FeSEM MRC Workshop

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. …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 Source Size The physical size of the tip is not necessarily the same as the source size

10. 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 LaB6 1.0eV Schottky 0.75eV Cold FEG 0.35eV Energy Spread 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 dpand ap are made small, ip is also reduced

16. Imaging modes • On the S4700 the convergence angle ais 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