam remains stationary on the specimen when the spot-size is changed.
•
Filtered: It is assumed that the user wants to monochromize the beam. The excitation control
is enabled. The spot-size dependent gun shift is aligned such that the beam remains
stationary on the monochromator slit and the C2 aperture when the spot-size is changed.
Potential
With the Potential control, the beam potential inside the monochromator is selected. Its voltage
supply can deliver any value between 300 and 3600 V. However, the software accepts only
those values for which the present Excitation can be realized.
Excitation
The Excitation control sets the strength of the deflection fields inside the monochromator. It is a
dimensionless number between and 0 and 2.3.
Normalize
Pressing the Normalize button starts a sweep of the magnetic deflection field of the
monochromator. The field goes from its present value to its maximum value, then to its
minimum value, and back to its present value. The total sweep takes about 10 seconds. The
monochromator alignments apply this normalization, so it is advised to always use the proper
normalization. This is most easily done by selecting the default option “Automatic
Normalization” of the Settings flap-out (see next section).
Find Beam
Pressing the Find Beam button starts an automatic sweep of the monochromator shift offsets.
The sweep halts as soon as the beam is detected on the viewing screen or when it has reached its
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maximum range. The sweep can be interrupted by pressing Find Beam again. Parameters
controlling the sweep pattern can be set in the FindBeam flap-out (see next section).
Undo Find
The Undo Find button becomes available after a Find Beam action. Pressing the Undo Find
button will reset the monochromator shift offsets to their values before the Find Beam action
was started.
Flap-out
Pressing the arrow button displays the flap-out containing the Settings flap-out, Gun lens flapout and Find Beam flap-out. These are described in the next subsections.
4.1.1 Settings flap-out
In the Settings flap-out options can be set which relate to the main operation controls of the
monochromator.
Potential offset
With this control an offset (in Volts) can be set on the monochromator potential. This offset can
be used to calibrate the dispersion of the monochromator since it gives a shifting of the beam in
the dispersed direction equal to the offset divided by the dispersion.
Potential ramp step
When the Potential control in the monochromator panel is set to a new value, the actual potential
ramps to this new value with steps of which the size is set by the Potential ramp step control (in
Volts). The Potential ramp step is also the step with which the potential is changed when the
spin button of the Potential control is pressed in the Monochromator panel.
Excitation ramp step
When the Excitation control in the monochromator panel is set to a new value, the actual
excitation ramps to this new value with steps of which the size is set by the Excitation ramp step
control (in radians). The Excitation ramp step is also the step with which the excitation is
changed when the spin button of the Excitation control is pressed in the Monochromator panel.
Maximum allowed Potential
This is the maximum monochromator potential (in Volts) that can be set at the present
excitation. The potential is limited in most cases by the maximum that the monochromator
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potential supply can deliver (namely 3600V). However, at high excitations it is limited by the
maximum that the supplies for the monochromator deflection fields can deliver.
Maximum allowed Excitation
This is the maximum excitation (in radians) that can be set at the present potential.
Automatic Normalization
When the Automatic Normalization option is selected, the magnetic deflection is automatically
normalized each time when the excitation is lowered. It is recommended to select this option.
4.1.2 Gun lens flap-out
When the energy selection slit is used, it is essential to focus the dispersed beam in the plane of
this slit. The focusing is done with the gun lens. The software which controls the
monochromator can calculate which gun lens strength is needed for this focus. In the Gun lens
flap-out options can be set which relate to this calculation.
Defocus at slit
For special applications it can be desirable not to focus in the plane of the energy selection slit.
With the Defocus at slit control one can choose the size of the defocus (in meters) at this plane.
Gun lens controlled by monochromator
When this option is not selected, the gun lens is set through the Gun lens control in FEG panel
(as in a standard Tecnai). When this option is selected (which is default), the gun lens is
automatically set by the monochromator software to the value which is needed having the
dispersed beam focused on the energy selection slit. This option is automatically selected when
the monochromator is switched to the unfiltered or filtered state.
Accelerating gun lens
When the gun lens is used as an accelerating lens, the potential of the gun lens electrode is
higher than the potential of the monochromator. As a consequence, most of the focusing action
of the gun lens takes place at the entrance plane of the monochromator. At this plane, the gun
lens aberrations are very small. Moreover, the Coulomb interactions are relatively small due to
the high potential in the gun lens. Therefore, this mode gives the highest brightness and highest
energy resolution. However, the total current in the beam is only a few nA. Due to the limited
voltage supply of the gun lens, the accelerating gun lens can only focus at the slit when the
monochromator potential is around or below 1kV. Use this mode for STEM or for (sub)-0.1eV
TEM experiments.
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Decelerating gun lens
When the gun lens is used as a decelerating lens, the potential of the gun lens electrode is lower
than the potential of the monochromator. As a consequence, most of the focusing action of the
gun lens takes place at the gun lens electrode. At this plane, the gun lens aberrations can be quite
noticeable. The total current in the beam can be up to 50 nA. Use this mode for all TEM
applications which do not require the ultimate resolution.
Wobble, Amplitude and Frequency
When the wobble button is pressed, the voltage on the gun lens will be wobbled according to the
parameters set with the Amplitude and Frequency controls. Wobbling the gun lens can be
helpful for stigmating the (dispersed) image at the energy selection slit or for finding the beam.
4.1.3
Find Beam flap-out
The beam can be missing due to wrong deflections in the monochromator. In this case, the Find
Beam routine can help finding the beam. Pressing the Find Beam button starts an automatic
sweep of the monochromator shift offsets which halts as soon as the beam is detected on the
viewing screen, or when the maximum ranges of the sweep have been reached. Options of this
sweep are set in the Find Beam flap-out.
Find Beam pattern
The user can select the following beam patterns:
•
Hysteresis: for finding a beam which is probably lost due to hysteresis. The beam is swept
up and down in the dispersive direction.
•
HT Change: for finding a beam which is probably not far away. The beam is swept over a
small square area around the present position.
•
Tip Change: for finding a beam which is at different position (e.g., because of a tip change).
The beam is swept over a large square area around the present position. This can take a few
minutes.
•
Custom: use the following controls to define one’s own beam pattern:
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Block spiral
The block spiral starts from the current position and spirals outward. The picture gives an
example with step sizes of one and maximums of 3.5 and 2.5 for x and y, respectively.
Horizontal swing
The horizontal swing starts from the current position and goes back and forward in the xdirection, each time at a different y-shift. The picture gives an example with step sizes of one
and maximums of 3.5 and 2.5 for x and y, respectively. Use this pattern if you expect that the
beam has moved away due to hysteresis.
Step size for shift x and shift y
These controls are for the shift sizes of the steps with which the Find Beam routine sweeps.
Since Shift-x is the step of the monochromator’s magnetic field (in mT) and Shift-y is the step of
the monochromator’s electric correction field (in V/mm), equal numbers in the x and y field do
not correspond to equal deflection angles. It is advised to use step-x =0.0010 – 0.0020 and
step-y=0.1000 – 0.2000.
Maximum for shift x and shift y
These controls determine the boundary of the pattern with which the Find Beam routine sweeps.
Since the hysteresis effect in shift-x is about 0.1000, it is not very useful to use maximum x
values larger than 0.2000. Entering very large values for these controls can lead to very long
search times, since the number of steps is about 3 per second.
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Monochromator Tune panel
The monochromator Tune panel contains the controls for aligning the monochromator.
•
The position with which the beam exits the monochromator (and consequently, with which
the beam hits the monochromizing slit) can be adjusted with the shift buttons. The x shift
adjusts the main magnetic field (displayed in mT) which results in a shift in the direction of
the dispersion of the monochromator. The y shift adjusts an electrostatic deflection
(displayed in V/mm) perpendicular to the dispersion.
•
The astigmatism of the beam at the monochromizing slit can be adjusted with the
monochromator stigmator buttons. The x stigmator stigmates in the direction of the
dispersion. The y stigmator stigmates in the direction rotated 45 degrees from the
dispersion. Both stigmating fields are in V/mm2.
