MRC SEM WorkshopIntroduce the FESEM microscope

Familiarize and Train experienced persons to use the Hitachi S4700 FESEM

Obtain a better understanding on how to utilize the FESEM

Workshop Outline

Introduction to the FESEMSample ContaminationLow Voltage SEMHigh Resolution ImagingX-ray MicroanalysisSample PreparationMRC Lab ProceduresOperation of the S4700 FESEMHands-on labs and testing for users

Introduction to the FEGSEMThe FEG SEM offers high performance

not just high resolutionThis means large probe currents (up to a

few nanoamps), and small diameter electron probes (from 1 to 3nm), over a wide energy range (1-30keV)

The FEG SEM package involves both the gun and the probe forming lens

The Electron Gun

The device which provides the electron beam is the called the ‘gun’

This is the single most important component of the SEM because it determines the level of performance that can be achieved

Electrons can be produced in several different ways ....

Electron Sources

W hairpin - 50µm diameter

LaB6 - 5µmThermal FEG -

250ÅCold FEG - 50ÅNano-FEG - 5Å

Nano tips - atom sized FEG

Nano-tips are field emitters in which the effective size of the tip has shrunk to a single atom.

They can be made by processing normal tungsten FE tips or from Pt-Ir, or from carbon nanotubes

They have exciting properties and may be part of upcoming SEMS but now they are still only a laboratory curiosity

Etched tungsten tip

Cut Pt-Ir tip

Comparing emitters

The various types of electron emitters can be compared by looking at three parameters - brightness, source size, energy spread

Other quantities are also important - e.g vacuum required, lifetime, cost, expected mode of use of SEM

Source Size

…is the apparent size of the disc from which the electrons come

Small is good - for high resolution SEM less demagnification

Big is sometimes good - e.g. for large probe sizes and high beam currents

The physical size of the tip is not necessarily the same as the source size

Energy Spread

Electrons leave guns with an energy spread that depends on the cathode type

Lens focus varies with energy (chromatic aberration) so energy spread spoils high resolution, and low energy, images

W hairpin 2.5eVLaB6 1.0eVSchottky 0.75eVCold FEG 0.35eV

co lde r

Summary of Electron Guns The cold FEG offers the best performance parameters in all three

categories for most purposes FEGs are best for high resolution, and low voltage operation Thermionic emitters have advantages when very high beam

currents and large spot sizes are required.

Brightness

At a typical imaging current FEG SEM spot size is set only by lens quality

Lower brightness guns must use bigger spots to give same beam - this is brightness limited imaging

S4700 Resolution Performance The best resolution

is always obtained at the smallest working distance (WD)

..but the minimum WD value varies with beam energy

At the eucentric/EDS WD of 12mm high quality imaging + analysis is possible on the 4700

Sharpness, Contrast, Depth of Field

These are dependent upon the three major electron-beam parameters:

Electron Probe Size dp

Electron Probe Current ip

Electron Probe convergence angle p

dp

ipp

Beam Performance

For the highest resolution beam diameter , dp ,

must be as small as possibleFor the best image quality and x-ray analysis,

emission current, ip, must be as large as possibleFor the best depth of field convergence angle, ap,

must be as small as possibleWhen dp and ap are made small, ip is also reduced

Imaging modes

On the S4700 the convergence angle is set by the operating mode of the microscope

No manual adjustment to the condenser aperture strip is required

Don’t change the aperture!

Gun behavior

The tip must be atomically clean to perform properly as a field emitter

Even at 10-7 torr a monolayer of gas forms in just 1 sec so the tip must be cleaned periodically

It is cleaned by ‘flashing’ - heating the tip to white heat for a few seconds. This burns off (desorbs) the gas

Flashing

The flash condition is set up at the factoryEach tip should show a consistent

emission current when it is flashedCompare the tip current with its own usual

value not with that from other tips Excessive flashing may blunt the tip

Gas production

The tip gets dirty... Gas molecules are

desorbed from 1st anode by electrons

Some of these stick on the tip making it less sharp

This causes the emission current to fall over time

The life cycle of an FEG tip

Typical characteristics

The tip is usually covered with a mono- layer of gas after 5-10 minutes of use

The emission then stabilizes for a period of from 2 hours (new machine) to 8 hours (mature machine).

On this S4700 the tip must be re-flashed after 92 hours of operation (the software gives a warning)

On the plateau, or stable, region the total noise + drift is only a few percent over any period of a few minutes

The secret of successful Field Emission Microscopy

Run the tip for at least a few hours every dayeven when the microscope is not otherwisein use

This keeps the first anode - which is the main sourceof gas - clean, reducing noise and drift.

Other care

Bake the system often - at least every six months - and on public holidays, long weekends

If tip noise is increasing and a bake is not possible raise the emission current to 20 or 30µA with the beam in the ‘freeze’ position for a couple of hours or more for Field Emission bombard

Drift in microanalysis

For normal EDS analysis drift is not a problem

For quantitative analysis using standards, and for line scans and X-ray maps which take significant time to record

Drift can be an issue, this is a reason to sue a thermonic SEM for quantitative analysis

Hidden benefits of FEGSEM

Reliability and reproducibility - no need to change tips or break vacuum. Control by computer ensures reset table values

Ease of use - one button operation, memory alignment settings

Longevity - with reasonable care time between tip changes 3-5 years (even with students operating)

We are on the NINTH year

What isn’t good?

The range of beam currents available is limited when compared to that from a thermionic emitter so an FEG is not ideally suited to such tasks as WDS where high currents are needed