scanning electron microscopy semcanfield.physics.iastate.edu/course/canfield 2018 sem.pdfscanning...
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How it works
• Create a focused electron beam
• Accelerate it
• Scan it across the sample
• Map detector output to screen
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Create an electron beam
Three main types of guns of increasing brightness and coherence
• Tungsten
• LaB6
• Field Emission
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Accelerate the beam
• Requires vacuum to support the voltageto prevent scattering
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Focus the beamlenses and apertures
• Wehnelt/Gun
• Condenser lens
• Objective lens
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Benefits of SEM
• Shorter wavelength – higher resolution (0.1 nm electron at 10 keV vs 500 nm for light)
• Longer working distance – greater depth of focus
• Generally intuitive image interpretation(super magnifying glass)
• Scanned beam – perfect parfocality
• Wealth of signals: SE BSE, X-ray, voltage
• Energetic beam - microanalysis
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Limitations of SEM
• First surface technique – limited penetration(can’t see through contamination)
• Vacuum requirement
• Conductivity “requirement”
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Sample preparation
• Generally minimal
• Clean and dry
• Cut sample to fit and show structure of interest
• Secure sample (tape, glue, clamp)
• Embed or polish for cross sections
• Coat with metal or carbon (optional)
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Available signals
• Auger electrons
• Secondary electrons
• Backscattered electrons
• Characteristic x-rays
• Continuum x-rays
• Cathodoluminescence
• Absorbed current
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Contrast mechanism:Topography
Secondary electrons have a limited escape depth – many are created but few escape
A tilted (more vertical surface) allows more to escape – brighter signal
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Extended depth of focus
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Magnification range of less than 50x to 100s of kx
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Continuous zoom from low magnification to high magnification
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By the way, this was a non-conductive sample
High-vac, low-vac (variable pressure), and environmental modes
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Image Interpretation• Illumination
• Detection
• Line of sight
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Contrast mechanism:Atomic number
Higher atomic number/electron density leads to
• greater secondary electron yield (coat samples with metal)
• greater backscattering coefficient
Atomic number is the only contrast mechanism for polished samples
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What signal should you use? SE or BSE
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What is the magnification?
240,000x . . .
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or 127,000x?
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Horizontal field width (HFW) = 1000um
Magnification = Display width/HFWMag * HFW = Display width
240,000 x 1000um = 240 mm = 9.5 inches127,000 x 1000um = 127 mm = 5.0 inches
I use a 5-inch Polaroid print as the standard.Other sizes are fake magnification.
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Resolution/Quality/SpeedPick two
• Small beam (spot size) leads to better spatial resolution but fewer electrons
• Bigger beam leads to more signal quality (less noise) but also less resolution
• Dwell time can be adjusted widely
High resolution images are worthless if you can’t see the detail through all of the noise
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Other issues
• Astigmatism - range of focal lengths
• Charging - unstable imaging
• Contamination – obscures features of interest
• Unstable specimens – moving targets
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Charging leading to flattening of SE image
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BSE image is somewhat more immune to charging
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Oil leftover from “cleaning”
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SEM images the first surface, be that sample, contamination, or surfactant
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The Importance of Cleanliness
Any organic residue left on the sample will build up and obscure the sample
with time.
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BSE imaging may be better at showing the true size
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Contamination layer builds up and shows in SE
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Contamination build-up after examination at 150kx
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Contamination is still visible at 15kx
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Residue is visible even at 5000x .
Longer exposure leads to more build up.
Even short exposures lead to contamination.
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As-received 100kx Cleaned with plasma
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As-received 150kx Cleaned with plasma
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Anatomic considerations:bit depth
Where can you distinguish gray levels?
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256 levels, 8-bit
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128 levels, 7-bit
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64 levels, 6-bit
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32 levels, 5 bit
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16 levels, 4-bit
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8 levels, 3-bit
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Anatomic considerations:Pixel density
• How many pixels are enough?
• What’s the difference between pixels and dots per inch?
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1024 pixels
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512 pixels
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256 pixels
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128 pixels
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PPI versus DPI(for when editors gets fussy)
• PPI = pixels per inch, how we describe images
• DPI = dots per inch, how printers describe files
• It takes about 8x8 dots to render 1 gray pixel
• Therefore a 1600 dpi requirement is met by a 1024-pixel image printed no more than 5 inches wide.
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Environmental Mode
• Variable pressure mode used for non-conductive or out-gassing samples
• Environmental mode used to maintain sample at equilibrium (micro-grapes)
• Various gas choices (water, air, reducing)
• Heating and cooling options (-25C to 1000C)
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