•
The position of the focus of the dispersed spot can be adjusted with the focus control. This
button sets an offset on the gun lens (in V).
The monochromator shift, stigmator, and focus can be stored in the alignment tables by pressing
the Store buttons. The section on the Offsets flap-out will explain this functionality in detail.
Shift
When the Shift button is pressed, the multi-function buttons get connected to the
monochromator User shift offsets.
Stigmator
When the stigmator button is pressed, the multi-function buttons get connected to the
monochromator User stigmator offsets.
If the value of the monochromator stigmator somehow gets outside the range of its electrical
supplies, the User offset is set back to zero in order to prevent clipping.
Store (next to Shift and Stigmator)
When this Store button is pressed, the User shift offsets and User stigmator offsets are made
permanent by replacing the stored offset values by the sum of the User offsets and the old stored
offsets. After this replacement, the User shift and User stigmator offsets are set to zero. The
Total shift and stigmator offsets do not change.
Focus
When the Focus button is pressed, the intensity button gets connected to the gun lens offset.
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Slit Wobbler
An easy way to focus the monochromizing slit is to switch on the Slit Wobbler (which wobbles
the gun shift) and to minimize the movement of slit by adjusting a condenser lens (=Intensity
button). This lens is either the C2 lens (in the C3off mode ) or the C3 lens (in Probe mode). In
TEM mode, this focusing is not possible because the illumination is parallel.
Store (next to Focus)
When this Store button is pressed, the User Gun lens offset is made permanent by replacing the
stored offset value by the sum of the User offset and the old stored offset. After this
replacement, the User Gun lens offset is set to zero. The Total gun lens offset does not change.
4.2.1
Offsets flap-out
The Offsets flap-out lists three columns of offsets. The first column shows the Stored offset, the
second column lists the User offsets, and the third column lists the Total offset which is the sum
of the Stored offset and the User offset.
The Stored offsets are saved in the alignment file (see File flap-out of the Alignment panel) and
are restored by selecting the monochromator part of the alignment file.
The User offsets are set in the Monochromator Tune panel, either by using their spin controls,
either by using the multi function buttons.
The Total offsets can be saved with the menu in the File flap-out (see section Error! Reference
source not found.) or with FEG Registers (see section 4.3)
When the Store button is pressed (as illustrated in the figure below), the Stored offsets are
replaced by the sum of the Stored offsets and the User offsets. Next, the User offsets are set to
zero. The Total offsets do not change.
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Initially, when no alignments have been stored, all Stored offsets are zero. When only one store
has been made at some particular setting A (characterized by the monochromator potential and
excitation), and the user changes to a different setting B, the routine will use the Stored offsets
from A at B (taking into account a scaling with the monochromator potential).
When two stores have been made at settings A and B, and the user changes to a different setting
C, the routine will linearly (intra- or) extrapolate the stored offsets at A and B to C (taking into
account a scaling with the monochromator potential).
When three or more stores have been made, and the user changes to a new setting, the routine
will quadratically (intra- or) extrapolate the stored offsets at the three nearest settings to get to
the new setting (again taking into account a scaling with the monochromator potential).
There are two different alignment tables for the monochromator offsets, one for the accelerating
gun lens, one for decelerating gun lens.
Reset
When the Reset button is pressed, all Stored offsets are deleted. The User offsets are adjusted
such that the Total offsets do not change for the present settings.
Default
When the Default button is pressed, a simple factory alignment (consisting of a few stored
values only) is set. The User offsets are adjusted such that the Total offsets do not change for the
present settings.
Set Default
This button is only available for Service and Factory. When this button is pressed, the present
Stored offsets are set as the default Stored offsets.
4.2.2 Outputs flap-out
The Outputs flap-out can be used by Service to check the supplies of the monochromator. These
are the supplies for the main magnetic deflection field (Icoil), the main electric deflection field
(V1x), the perpendicular deflection field (V1y), the two stigmator fields (V2x and V2y), and the
monochromator and gun lens potential. The flap-out also lists the present excitation.
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The first column lists the values that are sent to the DACs. The second column lists the
measured values that are sent from the DACs to the power amplifiers (these can be used to
check the communication between the PC and monochromator supplies at HT). The last column
lists the values that are actually measured at the outputs of the amplifiers (these can be used to
check the amplifiers).
4.3
FEG Registers
Note: FEG Registers is freeware. Please read the copyright and liability limitations in the on-line
help pages before using the software.
FEG Registers can be used to save and recall the settings of the FEG, monochromator,
condenser lenses, gun deflectors and beam deflectors. The precise list of settings is given in the
next section. FEG Registers makes it very easy to switch quickly between different operation
modes of the microscope, or to relocate the beam when it is lost.
Each user can store up to 200 settings under a user-defined label. These settings are saved in a
file (one for each user) under the Windows user name. Whenever the user logs in, the settings
are loaded from the file. Each of the settings can be selected in the list and set to the microscope.
Settings can also be updated and deleted. They can also be saved to and recalled from files with
user-defined names.
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If you want to save settings for different high-tension settings, identify them as such by the
label. High tension is neither stored not recalled automatically.
Set
When the Set button is pressed the settings from the register currently selected in the list will be
restored to the microscope. It is possible to restore only a specific part of the illumination (for
example, restore the FEG and monochromator settings but do not restore the condenser lenses)
by selecting specific categories in the Options flap-out (see Section 4.3.1).
Update
When the Update button is pressed, the selected setting is updated to the settings currently active
on the microscope. Typical use of this function would be to select a setting and set it back to the
microscope. Them modify any microscope settings needed (e.g. the gun alignment) and then
update. Update requires confirmation (since the old setting will be overwritten).
Delete
When the Delete button is pressed the setting currently selected in the list will be deleted. To
guard against accidental deletion, the Delete button will request confirmation. Multiple settings
can be selected for deletion (use Shift+Click to select a range or Ctrl+Click to select individual
settings).
Settings list
The setting list gives an overview of all settings of the current user. The settings are
automatically stored when the user logs off (it is not necessary to save them to file, settings are
added simply by Add and removed by Delete). The settings list initially will display the registers
in the sequence they have been defined. It is possible to sort the settings in the list differently by
pressing one of the buttons at the top of the list (Lbl, Pot, Exc, ...). At the first press, the settings
are sorted in normal order. When the same button is pressed again the sequence is reversed. The
width of the columns can be adjusted by moving the cursor to the boundaries between the
buttons and dragging the boundary to left or right.
The columns in the list have the following meaning:
Lbl : user-defined label for the setting.
Pot : monochromator potential
Exc : monochromator excitation
Mode : microscope mode
Spot : spotsize
Date (usually out of sight due to lack of space) : the date at which the setting was added.
These data are only a subset of the settings stored, but they are the important ones that allow the
user to see which setting should be recalled.
A setting is selected for setting to the microscope or updating by clicking the required row. The
label of the setting is automatically copied to the settings label edit field to ensure the proper
setting is used for updating.
Settings label
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The settings label defines the name of a new register setting (when Add is pressed). If no text is
entered, the software will add a label 'Register' with some number attached.
Add
When the Add button is pressed the settings currently on the microscope are stored in a new
register which is added to the list (all categories are saved to the FEG Register file, independent
of what is selected in Options flap-out).
4.3.1
Options flap-out
The Options tab allows to select which part of the
settings has to be restored to the microscope, and it
contains controls for saving the settings to file and
loading them from file.
Settings files
Settings files can be saved under any file name and in any location with the following
restriction: filenames with names that correspond to existing Windows NT usernames (that is,
the name under which users log in) are not allowed. These files will in fact exist for users who
have used FEG Registers before and contain their settings. You can open those files and use the
settings in there, but you cannot overwrite them.
Files do not contain a single setting but all settings together. When you load settings from file
you automatically remove all currently existing settings!
