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Lab report 4. Very important: if you raise the sample to the 10 mm line during pumpdown, the working distance will be approximately 10 mm. (How do you know what the WD is?) There is no “elephant” (pink or otherwise) for the Quanta. - PowerPoint PPT Presentation
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Lab report 4•Very important: if you raise the sample to the 10 mm line during pumpdown, the working distance will be approximately 10 mm. (How do you know what the WD is?)•There is no “elephant” (pink or otherwise) for the Quanta. •Some people still aren’t getting the hint about attaching a copy of their lab notes•Nobody reminded me to upload images!
Nanowires, 1 MP
Nanowires, 512x442
High resolution imaging
• Why?– To see really tiny stuff!
• Soot particles (Combustion research)• Gold nanoparticles and nanorods (Jennifer
Shumaker-Parry)– To get really sharp images of “fairly small” stuff
• Gold nanoparticles and nanorods (Marc Porter…)– Same considerations should apply to e-beam
lithography
Tin balls 2.4 Mx!
Soot on TEM Grid
High resolution imaging• How?
– FEG Why?
• dp2 = 4ip/( π2 * β * αp
2)– To see small stuff, you need a small spot.– Small spot requires
• Low probe current (small “spot size”)• High brightness• Large aperture angle
– αp = dA/2*WD– Short working distance – Large aperture? We’re still arguing over that one, but
nobody believes it!
Contributions to actual beam diameter
Optimum convergence angle
• The previous equation can be differentiated and the derivative set equal to zero
• Result is quadratic in α4
Optimum convergence angle• If chromatic aberrations can be neglected:
– αopt = [2/3Cs2)(8ip/βπ2 + 0.72 λ2)]1/8
– At 10 kV, λ = 0.01 nm, 100 pA, and β = 108 A/cm2sr Cs = 2 mm, then αopt = 6.4*10-3 rad
– αp = dA/2*WD• If dA = 30 microns: WD = 2.4 mm
– At 1 kv λ increases by a factor of 10• αp increases to 9 mrad• WD decreases to 1.7 mm
• Much shorter for W-filament!• Note that αopt was 4 mrad for W-filament
(Cs = 10 mm), 20 kV in Fig 2.22.
High resolution imaging• What’s the point of all this math for high
resolution imaging?• Short WD is important!• ETD is out of the game• Other options
– TLD– vCD– Helix
• Immersion mode increases resolution about another order of magnitude
High resolution imaging• Minimize emi (Ian’s lecture of last Friday!)• Minimize floor vibrations
– Each lab in INSCC is on an individually poured slab of concrete
• Minimize acoustic vibrations (audio noise)• Minimize temperature fluctuations• Try “Mains lock”• Replace carbon tape with silver or carbon
paste• Use single sample mount
Minimize Acoustic vibrations!
DCJ - High Resolution 14
When do we run out of resolution?
SE1 have a range of a few nm and create the ‘edge bright line’ effect
As a result when the feature size is close to the SE escape range the object is not resolved. This occurs at ~ 5-10nm for low Z materials
DCJ - High Resolution 15
In other samples...
When an object gets small enough to be comparable with the SE1 generation volume 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
Carbon nanotubes 5nm and 10nm wide
edge brightness
no edges
DCJ - High Resolution 16
SE image of Single Wall NanoTube
DCJ - High Resolution 17
..but how wide is this nanotube?
?
The image does not have defined edges - its width is indeterminate and equal to carbon
If these are the edges then width
= 5.8nm
If these are the edges then width
= 1.5nm
C =5.5nm
DCJ - High Resolution 18
Bypassing the SE1 limit
Metals have lower than carbon, and a higher SE yield
A thin metal film on a low Z, low density sample localizes all SE production within itself. The resolution now is a function of the film thickness only
Works even with very thin metal films (a few atoms thick)
We can exploit this effect to give interpretable contrast beyond the theoretical limit
Low SE yield
High SE yield
width ~ film thicknesseven when <
DCJ - High Resolution 19
Mass thickness contrast The SE1 yield varies
with the thickness of the metal
SE1 yield reaches the bulk value at a film thickness equal to about 3
The conformation of the film to surface topography provides contrast 1nm 2nm 3nm
Film thickness
SE
Yield
bulk value
mass thickness variation
DCJ - High Resolution 20
Metal builds contrast
The SE localization in the film provides edge definition
The mass thickness effect gives extra contrast enhancement
The feature is now ‘resolved’ since its size and shape are visible
5nm low Z object
2nm metal film
Beam position
SE profile with metal filmSE profile without metal
SE
DCJ - High Resolution 21
courtesy of Martin Müller and Rene Herrmann, ETH Zürich
T4 Phage coated with Cr
Cr coatings Cr films are smooth and
without structure even as thin 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
T4 Phage + Cr
DCJ - High Resolution 22
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 useful because of their higher SE yields and better S/N ratios
Above 100kx can use chromium or titanium or particulate coatings of W,Pt or Ir
Carbon is a contaminant not a coating
Helix Detector
• Looks just like the LowVac detector• Mounts just like the LowVac detector• Costs $28.5k• “You break it, you bought it.”• Runs in Immersion Mode, like the TLD• Gives super images in LowVac• Aperture is 62 microns
Helix Detector page
Helix detector settings• 1- why and when should Helix detector voltage be changed?• Helix detector voltage should be optimized after every parameter
change. This just means, from the previous Helix sweet spot, needing to bump it up or down a 1% or 2% after changing a parameter like water, FWD, beam current, kV, dwell.
• I believe Helix needs to be running at full current, just prior to arcing, at all times. A typical Helix session may run like this; 78.5% voltage = arcing, 77.5% voltage = no arcing and good signal contrast, 76.5% voltage = no arcing and poor contrast. I don’t expect you to see identical numbers as these but I’d expect you to experience a similar “narrow window” of good zone.
• For me, Helix sweet spots are 1%-1.5% off saturation, 3.2-mm FWD, 4kV- 7kV, as much as 30% less water than what was needed for LVD.
• Reducing Spot size just means increasing dwell times but I typically work between spots 2.0 and 3.0
Gold on glass, LowVac, Helix Detector
vCD Detector
• Backscatter detector• Mounts on pole piece• Has small aperture; will work in LowVac• Works with beam deceleration• Works in immersion mode
Landing Energy Page
Gold Nanoparticles, TLD
Gold Nanoparticles, vCD
Worm cross-section, vCD
Virus, vCD