Surface Emissions

Specimen current

X-raysCathodoluminescence

Pole Piece, etc SE3

≈ 1 nm for metals upto 10 nmfor insulators

Dependence of backscatter coefficient on backscattered electron energy

Backscattering increases strongly with Z independent of beam energy

Backscatter dependence on tilt and Z

Where are electrons coming from?

• Kanaya – Okayama equation for electron range in a material:

• Rk-o = (0.0276 A E01.67)/(Z0.89ρ) μm

– For E0 in keV, ρ in gm/cm3, A in gm/mole and Z = atomic number

Where are electrons coming from?

Material A Z ρ E0 R (μm)

PMMA 12.01 6 1.16 20 8.63295

Fe 55.26 26 7.87 20 1.58759

Fe 55.26 26 7.87 1 0.01066

Homework 2.2

Depth of originationof backscatteredelectrons

Backscatteredelectrons can Investigate deep into the sample

Distribution of scattered electron energies

Note that the distinction between SE and BSE is a definitionThere are only “scattered electrons”

Why do backscattered electrons give the most information about

chemical composition ?

Why don’t secondary electrons give much information about chemical composition?

Dependence of scattered electron yield on Z

50%!

10%Nearly independent

Why do secondary electrons give great topographic information?

Image formation and interpretation

Image formation and interpretation

Images from exactly the same area of the same sample taken with different detectors.

In case you thought the second image was just taken at higher

contrast…

And it’s not just your detector choice that can impact your image…

20 kV 500 V

Everhart-Thornley detector

Scanning and Data Collection

Transfer of image from sample to screen

Works for both topographic and elemental information

Pixels(Picture elements)

• NEC monitor: 380 x 300 mm; 1280 x 1024 pixels. Hence pixel size on monitor size is 297 x 293 microns...300 microns.

• Typical file size used in FEI Nova NanoSEM is 1024 x 884 pixels.

• Pixel size on sample is pixel size on monitor divided by magnification, about 15 microns (20X) to 0.6 nm (500kX).

So what; who cares?• Example: 10 keV beam at 100 pA viewing

at 100 X (neural array was taken at 118 X) saved into 1024 x 884 file using Leo and FEI.

• β = 4Ip/π2αp2 dp

2

• dp = (2WD/πrA)sqrt(Ip/ β) • FEI: dp = (2*5 mm/π*.015 mm)sqrt(10-10

Acm2sr/108A) = 0.5 nm!• Leo: (2*8 mm/π*.01 mm)sqrt(10-10 A

cm2sr/105A) = 40 nm!

So what; who cares? Cont’d

• So – Your pixel size varies from 0.6 nm to 15

microns– Your beam diameter can vary from 0.5 nm to

40 microns, at the smallest– Your interaction volume varies from 10

microns to 10 nm (BSE, SE2; last homework)

Implications

• If probe size is too small– You are wasting resolution: topography can

change between sampling points (Nyquist Theorem!)

– Resolution regained by sampling and saving more points

– You are wasting signal to noise– You are wasting contrast

How to make the spot larger?

• dp = (2WD/πrA)sqrt(Ip/ β)

– Increase working distance– Go to a smaller aperture– Increase probe current– Decrease accelerating potential

• Is this the dominant effect of decreasing the accelerating potential?