theory of scanning electron microscope

Lettuce Field(16M DRAM)

Theory of Scanning Electron MicroscopeTheory of Scanning Electron Microscope

Hitachi HighHitachi High--Technologies CorporationTechnologies Corporation

Image

SampleObjective

Lens

(Illumination Source)Lump

O M

CondenserLens

ProjectionLens

Screen

ImageImage

Sample

Sample

ObjectiveLens

Electron Source

CondenserLens

DeflectionCoils

SE Detector

C R T

T E M S E M

Fluorescentscreen

Scanning


Difference among OM, TEM and SEMDifference among OM, TEM and SEM

The Example of Observation of Alga (The Example of Observation of Alga (MesostigmaMesostigma) ) by Optical Microscope and SEMby Optical Microscope and SEM

Accelerating Voltage: 5kVAccelerating Voltage: 5kV

Optical MicroscopeOptical Microscope SEMSEM

（（αα22＝＝1010--22～～1010--33radrad））

SpecimenSpecimen

（（αα11≒≒1rad1rad））Optical MicroscopeOptical Microscope SEMSEM

Depth of FocusDepth of Focus(Shallow)(Shallow)

Depth of FocusDepth of Focus（（DeepDeep））

αα11

αα22

The Difference in the Depth of Focus The Difference in the Depth of Focus of Optical Microscope and SEMof Optical Microscope and SEM

Fine Electron BeamFine Electron Beamwith Small Incident Angle with Small Incident Angle

SE Detector

Specimen

CRT

Camera

Amplifier

Image Signal

High Voltage

DeflectionCoils

Deflection Amplifier

Vacuum Pump

FilamentWehnelt

Electron Gun

AnodeCondenser

LensDeflection

Coils

Objective Lens

SpecimenChamber

Scanning Electron Beam

Mag. Control

Configuration of a scanning electron microscopeConfiguration of a scanning electron microscope


Scanning(Y)

Scanning (X)

l

Scanning Electron Probe

S E MS E M

Specimen

Scanning Electron Beam of CRT

L

Pixel

C R TMagnification :( M)=L / l

Magnifying mechanism in the SEMMagnifying mechanism in the SEM


What is Magnification on Hitachi’s SEM

4 x 5 inchPolaroid unit

(120 x 90mm)

DD

Scan coil

DS

Sample

Magnification=DD／DS

Photomultiplier

Primary Electron Beam

Specimen

PhotonsLight Guide Signal

CRT

+10kV

SecondaryElectron

ScintillatorScintillatorPhosphors

Al Coating Layer


Secondary electron detection systemSecondary electron detection system

Photo Multiplier Tube

What is resolution on SEM?

Example : Digital Camera

MagnifiedMagnified

MagnifiedMagnified

Could not identify the gap between point and point


On SEM : Identify finest gap between 2 particles

Can not identify Can identify

Specimen

Vacc

Mag.

Resolution

: Pt particle

: 30kV

: 800kX

: 0.4nm

S-5500Specimen

Vacc

Mag.

Resolution

: Au particle

: 15kV

: 220kX

: 1.0nm

S-4800

100nm


↓

↓

↓

Primary Electron Beam

SecondaryElectron

BackscatteredElectron

Cathodeluminescence

Specimen Current

TransmittedElectron

Electron BeamInduced Current

Secondary Electron Detector

~10nm (Excitation Volume forSecondary Electron Emission)

Transmitted Electron

(Scattered)

Characteristic X-Ray

The primary electron beamThe primary electron beam--specimen specimen interaction ininteraction in the SEM the SEM



－

－

－

－

BackscatteredElectron Secondary Electron

Sample(Metal)Sample(Metal)

Vaccum

Simons,et.al

100 10,0001Energy of Electron (eV)

Qua

ntity

of

E

lect

rons

(Incident beam energy : 10,000eV)Energy spectrum of the electrons emitted from a specimen Energy spectrum of the electrons emitted from a specimen


