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Scanning Electron MicroscopySEM
Warren Straszheim, PhDMARL, 23 Town [email protected] 515-294-8187
How it works
• Create a focused electron beam
• Accelerate it
• Scan it across the sample
• Map detector output to screen
Create an electron beam
Three main types of guns of increasing brightness and coherence
• Tungsten
• LaB6
• Field Emission
Accelerate the beam
• Requires vacuum to support the voltageto prevent scattering
Focus the beamlenses and apertures
• Wehnelt/Gun
• Condenser lens
• Objective lens
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
Limitations of SEM
• First surface technique – limited penetration(can’t see through contamination)
• Vacuum requirement
• Conductivity “requirement”
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)
Available signals
• Auger electrons
• Secondary electrons
• Backscattered electrons
• Characteristic x-rays
• Continuum x-rays
• Cathodoluminescence
• Absorbed current
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
Extended depth of focus
Magnification range of less than 50x to 100s of kx
Continuous zoom from low magnification to high magnification
By the way, this was a non-conductive sample
High-vac, low-vac (variable pressure), and environmental modes
Image Interpretation• Illumination
• Detection
• Line of sight
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
What signal should you use? SE or BSE
What is the magnification?
240,000x . . .
or 127,000x?
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.
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
Other issues
• Astigmatism - range of focal lengths
• Charging - unstable imaging
• Contamination – obscures features of interest
• Unstable specimens – moving targets
Charging leading to flattening of SE image
BSE image is somewhat more immune to charging
Oil leftover from “cleaning”
SEM images the first surface, be that sample, contamination, or surfactant
The Importance of Cleanliness
Any organic residue left on the sample will build up and obscure the sample
with time.
BSE imaging may be better at showing the true size
Contamination layer builds up and shows in SE
Contamination build-up after examination at 150kx
Contamination is still visible at 15kx
Residue is visible even at 5000x .
Longer exposure leads to more build up.
Even short exposures lead to contamination.
As-received 100kx Cleaned with plasma
As-received 150kx Cleaned with plasma
Anatomic considerations:bit depth
Where can you distinguish gray levels?
256 levels, 8-bit
128 levels, 7-bit
64 levels, 6-bit
32 levels, 5 bit
16 levels, 4-bit
8 levels, 3-bit
Anatomic considerations:Pixel density
• How many pixels are enough?
• What’s the difference between pixels and dots per inch?
1024 pixels
512 pixels
256 pixels
128 pixels
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.
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)