Settings options
The following categories of settings can be separately be restored:
FEG settings
- Extraction voltage
- Gun lens
- Gun tilt and Gun tilt pivot points
- Gun shift and Gun shift pivot points
- Spotsize-dependent gun shift
- Gun cross-over and Gun stigmator
Monochromator
- Potential
- Excitation
- Monochromator settings (file)
Microscope mode
- TEM / STEM
- LM, SA, Mh, LAD, D
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- Microprobe / Nanoprobe/Lorentz
- Dark field
- Normal / EFTEM
Illumination
- C2 Aperture selected and position
- Illumination mode (Parallel / Probe)
- Condenser lens mode
- C2 cross-over mode
- Condenser free control mode
- Spot number
- Intensity
- Projection diameter
- Illumination diameter (parallel mode)/Convergence angle (probe mode)
Magnification
- Magnification (TEM or STEM)/ Camera length
Direct alignments
- Spot-size dependent intensity correction
- Beam shift
- Beam tilt pivot points
- Rotation center
- Diffraction shift
- Dynamic conical dark field pivot points
- Descan corrections
Stigmators (current channel only)
- Condenser stigmator (the complete list for all spots)
- Three-fold condenser stigmator
- Objective stigmator
- Diffraction stigmator
Open
The Open button brings up the standard Open file dialog from which a file can be selected for
loading. See the remark above about overwriting the currently existing settings.
Save
The currently existing settings can be saved to file. When the Save button is pressed the settings
are saved under a user-defined filename.
Note: Under normal circumstances it is not necessary to save settings in a file because a user's
settings are saved automatically. Saving files is only useful when a user wishes to have several
sets of settings of FEG Registers.
Save As
Save as is the same as Save except that another filename can be chosen.
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5
Version 1.2 and higher
March 16, 2011
Monochromator alignment procedures
The alignment procedures for the monochromator are accesible through the Alignments panel, in
the folders 'Gun' and 'Monochromator Table'.
5.1
Gun Alignment
5.1.1 Preparation
Purpose: Check that the beam is visible in the mode used to align the monochromator gun
(decelerating 3kV) and set up the gun lens.
Method:
- Remove objective and SA apertures and the C1 slit. Select largest C2 aperture size. If the
beam is not visible, perform the procedure 'Find Beam' (see Section 5.1.2) first.
- Turn the Automatic Normalization off in the Settings flap-out of the Monochromator panel.
- Center the beam on the screen with the monochromator shifts.
- Defocus the gun lens such that the beam fills the C2 aperture. Then center the C2 aperture.
- Insert and focus the monochromizing slit and focus it. This ensures that the operator is
looking precisely at the plane of the monochromizing slit. The focusing is done by adjusting
the C2 lens such that the movements of the slit are minimized when the gun shift wobbler is
on.
Next, the slit is taken out and the gun lens is used to focus the beam to a spot at the plane of the
slit. The gun lens focus is stored.
5.1.2 Find Beam
Purpose: Find the beam and set up the microscope for aligning the monochromator.
Method:
- Remove all apertures or select largest aperture size.
- Find the beam by defocusing the gun lens and scanning with monochromator shifts.
- Focus the gun lens such that the FEG is imaged at the plane of the monochromizing slit.
Procedure:
In the first step, the microscope is set in a condition in which it is likely that the beam is visible
(the monochromator is set to zero excitation and 3kV beam potential; all user offsets of the
monochromator are set to zero; the gun lens is set to decelerating mode; C1 is set to spot size 3;
C2 is set to a lightly defocused beam; the lowest SA magnification is set; the beam blanker is
switched off; the HAADF detector is retracted; the apertures are set to the largest holes; the user
offsets for beam and image deflectors are set to zero; all lenses are normalized).
If the beam is not visible, the operator is asked to try to find it by adjusting the shifts in the
monochromator (step 3). When the beam is not easily found, the routine jumps backward to step
2, where the gun lens is strongly defocused and wobbled. This makes the beam usually visible
but quite weak. When it is visible, the gun lens defocus (in the Monochromator Tune panel)
should be slowly increased to zero while keeping the beam centered with the monochromator
shift. When possible, the wobble amplitude should be reduced using the Gun lens wobbler
settings in the Gun Lens flap-out of the Monochromator panel.
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
It is should be noted that in step 2, when strong misalignments of the monochromator shifts and
strong defocus of the gun lens are present, it is possible that one does not find the central beam
but instead a secondary beam which is reflected somewhere in the monochromator. This can be
recognized by two things: First, it is absolutely not possible to get the reflected beam focused to
a bright spot. Secondly, the monochromator coil current (visible as Set value for Icoil in the
output flap-out of the Monochromator Tune panel) is around 0.8mA for the central beam, and
differs by more than 2 mA from this value for a reflected beam.
When the beam is found, the operator is asked in step 4 to set it in the middle of the C2 aperture
and to defocus the gun lens such that the narrow beam leaving the monochromator nicely
illuminates the whole C2 aperture. After this, the monochromator is normalized and the beam
centered again in the C2 aperture.
5.1.3 Gun shift pivot point
Purpose: Align shift tilt pivot point such that the beam does not move in the plane of the
monochromator slit when a gun shift is done.
Method: Minimize spot movement.
Procedure:
The monochromizing slit is inserted and focused. This ensures that the operator is looking
precisely at the plane of the monochromizing slit. The focusing is done by adjusting the C2 lens
such that the movements of the slit are minimized when the gun shift wobbler is on. Next, the
slit is taken out and the gun lens is used to focus the beam to a spot at the plane of the slit. The
gun shift wobblers are switched on and the operator minimizes the spot movements by adjusting
the position of the pivot point.
5.1.4 Gun tilt and monochromator shift
Purpose: Set the gun tilt and monochromator shift such that the beam is closely to the optical
axis of the monochromator. This minimizes higher order aberrations in the monochromator and
optimizes energy resolution.
Method: Use monochromator Shift to deflect the beam in the monochromator in four directions
all the way up to the edges of the large 1mm exit hole of the monochromator. Take the center of
these four points as the center of the monochromator.
Description:
Both the monochromator shift and gun tilt can be used to direct a tilted beam in the
monochromator back to the axis of the condenser system. Because of higher order aberrations in
the monochromator, it is important to keep the beam close to the axis of the monochromator.
FEI Electron Optics
Page 36 of 56
Monochromator manual Titan V1.2
Beam corrected by gun tilt only
Version 1.2 and higher
March 16, 2011
Beam corrected by monochromator shift and gun tilt
So, the trajectories in the left figure above give less good energy resolution as the trajectories in
the right figure above.
A good setting is found by ensuring that the beam goes through the centre of the large 1mm hole
at the exit of the monochromator. This position is found by deflecting the beam strongly in the
monochromator until it hits the sides of the exit hole (the beam is kept visible by simultaneously
compensating with the gun tilt). This is done most accurately when the beam is focused to a
fairly small spot.
Notes:
The exit hole of the monochromator is often easily recognizable by its very irregular borders.
However, in a few cases, the border manifests itself by deflection and distortion of the beam. In
that case, the position of the border should be taken as the position for which the spot begins to
deform.
It is important that the user does not exit this alignment before it is finished. Otherwise he can be
left with settings for the gun tilt for which the beam is only visible when it hits a border of the
large 1mm hole.
Procedure:
The alignment routine asks the operator to consecutively increase the monochromator shifts
such that the right border, left border, upper border and lower border of the exit hole are seen.
Next it sets the beam to the average of the border positions and normalizes.
FEI Electron Optics
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
5.1.5 Gun shift
Purpose: Make sure that the beam is as parallel as possible to the optical axis of the condenser
system. This keeps the beam near the same position on the C2 aperture and at the objective lens
for all spot sizes.
Method: Switch between spot size 3 and 9 and tune the gun shift until the position of the beam
changes as little as possible.
Procedure:
The gun shift alignment consists of two steps:
• A first step in which spot 9 is centered with the beam deflection coils.
• A second step in which spot 3 is centered with the gun deflection coils.
The two steps are repeated until the change in gun shift is very small. In the gun shift procedure
all spot-size dependent gun shift values are first reset to zero.
5.1.6 Spot-size dependent gun shifts
Purpose: Make sure that the beam remains centered for all spot sizes.
Method: For all spot sizes tune the spot-size dependent gun shift such that the beam remains
centered.
