Scanning Electron Microscopy SEM. Warren Straszheim, PhD MARL, 23 Town Engineering

Transcription

1 Scanning Electron Microscopy SEM Warren Straszheim, PhD MARL, 23 Town Engineering

2 How it works Create a focused electron beam Accelerate it Scan it across the sample Map detector output to screen

3

4

5 Create an electron beam Three main types of guns of increasing brightness and coherence Tungsten LaB6 Field Emission

6 Accelerate the beam Requires vacuum to support the voltage to prevent scattering

7 Focus the beam lenses and apertures Wehnelt/Gun Condenser lens Objective lens

8 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

9 Limitations of SEM First surface technique limited penetration (can t see through contamination) Vacuum requirement Conductivity requirement

10 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)

11 Auger electrons Secondary electrons Backscattered electrons Characteristic x-rays Continuum x-rays Cathodoluminescence Absorbed current Available signals

12 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

13 Extended depth of focus

14 Magnification range of less than 50x to 100s of kx

15 Continuous zoom from low magnification to high magnification

16

17

18

19

20

21

22 By the way, this was a non-conductive sample High-vac, low-vac (variable pressure), and environmental modes

23

24 Image Interpretation Illumination Detection Line of sight

25 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

26 What signal should you use? SE or BSE

27

28 What is the magnification? 240,000x...

29 or 127,000x?

30 Horizontal field width (HFW) = 1000um Magnification = Display width/hfw Mag * HFW = Display width 240,000 x 1000um = 240 mm = 9.5 inches 127,000 x 1000um = 127 mm = 5.0 inches I use a 5-inch Polaroid print as the standard. Other sizes are fake magnification.

31 Resolution/Quality/Speed Pick 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

32 Other issues Astigmatism - range of focal lengths Charging - unstable imaging Contamination obscures features of interest Unstable specimens moving targets

33 Charging leading to flattening of SE image

34 BSE image is somewhat more immune to charging

35 Oil leftover from cleaning

36 SEM images the first surface, be that sample, contamination, or surfactant

37 The Importance of Cleanliness Any organic residue left on the sample will build up and obscure the sample with time.

38 BSE imaging may be better at showing the true size

39 Contamination layer builds up and shows in SE

40 Contamination build-up after examination at 150kx

41 Contamination is still visible at 15kx

42 Residue is visible even at 5000x. Longer exposure leads to more build up. Even short exposures lead to contamination.

43 As-received 100kx Cleaned with plasma

44 As-received 150kx Cleaned with plasma

45 Anatomic considerations: bit depth Where can you distinguish gray levels?

46 256 levels, 8-bit

47 128 levels, 7-bit

48 64 levels, 6-bit

49 32 levels, 5 bit

50 16 levels, 4-bit

51 8 levels, 3-bit

52 Anatomic considerations: Pixel density How many pixels are enough? What s the difference between pixels and dots per inch?

53 1024 pixels

54 512 pixels

55 256 pixels

56 128 pixels

57

58 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.

59 Environmental Mode Variable pressure mode used for nonconductive 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)

60

61