1. FeSEM
MRC Workshop

2. MRC SEM Workshop Introduce the FESEM microscope
Familiarize and Train experienced persons to use the Hitachi S4700 FESEM
Obtain a better understanding on how to utilize the FESEM

3. Workshop Outline Introduction to the FESEM Sample Contamination
Low Voltage SEM
High Resolution Imaging
X-ray Microanalysis
Sample Preparation
MRC Lab Procedures
Operation of the S4700 FESEM
Hands-on labs and testing for users

4. Introduction to the FEGSEM
The FEG SEM offers high performance
not just high resolution
This 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

5. 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 ....

6. Electron Sources W hairpin - 50µm diameter LaB6 - 5µm
Thermal FEG - 250Å
Cold FEG - 50Å
Nano-FEG - 5Å

7. 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

8. 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

9. 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

10. Energy Spread W hairpin 2.5eV LaB6 1.0eV Schottky 0.75eV
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 2.5eV
LaB eV
Schottky eV
Cold FEG eV
colder

11. 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.

12. 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

13. 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

14. 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 ap ap ip dp

15. Beam Performance
For the highest resolution beam diameter , dp , must be as small as possible
For the best image quality and x-ray analysis, emission current, ip, must be as large as possible
For the best depth of field convergence angle, ap, must be as small as possible
When dp and ap are made small, ip is also reduced

16. Imaging modes
On the S4700 the convergence angle a is set by the operating mode of the microscope
No manual adjustment to the condenser aperture strip is required
Don’t change the aperture!

17. 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

18. Flashing The flash condition is set up at the factory
Each tip should show a consistent emission current when it is flashed
Compare the tip current with its own usual value not with that from other tips
Excessive flashing may blunt the tip

19. 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

20. The life cycle of an FEG tip

21. 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

22. The secret of successful Field Emission Microscopy
Run the tip for at least a few hours every day
even when the microscope is not otherwise
in use
This keeps the first anode - which is the main source
of gas - clean, reducing noise and drift.

23. 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

24. 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

25. 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

26. 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