Scanning Electron MicroscopySEM

Warren Straszheim, PhDMARL, 23 Town Engineeringwesaia@iastate.edu 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)