Theory Of Scanning Electron Microscope

Theory of Scanning Electron Microscope

Lettuce Field(16M DRAM)

Hitachi HighHigh- Technologies Corporation

Theory of Scanning Electron Microscope Lump (Illumination Source)

Electron Source

Condenser Lens

Condenser Lens

Objective Lens

Objective Lens

Projection Lens

O M

Scanning

SE Detector

Sample Fluorescent screen

Screen Image

Deflection Coils

Sample

Sample

Image

TEM

Image

SEM

Difference among OM, TEM and SEM

C R T

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

Accelerating Voltage: 5kV

Optical Microscope

SEM

The Difference in the Depth of Focus of Optical Microscope and SEM Fine Electron Beam with Small Incident Angle

α2

α1 Depth of Focus (Shallow)

Depth of Focus （ Deep Deep） ）

Specimen

Optical Microscope （ α 1≒ 1rad 1rad） ）

SEM （ α 2＝ 10 -2∼ 10 -3rad rad） ）

Theory of Scanning Electron Microscope High Voltage

Camera Filament

Electron Gun

Wehnelt Anode Condenser Lens Deflection Coils

CRT Mag. Control Deflection Coils Deflection Amplifier Scanning Electron Beam

Objective Lens Specimen Chamber

SE Detector

Image Signal

Amplifier

Specimen

Vacuum Pump

Configuration of a scanning electron microscope

Theory of Scanning Electron Microscope

Scanning Electron Beam of CRT L

Scanning Electron Probe Scanning (X) Scanning(Y)

l S E M

Pixel

Specimen

CRT Magnification :( M)=L / l

Magnifying mechanism in the SEM

What is Magnification on Hitachi’s SEM

Magnification=DD／DS

Scan coil

DD 4 x 5 inch Polaroid unit (120 x 90mm)

Sample DS

Theory of Scanning Electron Microscope Al Coating Layer Primary Electron Beam Phosphors

Scintillator Photomultiplier Photo Multiplier Tube

Secondary Electron

Photons Light Guide

Specimen

Signal

+10kV CRT

Secondary electron detection system

What is resolution on SEM? Example : Digital Camera

Magnified Could not identify the gap between point and point

Magnified

What is resolution on SEM? On SEM : Identify finest gap between 2 particles

Can not identify

Can identify

What is resolution on SEM?

100nm

S-4800

S-5500

Specimen Vacc

: Au particle : 15kV

Specimen Vacc

: Pt particle : 30kV

Mag.

: 220kX

Mag.

: 800kX

Resolution

: 1.0nm

Resolution

: 0.4nm

Theory of Scanning Electron Microscope Characteristic X-Ray Backscattered Electron

↓

Primary Electron Beam

↓ ↓

Secondary Electron

Cathodeluminescence ~10nm

Secondary Electron Detector (Excitation Volume for Secondary Ele ctron Emission)

Electron Beam Induced Current

Specimen Current Transmitted Electron Transmitted (Scattered) Electron

The primary electron beambeam-specimen interaction in the SEM

Theory of Scanning Electron Microscope

Backscattered Electron −

−

Secondary Electron −

Vaccum −

Sample(Metal)

Simons,et.al

Theory of Scanning Electron Microscope

Quantity of Electrons

Secondary Electrons Backscattered Electrons

1

100 Energy of Electron (eV)

10,000

(Incident beam energy : 10,000eV)

Energy spectrum of the electrons emitted from a specimen

Electron Beam generated from Sample Primary beam

BSE Detector

High resolution Surface feature

BSE information SE information Composition information Crystal Orientation

S E Detector

less than 10nm Sample de pth of SE generating

sample depth of BSE ~ more than 10nm gene rating

Sample

Theory of Scanning Electron Microscope Tungsten Filament

FE Tip

750μm Electron Source Type of Emission Operating Vacum (Pa) Brightness (A/cm ･str) Source Size (μm) Energy Spred (eV) Life Time (h) 2

Tungsten Filament Thermonic 10-5 5x105 30 2.0 50

Field Emission Cold FE ∼10-8 108 0.01 0.2 2000

Comparison of electron sources

Theory of Scanning Electron Microscope Tungsten Filament

FE Tip ΔV= ∼0.2eV

ΔV= ∼2eV

Crossover of High Energy Electrons

Crossover of Low Energy Electrons

Crossover of High Energy Electrons

Energy Spread Effect of chromatic aberration

Crossover of Low Energy Electrons

Tungsten (W)

Op. T emp

2300deg. Operation

Lanthanum Hexaboride (LaB6)