Description:
The optical axis through C1 and C2 varies slightly with the setting of C1 (or “spot size”) due to
the image rotations in these lenses. When C1 is changed, the beam moves slightly with respect
to the second condenser aperture and with respect to the specimen. In a conventional
microscope, the beam is very broad at the second condenser aperture and movements of the
beam with respect to this aperture are of no importance. However, in the monochromized
microscope, the beam can be very narrow due to the monochromator entrance aperture and slit.
The spot-size dependent corrections to the gun shift can compensate for this slight movement of
the beam. In a standard FEG microscope, these corrections are chosen such that focused probe
does not change position on the specimen. In the FEG column with monochromator, the spotsize dependent corrections depend on whether the monochromator is used or not.
1. When the monochromator is not used, the beam is usually fairly broad. Therefore the spotsize dependent gun shift is taken such that the focused probe changes not position on the
specimen (this is the conventional constraint).
2. When the monochromator is used, the beam is usually rather narrow (because of the inserted
slit). Therefore the spot-size dependent gun shift is taken such that the beam stays centered
on the second condenser aperture.
These two corrections are set in the alignments “Spot-size dependent gun shift for unfiltered
mode” and “Spot-size dependent gun shift for filtered mode”.
Procedure
The spot-size dependent gun shift alignments consist of three steps:
• A first step in which spot 3 is centered with the beam deflection coils.
• A second step in which spot 11 is centered with the beam deflection coils (spot 3 is done
first because spot 11 may be difficult to find, especially if the beam is defocused).
• A third step in which all spots down from 11 (10 to 1) are centered on the specimen (for the
unfiltered mode) or on the C2 aperture (for the filtered mode). In the filtered mode, the gun
lens is wobbled. This makes it more easy to recognize the center of the beam and to see
simultaneously the C2 aperture.
FEI Electron Optics
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
5.2 Monochromator Table alignments
The monochromator software can keep a list of offsets (for monochromator Shift,
monochromator Stigmator, and monochromator Focus) for different monochromator settings. At
first start-up of the microscope, this list is empty. The user can fill it by an arbitrary amount of
settings, which are identified by the monochromator potential, monochromator excitation and
gun lens mode (accelerating or decelerating). When three or more settings are stored, the
monochromator makes a quadratic interpolation on the three nearest by settings.
For the SFEG, the monochromator table section has three sections: (1) alignment of the standard
range of excitations for decelerating mode at 3000V potential, (2) transition to accelerating
mode, and (3) alignment of the standard range of excitations for the accelerating mode at 800V
potential. For the XFEG, only section (1) is done, because the accelerating mode is not present
for the XFEG.
5.2.1 Excitation alignment for decelerating mode
Purpose: Make sure that the beam remains centered when changing the excitation of the
monochromator.
Method: Gradually increase the excitation, while re-centering the beam after each change. Store
the monochromator settings at regular intervals.
Description:
The monochromator is set to decelerating mode and potential=3000V. For the SFEG, the
excitation alignment procedure fills the list at excitations = 0.0, 0.4, 0.8, 1.2, 1.6, 1.8, 2.0. For
the XFEG, the excitation alignment procedure fills the list at excitations = 0.0, 0.4, 0.8, 1.0.
Note:
The interpolation routine can give strong offset changes at small change of excitation when two
settings have been stored which differ much in offsets but differ little in excitation. This can
only occur when a new alignment is stored on top of an old alignment which was far off and
which was stored at different excitations. It is therefore advised to remove the old offset table
using the Reset button in the Offsets flap-out of the Monochromator Tune panel.
Procedure:
The excitation is changed in steps of 0.1. After each step the operator is asked to center and
focus the beam.
5.2.2 Accelerating gun alignment
Purpose: Make sure that the beam remains centered when changing the gun lens between
decelerating and accelerating.
Method: In decelerating mode, increase the gun lens defocus that much that the decelerating
gun lens becomes accelerating, while keeping the beam centered using the monochromator
Shift.
Note:
When the gun lens is brought to accelerating mode for the first time, the gun lens electrode is
baked-out by the fast electrons and the pressure near the tip increases. The pressure can be
monitored as the IGP3 current in the flap-out of the FEG panel. When the gun lens potential is
raised too fast, the pressure will rise too high and the FEG protection software will switch off
FEI Electron Optics
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
the FEG and high tension. If this happens, restart the FEG after a few minutes and re-try the
accelerating gun alignment.
Procedure:
The operator gradually decreases the monochromator potential from 3000V to 800V. Next, he
gradually increases the gun lens from decelerating focus (typically around 500V) to accelerating
focus (typically around 5000V), while keeping the beam on the C2 aperture using the
monochromator Shift. Between 1500V and 3000V the gun lens is far from focus and the beam is
very weak. In order to improve the visibility of the beam, the C2 lens is adjusted such that only a
small part of the screen is illuminated.
5.2.3 Excitation alignment for accelerating mode
Purpose: Make sure that the beam remains centered when changing the excitation of the
monochromator.
Method: Gradually increase the excitation, while re-centering the beam after each change. Store
the monochromator settings at regular intervals.
Description:
The monochromator is set to accelerating mode and potential=800V. The excitation alignment
procedure fills the list at excitations = 0.0, 0.4, 0.6.
Note:
The interpolation routine can give strong offset changes at small change of excitation when two
settings have been stored which differ much in offsets but differ little in excitation. This can
only occur when a new alignment is stored on top of an old alignment which was far off and
which was stored at different excitations. It is therefore advised to remove the old offset table
using the Reset button in the Offsets flap-out of the Monochromator Tune panel.
Procedure:
The excitation is changed in steps of 0.1. After each step the operator is asked to center and
focus the beam.
5.3
Direct Alignments for the monochromator
5.3.1 Focus Slit
The 'Focus Slit' alignment can help to fine-tune or check whether the slit is focused on the
screen. When this Direct Alignment is selected, the gun shift wobbler is switched on. If the slit
is focused on the screen, the shadow of the slit is stationary although the whole illumination is
wobbling. If the slit is not focused on the screen, the shadow of the slit moves with an amplitude
proportional to the defocus.
FEI Electron Optics
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
5.3.2 Fine Mono Shift and Reset Fine Mono Shift
The two Direct Alignments described in the section can be helpful for making very small
monochromated probes.
Very small probes require a high spot number. At such high spot number, the monochromated
beam is considerably larger than the condenser aperture (see Sections 1.3.3 and 1.3.5). The
center of the monochromated beam must be well positioned with respect to the condenser
aperture in order to maximize the current in the probe.
Normally, this position is tuned using the deflectors of the monochromator (Monochromator
Shift-x and Monochromator Shift-y). However, the smallest step of the x-shift in the
monochromator is relatively large and can be too large for fine-tuning the position of the
monochromated beam, especially when the monochromator is operated at low potential (800V).
An alternative way to position the monochromator beam with respect to the condenser aperture
is to use the gun shift, which has much finer resolution. The use of the gun shift is enabled with
the following two Direct Alignments.
Selecting 'Fine Mono Shift' connects the gun shift deflectors to the multifunction knobs. The
multifunction knobs do not change the 'alignment value' of the gun shift, which is set in the gun
shift alignment (section 5.1.5), but instead they are connected to an offset on the gun shift. This
offset can be set back to zero by selecting the Direct Alignment 'Reset Fine Mono Shift'.
FEI Electron Optics
Page 41 of 56
Monochromator manual Titan V1.2
6
Version 1.2 and higher
March 16, 2011
Practical guidelines
6.1 General way of adjusting monochromator
For a well-aligned system, the user can easily switch between the various monochromator
modes by using settings stored in FEG registers. When these have not yet been made or when
the user wants to add new ones, or when the FEG registers settings are very old and no longer
re-call the beam in a reliable way, then the user has to adjust the monochromator. This requires
some care because the beam after the monochromator is relatively small and it is therefore
possible to lose the beam. Sections 6.3 to 6.5 describe how to adjust the monochromator without
losing the beam. The general way of adjusting is sketched in the following figures for the
monochromator with SFEG and with XFEG:
SFEG:
‘Start’
(decel,
pot=3000,
exc=0.0)
gradually
‘line decel’
(decel,
pot=3000,
exc=1.8)
gradually
(decel,
pot=800,
exc=0.0)
gradually
‘Start accel’
(accel,
pot=800,
exc=0.0)
gradually
‘line accel 0.2’
(accel,
pot=800,
exc=0.2)
gradually
‘line accel 0.4’
(accel,
pot=800,
exc=0.4)
How to gradually navigate to new monochromator settings for the SFEG.
