chapter 2 electron optics/ operation modes electron gun
TRANSCRIPT
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Chapter 2 Electron Optics/ Operation Modes
Electron gun- thermionic source- Field emission source- Brightness of source- Coherence of source
Magnetic lenes- Spherical aberration- Chromatic aberration- Astigmatism
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• 我們必須使⽤用透鏡(聚光鏡,物鏡,投影鏡)及光闌(聚光光闌,物鏡光闌,擇區光闌)的組合來控制成像(imaging),繞射(diffraction)及
成份分析的號。
電子顯微鏡夠構造圖
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• 我們必須使⽤用透鏡(聚光鏡,物鏡,投影鏡)及光闌(聚光光闌,物鏡光闌,擇區光闌)的組合來控制成像(imaging),繞射(diffraction)及
成份分析的號。
電子顯微鏡夠構造圖
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• 我們必須使⽤用透鏡(聚光鏡,物鏡,投影鏡)及光闌(聚光光闌,物鏡光闌,擇區光闌)的組合來控制成像(imaging),繞射(diffraction)及
成份分析的號。
電子顯微鏡夠構造圖
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2.1Electron Source
• thermionic source
• field-emission source
no#
keV
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A. Thermionic Source• Work function: energy barrier required for an electron to escape from the metal
surface
�Polarization effect�Image effect)
+
e+
+-
-
-
surface
To extract electrons from the “tip”, work is needed.
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Thermionic tip
WLaB6
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Thermionic tip
WLaB6
50µm
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Thermionic tip
WLaB6
50µm
10µm
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• J:current density (Amp/cm2)• A:Richardson’s constant (A/cm2
* k2):• T:operating temperature (ok)
• Requirement for a filament
1. low work function 2. high melting temperature 3. stable at high T
Richardson’s Law
e
e
e
field emission
thermionic
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Electron Source
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Thermionic gun
electrostatic lens
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temperature limited current
space charge limited current
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Space Charge/ Boersch Effect
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Space Charge/ Boersch Effect
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Space Charge/ Boersch Effect
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E-gun
pannel
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E-gun
pannel
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E-gun
pannel
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E-gun
pannel
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E-gun
pannel
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B. Field Emission Gun
r : radius of curvature of tip。
Generally, r <0.1µm(1000Å), V=kv(1000ev)--->E=106 V/m strong electric field makes electron tunneling
E=V/r
V: applied voltageE: electric field
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B. Field Emission Gun
r : radius of curvature of tip。
Generally, r <0.1µm(1000Å), V=kv(1000ev)--->E=106 V/m strong electric field makes electron tunneling
E=V/r
V: applied voltageE: electric field
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Field Emission Gun
V1=幾kV:extraction voltage萃取電壓
V0=100kV or more:accelerating
voltage(加速電壓)
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•Thermionic Source vs. Field Emission Source
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•Thermionic Source vs. Field Emission Source
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•Thermionic Source vs. Field Emission Source
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2.2. Brightness
A brightness vs intensity
Intensity: detector
(Amper/cm2*sec) detector
Brightness; Source
(Amper/cm2*sr) source
Drawback of FEGstability is poorer, Flash is needed
Advantage of FEG Brightness is 1000 times higher
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Brightness(β)
• do 燈絲半徑(ro) 交叉點半徑(do) W = 50µm 100 µm LaB6 = 10 µm 10 µm FEG < 10 nm 10 nm
• current : ie
• semi-angle : αo
• Cross-Over in electron gun
current density
solid angle αo
do
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Energy band gap, phonon scattering, electronic structure
SrTiO3
Importance of BrightnessSingle atom spectroscopy
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ZrO2/W: Work function f ZrO2 is low. It takes advantage of both thermionic gun (stability) and FEG (high brightness)
Schottky
功函數φ
Φ θ
θ
θ
Fermi距離表⾯面 X
Schottky
場發射
熱游離
Schottky Gun
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場發射光源具有點光源(好空間相干性)和⼩小能量發散(好能量相干性)
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C2 aperture
SAD aperture
objective aperture
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2. 4. Magnetic lens
• The magnetic lens in an EM is like 凸 glass lens. It is always a convergent lens.
