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

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

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

4. Pinhole Lens zThe original SEM lens - designed to produce no magnetic field in the sample chamber zGood sample access zLong focal length and a big working distance so high aberrations zPoor EM screening zAsymmetric SE collection due to position of ET

5. Immersion Lens zShort focal length - so low aberrations zGood EM screening zVery stable specimen mounting in lens zSymmetric SE collection using the through the lens (TTL) detector system zRestricted to small samples (3mm disc)

6. Snorkel Lens zShort focal length - so low aberrations and high performance zGood EM screening zThe sample is outside the lens so there is no limitation on the size of the specimen

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

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

9. z 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  convergence angle zPerformance improves with higher energies zOn the S4700 the aperture size is set automatically zChanging the CL (spot size) does not affect resolution much Variation of probe size with energy and beam convergence for S4700

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

12. Beam current zTypical contrast levels are 3-10% on most samples zImproving contrast lowers required I B, beam current, and improves resolution  Increase I B by raising the tip emission current from 10  A to 20 or 30  A if necessary

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

14. Image Content zSE1 - high resolution zSE2 - low (BSE) resolution zSE3 - tertiary signal, interactions of the BSE with the pole piece and chamber walls zET sees ­ 40% SE3, 45% SE2, 15% SE1 zTTL sees 75% SE2 and 25% SE1

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

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

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

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

19. Pixels - a summary zHigh resolution requires the use of a high magnification to keep the pixel size at a small enough value not to limit the resolution zHigh 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 zOn the S4700 imaging in SE mode with a resolution into the nanometer range is readily possible zWhat is the ultimate resolution limit? zOptical performance, signal origination, and current to establish sufficient signal quality Imaging a 10nm thick oxide layer

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

22. SE resolution zThe 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 zThe 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 zWhen 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 20nm Width = 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 zThis is Gabor’s resolution limit for SE imaging  The resolution in SE mode therefore depends on the value of Particle contrast 5 nm width =

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

26. “Lattice” fringes zIn 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 zImage recorded at 20keV on an Hitachi S-900 FEGSEM zThe probe size for this image was about 0.9nm SurfaceConfiguration

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

28. The resolution limit zThe 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 6In addition the signal to noise ratio is always worse for the smallest detail in the image

29. Improving the resolution zImproving SEM resolution therefore requires two steps: 4minimizing or eliminating the spread of secondary electrons 4improving the signal to noise ratio so that detail can be seen

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

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

32. Decoration zIn some cases the sputtered particles decorate active features on a structure, making them more visible zHigh 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 zWorks even with very thin metal films (few atoms thick) zCan exploit this effect to give interpretable contrast at high resolution Low SE yield High SE yield width ­ film even when < 

34. Mass thickness contrast zThe SE1 yield varies with the thickness of the metal film  This effect saturates at a thickness equal to about 3 zThe conformation of the film to surface topography thus provides contrast 1nm2nm3nm Film thickness SEYieldSEYield bulk value mass thickness variation

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

36. Cr coatings zCr films are smooth and without structure even at thicknesses as low as 1nm zThe mass thickness contrast resolves edges and make the detail visible down to a nanometer scale zThe high SE yield of the Cr improves the S/N ratio zHowever 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 zCoatings are an essential part of the technique of high resolution SEM because they generate interpretable contrast, improve resolution, and enhance the S/N ratio zThin coatings are better than thick coatings - do not make your sample a piece of jewelry zBelow 100kx particulate coatings are superior because of higher SE yields zAbove 100kx use chromium or titanium zMRC lab uses Au/Pd coatings on most samples zCarbon is a contaminant not a coating

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

39. Clean Power zMany cases of ‘jaggies’ are due to dirty mains lines not EM pickup zCheck waveform at your wall plug zUse clean power from a UPS for critical electronics zAvoids surges

40. Operating tips zAllow 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) zUse the stage lock - but don’t forget to turn it off before unloading sample zUse the beam shift rather than stage motion - but remember to recenter the beam before taking a critical image zLook for the scan speed which minimizes ‘jaggies’ when viewing the image live

41. Getting the best image zWhenever possible take a single slow speed scan rather than accumulating multiple high speed scans zThis 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