What is needed for High Resolution SEM?

1. What is needed for High Resolution SEM? • A small probe size • High beam current • A mechanically stable microscope and a quiet lab environment • A skilled operator

2. Lens performance • The probe size is determined by the aberrations of the lens • The magnitude of the aberrations vary with the focal length of the lens - which is about equal to the working distance • Some lens’ designs are more capable than others at combining both high performance and good sample access

3. Lens performance cont. • Aberrations including spherical and chromatic are correctable to varying degrees • Corrections depend on a variety of factors including pole piece quality, aperture size, aperture, angle and electron wavelength

4. Pinhole Lens • The original SEM lens - designed to produce no magnetic field in the sample chamber • Good sample access • Long focal length and a big working distance so high aberrations • Poor EM screening • Asymmetric SE collection due to position of ET

5. Immersion Lens • Short focal length - so low aberrations • Good EM screening • Very stable specimen mounting in lens • Symmetric SE collection using the through the lens (TTL) detector system • Restricted to small samples (3mm disc)

6. Snorkel Lens • Short focal length - so low aberrations and high performance • Good EM screening • The sample is outside the lens so there is no limitation on the size of the specimen

7. S4700 Snorkel Lens • Up to 45 degrees of sample tilt even at short WD and permits EDS operation at WD of 12mm • Biased deflector plates optimize SE collection for either or both detectors • Improved magnetic screen and stronger stigmators can image magnetic samples at all WD • The lens also acts to filter the SE signal to the TTL S4700 lens configuration Excitation - 1000 amp.turns

8. S4700 detector • Snorkel lens permits multiple detectors to be used • In-lens (TTL) detector gives a shadow free image with ultra-high topographical resolution. Super efficient • Lower (ET) detector gives SE images with material contrast information and high efficiency at high tilt angles • These detectors can be used separately or combined as desired for maximum flexibility Snorkel lenses allow multiple detectors

9. Detector Flexibility DRAM with both Upper and Lower detectors MO layer in BSE mode (DRAM stands for dynamic random access memory, a type of memory) Multiple imaging modes provide flexibility and problem solving power

10. What determines spot size? • The spot size depends on the beam energy, WD, and the final aperture a, convergence angle • Performance improves with higher energies • On the S4700 the aperture size is set automatically • Changing the CL (spot size) does not affect resolution much Variation of probe size with energy and beam convergence for S4700

11. Working Distance • Working distance is the most important user controlled parameter • Always use the smallest WD that is possible for a given specimen • Note also that the image resolution is almost independent of the beam energy Microanalysis Imaging

12. Beam current • Typical contrast levels are 3-10% on most samples • Improving contrast lowers required IB , beam current, and improves resolution • Increase IB by raising the tip emission current from 10mA to 20 or 30 mA if necessary

13. Resolution • The pixel size is equal to the CRT pixel size divided by the actual magnification e.g a 100µm pixel at 100x gives 1µm resolution • Probe size only limits resolution at high magnifications Image at 1kx magnification has 0.1µm pixel resolution

14. Image Content • SE1 - high resolution • SE2 - low (BSE) resolution • SE3 - tertiary signal, interactions of the BSE with the pole piece and chamber walls • ET sees ­ 40% SE3, 45% SE2, 15% SE1 • TTL sees 75% SE2 and 25% SE1

15. SE1/SE2 interaction volumes • The SE1 signal comes from a few nm area at all energies • The SE2 signal comes from an area that can be up to a few microns in diameter at high energies

16. Pixel size and SE2 • At low and medium magnifications the pixel size ( a few µm) is comparable with SE2 interaction volume • So the image is mostly from the BSE generated SE2 component • The SE1 are not a significant contributor

17. Medium magnification • Medium magnification images have a resolution limited by SE2 interaction volume • SE and BSE images will look similar but not necessarily identical Image at 20kx - 50Å pixels

18. High magnification images pixel • Field of view is about size of the SE2 interaction volume so that signal remains about constant as beam scans • The pixel size is about equal with the SE1 area so the SE1 component now provides the image detail field of view

19. Pixels - a summary • High resolution requires the use of a high magnification to keep the pixel size at a small enough value not to limit the resolution • High resolution at high beam energies also requires a high magnification so as to separate the SE1 signal from the lower resolution SE2 signal

20. High resolution imaging • On the S4700 imaging in SE mode with a resolution into the nanometer range is readily possible • What is the ultimate resolution limit? • Optical performance, signal origination, and current to establish sufficient signal quality Imaging a 10nm thick oxide layer

21. How good is SE resolution? • The production of SE occurs over a finite volume of space • The initial SE event produces additional SE and so on, leading to a diffusing cloud of SE around the impact point • How far do they travel? Depends on the MFP (mean free path)

22. SE resolution • The diffusion effect is visible at the edges of a sample as the ‘bright white line' due to extra SE emission • The width of this line is a measure of the SE MFP  • The presence of this SE1 edge effect sets an initial limit to the achievable SE image resolution Molybdenum tri-oxide crystals Hitachi S900 25keV SE diffusion volume

23. Classical resolution limit • When the object is large its edges are clearly defined by the ‘white lines’ • But as the feature reaches a size which is comparable with  the edge fringes begin to overlap and the edge contrast falls Width = l 20nm 10 nm

24. Classical resolution limit • When the feature size is equal to or less than the edge lines overlap and the object is not resolved at all since it has no defined size or shape • This is Gabor’s resolution limit for SE imaging • The resolution in SE mode therefore depends on the value of l width = l 5 nm Particle contrast

25. High Resolution Imaging • On a high atomic number, very dense , material such as tungsten the SE MFP is only a nanometer or so • So a spatial resolution of about 1nm is likely to be possible • In fact ...

