FEG Alignment in the Tecnai F30

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FEG Alignment in the Tecnai F30

• Chris Deeb
• Case Western Reserve University
• Dept. of Materials Science and Engineering

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Outline

• Overview-Operation
• Important Operation Parameters
• FEG Registers
• Basic Alignment
• Advanced Alignment Procedures
• TEM Modes and Alignment

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Outline

• Overview
• Field Emission Gun (FEG)
• FEG Assembly
• Advantages
• Important Operation Parameters
• FEG Registers
• Basic Alignment
• Advanced Alignment Procedures
• TEM Modes and Alignment

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Overview-FEG

• Tungsten (100) Tip
• F30 has a Schottky FEG
• ZrO2 coated tip
• Electrons originate from the center of the tip
• yields smaller apparent (virtual) source
• Gives high brightness and sharp angular current
  distribution

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FEG Assembly
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FEG Assembly

• Suppressor Cap
• limits the electron emission to the desired area
  of the tip
• actually blocks electrons from the heater and
  shaft
• Heating Filament-tungsten hairpin
• heats the tungsten tip to enhance emission
  (1800K)
• Emitter

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FEG Assembly

• Extractor Anode
• applies voltage to the filament to extract
  electrons from the tip (1.8 - 7 keV)
• Gun Lens
• Electrostatic lens
• forms a crossover of the electron source
• acts similar to the C1 lens

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FEG Assembly
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FEG Advantages

• Small apparent source
• helps obtain small probes with high temporal
  coherence
• Decreased energy spread

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Outline

• Overview-Operation
• Important Operation Parameters
• Extraction Voltage
• Gun Lens
• C1 Lens (spot size)
• C1 aperture
• FEG Registers
• Basic Alignment
• Advanced Alignment Procedures
• TEM Modes and Alignment

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Extraction Voltage

• Determines the emission current
• Typical extraction voltages are lt 4 keV
• yields average emission current lt 100mA
• Extractor Limit set to 4.55 keV
• Emission currents gt150 should be avoided
• Highest extraction voltage is used for
  microanalysis and to eliminate side lobe emission

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Extraction Voltage

• Lower extraction voltages (3.5 - 4 keV)
• used for high resolution and BF imaging
• FEG registers optimized for spatial coherence and
  energy spread
• High extraction voltages
• used for microanalysis where probe current is
  most important
• Extraction voltages gt4.55 keV dont increase
  emission (suppressor limits current)

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Gun Lens

• Gun lens forms first cross-over of the gun
• depends on the extraction voltage
• Stronger gun lens voltages form the cross-over
  higher and higher up the column
• Weak gun lens forms cross-over near the C1 lens
• Stronger gun lens settings lead to reduced gun
  aberration

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Gun Lens

• Left
• Weak gun lens
• High intensity
• High effect of gun aberrations
• Right
• Strong gun lens
• lower intensity

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Condenser (C1) Lenses and Apertures

• C1 Lens
• spot size (similar effect as gun lens)
• C1 Aperture
• smaller apertures used for microanalysis
• better vacuum than C2 (usually cleaner than C2)
• Used to limit current, improve in hole
  performance and eliminate side lobe effect

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Outline

• Overview-Operation
• Important Operation Parameters
• FEG Registers
• HREM
• Microanalysis
• Basic Alignment
• Advanced Alignment Procedures
• TEM Modes and Alignment

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FEG Registers

• Used to save combinations of parameters for use
  in different situations
• Extraction voltage
• Gun lens
• C1-spot size
• related parameters

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HREM

• Mode TEM BF (microprobe)
• C1 aperture 2 mm
• C2 aperture 100/50 um (largest not recommended)
• Objective aperture 100 um
• Extractor 3.5 - 4 kV
• Emission lt100 mA
• Gun Lens 1 - 3
• Spot size 1

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HREM Important Points

• Weak gun lens and C1 maximizes current
• Extraction voltage kept low to minimize energy
  spread
• Maximum C1 aperture used
• no need to limit the current
• Microprobe allows large illuminated area
• for contamination, defocus intense beam

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Microanalysis Important Points

• High Extraction voltage used
• maximize current since apertures will be used to
  form fine probe
• Strong gun lens necessitated by extraction
  voltage
• high extraction voltage and strong gun lens
  situation very sensitive to gun alignment
• Strong gun lens allows fine probe
• spot size and gun lens combine to form finest
  probe
• If one is changed, the other should also be
  changed

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Microanalysis Important Points

• Nanoprobe used to form fine probe
• also limits the effect of spurious x-rays
• C1 used to form probe
• occurs in higher vacuum than C2
• less scattering at the edge of C1
• C2 can be used to block any electrons scattered
  by C1 edge

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Microanalysis

• Mode Nanoprobe
• C1 aperture 30 um
• C2 aperture 150 um
• Extractor lt4.6 kV (i.e. crossover mode)
• Gun Lens 5 - 8 (depending on current
  required)
• Spot size 3 (S-TWIN)