Schottky FE

ColdColdCathode FE

1800deg. Operation

1800deg Operation

Ambient Temp

Energy spread

less than 2.0eV

less than 1.5eV

Less than 0.8eV

Less than 0.2eV

Brightness

5x105 A/cm2sr

5x106 A/cm2sr

5x10 8 A/cm2sr

2x109 A/cm 2sr

less than 100hr

500 – 1000 hr

1 year

More than 2 years

Tip change

Gun exchange

No Gun exchange

Tip activation

N eed continuously

Life time

Comparison Of Electron Sources

Need at time of use

Theory of Scanning Electron Microscope Primary beam

S-4300 SE Detector

Lens Specimen

１)Conventional type(Out-Lens)

S-4500 SE Detector (Upper)

S-5500 Primary beam SE Detector

Primary beam Lens

SE Detector (Lower)

Specimen

２)S nokel type

Lens

Specimen

３)In-lens type

Theory of Scanning Electron Microscope 20

Resolution (nm)

10

W filament SEM

Out lens FE-SEM

1.0

Snorkel lens FE-SEM 0.5 0.5

In-Lens FE-SEM 1.0

10

30

Acc.(kV)

Comparison of resolution

Reliable FE-Gan

1st anode FE tip

- Hitachi is manufacturing both FE Gun and FE Tip. - Baking is almost unne cessary afte r the installation.

2nd anode INNER baking OUTER baking

- Even if re quire d, it is very easy including inne r baking. - Heate d Obj. aperture can eliminate contamination for long life-time . - Hitachi’s FE tip long life has been well known as 3-7 years.

Adjustment of Accelerating voltage High Vacc（10ｋV∼30ｋV） 1）When thick metal layer is coated on the sample 2）When high resolution observation 3）When internal information of sample is required

Middle range Vacc（3ｋV∼10ｋV） 1）When high resolution and surface info rmation is required 2）When need good contrast on uncurved surface sample

Low Vacc（0.5ｋV∼3ｋV） 1）When surface feature observation 2）When need to avoid Charge-up effect and sample-damage

Beam spreading in the sample Vacc 1kV

Vacc 15kV

Magnified

20 nm

1μm 1μm

20 nm

a) Sample ：Carbon Monte Carlo Simulation

Beam spreading in the sample Vacc 1kV

Vacc 15kV

Magnified

0.2μm 5 nm 5 nm

0.2μm

b) Sample ：Gold

Monte Carlo Simulation

Theory of Scanning Electron Microscope Vacc:1.5kV

Chage-up Phenomena

Vacc:0.7kV

Eliminate Chage-up Phenomena Specimen : SiO2 on Photo Resist Line Pattern

Observation at lower accelerating voltages

Theory of Scanning Electron Microscope Vacc : 15kV

High Acclerating Voltage

Vacc : 1.0kV

Low Accelerating Voltage Specimen : Solar Battery

Comparison of high and low acclerating voltage

Electrical field effect to primary electron(E)

Primary elec tron Primary elec tron

Pr im ary e lect ron Pr im ary e lect ron

SE SEDetector Detector

ee

FFE E

Sample Sample

+1 0k V +1 0k V

Sample Sample

History of Upper SE Detector (+10kV has been applied to detector surface ) Instrument

Semi-inlens S EM

Inlens S EM

TEM

H V >75kV

Countermeasure No problem due to high HV

S-900

0.5 -30kV

Keep the detector away from axis

S-5000

0.5 -30kV

Insert shielding cylinder

S-5200

0.5 -30kV

S-4500

0.5 -30kV

S-4700

0.5 -30kV

S-4800

0.5 -30kV

ExB equipped

Patent No. : P3081393

What is ExB ? Current

I

Magnetic Field

B

Force

Primary electrons

E

SE Detector

B FB

e

FE +10kV

+V

FB Fleming’s left-hand rule



|FE|=|FB| Current(I) direction is opposite to electrons

OBJ Lens Sample

Patent No. : P3081393

What is ExB ?

Primary electrons

Magnetic Field B

E

SE Detector

B Force

FB Current

I

FB

e

FE +10kV

FE +V

e

Secondary Electrons

FB

Fleming’s left-hand rule

<Secondary electron case>

OBJ Lens Current(I) direction is opposite to electrons

Sample

Adjustment of Optics conditions d1（Beam spot size）＝ Gun

d0

d０ ・ ba１１・ ba22

b1：Adjusted by Cond. lens

a1 Cond. Lens b1 Obj. Lens

a2 b2

Obj. lens aperture

d1 Sample

b2：Adjusted by WD

Relation between Cond. Lens and image Beam spot size can be adjusted by changing electric current of Condenser Lens.