XFEG:
‘Start’
(decel,
pot=3000,
exc=0.0)
gradually
‘line decel 0.6’
(decel,
pot=3000,
exc=0.6)
gradually
‘line decel 1.0’
(decel,
pot=3000,
exc=1.0)
How to gradually navigate to new monochromator settings for the XFEG.
Start from the basic setting 'Start' (decelerating gun lens mode, monochromator potential 3000V,
excitation zero). This setting is a good starting point, because it is the most stable over time. All
other settings can be created from this basic setting by gradually changing Potential or
Excitation or Gun Lens, while observing the beam and if it moves away use Monochromator
Shift to keep it visible.
FEI Electron Optics
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
If an old or invalid monochromator alignment is loaded, then it can be that you have to re-center
the beam after almost every little step during the gradual change of the monochromator settings.
In that case, remove the invalid alignment by pressing 'Monochromator Tune' Æ Offsets Æ
Reset. This should set the column 'Stored' displayed in the Offsets flapout to zero or to a more
valid alignment. If the more valid alignment is still too far off, then start the registry editor
(runÆ regedit) and delete the following branches 'SOFTWARE\FEI\BrickBox\Userdata\tem\
Username\MdlMonochromator', where Username is the name of all users which are higher in
hierarchy (like Factory and Service). It is advised to backup these branches, for example by
renaming them to MdlMonochromator2.
6.2 Recommended settings
General guidelines for setting the excitation of the monochromator are
•
increase excitation for more energy resolution
•
decrease excitation for more beam current.
With an SFEG, it is also possible to switch the gun lens between accelerating and decelerating
mode, see Section 1.3.2. Guidelines for these modes are
•
decelerating mode for more beam current
•
decelerating mode for more ease of use
•
accelerating mode for ultimate energy resolution and ultimate brightness.
The exact energy resolution of the monochromator depends on the intrinsic aberrations of the
monochromator (wich mostly increase with excitation) and on parasitic aberrations due to
mechanical imperfections (which mostly decrease with excitation). The three graphs below plot
the calculated optical energy resolution of the monochromator, for the range of mechanical
imperfections normally encountered. The total resolution of the system is also determined by the
contributions of the aberrations of the spectrometer, of the noise of the various supplies for the
high tension and spectrometer, of the point-spread-function of the spectrometer's CCD (typically
~2 pixels), of the Coulomb interactions in the beam, and of environmental disturbances such as
stray fields and mechanical vibrations. These contributions add between 0.07eV and 0.15eV in
quadrature to the optical resolution of the monochromator, depending on the type of spetrometer
and on the environment.
FEI Electron Optics
Page 43 of 56
Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
D50 optical energy resolution (SFEG, decelerating gun)
1
optical energy resolution [eV]
typical variation between
monochromators
resolution Tridiem865 (roughly)
0.1
resolution Tridiem866 (roughly)
optimum setting of
monochromator
0.01
0.0
0.5
1.0
1.5
2.0
2.5
excitation
D50 optical energy resolution (SFEG, accelerating gun)
0.3
optical energy resolution [eV]
typical variation between
monochromators
resolution Tridiem865 (roughly)
0.1
resolution Tridiem866 (roughly)
0.01
optimum setting of
monochromator
0.003
0.0
FEI Electron Optics
0.5
1.0
excitation
Page 44 of 56
1.5
2.0
Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
D50 optical energy resolution (XFEG)
optical energy resolution [eV]
1
resolution Tridiem865 (roughly)
0.1
resolution Tridiem866 (roughly)
optimum setting of
monochromator
typical variation between
monochromators
0.01
0.0
0.5
1.0
1.5
2.0
excitation
Calculated optical resolution of the monochromator for typical range of mechanical
imperfections. The total energy resolution is the sum of this optical resolution and of the
contributions of spectrometer, environemental disturbances, and electrical noise. These add
about 0.07…0.15eV in quadrature to the optical resolution of the monochromator.
Below is a table of applications with recommended settings.
FEI Electron Optics
Page 45 of 56
Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
SFEG:
gun lens,
Potential,
Excitation,
Reference
decel,
3000, 0.0,
Sect 6.3
'Start'
Microscope Remarks
XFEG:
Potential, mode
Excitation,
Reference
3000, 0.0,
Sect 6.3
'Start'
imaging,
spot 1..3,
microprobe
Non-monochromized
STEM
decel,
3000, 0.0,
Sect 6.3
'Start'
3000, 0.0,
Sect 6.3
'Start'
STEM,
spot 4..8,
nanoprobe
Non-monochromized HRSTEM
accel,
800, 0.0,
Sect 6.5
'Start accel'
3000, 0.0,
Sect 6.3
'Start'
STEM,
spot 6..8,
nanoprobe
Monochromized
illumination in TEM for
recording EELS spectra of
relatively large (>100nm)
areas
EELS spectra of medium
sized (∼10nm) areas
decel,
3000, 1.8,
Sect 6.4
'line decel'
3000, 1.0,
Sect 6.4
'line decel'
imaging,
spot 3..7,
microprobe,
C3off
decel,
3000, 1.8,
Sect 6.4
'line decel'
accel,
800, 0.4,
Sect 6.8
'line accel'
3000, 1.0,
Sect 6.4
'line decel'
diffraction,
spot 6..8,
nanoprobe
3000, 1.0,
Sect 6.8
'line decel'
STEM,
spot 12..16,
nanoprobe
accel,
800, 0.2,
Sect 6.5
'line accel'
accel,
800, 0.6,
Sect 6.7
'line accel'
accel,
800, 0.4,
Sect 6.8
'line accel'
3000, 0.4,
Sect 6.4
'line decel'
3000, 1.2,
Sect 6.7
'line decel'
imaging,
spot 3..6,
microprobe,
Probe mode
imaging,
spot 1,
microprobe
3000, 0.8
Sect 6.8
'line decel'
diffraction,
spot 15..17,
nanoprobe
Application
Non-monochromized
TEM
EELS spectra of small
(<2nm) areas
Monochromized
illumination for reducing
effect of chromatic
aberration in HR-TEM
Ultimate energy resolution
Ultimate spatial resolution
in monochromized STEM
FEI Electron Optics
Page 46 of 56
Underfocus 'Mono
Focus' such that the
largest C2 aperture is
fully illuminated at
spot 1
Overfocus
'MonoFocus' such that
the largest C2 aperture
is fully illuminated at
spot 1
Underfocus 'Mono
Focus' such that the
largest C2 aperture is
fully illuminated at
spot 1
Focus dispersed beam
on specimen; select
area of interest with
entrance aperture of
GIF
Use monochromator
slit; focus beam to a
small probe using
condenser lenses
Use monochromator
slit; use
monochromized
STEM to find area of
interest
only with image Cs
corrector; focus
dispersed beam on
specimen
Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
6.3 How to find the beam
Normally, the beam can be found by selecting a reliable setting from FEG Registers. When this
does not help, one can try and check the following:
1. Press Monochromator Normalize; change Monochromator Shift x up and down by about
0.02.
2. Check that the valves are open.
3. Check that the FEG emission is on.
4. Check that the high tension is on.
5. Check that the largest C1 aperture is in (thus no slit in), that the largest C2 and C3
apertures are in, that the objective aperture is out, and that the SA aperture is out.
6. Check that no specimen is stopping the beam (preferably take the specimen out).
7. Check that the gun lens is decelerating and controlled by the monochromator (see Gun
lens flap-out of Monochromator panel)
8. Set the monochromator excitation to zero and potential 3kV.
9. Check that all monochromator supplies are on: the Ref. and Readback values must not
differ significantly from the Set values (in the Outputs flap-out of the Monochromator
Tune panel).
