electron optics two essential components:
DESCRIPTION
Electron gun Cathode Anode Alignment coils. Lenses condensers objective. Objective aperture assembly. Electron Optics Two essential components:. Electron source (gun) Focusing system (lenses) Add scanning apparatus for imaging. sample. → → →. F = -e(VxB). - PowerPoint PPT PresentationTRANSCRIPT
Electron Optics
Two essential components:
1) Electron source (gun)
2) Focusing system (lenses)
Add scanning apparatus for imaging
Electron gunCathodeAnodeAlignment coils
Lenses condensers
objective
Objective aperture assembly
sample
Electron LensesPrinciple of electromagnetic focusing
Interaction of electromagnetic field on moving charge (electron)
Vector equation
Magnitude of F will be
F = -eVBsinθwhere θ is the smaller of two angles between V and B
Direction of F determined by “right hand rule”
F = Force on electronV = electron velocityB = Magnetic field strength (rotationally symmetric)e = charge of electron
→ → →F = -e(VxB)
Axially symmetric electromagnetic lens
Fe shielding to prevent flux leakage from solenoid
Lens strength: intensity of field in gaps
proportional to (NxI) N = # turns of solenoid winding
I = current flowing through windings
Focal Length (f):Point on Z axis where initially parallel rays cross the axis after
passing through the lens
HIGHER LENS STRENGTH = SHORTER FOCAL LENGTH
B = vector parallel to fieldBr = radial component of field (vector perpendicular to axis)Bz = axial component of field (vector parallel to axis)B varies depending on position in lens
Results:
Force strongest in the center of the gap
Actual electron paths spiral through lens
Low lens strength = long focal length
High lens strength = short focal length
f
f
Upper polepiece
Lower polepiece
Usually two-stage condenser system
Purpose:
1) Further demagnification of the gun crossover image
2) Control amount of beam entering objective
major effect on beam current at sample
High Lens strength
Small crossover image
Less current entering the objective
Low Lens strength
Large crossover image
More current entering the objective
Sample
Objective
Condenser
Electron gun
f (low condenser lens strength)
f (high condenser lens strength)
Low condenser strengthLong focal lengthSmall divergence angleHigh current into objective
High condenser strengthShort focal lengthLarge divergence angleLow current into objective
Objective lens
Purpose: control the size and shape of the final beam spot
Considerations:
Produce small beam diameter
Magnetic field must not hinder detector system from collecting low eV electrons
Must allow clear path from sample to detectors
Bore must be large
Contain scan coils and Stigmators
Beam-limiting apertures
Aberrations usually increase with focal length – minimize aberrations
For quantitative analysis
long focal length = long working distance
high takeoff angle and low absorption
For objective
Increase demagnification by increasing lens strength to produce small spot size
→ Short working distance due to short focal length
→ Short depth of field
Objective lens, therefore,
Controls image focus = beam size
Objective lens design
Pinhole lens
Snorkel lensImmersion lens
Depth of field
Depth of field
Sample
Objective
Condenser
Electron gun
f (high objective lens strength)= short working distance
f (low objective lens strength)= long Working distance
Effective beam diameter
High objective strengthShort focal lengthLarge divergence angleShort working distanceShort depth-of-fieldHigh resolution
Low objective strengthlong focal lengthSmall divergence angleLong working distanceLarge depth-of-fieldLow resolution
Current in final beam spot is a function of:
1) Condenser lens strength2) Objective aperture size
Objective aperture affects:1) Current reaching sample2) Beam size passing through
objective3) Depth of field
Tradeoff: Best image resolution with smallest spot size (smaller aperture)Direct cutting of current compromises signal produced as current reduced in final spot
Depth of field
Plane of focusD
Region of image in effective focus
Long working distance
Sample surface
Short working distance Insert smaller objective aperture to improve D
Pixel size
Objective aperture assembly = beam regulating assemblySense current drift on aperture and adjust condenser lens strength to compensate
Beam current regulation
Feedback to condensers
Production of minimum beam sizeMagnification:
Mc = S0/Si = magnification of condenser
S0 = distance from gun crossover to lens gap
Si = distance from gap to where imaged
(related to focal length)Diameter of beam after passing through condenser
di = d0/Mc
S0 is constant, so
Increase condenser current → decrease in focal length and increase in demagnification (smaller crossover diameter)
Note that divergence angle also increases so less current enters objective
Sample
Objective
Condenser
Electron gun
S0
S1
For objective:Increase demagnification to produce small beam diameter, results in:- short working distance- short depth of field
Final beam size:
df = d0 / (McMobj)
Current in final beam is a function of:- condenser lens strength- objective aperture size
Minimum beam size (and highest resolution) achieved by increasing lens strength – sacrifice current (and signal / noise)Quality of final beam at the sample will be affected by:
