Low-Voltage Microscopy

When electron beams impinge on non-conducting samples a charge can build up which can make SEM imaging difficult or impossible

By operating at low beam energies this problem can often be minimized or eliminated

Charge balance

I bI b

I b

sc

Electrons cannot becreated or destroyedso currents at a pointmust sum to zero. Thecurrent flow to earth Isc

is the difference betweenthe in and out currents

If the sample is a conductor Isc can take any value (+ve or -ve) to achieve charge balance

Non-conductors

Sample can accumulate negative charges or positive charges

There can be a dynamic charge balance

For a non-conductor Isc is zero so charge accumulates

+ -

Complex materials

In the case of complex materials (e.g. layered) then the charge balance must be considered separately for each component

If a beam penetrates a layer then it will charge positively, a net electron emitter. substrate

SE BS

Imaging non-conductors

On a new SEM this will be the lowest available energy

On older machines you must decide how low to go before the performance becomes too poor to be useful for the purpose intended

Set the SEM to the lowest operating energy

Negative charging

-ve charging

E > E20

If the scan square is brighter than the background then the sample is charging negative

Positive Charging

+ve charging

E < E20

If the scan square is dark compared to the background then the sample is charging positive

Is that all there is to it?

No - charging is a complex phenomena and simply running the SEM at a low energy does not guarantee an image that is free from charge artifacts

To understand why we must look in more detail at what happens when a poorly conducting specimen is hit with an electron beam

Mechanisms for Charging

STATIC CHARGEStatic charging depends

on the net charge balance in the sample

DYNAMIC CHARGINGDynamic charging comes

from charge generated in the sample itself from electron-hole pairs

There is no global condition where this term is zero

Combining these two contributions we can synthesize a detailed model of the charging process

Charge Distribution The net amount of

negative charge injected = 1-. This is deep in the sample

The net charge that is emitted = and gives a positive region at the surface

Induced charge occurs throughout the interaction volume and could be of either sign

+ve

-ve

Incident beam Ib

Even at charge balance there is still stored charge and fields in the sample

Conductivity and Charging

The +/- charge separation produces a field which moves the induced carriers producing conductivity (EBIC)

Traps reduce the number of electrons. If the escape time from the traps is >> than the time between electron arrivals so the charge builds-up. A charged region is therefore like a leaky capacitor

EBIC is the key to dynamic charging effects

VbiasL Charge e

dQ

Ramo s Theorem

force eV

Lso work done e

V

Ldx VdQ

Thus IdQ

dteV

Lvelocity

If velocity k field kV

Lthen by Ohm s law R

V

Iand

resistivityL A

e k

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.

.

. '

.

.

Area A

Surface Potential and Electric FieldsSurface Potential and Electric Fields

-4E6

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-2.2E6

-2.1E6

-1.6E6

-1.5E6

-1.3E6-1.2E6

-1.1E6

-1E6

-9E5-8E5

-7E5

-6E5

-5E5

-4E5

-3E5

-2E5

-1E50

9.7E4

2E5

3E5

4E5

5E5

6E5

7E5 8E59E5 1E6

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1.2E6

1.3E6

1.4E6

1.5E6

1.6E6

1.7E6

2.4E6

1.3E6

1.4E61.4E6

1.5E6

1.8E6

8E5

8.4E5

8.8E5

9.2E5

9.6E5

1E6

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

0.8

0.6

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0.0

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-0.4

-0.6

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X-electric field distribution with EBIC

Cr

PMMA

Th

ickn

ess

(m

)Position (m)

The fields produced by even small amounts of charging are very high.

