Scanning Electron Microscopy EMSE-515 F. Ernst 1
2 Scanning Electron Microscopy Max Knoll Manfred von Ardenne Manfred von Ardenne
Principle of Scanning Electron Microscopy 3
Principle of Scanning Electron Microscopy 4
Animated Demonstration of SEM 5
Principle of Scanning Electron Microscopy focused electron beam specimen: emits electrons and photons detector: electrons, photons, amplifier brightness of corresponding pixel deflection coils: scan the beam image scanning electronmicroscopy SEM no imaging electron optics! 6
Magnification 7
Advantages of SEM easy to change magnification can use manifold of emissions from the specimen, e. g.: electrons: secondary electrons (S), backscattered electrons (B) photons: X-rays, light, heat, complementary information from the same region of the specimen electronic processing: contrast amplification noise reduction 8
Realization
Field-Emission Gun Scanning Electron Microscope Hitachi S4500 10
Field-Emission Gun Scanning Electron Microscope Hitachi S4500 field-emission gun two secondary electron detectors backscatter detector infrared chamber scope Noran XEDS (X-ray energy-dispersive spectrometry) system spatial resolution: < 1.5 nm at 15 kev electron energy also performs well at reduced beam energies (e. g. 1 kev) facilitates observation of insulating materials emphasizes near-surface structure of the specimen 11
Environmental Scanning Electron Microscope Philips XL30 12
Environmental Scanning Electron Microscope Philips XL30 usual SEM capabilities for imaging and microanalysis, plus: imaging in various gases including water vapor pressures up to 20 mbar cooling stage tensile stage (capable of heating up to 1000 C) heating stage, up to 1500 C OIM (orientation imaging microscopy) EBSP (electron backscatter pattern) detector automated determination of crystal orientation 13
ESEM Applications pharmaceutical dynamic physio-chemical properties e. g. swelling, disintegration and dissolution of drugs fiber technology: wetting and drying of wool, cotton and synthetic fibers cement science observe how water reacts with cements e ects of additives on the cementing process medical: observe fresh and wet tissue biology/botany/entomology: live (!) plant seedlings, ants, fleas 14
Electron Beam Formation electron gun: thermal emission tungsten filament or LaB 6 single crystal requires vacuum < 10 5 Pa field-emission (tunneling e ect): pointed tungsten tip ( strong electric field) requires UHV (< 10 7 Pa), to keep tip clean anode ground potential potential di erence versus cathode: 0.5..30 kv
Electron Beam Formation 16
Tungsten Filament 17
LaB 6 Filament 18
Field-Emission Tip 19
Instrument Capabilities and Limitations brightness of the electron gun definition: current density (A/m 2 ) per solid angle (sr) (steradian) LaB 6 cathode: 10 times brighter than tungsten cathode field-emission gun: 100 times brighter than LaB 6 typical brightness of a field-emission gun: > 10 12 A m 2 sr 1 20
Instrument Capabilities and Limitations minimum beam diameter d small beam diameter requires reduction of source image by condensor system typical condensor system: two electromagnetic lenses minimum beam diameter in practice: 20..50 µm for thermal emission (LaB 6 ) 10 nm for field-emission gun 21
Ray Path 22
Instrument Capabilities and Limitations beam convergence (semi-angle of the beam cone) aperture in the plane where beam position independent of the scan point (tilt angle) aperture diameter determines beam convergence resolution limit : > d high resolution requires small beam diameter but: current I 0 into the specimen decreases rapidly with decreasing beam diameter d: I 0 2 d 2 : semi-angle of the beam cone ( convergence angle). 23
Instrument Capabilities and Limitations aberrations of the illumination system increase the theoretical beam diameter d to d 2 1 2 0 = d2 + 2 C s 3 2 + d 0 : true beam diameter; C s : coe cient of spherical aberration; : de Broglie wavelength of the electrons. both, very large and very small beam convergence are disadvantageous 24
Instrument Capabilities and Limitations general problem: SEM does not make optimum use of illumination to optimize SEM performance: choose beam convergence for maximum beam current I 0 (for given beam diameter d, determined by required resolution) depth of focus: D f := pixel diameter in specimen plane typical value: pixel diameter 100 very large! 25
Signal-to-Noise Ratio definition of contrast C of image detail: C := S S S: average signal level; S: increase at region of interest. noise: random fluctuations of signal level around S visibility of a detail (empirical criterion): contrast C must exceed noise by factor 5 detail with contrast C visible only if I B > Q 0 C 2 : readout time per image; Q 0 = 4 10 12 As. 26
Signal-to-Noise Ratio definition of contrast C of image detail: observation of details with weak contrast: long exposure time, or large beam current fundamental limit of resolution imposed by: gun brightness aberrations 27