Scanning Electron Microscopy (SEM) and Focused Ion Beams (FIB) in Materials Research Jim Mabon and Wacek Swiech Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign 1 © 2012 University of Illinois Board of Trustees. All rights reserved. Comparison to Optical Microscopy 2 Optical SEM (secondary electron) The higher resolution and depth of focus available with the SEM are clearly observed, The SEM also provides a very wide, easily adjustable range of magnifications. For most imaging applications minimal or no sample preparation is required. The high resolution attainable (very small probe size) is due to very low mass and short wavelength of energetic electrons (0.007nm @30kV). The combination of high brightness sources of electrons and electron optics allow the formation and manipulation of very fine focused electron beams to probe the sample surface for imaging and analysis. Micrographs from Scanning Electron Microscopy and X-Ray Microanalysis, Joseph I. Goldstein et al. Plenum Press 3 Exceptionally High depth of Focus 2,200X original magnification 100,000X original magnification “rat’s hair” psilomolene (Mn, Ba Carbon Nanotube oxide natural mineral sample) Extremely Wide Range of Magnifications 4 12X original magnification 500,000X original magnification Miniature Sensor Device - Sputtered Au-Pd on Magnetic Tape Calorimeter 5 What does the SEM do? • A Scanning Electron Microscope is an instrument for observing and analyzing the surface microstructure of a bulk sample using a finely focused beam of energetic electrons. • An electron-optical system is used to form the electron probe which may be scanned across the surface of the sample in a raster pattern. • Various signals are generated through Many Applications: the interaction of this beam with the sample. These signals may be collected One of most widely employed microscopy or analyzed with the application of techniques other than optical microscopy. appropriate detectors. • For imaging, the signal amplitude Surface topography / morphology Composition analysis obtained at each position in the raster Crystallography (electron diffraction and pattern may be assembled to form an channeling techniques) image. Optical/Electronic properties (cathodoluminescence, EBIC) Animation from A Guide to X-Ray Microanalysis, Many other more specialized applications Oxford Microanalytical Instruments 6 Sequential Image Acquisition in SEM • The scan of the electron beam and the digitization of the image pixel value are synchronized with intensity proportional to the collected signal. • Typically electrons emitted from the sample are detected to M = Ldisplay/Lspecimen assemble the image. Caution: • Magnification is given by the Instrument magnification value ratio of the length of the line on is based on reference display device to length scanned image size which may vary on the real sample. Animations from, The Oxford Guide to X-Ray Microanalysis, Figure from Scanning Electron Microscopy and X-Ray Oxford Instruments Microanalysis Group Microanalysis, Joseph I. Goldstein et al. Plenum Press 7 Generalized Construction of an SEM • Vacuum System • Electron Source and Accelerating Voltage • Electron Lenses (electromagnetic) – Condenser Lens(es) – Objective Lens • Stigmator Coils • Beam Deflectors (electromagnetic) – Alignment – Scanning (raster) • Objective Aperture • Multi-Axis Specimen Stage • Detectors – Imaging detectors – Analytical detectors • Operating / Display Systems 8 Major Electron Beam Parameters Four electron beam parameters define the probe: • Probe diameter – dp • Probe current – Ip • Probe convergence angle – ap • Accelerating Voltage – Vo These interdependent parameters must be balanced by the operator to optimize the probe conditions depending on needs: Electron optical brightness, b, of the probe is essentially equal to the • Resolution brightness of the source, thus is a very • Depth of Focus important electron source parameter. • Image Quality (S/N ratio) • Analytical Performance From Scanning Electron Microscopy and X-Ray Microanalysis, Joseph I. Goldstein et al. Plenum Press 9 Electron Sources - Thermionic Sources Tungsten Filament LaB6 http://www.a-p-tech.com/lab6.htm Major Advantages: • Very high probe currents obtainable • Stable probe, especially W • Less complex vacuum system • Lowest overall cost / easy to maintain Disadvantages: • Lower brightness Schematic of a generalized thermionic • Relatively short lifetimes electron source for electron microscopy. From Scanning Electron Microscopy and X-Ray Microanalysis, Joseph I. Goldstein et al. Plenum Press 10 Electron Sources -Cold Field Emission Sharp Single Crystal (310) Tungsten Tip (310) 300 K Courtesy Hitachi Instruments Major Advantages: • Highest brightness SEM source available • Very long potential source lifetime – many years Disadvantages: • Lowest maximum probe current • Poor short and long term probe current stability • Requires ultra-high vacuum in gun area • Cost (initial) Courtesy Hitachi Instruments 11 Electron Sources - Thermal-Field (Schottky) Emission Sharp Single Crystal (100) Tungsten Tip with Zr02 Film ~1800 K From Rooks and McCord, SPIE Handbook of Microlithography Major Advantages: • Very high brightness source • High probe currents obtainable (few hundred nA) • Long potential source lifetime (few years) • Excellent short and long term stability (100) Disadvantages: • Requires ultra-high vacuum in gun area • Source heating is continuous, 24/7 (finite life) ZrO2 • Cost (initial and maintenance) http://www.fei.com/products/components/electron-ion-sources.aspx 12 Comparison of Electron Sources TUNGSTEN LaB6 SCHOTTKY COLD FIELD Effective Source Size (nm) 15,000 5000 15 3 Brightness 5- 6 6 7 8 9 (A/cm2SR) 10 10 10 -10 >10 10 Energy Spread (eV) 1.0 1.0 0.5 - 1.0 0.3 Emission Current <150mA <100mA <150 mA <20mA Maximum Probe Current (SEM) 1000+ nA <1000 nA 10-500 nA <2.0 nA SEM resolution (typical specs range) 3.0 -4.0 nm 2.0-3.0nm <1.0 – 2.0 nm <1.0-1.5 nm Probe Current Stability (%/hour) <1 <2 <1 >10 Operating Temperature (K) 2800 1850 1800 300 Operating Vacuum -2 -5 -7 -7 (Pa) <10 <10 <10 <10 Typical Service Life 100 hrs 1000 hrs >>1 year >>>1 year Cost $ $$ $$$ $$$ 13 Lenses in Electron-Optical Columns Graphic from A Guide to X-Ray Microanalysis, Oxford Microanalytical Instruments Limiting Parameters Magnetic Lens Spherical Aberration Electromagnet coil Chromatic Aberration Precision machined soft Astigmatism iron “pole piece” Aperture diffraction 14 Lens Aberrations / Optimum Aperture Angle Spherical Aberration Aperture Diffraction causes a fundamental limit to the achievable probe size. Optimum aperture angle determined by combined effect of spherical aberration and aperture diffraction. Adapted from Scanning Electron Microscopy and X-Ray Chromatic Aberration Microanalysis, Joseph I. Goldstein et al. Plenum Press Lens Aberrations / Astigmatism 15 Astigmatism is causedLens byAberrations imperfections in– theAstigmatism lens or other interference. It can be corrected using additional elements called stigmators contained inside the objective lens. Magnetostatic quadrupole lens is basis of a stigmator Octupole lens stigmator Adapted from Scanning Electron Microscopy and X-Ray Microanalysis, Joseph I. Goldstein et al. Plenum Press 16 Function of Condenser Lens(es) De-magnify the beam extracted from the source to enable a small spot to be obtained on the sample. Multiple lenses may be used in the condenser lens system. Weak Lens Strong Lens • Longer focal • Short focal length, Larger a1, length, Small a1, Smaller d1 Larger d1 • More beam • Less beam accepted into accepted into objective objective aperture aperture • Higher probe • Lower probe current at current at specimen specimen • Larger focal spot • Smaller focal at specimen spot at specimen CondenserLens • Lower • Higher Objective Resolution Aperture resolution • Higher Signal • Lower Signal Levels Levels Objective Lens u v 1 1 1 M v/ u Object u v f Adapted from Scanning Electron Microscopy and X-Ray Image Microanalysis, Joseph I. Goldstein et al. Plenum Press Objective Lens / Working Distance 17 Strong Lens Weak Lens i.e. short WD i.e. long WD Electron • Focus the electron beam on Gun the specimen with minimal lens aberrations, astigmatism is corrected Condenser • Short Focal Lengths(W1) –> smaller d2, larger a2 -> Lens better resolution, poorer depth of focus • Longer Focal Lengths(W2) –> larger d2, smaller a2 -> better depth of focus, poorer resolution Objective • Smaller Apertures Lens –> smaller d2, smaller a2 -> better resolution & better depth of focus, limited by aperture diffraction or S/N Adapted from Scanning Electron Microscopy and X-Ray Microanalysis, Joseph I. Goldstein et al. Plenum Press Secondary and Backscattered Electrons 18 Energy Distribution of Emitted Depth/Lateral Distribution of Emitted Electrons Electrons (and other emissions) Secondary electrons Secondary electrons (<~10nm) Auger electrons Backscattered electrons (<~3nm) Backscattered electrons Secondary electrons are low energy Characteristic electrons (<50eV) ejected from the X-rays specimen atoms by the energetic primary beam. Continuum X-rays Backscattered electrons are primary beam electrons scattered back out Fluorescent of the sample. X-rays There is a purely elastic peak and a continuum of BSE that have lost energy. Monte-Carlo simulations of electron scattering 19 C 15 keV Al PMMA @ 20kV Everhart et.al. (1972) 15 keV Ti Ti Ti 1 mm 5 keV 15 keV 25 keV • Determine effective lateral or W Simulations are very useful for