10. Normalize the monochromator. While normalizing, check that the Readback value of
Icoil is actually going up and down.
11. In case you doubt the quality of the condenser alignment: switch of the C3 lens (Beam
Settings panel Æ Free Control Æ C3off).
12. Go to microprobe with the lowest SA magnification (about 4000x).
13. Choose spot number 1.
14. In the Monochromator Tune panel, set all controls (Monochromator Shift,
Monochromator Stigmator, and Focus) to zero.
15. Check that the Total offset (in the Offsets flap-out of the Monochromator Tune panel)
are not too large (Shift x =0.1000±0.1000; Shift y=0±3; Stigmator x,y=0±1; Focus
=0±100).
16. Set the beam shift to zero (by pressing Reset Beam in the Beam Settings panel).
17. If the beam is still not visible, one should redo the “Monochromator preparation
alignment” in the Alignment panel.
18. Defocus/focus the gun lens (using the Focus control in the Monochromator Tune panel)
and center the beam (using Monochromator Shift x/y) such that the C2 aperture is fully
illuminated.
19. Center the C2-aperture (focus the beam to a point using intensity-knob; center the point
using the left tracker-ball; defocus the beam again; center the shadow of the C2-aperture.
Repeat this procedure once). Maybe re-center the beam on the C2-aperture again (using
Monochromator Shift x/y).
20. Make sure that the objective lens is focusing at the eucentric height: Insert a specimen,
press eucentric focus and adjust the height of the sample such that it is in focus.
Alternatively, wobble the tilt of the stage and adjust the height of the sample such that
the movement of the sample is minimized, and focus the image.
21. If the condenser is in C3off mode (otherwise skip this step): Preferably select a hole in
the sample. Insert the selection slit and focus the image of the slit (using the C2 lens
which is controlled by the intensity knob). Remove the slit.
22. Maybe refocus the gun lens (using the Monochromator Focus) such that the C2 aperture
is fully illuminated.
FEI Electron Optics
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Monochromator manual Titan V1.2
Version 1.2 and higher
March 16, 2011
At this point, the setting of the monochromator may be saved using FEG Registers or the File
flap-out of the Monochromator Tune panel. A logical name for this setting could be 'Start'. The
user offsets may be made more permanent by turning them in to alignments by pressing the two
Store buttons on the Monochromator Tune panel.
6.4 How to switch on the monochromator
The microscope is assumed to be set as described in the section 6.3, that is, aligned for its offstate. The excitation of the monochromator can be switched on by following the
“Monochromator excitation alignment” in the Alignment panel, or by doing the following:
1. Go to microprobe with the lowest SA magnification (about 4000x). Select largest C2
aperture. Choose spot number 1. Adjust intensity such that 2/3 of the screen is illuminated.
2. Set the Excitation ramp step to 0.02 (in the Settings flap-out of the Monochromator panel).
3. Click on the spin button of the Excitation control (in the Monochromator panel). The
excitation will increase by 0.02. If the beam moves away from the C2-aperture, re-center it
using Monochromator Shift x/y.
4. Repeat the previous step until the excitation has reached its desired value. It is advised to use
to go to excitation=1.8 in decelerating gun lens mode for the SFEG, and to excitation 0.8 in
decelerating mode for the XFEG. This value gives a good balance between energy resolution
and monochromated beam current. Higher excitation gives slightly better energy resolution,
and lower excitation gives slightly higher monochromated beam current.
5. When large corrections of Monochromator Shifts x/y are required after each increase of
excitation, it is advisable to press Store (in the Monochromator Tune panel) at some few
excitations (e.g. at excitation is 0.0, 0.4, 0.8, …). When really large corrections of
Monochromator Shift x/y are required, it can be useful to remove old monochromator
alignments as described in the last paragraph of Section 6.1.
6. Use the Focus control (in the Monochromator Tune panel) to focus the spot to a line as sharp
as possible.
7. Remove the astigmatism in the y-direction by sharpening the line further using
Monochromator Stigmator y.
Now remove the astigmatism in the x-direction. This can be sometimes difficult to see.
8. First check the total x-astigmatism offset (in the Offsets flap-out of the Monochromator
Tune panel) which should be approximately –0.65±0.1 at 3 kV monochromator potential and
excitation=1.8; the offset is roughly linear in potential and excitation.
9. Defocus the spot (using Monochromator Focus) and make it as square as possible, with
sharp straight structure, as in figure (a) below. If the spot looks as in figure (b) or (c), use
Monochromator Stigmator y to make it as in (a). In (a), the spot is sharply focused in the
dispersive (horizontal) direction, and not focused in the vertical (non-dispersive) direction.
(a)
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(c)
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10. Now use Monochromator Stigmator x to make the structures inside the spot as round as
possible, as in figure (d). If you have done this correctly, you have reduced the size of the
spot in the vertical direction by a factor two [figure (a) is two times higher than figure (d)].
(d)
11. Use Monochromator Focus to focus the spot to a line.
12. With an XFEG, the focused line can show the effect of chromatic aberration of the gun lens.
This is illustrated in the sketch below. In the center of the dispersed line, thus at the central
energy, the line is well focused; at its outer ends, the line gets defocused by the chromatic
aberration. The point of best focus (indicated by the arrow below) moves as the
Monochromator Focus is changed. Of course, the best energy resolution is obtained from the
point of best focus.
under focus
in focus
over focus
Note: This chromatic aberration of the gun lens is not visible when the monochromator is
used with the SFEG. Because of the lower current density of the SFEG compared to XFEG,
the beam diameter must be larger with the SFEG. At this larger diameter, the contributions
of higher order aberrations (such as coma) dominate over the contribution of the chromatic
aberration.
13. Small x-astigmatism offsets can only be tuned by using the spectrometer and by observing
the energy resolution. This requires that the GIF is well-tuned. If it is not well-tuned, then
first proceed with Section 6.6 ("How to find the beam on the GIF") and Section 6.7 ("How to
tune the GIF for the highest resolution").
14. Tune the spectrometer and measure the energy resolution. Increase the Monochromator
Astigmatism x (and focus again to a line with Focus) in steps of approximately 0.015 and
measure the energy resolution for each step. Resolution loss is well noticeable when the
Monochromator Stigmator is more than approximately 0.04 from its optimal value. Try to
find the stigmator values 0.04 below and 0.04 above the optimal value. Set the stigmator
halfway those two values.
15. The present settings of the monochromator can be saved in the alignments by pressing the
Store buttons in the Monochromator Tune panel.
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March 16, 2011
At this point, the setting of the monochromator may be saved using FEG Registers or the File
flap-out of the Monochromator Tune panel. A logical name for this setting could be 'line decel'.
The user offsets may be made more permanent by turning them in to alignments by pressing the
two Store buttons on the Monochromator Tune panel.
6.5 How to switch to accelerating mode
If the accelerating mode has not yet been aligned, then use this fast procedure for getting the
microscope in accelerating mode. Accelerating mode is only possible for the SFEG, not for the
XFEG.
1. Set the microscope in the 'Start' setting described in Section 6.3 (that is decelerating mode,
potential=3000V, excitation=0).
2. Go to microprobe with the lowest SA magnification (about 4000x). Select largest C2
aperture. Choose spot number 1. Adjust intensity such that 2/3 of the screen is illuminated.
3. For ease of use, deselect automatic normalization (in Monochromator -> Settings flapout).
4. Gradually decrease the potential to 800V. If beam shifts away, re-center it with
Monochromator Shift.