Secondary Electrons

BackscatteredElectrons

Electron Beam generated from SampleElectron Beam generated from Sample

Sample

Primary beamPrimary beam

High resolutionSurface feature

Composition information Crystal Orientation

less than 10nm Sample depth of SE generating~ more than 10nmsample depth of BSE

generating

SE information

BSE information

BSE Detector

SE Detector

Comparison of electron sourcesComparison of electron sources


FE TipTungsten Filament

750μm

Electron SourceType of EmissionOperating Vacum (Pa)Brightness (A/cm2･str)Source Size (μm)Energy Spred (eV)Life Time (h)

Tungsten FilamentThermonic

Field EmissionCold FE

10-5 ～10-8

5x105 108

30 0.012.0 0.250 2000


Energy SpreadEnergy SpreadEffect of chromatic aberration Effect of chromatic aberration

ΔV=～2eV

Tungsten Filament

Crossover ofCrossover ofLow EnergyLow EnergyElectronsElectronsCrossover of Crossover of

High EnergyHigh EnergyElectronsElectrons

ΔV=～0.2eV

FE Tip

Crossover ofCrossover ofLow EnergyLow EnergyElectronsElectrons

Crossover ofCrossover ofHigh EnergyHigh EnergyElectronsElectrons

ColdCold--CathodeCathode

FEFE

Tungsten(W)

LanthanumHexaboride

(LaB6)

SchottkyFE

Comparison Of Electron SourcesComparison Of Electron Sources

2300deg. Operation 1800deg. Operation 1800deg Operation Ambient TempAmbient Temp

less than 2.0eV less than 1.5eV Less than 0.8eV Less than 0.2eVLess than 0.2eV

5x105 A/cm2sr 5x106 A/cm2sr 5x108 A/cm2sr 2x102x109 9 A/cmA/cm22sr sr

less than 100hr 500 – 1000 hr 1 year More than 2 yearsMore than 2 years

Gun exchange No Gun exchangeNo Gun exchange

Need continuously Need at time of useNeed at time of use

Op. Temp

Energy spread

Brightness

Life time

Tip change

Tip activation

Primary beam

LensSE Detector

Specimen

３)In-lens type

Primary beam

SE Detector Lens

Specimen１)Conventional type(Out-Lens)

SE Detector(Upper)

SpecimenLens

Primary beam

２)Snokel type

SE Detector(Lower)


S-4300

S-4500 S-5500


W filament SEM

Out lens FE-SEM

Snorkel lens FE-SEM

In-Lens FE-SEM

0.5 1.0 10 30

0.5

1.0

10

20

Acc.(kV)

Res

olut

ion

(nm

)

Comparison of resolutionComparison of resolution

YGL

已批准

OUTERbaking

INNERINNERbakingbaking

1st anode

2nd anode

FE tip

- Hitachi is manufacturing both FE Gun and FE Tip.

- Baking is almost unnecessaryafter the installation.

- Even if required, it is very easyincluding inner baking.

- Heated Obj. aperture can eliminatecontamination for long life-time.

Reliable FE-Gan

- Hitachi’s FE tip long life has beenwell known as 3-7 years.

High Vacc（10ｋV～30ｋV）

1）When thick metal layer is coated on the sample2）When high resolution observation3）When internal information of sample is required

Middle range Vacc（3ｋV～10ｋV）

1）When high resolution and surface information is required2）When need good contrast on uncurved surface sample

Low Vacc（0.5ｋV～3ｋV）

1）When surface feature observation2）When need to avoid Charge-up effect and sample-damage

Adjustment of Accelerating voltageAdjustment of Accelerating voltage

Vacc 1kV Vacc 15kV

1μm

Magnified

20 nm

1μm20 nm

a) Sample ：Carbon

Beam spreading in the sample Beam spreading in the sample

Monte Carlo Simulation

0.2μm

0.2μm5 nm

5 nm

Beam spreading in the sample Beam spreading in the sample

Vacc 1kV Vacc 15kV

Magnified

b) Sample ：Gold Monte Carlo Simulation


Chage-up Phenomena Eliminate Chage-up Phenomena

Vacc:1.5kVVacc:1.5kV Vacc:0.7kVVacc:0.7kV

Specimen : SiO2 on Photo Resist Line Pattern

Observation at lower accelerating voltagesObservation at lower accelerating voltages