26. “Lattice” fringes • In this image by Kuroda et al (J.Elect.Micro 34,179, 1985) fringe structures with a spacing of 1.4nm are clearly visible in the SE image • This resolution is consistent with the diffusion model for SE production with =1nm • Image recorded at 20keV on an Hitachi S-900 FEGSEM • The probe size for this image was about 0.9nm Surface Configuration

27. In other samples... • When an object gets small enough to be comparable with l then it becomes bright all over and the defining edges disappear. • For low Z, low density materials, this can happen at a scale of 5-10nm edge brightness no edges Carbon nanotubes

28. The resolution limit • The resolution of the SEM in SE mode is thus seen to be limited by the diffusion range of secondary electrons, especially in low Z materials • In addition the signal to noise ratio is always worse for the smallest detail in the image

29. Improving the resolution • Improving SEM resolution therefore requires two steps: • minimizing or eliminating the spread of secondary electrons • improving the signal to noise ratio so that detail can be seen

30. Improving the S/N ratio • Use a metal coat as all metals give more SE than carbon • SE yield tends to rise with Z value • But high Z materials are denser and cause more scatter • Usually consider Cr, or Ti as best choices but W, Pt are also good Computed SE1 yield at 2keV

31. Particulate Coatings • Au produces very big particles (30nm) • Au/Pd and W make much smaller (3nm) particles • These have a very high SE yield • Can be deposited in a sputter coater • Coatings are stable • Good below 100kx 3nm of Au/Pd at 100kx

32. Decoration • In some cases the sputtered particles decorate active features on a structure, making them more visible • High Z materials, such as tungsten also permit BSE imaging Tungsten decorated T4 polyheads 25nm ring diameter 30keV Hitachi S900

33. Bypassing the limit • Since metals have much lower than carbon, and a higher SE yield, a thin metal film coating on a low Z, low density sample effectively localizes all SE production within itself. The resolution now is a function of the film thickness only and not of  • Works even with very thin metal films (few atoms thick) • Can exploit this effect to give interpretable contrast at high resolution High SE yield Low SE yield width ­ film even when <

34. Mass thickness contrast • The SE1 yield varies with the thickness of the metal film • This effect saturates at a thickness equal to about 3 • The conformation of the film to surface topography thus provides contrast bulk value S E Y i e l d mass thickness variation 1nm 2nm 3nm Film thickness

35. Metal builds contrast • The SE localization in the film provides edge resolution • The mass thickness effect gives extra contrast enhancement • The feature is now truly ‘resolved’ since its size and shape are visible 5nm low Z object 2nm metal film SE Beam position SE profile with metal film SE profile without metal

36. Cr coatings • Cr films are smooth and without structure even at thicknesses as low as 1nm • The mass thickness contrast resolves edges and make the detail visible down to a nanometer scale • The high SE yield of the Cr improves the S/N ratio • However these coatings are not stable - so use Cr coated samples immediately after they have been made AIDS virus on human cells 500kx 2nm Cr at 20keV Hitachi S900

37. Coating Summary • Coatings are an essential part of the technique of high resolution SEM because they generate interpretable contrast, improve resolution, and enhance the S/N ratio • Thin coatings are better than thick coatings - do not make your sample a piece of jewelry • Below 100kx particulate coatings are superior because of higher SE yields • Above 100kx use chromium or titanium • MRC lab uses Au/Pd coatings on most samples • Carbon is a contaminant not a coating

38. Getting the most from your SEM • Alignment is crucial. Check aperture alignment every time you change areas or imaging conditions and ensure that the stigmators are properly balanced • Minimize vibrations by choice of SEM location. Move pumps away etc. • Keep the room quiet, noise dampening material on the walls. • Check for stray fields. Remove fluorescent lights and dimmer controls. • Keep computer monitors away - use flat screens • Beware of ground loops

39. Clean Power • Many cases of ‘jaggies’ are due to dirty mains lines not EM pickup • Check waveform at your wall plug • Use clean power from a UPS for critical electronics • Avoids surges

40. Operating tips • Allow the SEM to thermally stabilize and the cold finger to cool down before attempting high resolution - this may take > 1 hour (seldom used at MRC) • Use the stage lock - but don’t forget to turn it off before unloading sample • Use the beam shift rather than stage motion - but remember to recenter the beam before taking a critical image • Look for the scan speed which minimizes ‘jaggies’ when viewing the image live

41. Getting the best image • Whenever possible take a single slow speed scan rather than accumulating multiple high speed scans • This eliminates blurring due to drift, and distortions in the video amplifier chain and usually produces a higher signal to noise ratio and better contrast 32 high speed frames single 20 second scan