Scanning Electron Microscope (SEM)

1. Scanning Electron Microscope (SEM) Major components: Vacuum system Electron beam generation system Electron beam manipulation system Beam specimen interaction system Detection system Signal processing system Display and recording system SEM operation principles: http://www.youtube.com/watch?v=PNHn4YM7yfc&feature=related SEM introduction http://www.youtube.com/watch?v=c7EVTnVHN-s&feature=related

2. Vacuum system Low vacuum pump (<10-3 Torr) (remove 99.99% of air), high vacuum pump(>10-3 Torr) SEM operation need 10-4 to 10-6 Torr

3. E beam generation • Three components: • 1) cathode (filament or field emission) • 2) grid cap that control the flow of electrons • 3) anode that attracts and accelerates electrons • Voltage ranges from 0.1 to 40 Kv, (10kV the most common for biological specimen) • the higher the voltage the better the resoution (but • greater heat will be generated on the specimen) • On/off switch for high voltage (HV) • Beam current (the flow of electrons that hit a sample), controlled by bias voltage between filament and grip cap, • Increasing beam current results in deeper penetration of electron and a larger diameter spot) • Filament current control (adjustment of current filament, providing necessary heating current to filament) • Filed emission: • advantages: cool cathode, emitted beam is smaller in diameter (better resolution), longer life time • disadvantages: higher vacuum need (~10-7 Torr), cleaner microscope needed, not many x-rays generated (due to low beam currents and small beam diameters)

4. E beam manipulation E-gun is controlled by electrostatic field, while the rest of SEM is controlled by magnetic lenses. Electromagnetic lenses: - condenser lenses: reduce spot size spherical aberration limit resolution - correct astigmatism (non-circular beam spot), due to beam formed by filament is elliptical, dirt in column, beam distorted on the aperture using stigmator (control strength and azimuth) - correct alignment - two sets of magnetic coils (raster coils) that move the seam scanning in the X and Y direction - magnification: ratio of dimension of CRT to dimension of the area being scanned two ways of magnification adjustments: 1) change scanned area of the specimen 2) adjust focal point of the beam and working distance (move Z-axis to bring sample to focal point) Aperture: a round hole that control the passing through of scattered electrons - small aperture for high resolution - big aperture for low resolution with more electrons needed

5. Beam interaction Backscattered electrons: original beam electrons, high energy level, useful when relative atomic density information with topographical information is displayed Secondary electrons: generated by dislodging specimen e or other secondary e a few eV, detected near the surface, can obtain topographical and high solution, contrast and soft shadows of image resemble specimen illuminated with light X-rays: can obtain elemental information (from wavelength and energy characteristic of elements) measurement of wavelength (wavelength dispersive spectrometer (WDS)) or energy level (energy dispersive spectrometer (EDS)) wavelength range: 10 ~ 0.01 nm (3x1016 ~ 3x1019 Hz), soft x-ray:0.12ev~12 keV, hard x-ray:12kev~120 keV, http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html http://www.physics.isu.edu/radinf/xray.htm

6. Cathode Luminescence: • specimen molecule’s florescence that produces light photon Specimen current: • e energy decreases after scattering and e absorbed by sample • can build up negative charge and lead to charging Transmitted electrons: • primary e pass through specimen • provide atomic density information displayed as a shadow higher the atomic number the darker the shadow

9. Review for SEM Operation • Contamination : image become dark when irradiated on a portion for a long time, cause by residual gas being struck by e. probe (see the guide on p.17) • Charge-up: negative charge collected that leads to prevent normal emission of second e.

10. Signal Detection Detector Structure • secondary e. attracted by 200V applied to ring around the detector (Faraday Cup) • e. accelerated by 10 Kv applied to hit scintillator and produce photons • photons travel down to photomultiplier (PM) where signal is increased or amplified (PM control contrast) Topological Features • density of emissive area determine signal strength • emissive area affected by topology such as flatness, pointed structure, edge (see the guide on p. 9) X-ray detection • x-ray loss energy by hitting another particles that produce background • EDS detector gather spectrum from 0 to 30 ev • WDS is set to detect a small a range (more sensitive than EDS)

11. Signal Manipulation • brightness and contrast are main control of signal manipulation • brightness: actual value of each pixel control by adding or subtracting value for each pixel • contrast: difference between two pixels control by the amplifier of secondary e. (PM)

12. Display and Record System • Brightness • Contrast • Resolution: ability to distinguish between two points determined factors: beam spot size, working distance, aperture size, beam bias current/voltage, how cylindrical the beam is • Magnification: a function of area scanned and viewing size adjusted by raster coils and location of focal point of the primary beam to final lens • Depth of field: region of acceptable sharpness in front of and behind focus points • Noise: can be controlled by increasing signal (aperture size, bias voltage, etc) decrease noise increasing scanned time

13. Waveform Monitor used to set appropriate brightness and contrast