5. When at 800V, press Normalize. If necessary, re-center beam with Monochromator Shift x.
6. Re-select automatic normalization.
7. Press both 'Store' buttons in 'Monochromator Tune'.
Now you have aligned the monochromator for Potential=800V, Excitation=0, and you can
directly go to the accelerating gun lens alignment, skipping the previous 'monochromator
potential' alignment:
8. Do 'Alignments -> Monochromator Table -> Accelerating Gun' alignment. This one has four
steps. In the second step, you should adjust Intensity such that the beam (in this case, the C2
aperture shadow) is condensed to a circle of about 5mm.In the third step, you gradually
increase the gun lens potential from about 700V to about 4700V. This corresponds to
Monochromator Focus from about 0 to 4000. At both values, the probe is in focus (the first
is the decelerating mode, the second is the accelerating mode). In between, the beam is not in
focus, and the intensity on the screen will be very low. However, if you use the small screen
and the binoculars then you can easily see what the beam is doing. If the beam moves away
then adjust the Monochromator Shift to keep it visible.
9. When you reach the second focus at Monochromator Focus of about 4000V, you can have
large astigmatism. It is not exceptional that you see that the two line foci are separated by
several hundreds of volts of the gun lens. Use the Monochromator Stigmator to minimize
this astigmatism (available in the alignment routine through user button R2). An relatively
easy way to correct very large astigmatism is the following: go to the line focus at the lower
Monochromator Focus (e.g. 3800). Increase the focus by 50 (e.g. 3850). Adjust the
Monochromator Stigmator such that you have again a line focus. Increase the focus by 50
(e.g. 3900). Adjust the Monochromator Focus such that you have again a line focus.
Etcetera.
10. In the fourth step of the 'Accelerating gun lens' alignment you defocus the gun lens such that
the C2 aperture is fully illuminated.
You have now created a setting for the accelerating mode. This setting of the monochromator
may be saved using FEG Registers or the File flap-out of the Monochromator Tune panel. A
logical name for this setting could be 'Start accel'.
You can now switch on the monochromator in accelerating mode:
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11. Check that you are looking at the plane of the monochromator slit (insert slit, adjust intensity
to focus slit, maybe use 'slit wobbler' to help find focus, extract slit)
12. If the monochromator is in the 'unfiltered' state, switch it to 'filtered'. If the beam disappears
at this point, it can only be due to a changing C2 lens (=intensity), to a changing gun shift, to
a changing beam shift, or to hysteresis in the monochromator (check by slightly changing
Monochromator Shift x). If you have problems finding the beam after switching from
'unfiltered' to 'filtered', then do the following: Go back to 'unfiltered'. Open the System Status
window. Make a screendump of this window or write down the values in it. Switch again to
'filtered'. Look for differences in System Status window. Minimize differences by adjusting
intensity, beam shift and/or gun shift.
13. Gradually (steps of 0.01) increase the excitation of the monochromator to about 0.4. Keep
beam centered with Monochromator Shift x,y. This excitation gives an optimum balance
between energy resolution and monochromated beam current. Higher excitation (e.g.
excitation=0.6) gives slightly better energy resolution, and lower excitation (e.g.
excitation=0.2) gives higher monochromated beam current.
14. Focus monochromator to a line with Monochromator Focus (not with C2=Intensity). Use
MonoStigmators to optimize line shape as described in steps 7 to 11 of section 6.4.
15. Small x-astigmatism offsets can only be tuned by using the spectrometer and by observing
the energy resolution. This requires that the GIF is well-tuned. If it is not well-tuned, then
first proceed with Section 6.6 ("How to find the beam on the GIF") and Section 6.7 ("How to
tune the GIF for the highest resolution").
16. Tune the spectrometer and measure the energy resolution. Increase the Monochromator
Astigmatism x (and focus again to a line with Focus) in steps of approximately 0.0050 and
measure the energy resolution for each step. The Resolution loss is well noticeable when the
Monochromator Stigmator is more than approximately 0.02 from its optimal value. Try to
find the stigmator values 0.02 below and 0.02 above the optimal value. Set the stigmator
halfway those two values.
This setting of the monochromator may be saved using FEG Registers or the File flap-out of the
Monochromator Tune panel. A logical name for this setting could be 'line accel'.
6.6 How to find the beam on the GIF
1. Check that sufficient beam intensity is concentrated in the small circle on the viewing
screen.
2. Check that the GIF slit is not inserted.
3. Select the middle-sized GIF entrance aperture.
4. Switch the GIF to spectroscopy mode.
5. Select a small dispersion (e.g. 0.1 eV/pixel).
6. Check that the Energy Offset and Energy Shift and Drift Tube are zero.
7. Start DM Æ Filter Æ Turbo View Spectrum
8. Raise the viewing screen.
9. Find the zero-loss peak in the spectrum by changing Adjust (in Filter Control). The Adjust
value can conveniently be adjusted using the ↑ and ↓ keys on the keyboard (use Shift+↑↓ for
steps of 10eV and Alt+Shift+↑↓ for steps of 100eV). Set the beam near the left edge of the
spectrum at zero energy.
6.7 How to tune the GIF for the highest resolution
10. Switch off the illumination of the control pads (in the Control Pads panel).
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March 16, 2011
11. Switch off all TL illumination near the microscope.
12. Switch the GIF to Imaging mode.
13. Select the largest GIF entrance aperture.
14. Select a GIF slit of approximately 20 eV.
15. Ensure that Results window of Digital Micrograph (DM) is open and visible.
16. Make a small (approx. 5 mm) and fairly bright beam on the viewing screen and center it at
the position of the GIF entrance aperture.
17. Raise the viewing screen.
18. Select the GIF CCD, full CCD. The CCD should not be in cinema mode. Start image
acquisition. Check that the beam fills the complete CCD of the GIF. Stop image acquistion.
19. If you are sitting on a metal chair, do not move the chair during the next step.
20. In DM, select Tune Spectrum Aberrations. The routine may complain that the intensity is too
high or too low. When the intensity is too high, increase the magnification of the
microscope. When the intensity is too low, decrease the magnification of the microscope.
21. Insert the smallest GIF aperture.
22. Switch to Spectroscopy mode.
23. Select dispersion 0.01 eV/pixel (use the upper one if you have two 0.01eV/pixel settings).
24. Select the GIF CCD. Set the CCD to binning 1, full CCD, about 0.1s exposure. The CCD
should not be in cinema mode. Start CCD image acquisition.
25. If necessary, reduce exposure time or beam intensity such that the CCD does not saturate.
26. If necessary, use Adjust to bring the zero-loss peak to the left of the image.
27. Tune Focus X (of Filter Control) such that the zero loss peak is focused to a thin line.
28. Tune Focus Y (of Filter Control) such that the line becomes nicely vertical.
29. Insert the smallest GIF aperture.
30. If possible, use Focus X/Y to make the line image even sharper and more vertical.
31. Stop the image acquisition.
The aberrations of the GIF are now well-tuned. In the next steps, a 'streak-image' is made to
visualize and minimize the effects of stray magnetic AC fields. This requires three things: no
shutter, double-focused dispersion, and zero exposure time:
32. Lower the viewing screen. Disable the shutter; an easy way to disable the shutter is to switch
the Shutter Control on the 'FirstLight' controller in the Gatan electronics cabinet from 'Auto'
to 'Open'. If you have a Quantum GIF with fast shutter, then you can disable the shutter by
DM Æ Fast Shutter Control Æ deselect 'Enabled'.
33. Select the 0.00 eV/pixel setting (select the lower 0.01eV/pixel setting if you have two
0.01eV/pixel settings). This setting is identical to the 0.01 eV/pixel setting, except for that it
also focuses the beam in the vertical direction.
34. Set the exposure time to 0 seconds.
35. Start the CCD image acquisition
36. Wait until the first image is displayed by DM. Then raise the viewing screen. The image on
the CCD will be a “streak image”, that is a recording of energy versus time, in which 50 Hz
(or 60 Hz) stray fields and other irregularities can be easily seen.
37. Adjust AC Comp A/B (of Filter Control) to minimize the 50 Hz (or 60Hz) effects.
38. You can use the AnalyzeStreaks.s script in DM to make a Fourier transform of the energy
versus time recording. The script is available through DMÆ Install GIF Æ Analyze Streak
Image (Alternatively, select the streak image; open AnalyzeStreaks.s using the File Open
menu; press Ctrl+Enter). The Results window will show the contributions to the resolution
of noise and the various harmonics of the stray fields.
