MRC SEM Workshop zIntroduce the FESEM microscope zFamiliarize and Train experienced persons to use...
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Transcript of MRC SEM Workshop zIntroduce the FESEM microscope zFamiliarize and Train experienced persons to use...
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