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SAED

• Mode Microprobe
• C1 2 mm
• C2 150/100/50 um
• Extractor lt4.6 kV
• Gun Lens 1-3
• Spot size any

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Microdiffraction

• Smallest Angle
• Mode Microprobe
• C1 2 mm
• C2 30 um
• Extractor lt4.6 kV
• Gun Lens 5 - 8
• Spot size 7 - 11

Smallest Probe Mode Nanoprobe C1 2 mm C2 30
um Extractor lt4.6 kV Gun Lens 5 - 8 Spot
size 7 - 11
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Outline

• Overview-Operation
• Important Operation Parameters
• FEG Registers
• Basic Gun Alignment
• Advanced Alignment Procedures
• TEM Modes and Alignment

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Gun Tilt/Shift

• Gun Tilt- put beam parallel to the optic axis
• Gun Shift- put beam along the optic axis

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Basic Gun Tilt

• Maximize the intensity of the beam using the gun
  tilt coils
• use the exposure time as a measure of the beam
  intensity
• image of the cross-over cant be used as when a
  LaB6 filament is used

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Gun Tilt Pivot Point

• Use this alignment to alleviate beam motion when
  the gun tilt is corrected
• Align the gun tilt
• Correct the gun tilt pivot points
• Correct the gun tilt again since the pivot point
  alignment affects the gun tilt
• This alignment is in the Alignment procedure (not
  direct alignments)

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Basic Gun Shift

• Misalignments are demagnified when stronger C1
  lens settings (smaller spot sizes) are used
• Use small spot size as a reference using beam
  shift coils
• misalignment free condition
• Align the gun shift coils on a larger spot size
  relative to the smaller spot

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Basic Gun Shift

• For the F30, use spot 6 to set the beam shift
• For the F30, use spot 2 to correct the gun shift
• For gun tilt and shift, the direct alignments can
  be used

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Spot Size Dependent Gun Shift

• Used to correct the gun shift for each spot size
• also ensures that each spot is aligned along the
  optic axis
• Use the alignment procedures
• Align spot 11 using the beam shift
• Align all other spots relative to spot 11 using
  the gun shifts

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When is Re-alignment Necessary?

• The gun coils are located after the accelerator
• this is after the extractor and the gun lens
• The gun tilt and shift must be re-aligned any
  time
• the mode is changed
• the gun lens is changed
• the extractor is changed

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Saving Alignments

• In the alignment CP
• flap-out contains alignment file
• if alignment procedure is performed, it is saved
  here

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Outline

• Overview-Operation
• Important Operation Parameters
• FEG Registers
• Basic Gun Alignment
• Advanced Alignment Procedures
• TEM Modes and Alignment

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Illumination Fine Tuning in HREM

• Slightly overfocus the beam in Mh mode
• Observe bright spot in the beam
• Using fine gun tilt adjustment, maximize current
• pivot point alignment will make this easier
• change fine gun shift to put bright spot in the
  center of the beam
• important for even illumination in HREM

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Spot Size-Intensity Calibration

• Provides a reference condenser setting for each
  spot size
• lets the microscope know the C1/C2focus of each
  spot size
• Focus Spot 3 and then focus all other spots
• microscope notes the deviation in C1/C2 for each
  spot
• This is necessary for intensity zoom and
  intensity limit operation

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Side Lobe Emission

• At high extraction settings, emission occurs from
  the faces of the filament

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Side Lobe Elimination Procedure

• Procedure
•          Remove any C1 aperture and select the
  largest C2 aperture. Select the spot size to be
  used.
•          Turn up the extraction voltage to 4.55
  - 4.62 kV.
•          Switch the gun lens to 5. Typically the
  beam will now disappear altogether so watch the
  direction in which the beam disappears.
•          Switch back to gun lens 6 and observe
  which multifunction knob changes the gun tilt
  mostly in the same direction as the one in which
  the beam disappears when the gun lens is set to
  5.
•          Change the gun tilt a bit and go back
  to gun lens 5. Repeat switching to 6 and changing
  the gun tilt until there is light visible for gun
  lens 5.
•          Optimize gun tilt and gun shift for gun
  lens 5. Typically what is observed is a bright
  spot (which may as yet be quite malformed) and a
  bright ring showing the outline of blobs. The
  bright line is the image of the gun lens (the
  blobs are charging effects of the - still -
  low-energy electrons at the edge of the gun
  lens). Center the bright spot inside the ring and
  optimize gun tilt and gun shift as well as
  possible. If the bright spot inside the beam
  cannot be centered with the gun controls, try
  using the rotation center to center it.
•          Insert the C1 aperture (the smallest)
  around the beam. This will cut off the image of
  the gun-lens ring.
•          Insert the required C2 aperture so that
  is centered around the beam and no edge of the C1
  aperture remains visible.
•  

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Changing Modes

• When FEG operating modes are changed the gun must
  be realigned for best operation
• C2 aperture also must be realigned due to changes
  in the minicondenser