It is these fields which deflect the incident beam, push the secondary electrons around, move the electron-hole pairs and may even change the yield of electrons

This is seen as a drifting image

Monte Carlo calculation of fields in and above a resist sample

Minimizing dynamic charging

Reduce the beam current as the charging varies directly with IB

Change to Ultra-High resolution operating mode and lower the emission current

Reduces S/N

Dynamic Charging

Reduce the magnification

Dynamic charging depends on dose and on the magnification

Limits resolution by limiting magnification

Time dependent charging

Dynamic charging is time dependent because of the leaky capacitor effect (EBIC)

Scanning at a high speed extracts a signal before charging occurs

The whole scanned area now floats to a uniform potential allowing stable focussing and stigmation

Coating specimens

Coating should be as thin as possible, a good conductor, and a good emitter of SE

Au/Pd, Cr are good Carbon is bad (the

filler contaminates ) and the evaporator heats the sample

Coating is effective but may hide real surface detail

May be only route if high beam energy is required e.g for EDS

How coatings work

Coatings do not make the specimen conductive

They form a ground plane - eliminate fields due to charge

Increase SE yield - reduce charging

-----------------

-----------------

Charge in sample

Field deflectsincident and exit electrons

++++++++

coating

'image charge'+++++

metal is equipotential ground plane

NO EXTERNAL FIELDS

Charge in sample

Field deflects electrons

ground plane

Result of coating

Both Au-Pd and Cr effectively eliminate charging up to about 8keV

Even at higher beam energies charge-up is minimal

Thin coats do not affect EDS analysis

Other options

Heating the sample - effective for ceramics, oxides etc

Use a low pressure of a gas (VP-SEM mode or from a gas jet)

Low energy electron or ion flood beam to neutralize the charging

Use BSE detector for imaging- much less sensitive to charging

Try different SE detector, mixed or upper

Try high energy if sample is thin or on a substrate-depends on what you want to examine

Choice of detector

The choice of the detector that is used can be very significant in determining how seriously charging will appear to be

Try biasing the sample stage

Try mixing the detector signals, or switching to the lower detector if possible

S4700 TTL detector

This detector is very efficient and gives a symmetric view

These electrons are very sensitive to chemistry and to charging effects

High energy SE, BSE, and SE3 are excluded from the signal - this improves contrast

S4700 detectors

Lower (ET) and upper (TTL) detectors on S4700 have different characteristics

Lower (ET) detector accepts SE1,II and III as well as some BSE

Upper (TTL) detector accepts only low energy SE1 and 2

SE spectra

The upper detector accepts SE with energies around the peak of the SE spectrum. Peak position depends on amount of charge, chemistry, electronic structure, so these effects cause image contrast on TTL detector

Lower (ET) detector accepts everything below 50eV. Much less sensitive to charging

SE spectrum from Aluminum

Nonconducting samples

Latex paint at 1keV in Hitachi S4500

Uncoated, slow scan image at E2 energy

30kx original magnification

Lower (ET) detector for topography, reduces visibility of charging

Lower detector

Individual polymer macro- molecules on a silicon substrate imaged at 1.5keV

The lower detector shows little or no contrast

Upper detector

The upper detector easily reveals the macro- molecules

This is because they are charging negative and the TTL detector is highly sensitive to charging effects

Charging can be a useful form of contrast

Doping contrast

Chemical contrast in the SE mode

Sensitive to both P- and N-type dopants

Only visible on upper (TTL) detector

Boron doping in Si 1.5keV

Upper detector

Birds-beak dopant contrast in a device

S4500 at 1keVThis is a unique

imaging capability - 2 dimensional dopant profiling at high resolution and sensitivity (1ppm)

Damage at low energies

It is often stated that operation at low beam energies minimizes or eliminates beam induced damage

From casual observation this may appear to be true, but measurements show that the truth is just the opposite

Damage and beam energy

At high energies the damage rate is low

Damage rate rises as the energy is reduced, reaching a peak at about 100eV

At still lower energies the stopping power falls again

Experimental Stopping Power Data for Copper

Damage while scanning

If the beam is scanning then the rise in damage rate is less drastic but still considerable

however damage is confined to the near surface and not spread through a volume