- lens aberrations- vacuum quality- sample effects
Lens aberrationsSphericalChromaticDiffractionAstigmatismComa
Electrons moving in trajectories further from optic axis are focused more strongly than those near the axisCauses image enlargement (disk = ds)
Diameter of ds = ½ Cs α3
Cs = spherical aberration coefficientα = angle of outer ray through lensCs minimized in short focal length lenses (e.g., immersion)For pinhole lenses, can decrease spherical aberration by decreasing α using aperture
Lens aberrationsSphericalChromaticDiffractionAstigmatismComa
If energies of electrons passing through the lens differ, they will follow different ray paths
Diameter of dc = CCα (ΔE / E0)
Cc = chromatic aberration coefficient
α = convergence angleDirectly related to focal lengthMuch less significant at high E0
Always some spread in initial electron energies as leave cathodeW ~ 2 eVLaB6 ~ 1 eVFE ~ 0.2 to 0.5 eVMinimize by decrease in α
Lens aberrationsSphericalChromaticDiffractionAstigmatismComa
Diameter of dd = 0.61 λ / α
λelectrons = 1.24 / (E0)1/2
Larger divergence angle (α) = smaller contribution of dd
Wave nature of electrons
Spherical aberration disk ds and aperture diffraction disk dd vs. aperture angle
Lens aberrationsSphericalChromaticDiffractionAstigmatismComa
Magnetic lenses have imperfect symmetryEnlarges beam diameter and changes shape
Use stigmators in objective lenssupplies weak correcting fieldtypically use octupole arrangement
Lens aberrationsSphericalChromaticDiffractionAstigmatismComa
Different focal lengths of electron paths with different incidence anglesGenerally eliminated by proper lens alignment
Aberrations most significant in objectiveFinal probe size = quadrature sum of disk diameters…
dp = (dg2 + ds
2 + dd2 + dc
2)1/2
Importance:At 30 kV, tungsten filamentimax = 1.64 x 10-12 A
diameter dd = 1.4 nm
ds = 2.5 nm
chromatic aberration especially significant for W sources where ΔE ~ 2-3 eV
Increase effective diameter1) 30 kV dp 5nm → 6.5nm
2) 15 kV → 9.5 nmlower energy spread will make a big difference…
dg = gaussian probe size
Sample
CP
CP
Schottky - UC, hot swap gun
Double magnetic shielding
Electrostatic scanning
Fast beam
Bias
2 - mode objective lens with field - free and immersion capability
Sample
CP
CP
Schottky - UC, hot swap gun
Double magnetic shielding
Electrostatic scanning
blanker
Bias Beam deceleration
2 - mode objective lens coils with field-free and immersion capability
Beam at HV Beam at landing V
vCD New TLD design
Elstar electron column
Key design elements:• Large travel/tilt stage compatible
• “DualBeam” SEM-FIB• Minimum lens aberrations
• Automated aperture selection• Default small beam current
• Small gun apertures• High current for analytics >20 nA
• Thermal & environmental stability• Constant power lenses• Double magnetic shielding• Electrostatic scanning
CPI1I2
CPI1I2
CPI1I2
Magellan XHR SEM: three beam modes available
Schottky-FEGextractor,2 apertures
segmentedgun lens
aperture and slit
deflector
Standard High current Monochromated (UC)
Performance improvement with monochromator
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 10 20 30 40alpha [mrad]
25-7
5% e
dge
reso
lutio
n [n
m]
total, UC offtotal, UC onCc, UC offCc, UC ondiffractionCs
Beam voltage 1 kV
Improved resolution
More current (better S/N)
Chromatic
Spherical
UC allows larger α therefore lower diffraction (dd)
Diffraction effects dominate everything at low aperture angles…
The chromatic and spherical aberration contributions dominate at high aperture angles…
Magellan: Resolution improvement UC off and on
• 1 kV, no beam deceleration• 256 nm HFW in both cases
UC off UC on
Beam deceleration: enhancing resolution and contrast
If Bias=0 (no BD):Landing V = HV
What is beam deceleration?New optics mode enabling high resolution imaging and high surface sensitivity at very low kV
BD specifications:• Landing energy range: 30 keV down to 50 eV• The deceleration (Bias) can be continuously
adjusted by the userBenefits:
• Enhances the resolution • Provides additional contrast options• Greatest benefit at 2kV and below
Bias
HVLanding V
Beam
vCD
Sample
TLD
2-mode final lens
Extending low kV resolution via UC and BD
0
0.5
1
1.5
2
2.5
3
3.5
10 100 1000 10000 100000Landing voltage [Volt]
25-7
5% e
dge
reso
lutio
n [n
m] BD on, UC on
BD off, UC onBD off, UC off
50 V with 1 kV beam deceleration850 nm HFW
50 V
Practical considerations:
Must optimize: Instrument alignment (gun, lenses, apertures) Minimum vibration Minimum stray fields Specimen contamination Clean apertures
Then go for minimum spot size if resolution is the goal
However, ultimate resolution depends on the detected signal, a function of:
Minimum spot size AND
Emission volume of signal… beam-specimen interactions
SEM variables and secondary electron imaging – essential tradeoffs
Res. DOF S/N charge Res. DOF S/N charge Res. DOF S/N charge Res. DOF S/N chargeHigher Voltage (kV) Higher Current Longer WD Larger Obj. Aperture
decrease
increase
Arrows illustrate result (increase or decrease) on increasing kV, current, working distance, or objective aperture diameter
Metal coatC-coat
Res. = resolutionDOF = Depth-of-FieldS/N = Signal/noiseCharge = propensity for charging
Complex