•in fact, the trajectory of electron is not straight line in lens
•stronger lens means
smaller focal length。
•In typical EM,βis about 10 mrad ~
0.57o。α
β
optical axis
aperture
object
focal point
imageinvert real image
f
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Induced Magnetic field
-
+
B
I S
+- i
N
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2.4.1 magnetic lens
iron
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2.4.2 trajectory of electron
• F (Lorentz force)
θ angle between optical axis and trajectory
θ=0 , No force
θ=90 max. force
velocity of electrons
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• Higher magnetic field is needed for higher energy electron
•electron trajectory is hellical .
velocity of electrons
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NTHU
z
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NTHU
focal plane
IMAGE plane
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• Principal Optical Elements
--- object plane--- back focal plane--- image plane• image-object
(conjugate planes)
• u object position υ image position f focal length M magnification
object plane
(Back focal plane BFP)
θ
(Gaussian image plane)
optical axis
Geometric Optics
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image(2)
image (1)
object
f2f1
v1
v2
u
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NTHU
n 若透鏡變弱,焦距變⻑⾧長使得聚焦及成像在特定的焦⾯面及像⾯面底下,我們稱為⽋欠焦(underfocus) 。
• 若透鏡變強,焦距變短使得聚焦及成像在特定的焦面及像面底下,我們稱
為過焦(overfocus) 。
underfocus
focus像overfocus
像⾯面
焦⾯面
透鏡
物⾯面
像
像
αα
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NTHU
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NTHU
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NTHU
2.4.3. Lens aberration
• A.球⾯面像差(Spherical aberration) --透鏡對不同⾓角度的光束聚焦能⼒力不同, ⽽而形成⼀一模糊區。• B.⾊色像差(Chromatical aberration) --透鏡對不同頻率的光束聚焦能⼒力不同, ⽽而形成⼀一模糊區。• C.散光(Astigmatism) --透鏡對⽔水平及垂直⽅方向的聚焦能⼒力不同, ⽽而形成⼀一模糊區。
♦幾種重要的透鏡像差
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• A.球⾯面像差(Spherical aberration)
--透鏡對不同⾓角度的光束聚焦能⼒力不同, ⽽而形成⼀一模糊區
透鏡的聚光能⼒力在外緣⽐比中間要強,
也就是⼊入射⾓角度愈⼤大的光束被透鏡偏折得較利害。
•影響聚光鏡的光斑⼤大⼩小 物鏡的分辨率
•Cs : 球⾯面像差係數 ~ 1mm ~ 3mm⽐比電⼦子波⻑⾧長⼤大太多
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Chromatic Aberration Corrector
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Hubble Telescope
aberration corrected
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C. 散光(Astigmatism)
n 散光最主要的原因是磁場的不均勻匀性。 當電⼦子感到這不均勻匀性 會被聚焦在光軸的不同位置(因為軟鐵不易加⼯工的完美對稱) 。
nΔf是因散光⽽而引起的焦距發散。
n散光可很容易由散光器來補償
(最主要用於補償聚光鏡及物鏡)n散光器(Stigmator)是一個四極透鏡。
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505mm
桌上型分析式掃描電鏡之技術
65kg
550mm
475mm
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electron Gun
1st condenser lens
2nd condenser lens
Deflector
stigmaterObjective lens
Established Technologies Table Top SEM
- Electron Optics design - E-gun: Thermionic/ FEG gun - Lenses: Condenser/ Objective lenses - Multipole: Deflector/ Stigmator- control electronic boards/ software- image acquizition- BSED Detectors
FEG gunRay TracingThursday, March 21, 13
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Established Technologies Control Electronics Boards
(a) (b) (c)
Voltage (gun)vacuum (pump)condenser lenses (spot size, resolution)objective lenses (focus, magnification)scanning (deflectors, magnifiaction)
stigmatism (stigmators)image acquizition (processing)Auto-tuning and alignment (future). etc
20mm
BSE detector
Home Made Electron Detector e-beam
electron detector
sample
back-scattered electron
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Control Software Panel
1000x 5000x 10,000x
5000x 10,000x1000x
fly
Al-Ag
Voltage: 7, 15, 30keVmagnification: 30x-10,000x
resolution: 20nmimage size: 1280x960
scan time: 2.5secõ 3 minutes from Air to Readyõ Fully automatic control
Cr2O3 NW
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Control Software Panel
1000x 5000x 10,000x
5000x 10,000x1000x
fly
Al-Ag
Voltage: 7, 15, 30keVmagnification: 30x-10,000x
resolution: 20nmimage size: 1280x960
scan time: 2.5secõ 3 minutes from Air to Readyõ Fully automatic control
Cr2O3 NW
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Control Software Panel
1000x 5000x 10,000x
5000x 10,000x1000x
fly
Al-Ag
Voltage: 7, 15, 30keVmagnification: 30x-10,000x
resolution: 20nmimage size: 1280x960
scan time: 2.5secõ 3 minutes from Air to Readyõ Fully automatic control
Cr2O3 NW
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Control Software Panel
1000x 5000x 10,000x
5000x 10,000x1000x
fly
Al-Ag
Voltage: 7, 15, 30keVmagnification: 30x-10,000x
resolution: 20nmimage size: 1280x960
scan time: 2.5secõ 3 minutes from Air to Readyõ Fully automatic control
Cr2O3 NW
Thursday, March 21, 13