Lens current

Small excitation

Large excitation

Long

Short

Resolution

Low

High

Beam current

Large

Small

S/N of image

Large

Small

b1

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

Long

b2

Long

Short

Resolution

Low

High

Lens excitation

small

large

Focus depth

Deep

Short

Shallow

Relation between WD and image Focus depth: Shallow Resolution : High

WD：5mm

Focus depth : Deep Resolution : Low

WD：20mm

Relation between Obj. lens aperture and image 1) Adjusting beam spreading angle 2) Adjusting beam current eradiating to sample Gun Shielded beam by Obj. lens aperture

Obj. Lens

Obj. Lens aperture W.D

Beam angle(α)

Sample

B2

Relation between Obj. lens aperture and image

Aperture Hole size

Large

Small

Resolution

Low

High

Beam current

Large

Small

S/N of image

Large

Small

Focus depth

Shallow

Deep

Theory of Scanning Electron Microscope Aperture Size : Small

Focus Depth → Deep

Aperture Size : Large

Focus Depth → Shallow

Specimen : Si on Photo Resist Pattern

Comparison of objective movable aperture hole size

Detecting Affection under SEM imaging Image detecting affection and countermeasures Affection

Charge-up effect

Contamination

Beam damage

Phenomenon

Countermeasures

*Extraordinary contrast *Drift of sample

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

*Less contrast

*Heating / Cooling *UV eradiation *Plasma cleaning

*Change of sample shape

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

Charge-up effect

What is Charge-up effect? Iｉｎ ：electron flow entering sample = IP Iｏｕｔ ：electron flow radiating from sample = IS E + IBS E ＋ Iａｂ Probe current (IP)

If the sample is conductive Iｉｎ ＝ Iｏｕｔ となり Electric charge of sample is balanced

BSE flow (IBSE)

SE flow (ISE)

Sample

Absorbed electron flow (Iａｂ)

What is Charge-up effect? Iｉｎ ：electron flow entering sample = IP Iｏｕｔ ：electron flow radiating from sample = IS E + IBS E ＋ Iａｂ Probe current (IP)

If the sample is non-conductive Iｉｎ ≠ Iｏｕｔ となり Charge-up effect

BSE flow (IBSE) Electric charge

SE Flow (ISE) ee- e- ee- e-

Sample

Absorbed electron flow (Iａｂ)

Countermeasures against charge-up Metal coating Metal coating by vacuum elaboration system or Ion sputtering system Coating Material Need to select optimum material for SEM observation 1. Can coat at homogeneous distribution and fine particle 2. Good efficiency of SE generating 3. Stable against oxidization

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

Coating method Focus point in coating work Need to avoid affecting sample shape Artifact Changing sample size Contamination / Damage

Need to minimize

Coating layer

Sample Homogeneous distribution

Not Homogeneous distribution

Coating method Artifact by coating treatment

200nm

Non coating

Pt coating

Vacc ：2kV

Vacc：3kV

Coating method Less than 3nm coating layer is ideal to avoid artifact

100nm

Less than 3nm

How about odd-shaped sample?

Coating method a) Coating from only overhead

Coating layer

b) Coating from several direction

Coating direction

Sample

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

* Surface feature is accurate

Imaging technique What is “ Just focus” focus”?

Condensed electron beam to finest beam spot is eradiated at sample surface

Spot shape should be “perfect circle” circle ”

Theory of Scanning Electron Microscope Before correction

Y Beam Diameter X

Electron Source

Electron Beam Objective Lens

After correction

Stigmator

Y

Beam Diameter X

Electron Source

Electron Beam Stigmator

Objective Lens

Astigmatism correction method

Theory of Scanning Electron Microscope Under focus

Just focus

Over focus

Before correction Just focus

After correction

Specimen:Trachea of rat

Astigmatism correction method

Ideal adjustment of focus and stigma stigma

X

focus

Y

Beam Beamspot spotshape shape Ideal beam spot shape

Present beam spot shape

coarse

fine

Ideal adjustment of focus and stigma stigma

X

Beam Beamspot spotshape shape

focus

Y

coarse

fine

Ideal adjustment of focus and stigma stigma

X

Beam Beamspot spotshape shape

focus

Y

coarse

fine

Ideal adjustment of focus and stigma Adjust knob to center where image is not drifting

stigma

X

Beam Beamspot spotshape shape

focus

Y

coarse

fine

Ideal adjustment of focus and stigma stigma

X

Beam Beamspot spotshape shape

focus

Y

coarse

fine

Ideal adjustment of focus and stigma stigma

X

Beam Beamspot spotshape shape

focus

Y

coarse

fine

Summary: Imaging technique for high resolution Gun

d1＝ d０ ・ ba１１・ ba22 ・ ba33

d0 a1 1st Cond. lens b1

How to minimize d1?

a2 2nd Cond. lens b2

Shorten b1： Increasing excitation of Cond. lens

a3 b3

d1 Sample

Obj. Lens (Focus knob)

Shorten b3： Shorten WD

Summary: Imaging technique for high resolution

Cond. Lens：１notch ＷＤ：12mm

Cond. Lens ：8 notch Ｗ Ｄ：2.5mm

Sample：ITO layer Vacc ：3.0ｋＶ Mag. ： x 100,000