advanced characterization of materials
TRANSCRIPT
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Microstructural CharacterizationMicrostructural Characterizationof of
MaterialsMaterials
EunEun SooSoo ParkPark
Office: 33-316 Telephone: 880-7221Email: [email protected] hours: by an appointment
2009 spring
04. 13. 2009
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Scanning TEMcroscopy TEM
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S E M Operation
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Scanning Technology
MonitorSample
Secondaryelectrons
ElectronBeam
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• Electron gun produces beam of monochromatic electrons.
• First condenser lens forms beam and limits current ("coarse knob"). – Condenser aperture eliminates
high-angle electrons.
• Second condenser lens forms thinner, coherent beam ("fine knob" ).– Objective aperture further elimi-
nates high-angle electrons from beam.
SEM: Optics #1
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• Beam "scanned" by deflection coils to form image.
• Final objective lens focuses beam onto specimen.
• Beam interacts with sample and outgoing electrons are detected.
• Detector counts electrons at given location and displays intensity.
• Process repeated until scan is finished (usu. 30 frames/ sec).
SEM: Optics #2
http://www.unl.edu/CMRAcfem/semoptic.htm
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Electron Gun (전자빔)
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Electron Emission-electron gun
• Cathode of the gun is the source of electrons for the beam in the electron microscope
• Thermionic emission – heat• Field emission – strong electrical field• Photoelectric emission – electromagnetic radiation
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Work Function: WF
W.F 낮을 수록 전자방출이 쉽다
W.F 낮은 금속: 원자반경(atomic radius)이 크다
결정구조에서 원자간 간격이 크다
: 전자가 금속표면을 탈출할 때
넘어야 하는 potential energy(V)
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Field emission of electronField emission of electron
Schematic view
W 원자의 가장 높은 궤도의 전자들은 실온에서 energy barrier를 넘기 힘들다.
그러나 금속이 강한 전기장(electric field) 이 놓이면 궤도전자들은 potential barrier energy를 통해 tunneling해서 금속 표면에서 방출된다.
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Electron Source Physics
• Electrons need to have more energy than the work function (WF) toleave the emitter. Their energy depends on the temperature, so heatingcan be used to cause emission (Thermionic emitter like W or LaB6)
• The WF depends on the material, so choosing a material with a low WFhelps emission (LaB6 or ZrO layer on FEG emitter). It also depends onthe crystal orientation (FEG).
• The Schottky effect is that the WF becomes lower when a strong electricfield is applied on the emitting surface (used in the FEG).
• Field emission occurs at even higher electron fields. In this case the elec-trons tunnel through the now very thin barrier.
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Thermionic Emission
LaB6 single crystal
Tungsten Hairpin
Self biasing from a resistor
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1. Tungsten Hairpin Electron Gun
Inexpensive, low mag. high current (x-ray microanalysis), low vacuum(10-5torr), t=30-100h
V-shaped hairpin tip of d = 100μm대부분의 금속 : 열전자 방출 전에 용융, 융점이 높은 W 사용
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High brightness(5-10times) : lower work function(2.5eV : 4.5eV for W), expensive but longer lifetime(1000hr), high vacuum(10-7torr)
직접 가열 방식
srA/cm 25max 10=β
2. LaB6 Electron Gun
간접 가열 방식
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Thermionic Electron Emission Gun
가속전압(Accelerating Voltage): cathode와 Anode 사이의 potential 차이
- 보통 30,000V (30kV)
Cathode(Filament): 열전자 방출(W, LaB6)
: 열전자 집속, 1~3 mm 직경의 aperture (hole)
Anode: 열전자 가속
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Self Biased Electron Gun
Richardson’ s law
)kT/Eexp(TAJ W2
C −=
work function
: 방출 전자 전류밀도 (A/cm2)
Filament current(if)
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♦ Brightness : concept of current density
srA/cm 4current 2222 απ
βdi
anglesolidareab=
×=
Brightness ↑ ⇒ current ↑ in same beam size⇒ beam diameter ↓ in same current
Maximum theoretical Brightness (Langmuir eq’n)
srA/cm 2max kT
eVJ oc
πβ = for thermionic gun ~ 9.2 ×104A/cm2sr
srA/cm 2max E
eVJ oc
Δ=π
β for field emission gun
→ 0.3eV for cold emission~ 2 ×109A/cm2sr
EΔmaxβ
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3. Field Emission Gun
<301> W single x-tal→ small work function
Disadvantages of thermionic gun : low brightness, evaporation of gun,thermal drift
Small tip radius 100nm
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Flashing (burn out) 필요:
advantages: small spot size (1-2nm) high resolution
low ΔE improving of low volt. Operation.
tip 수명 단축
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(larger ΔE)
: flat emitting area on <100> facet
advantages: No flashing, brightness is similar to cold FE
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V1 : extraction volt.(방출전압)3-5kV
Vo : acceleration volt.(가속전압)
Work Function Barrier decreasing due to electric field (Schottky effect) and electron tunneling → Electron emission
During long-term operation, the tip blunts → more V1 is required.
Electron emission from FE Gun
srA/cm102 27max ×=β
Brightness
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Control of electron emission
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Electron sources
W
LaB6
FEG
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Comparison of Electron Source
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Condenser lense :
전자총에서 형성된 crossover를 줄이는 역할과 spot size의 조절
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집속렌즈 세기변화 Cond. Lens
• 집속렌즈를 세게하면– 프로브 크기(↓)
– 분해능(↑)
– 프로브 전류(↓)
– 화질(↑)
Weak condenser lens Strong condenser lens
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Object lense :probe crossover의 위치를 광축을 따라 변화시키면서 image를 focusing 하는 역할
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Electrostatic Stigmator
Stigmator: 전자빔의 비점수차(astigmatism)를 보정하는 장치
-찌그러진 전자빔을 circular beam으로 만든다.
-정전기적(electrostatic), 자기적(magnetic) stigmator로 나누어짐
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Specimen and Detectors
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Electron Interactions: All Types
Cathodoluminescence
Secondary e–
Backscattered e–
Incident e–
Auger e–
X-rays
ElasticallyScattered e–
InelasticallyScattered e–
Unscattered e–
Electron/Specimen InteractionsWhen the electron beam strikes the sample, both photon and electron signals are emitted.
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Auger e–
Through thicknessComposition information
Atomic number and topographical information
Electrical information Topographical information
Surface sensitivecompositional information
Specimen currentElectrical information
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