Comparison of high and low Comparison of high and low accleratingacclerating voltagevoltage


High Acclerating Voltage Low Accelerating Voltage

Vacc : 15kV Vacc : 1.0kV

Specimen : Solar Battery

Primary electron

Sample

Primary electron

Sample

SE Detector

+1 0kV

eFE

Prim ary e lectron

Sample

SE Detector

+1 0kV

eFE

Prim ary e lectron

Sample

Electrical field effect to primary electron(E)Electrical field effect to primary electron(E)

History of Upper SE DetectorHistory of Upper SE Detector

TEM >75kV

S-900 0.5 -30kV

S-5000

S-5200

S-4500

S-4700

S-4800

ExB equipped

Keep the detector away from axis

Insert shielding cylinder

No problem due to high HV

0.5 -30kV

0.5 -30kV

0.5 -30kV

0.5 -30kV

0.5 -30kV

Instrument H V Countermeasure

(+10kV has been applied to detector surface)Se

mi-i

nlen

sSEM

Inle

nsS

EM

Primary electrons

SE DetectorB

Sample

eFE

OBJ Lens

FB

|FE|=|FB|

E

+V

+10kV

What is ExB ?

Current(I) direction is opposite to electrons

I

B

FB

Field

Current

Force

Fleming’s left-hand rule

<Primary electron case>

Magnetic

Patent No. : P3081393

B

eFE

FB

E

+V

+10kV

Secondary ElectronseFE

FB

B

ICurrent

FB

Force

<Secondary electron case>

Primary electrons

SE Detector

Sample

OBJ Lens

What is ExB ?

Current(I) direction is opposite to electrons

Fleming’s left-hand rule

FieldMagnetic

Patent No. : P3081393

Sample

Cond. Lens

Obj. Lens

a1

a2

b2

b1

d0

d1

Gund1（Beam spot size）＝・ a2

b2d０・ a１b１

Obj. lens aperture

b1：Adjusted by Cond. lens

b2：Adjusted by WD

Adjustment of Optics conditionsAdjustment of Optics conditions

Relation between Cond. Lens and imageRelation between Cond. Lens and image

Beam spot size can be adjusted by changing electric current of Condenser Lens.

Lens current Small excitation Large excitation

Resolution

Beam current

S/N of image

Low High

Large Small

Large Small

b1 Long Short

WD (Working Distance) :length between Obj. lens and sample surface

WD Long Short

Resolution

Lens excitation

Focus depth

Low High

small large

Deep Shallow

Relation between WD and imageRelation between WD and image

b2 Long Short

WD：5mm WD：20mm

Relation between WD and imageRelation between WD and image

Focus depth: ShallowResolution : High

Focus depth : DeepResolution : Low

1) Adjusting beam spreading angle2) Adjusting beam current eradiating to sample

Obj. Lens aperture

Obj. Lens

Gun

Shielded beam by Obj. lens aperture

W.D Beam angle(α)

Sample

B2

Relation between Obj. lens aperture and imageRelation between Obj. lens aperture and image

Aperture Hole size Large Small

Resolution

Beam current

S/N of image

Low High

Large Small

Large Small

Focus depth Shallow Deep

Relation between Obj. lens aperture and imageRelation between Obj. lens aperture and image

Theory of Theory of ScanningScanning Electron MicroscopeElectron Microscope

Comparison of objective movable aperture hole sizeComparison of objective movable aperture hole size

Focus Depth → Deep Focus Depth → Shallow

Aperture Size : SmallAperture Size : Small Aperture Size : LargeAperture Size : Large