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39. Make a line scan across the streak to find the energy resolution for very short times. One
pixel in the vertical direction of the streak image corresponds to about 1 millisecond
exposure time. For such an exposure, the resolution is only limited by aberrations and the
high-frequency ripple (typically 0.01-0.03eV) of the HT supply.
40. At this point, it is useful to fine-tune the x-astigmatism of the monochromator, as described
in step 14 of Section 6.4 for the decelerating gun lens mode and in step 16 of Section 6.5 for
the accelerating mode.
41. Try to optimize the resolution further by fine-tuning Focus X/Y.
It is worthwhile to try to improve the resolution further by iterating steps 37 to 41. The
attainable resolution in a 1 millisecond exposure is typically 0.08…0.13eV for a Tridiem866 and
0.15 … 0.20eV for a Tridiem865 at 300kV, depending on the quality of the environment and the
setting of the monochromator.
42. Leave streak image mode by undoing steps 32 to 34: Enable the shutter of the GIF by
switching the Shutter Control on the 'FirstLight' controller in the Gatan electronics cabinet
back from 'Open' to 'Auto'.
43. Select dispersion 0.01 eV/pixel (use the upper one if you have two 0.01eV/pixel settings).
44. Set the exposure time to (for example) 0.1 second.
45. Start spectrum acquisition.
46. Try to optimize the resolution by fine-tuning Focus X/Y.
Important notes:
•
Due to the point spread function of the CCD of the GIF, all measured peaks will be
broadened by about 2 – 3 pixels. Therefore, select a dispersion setting which gives at least 4
pixels per required resolution (e.g. 0.05 eV/pixel at 0.20 eV required resolution).
•
Do not use the drift tube to shift the energy to core losses, since the ends of the drift tube act
as electrostatic lenses which can shift and deform the beam which leads to resolution loss
(typically 0.5eV resolution loss at 500eV core loss). Instead use Energy shift.
•
When the microscope is in imaging mode, the energy resolution is limited by the size of the
diffraction pattern at the GIF energy selection plane. The full size β of the diffraction pattern
at this plane is inversely proportional to the magnification. At magnifications M below 10kx,
diffraction patterns with distinct spots can lead to spurious distinct peaks in the spectrum.
The resolution loss due to this effect is ΔE = cβ /M, with c = 175 keV at 300kV.
•
When the microscope is in diffraction mode, the energy resolution is limited by the size s of
the illuminated specimen since this area is imaged at the GIF energy selection plane. The
size of the illuminated area at this plane is inversely proportional to the camera length L. The
resolution loss due to this effect is ΔE = c s/L, with again c = 175 keV at 300kV.
•
When the microscope is in STEM mode, and when de-scan is not used, then a shift x of the
probe results in a shift of the cross-over in the energy selection plane. This results in a shift
of the energy by ΔE = c x/L, with again c = 175 keV at 300kV. De-scanning can reduce this
by at least a factor of 10.
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6.8 How to make small monochromized probes and STEM
First switch on the monochromator, for example by setting it to the setting 'line decel' described
in Section 6.4 or to the setting 'line accel' described in Section 6.5.
1. Go to microprobe with the lowest SA magnification (about 4000x). Select largest C2
aperture. Choose spot number 1. Adjust intensity such 2/3 of the screen is illuminated.
2. Switch to STEM mode. Switch out of diffraction. Select a low magnification (about 4000x)
Select a low spot number (3 or lower).
3. If the beam has disappeared, proceed as follows. Beam Settings panel Æ Free Control Æ
TEM mode or C3off mode; decrease spot number to 1; switch to microprobe mode. The
beam should be visible again. Switch back to nanoprobe, switch back to probe mode. If the
beam disappears again, find it back by adjusting Beam Shift and/or Monochromator Shift.
4. Insert the monochromator slit and ensure that it is reasonably centered. Ensure that the slit is
in focus (if not adjust Intensity). Select a slit width comparable to the width of the beam in
the non-dispersed direction (as in the second sketch in step 18 below). The geometry of the
slits is shown in the figures in Section 3.4. The width of the wedge-shaped slit can be
adjusted through Apertures Æ C1 condenser Æ Adjust Æ Multifunction x.
5. Take the slit out again.
6. Use the graph below to estimate which spot number you need. For each increase by one of
the spot number, the probe diameter decreases by a factor √2 and the probe current decreases
by a factor 2.
Titan nanoprobe (no probe corrector)
STEM probe [nm]
100
10
1
0.1
3
5
7
9
11
spot number
13
15
17
7. Increase the spot number by one. When necessary, re-center the dispersed beam on the C2aperture using Monochromator Shift x/y.
8. Repeat increasing the spot number until the desired spot number is reached or until you are
at spot 11. At the same time, increase the magnification gradually to about 100kx --- 400kx.
9. If you need a spot number ≤11, insert the (one but) smallest C2 aperture and center it.
Continue with step 14.
10. If you need a spot number ≥12, proceed as follows:
11. If not selected already select 'Probe' in the main Beam Settings panel. Open the 'Tune' flapout. Select Monochromator spot number 12.
12. Repeat increasing the spot number until the desired spot number is reached. When necessary,
re-center the dispersed beam on the C2-aperture using Monochromator Shift x/y.
13. Select the (one but) smallest C3 aperture. When necessary, re-center the C3 aperture.
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14. Write down the Monochromator Focus value. Defocus the Monochromator Focus such that
the whole condenser aperture is homogeneously illuminated.
15. Focus the beam to a small spot using the condenser lenses (Intensity knob).
16. Use the condenser stigmator to minimize astigmatism in the probe. Use ‘objective rotation
center’ from the Direct Alignments menu to minimize coma in the probe.
17. Refocus the Monochromator Focus to the value written down in step 14.
18. Insert the monochromator slit.
The following two figures sketch how the slit and the condenser aperture limit the beam.
Imperfect alignment of
dispersed line, slit and
condenser aperture.
Good alignment of dispersed
line, slit and condenser
aperture.
The actual image of the beam can be different for two reasons. First, the height of the dispersed
line is larger than the condenser aperture for the higher spot numbers (starting at approximately
>8), so then the upper and lower edges of the dispersed line are not visible. Second, for small
probes, the shape of the beam will be dominated by diffraction, and by the spherical aberration
of the objective lens, and maybe by remaining coma and astigmatism in the beam. Under such
conditions, it is difficult to recognize the positions of the beam and slit with respect to the
aperture.
19. If the beam disappears when the slit is inserted, then adjust the slit (Apertures Æ C1
condenser Æ Adjust Æ Multifunction y).
20. Adjust the position of the slit such that it is centered with respect to the condenser aperture
(Apertures Æ C1 condenser Æ Adjust Æ Multifunction y). If you cannot recognize the
edges of the slit, then adjust the slit position to maximize the symmetry in the probe.
21. Adjust the position of the beam in the non-dispersive direction such that it is centered with
respect to the condenser aperture (Monochromator Shift y). If you cannot recognize the
position of the dispersed beam, then adjust Monochromator Shift y such that the symmetry
in the probe is maximized and that the intensity is maximized.
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22. If recognizable, and if you are using the wedge-shaped slit, adjust the width of the slit
(Apertures Æ C1 condenser Æ Adjust Æ Multifunction x) such that the width of the slit and
the width of the beam in the non-dispersive direction are comparable.
23. Adjust the position of the beam in the dispersive direction (Monochromator Shift x) such
that the intensity of the beam is maximized.
24. Sometimes the size of the smallest steps of Monochromator Shift x,y are too large for a
perfect fine-tuning of the beam position. In that case, use Direct Alignments Æ Fine Mono
Shift (the Fine Mono Shift is a small offset on the Gun Shift deflectors).
25. Go to diffraction. Focus the beam by observing the Ronchigram.
26. Start TIA. Press Search in the STEM panel. Fine-tune the probe focus and probe astigmatism
with the condenser stigmator.
27. When the monochromator has recently been switched on, its electronics will start to heat up
and this causes a slow drift of the beam, mostly in the dispersive direction. After 5…20
minutes it is advised to correct the drift with Monochromator Shift x. The electronics will be
stable after a few hours.
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