电子光学性质简介Basic Theory

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Introduction to Electron Optics
电磁透镜
用于实现： 1. 电子束在样品表面聚焦， 2. 图像聚焦， 3. 变换放大倍率， 4. 成像模式与衍射模式的转换
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Introduction to Electron Optics
显微镜 / 分辨率
Rayleigh Criterion:
0.61λ δ = n sin α
δ α
光学显微镜: δ= 200 nm 电子显微镜(TF30): δ= 0.2 nm
灯丝 控制电极 • 偏压电极 阳极 -+
Cross-over
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Introduction to Electron Optics
电子光学系统----热场枪
能量发散 • imperfections of filament • instability of high tension • surface temperature • Boersch effect (mutual interaction) 电子源束斑大小 • 30 μm for W • 5 μm for LaB6
-e
Negative charge Strong interaction electron-matter
Disadvantages
Strong interaction electron-matter Excellent vacuum and Thin specimen are required
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Introduction to Electron Optics
电磁透镜
Working Principle: Lorenz Force F = q (v x B) • electrons are only deflected by magnetic fields
Introduction to Electron Optics
电子光学系统----电子枪比较
W Maximum Current (nA) Normalised Brightness (-) Energy spread (eV) Source spotsize Required Vacuum (Pa) Temperature (K) Life time (hr) Normalised Price (-) 1000 1 3-4 30-100 μm 10-3 2700 60-200 1 LaB6 500 10-30 1.5-3 5-50 μm 10-5 2000 1000 10 FEG (Schottky) 300 2500 0.6-1.2 15-30 nm 10-7 1800 >2000 100
object plane image plane
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Introduction to Electron Optics
显微镜 / 分辨率
电子波长 0.61λ λ∼0.002 nm δ= 电子显微镜分辨率 α δ∼0.1 nm 为什么? • Lens aberrations (e.g. Cs, Cc): can be corrected by multipole lenses or focus-series reconstruction • Instabilities (e.g. HT, electronic, mechanical / acoustic, thermal)
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Introduction to Electron Optics
电子光学系统----场发射枪 (FEG)
Heating Filament Single Crystal Emitter Suppressor Cap Extraction Anode Electrostatic lens
SW
-e
Short wavelength
High resolution (compared to photons)
Low mass
Non-destructive for most specimens (compared to neutrons) Beam focused / deflected via electric coils Different signals available - Elastic scattering: imaging - Phase shift: weak-scatter imaging - Diffraction: crystallography - Inelastic scattering / ionisation: chemical analysis
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Introduction to Electron Optics
电子显微镜历史
1897: Thompson describes the existence of negatively charged particles (electrons) 1925: De Broglie theorised that electrons have wave-like characteristics, addressing the wave/particle duality 1927: Thompson and Reid demonstrated the wave nature of electrons by diffraction experiments 1931: Ruska et al. build the first electron microscope (Nobel Prize in 1986)
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Introduction to Electron Optics
FEI透射电镜历史
6
EM100
5
4
nm 3
2
EM75 EM300
1
EM430
CM
Tecnai Titan
0 1940
1950
1960
1970
1980
1990
2000
Period
Introduction to Electron Optics
电子光学系统----电子枪
热场枪 • 钨灯丝 • LaB6灯丝(或CeB6) 场发射枪 • 单晶钨
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Introduction to Electron Optics
电子光学系统----热场枪
0.2mm
0.2 μm
0.1nm
100μm 10μm μm 100nm 10nm 1nm 0.1nm
Sewing needle diameter Razor blade edge thickness Most cells Bacteria Viruses Macromolecules Atoms
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Introduction to Electron Optics
为何用电子?
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Introduction to Electron Optics
为何用电子?
不同高压下的电子波长
U 100kV 120kV 200kV 300kV
Relativistic λ= 3.7 pm λ= 3.4 pm λ= 2.5 pm λ= 2.0 pm
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Introduction to Electron Optics
为何用电子?
Advantages
Electron seemingly originating from tip itself
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Introduction to Electron Optics
电子光学系统----场发射枪 (FEG)
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Introduction to Electron Optics
电磁透镜
An electromagnetic lens is based on the fact that moving electrons are forced by its magnetic field into a spiral trajectory, i.e. focused into one point
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Introduction to Electron Optics
显微镜 / 分辨率
1 meter 100mm 10mm 1mm Light Microscopy
Transmission Electron Microscopy
Human eye
电子光学简介 基本理论 Tecnai 基础课程
史为
Introduction to Electron Optics
Plan
• • • • • 为什么用电子? 电镜历史简介 分辨率 光学显微镜 vs. 电子显微镜 电子光学系统
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Introduction to Electron Optics
为何用电Байду номын сангаас?
电子的波动性
• images (phase / amplitude contrast) and diffraction patterns • wavelength can be tuned
电子的粒子性
• electron specimen interactions • chemical analysis is possible
FEI Confidential Copyright © 2004
Introduction to Electron Optics
电子光学系统 Electron Sources 电子源 Lenses 透镜 Deflection Coils 偏转线圈 Stigmators 消像散器
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Courtesy of Dong Tang, FEI
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Introduction to Electron Optics
显微镜 / 分辨率
显微镜: • the technique of magnifying things too small to see with the naked eye 分辨率 (或分辨力): • the smallest distance between two points that can be distinguished
Introduction to Electron Optics
电子显微镜历史
19th century: Ernst Abbé 发现光学显微镜受限于可
见光的波长
“According to our present scientific knowledge, the range of the human eye is limited by the nature of light itself […] The human intellect of future generations will perhaps find ways to pass these borders which we consider insurmountable by making still unknown processes and forces to serve this purpose” (1876)