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Microstructural CharacterizationMicrostructural Characterizationof of

MaterialsMaterials

EunEun SooSoo ParkPark

Office: 33-316 Telephone: 880-7221Email: espark@snu.ac.krOffice 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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