Specimen : Si on Photo Resist Pattern

*Metal coating*Observe at ultra Low Vacc.*Cooling Observation

*Change of sample shapeBeam damage

*Heating / Cooling*UV eradiation*Plasma cleaning

*Less contrastContamination

*Metal coating*Observe at Low Vacc* Observe with BSE

*Extraordinary contrast*Drift of sample

Charge-up effect

CountermeasuresPhenomenonAffection

Image detecting affection and countermeasures

Detecting Affection under SEM imagingDetecting Affection under SEM imaging

Charge-up effectCharge-up effect

If the sample is conductiveIｉｎ＝ Iｏｕｔとなり

Electric charge of sample is balanced

Iｉｎ：electron flow entering sample = IP

Iｏｕｔ：electron flow radiating from sample = ISE + IBSE ＋ Iａｂ

Probe current (IP)

Sample

SE flow(ISE)

SE flowSE flow((IISESE))

BSE flow (IBSE)

Absorbed electron flow(Iａｂ)

What is Charge-up effect?What is Charge-up effect?

If the sample is non-conductiveIｉｎ ≠ Iｏｕｔとなり

Charge-up effect

Sample

SE Flow(ISE)

SE Flow(ISE)

Probe current (IP)

BSE flow (IBSE)

e-e-e- e-

Electric chargeElectric charge e- e-

Absorbed electron flow(Iａｂ)

What is Charge-up effect?What is Charge-up effect?

Iｉｎ：electron flow entering sample = IP

Iｏｕｔ：electron flow radiating from sample = ISE + IBSE ＋ Iａｂ

Countermeasures against charge-upCountermeasures against charge-up

Metal coating

Metal coating by vacuum elaboration system or Ion sputtering system

Coating Material

Need to select optimum material for SEM observation1. Can coat at homogeneous distribution and fine particle2. Good efficiency of SE generating3. Stable against oxidization

Observation：Au、Au-Pd、Pt-Pd、Pt ....Analysis：C、Al....

Need to avoid affecting sample shape

ArtifactChanging sample sizeContamination / Damage

Need to minimize

Coating layer

Sample

Focus point in coating work

Homogeneous distribution Not Homogeneous distribution

Coating methodCoating method

200nm

Non coating

Vacc ：2kV

Pt coating

Vacc：3kV

Artifact by coating treatment


100nm

Less than 3nmLess than 3nm

Less than 3nm coating layer is ideal to avoid artifact

How about odd-shaped sample?


b) Coating from several directiona) Coating from only overhead

Coating direction

Sample

Coating layer

*Coating layer become thick when you want to avoid charge-up*Surface feature is not accurate

* Surface feature is accurate


What is What is ““ Just focusJust focus””??

Condensed electron beam to finest beam spotCondensed electron beam to finest beam spotis eradiated at sample surfaceis eradiated at sample surface

Spot shape should be Spot shape should be ““perfect circleperfect circle””

Imaging techniqueImaging technique

Astigmatism correction methodAstigmatism correction method

Beam DiameterBefore correction

Objective Lens

Electron Source Electron Beam

X

Y

Electron BeamElectron Source

Objective LensStigmator

After correctionY

X

StigmatorBeam Diameter



After correction

Before correction

Under focusUnder focus Just focusJust focus Over focusOver focus

Just focusJust focus

Astigmatism correction methodAstigmatism correction method

Specimen:Trachea of rat

stigma focus

coarse fineX Y

Beam spot shapeBeam spot shape

Ideal adjustment of focus and stigmaIdeal adjustment of focus and stigma

Present beam spot shape

Ideal beam spot shape

stigma focus

fineX Y


coarse


stigma focus

fineX Y

Adjust knob to center where image is not drifting


coarse


stigma focus

fineX Y


coarse


Shorten b1：Increasing excitation of Cond. lens

Sample

1st Cond. lens

2nd Cond. lens

Obj. Lens(Focus knob)

a1

a2

b2

a3

b3

b1

d0

d1

Gun

d1＝・ a2b2 ・ a3

b3d０・ a１b１

Summary: Imaging technique for high resolutionSummary: Imaging technique for high resolution

How to minimize d1?

Shorten b3：Shorten WD

Cond. Lens：１notchＷＤ：12mm

Cond. Lens ：8 notchＷＤ：2.5mm

Sample：ITO layerVacc：3.0ｋＶMag. ： x 100,000

Summary: Imaging technique for high resolutionSummary: Imaging technique for high resolution

theory of scanning electron microscope

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