Scanning Electron Microscopy Field emitting tip Grid 2kV 100kV Anode ZEISS SUPRA Variable Pressure FESEM Dr Heath Bagshaw CMA bagshawh@tcd.ie
Why use an SEM? Fig 1. Examples of features resolvable using different imaging techniques
Improving Resolution Firstly, the wavelength of the imaging source is important. In an optical microscope white light is used(λ 380-700-nm) In an Electron Microscope the imaging source is a beam of electrons which has a shorter wavelength(λ ~0.0025nm at 200kV). This is approximately five times smaller than visible light and twice as small as a typicalatom thisiswhyelectronscan see atomsbutwhitelightcan t:- the analysis probe must be smaller than the feature being analysed The wavelength of electrons is dependent on the accelerating voltage, i.e.:- kv Wavelength λ (pm) 20 8.588 100 3.702 200 2.508 300 1.968 The higher the accelerating voltage the shorter the wavelength.
The Parts of an EM Electron Microscopes (EMs) are similar in operation to optical microscopes except thatthey use a focused beam of electrons instead of lightto"image"the specimen and gain information about its structure and composition. There are four major regions in an Electron Microscope:- (1) A stream of electrons is formed (by the electron source/gun) and accelerated toward the specimen using a positive electrical potential (2) This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam. (3) This beam is focused onto the sample using a magnetic lens. In an SEM thebeamisthenalsoscannedacrossthesurfaceofthesample. (4) Interactions occur inside the irradiated sample, affecting the electron beam which are detected and transformed into an image or signal. The above happens in all EMs regardless of type.
Layout of a Generic SEM Gun 1 Aperture Holder 2 Deflection coils 3 Objective Lens 4
Electron Gun Therearetwomaintypesofgun ThermionicandFieldEmission(FEG). Thermionic gun :- Simplistically, a material (often a piece of twisted tungsten) is heated toahightemperaturesothatitwillemitelectrons. CanalsouseLaB 6 crystalgrowntoatip givesabrighterbeamthanwforsamekv. Tungsten filament Tungsten filament assembly LaB 6 filament tip
Thermionic Gun Filament Wehnelt cylinder - 10-10000kV + Anode earth Filament is heated and begins to produce electrons. Electrons leave the filament tip with a negative potential so accelerate towards the earthed anode and into the microscope column. A smallnegative bias on the Wehneltthen focuses the beam to a crossoverwhich acts as the electron source.
Field Emission Gun (FEG) FEG source W tip A very strong magnetic field (~10 9 Vm -1 ) draws electrons from a very fine metallic tip (usually W). An extraction voltage of around 2kV is applied to the first anode to create an intense electric field to allow electrons to escape from the tip. The second anode is then used to accelerate the electrons into the microscope at the required energy. Combination of the two anodes focuses the beam into a crossover creating a fine beam source.
Comparison of Sources W filaments are very simple and inexpensive. LaB 6 filamentsgivegreaterbrightnessthanw(approximatelyx10),butcostmore. FEG s give much more brightness than thermionic systems. FEG s give a more monochromatic electron source and finer probe(i.e. better resolution). Temperatures used are much lower than for thermionic sources (particularly cold cathode FEG s). FEG s require better vacuum systems and are more expensive. Comparison of the three types of source operating at 100kV
Focusing the Beam Afterthebeamisformeditisfocusedbyacondenserlenssystemtoforma probe. The lenses are electromagnetic the focal length changes as current in the coil changes. After focusing, the beam is passed through an aperture which excludes electrons which are not on the optical axis improving resolution. Inconsistencies in the beam are corrected by stigmators and the beam focused onto the sample. A typical Electro Magnetic Lens
Scanning the Beam\Beam Interactions Deflector coils move the beam back and forth over the sample and the signal generated from each area is collected simultaneously, building up the final image shown on the monitor. Many signals are generated at the surface of the sample and many different forms of analysis may be performed. The interaction volume is the area of the sample excited by the electron beam to produce a signal. Incident Electrons (Electron Probe) Auger Electrons Secondary Electrons Backscattered Electrons Continuum X-Rays Fluorescence X-Rays Characteristic X-Rays Signals generated in the interaction volume
Interaction Volume The interaction volume is the area of the sample excited by the electron beam to produce a signal. The penetration of the electron beam into the sample is affected by the accelerating voltage used, the higher the kv the greater the penetration. The effective interaction volume can be calculated using the electron range, R:- 1.67 0.0276 A E0 R = ( µ m) 0.89 ρ Z Where A is the atomic weight (g/mole), Z is the atomic number, ρ is the density (in g/cm 3 ) and E o is the energy of the primary electron beam(in kv). Take the example of iron: A=55.85, Z=26, r=7.87 g/cm 3 Accelerating voltage (kv) Primary Electron Range (µm) 30 3.1 15 0.99 5 0.16 1 0.01 (10nm!) Signals generated in the interaction volume
Signal Detection The Everhard Thornley Detector (ETD) is the most common detector used to detect secondary electrons to image surface topography. Electrons are attracted to a +ve charge on a grid in front of the detector. The captured electrons are then amplified by a photo-multiplier before being digitised and sent to a screen. The signal detected is transferred to a viewing screen as the beam is scanned building up the image. Everhard Thornley Detector Scanning to produce an image
Biological Samples Biological samples are not conductive and are particularly vulnerable to beam damage and other heating effects when examined in an Electron Microscope. The level of exposure is referred to as Electron Dose and is a measure of the number of electronsperunitarea(e/nm 2 ). Samplesareeitherstainedwithconductivematerials(e.g.OsO 4 )orcoatedwithauorc. Samplesareviewedundervacuum,sotheyaredriedtoremoveallwater. a) b) a)semimageofpneumonia,andb)semimageofdiatom (PicturesfromUniversityofIowa) Preparing the samples fixes, and alters them need a way to look at samples whilst they are still wet.
Variable Pressure SEM Localised charging is removed by the presence of gas in the sample chamber, effectively allowing the examination of non conductive samples In Low vacuum mode the chamber is isolated from the high vacuum system(a) and is instead pumped by the additional rotary pump system(b).
Variable Pressure SEM (2) Thisallowsanalysisof nonconducting samplesaschargeiscompensatedbygasinthe chamber. UseofanSEMinVPmodedoesleadtosomelimitationsinit soperation:- BecausethevacuumislowerinaVPSEMchamber,someresolutionoftheinstrumentis lostduetoscatteringoftheelectronbeambythegasparticlesinthechamber. In situ heating and or cooling (with the appropriate sample stage) is possible in VPSEM to allow direct observation of sample changes. Compositional analysis is still possible.
Compositional Analysis Back Scattered Imaging As mentioned previously, when the electron beam hits the sample a number of signals are generated. Secondary electrons are used for looking at surface detail(topography). EM is also a very powerful technique for analysing composition and compositional distribution in a material\sample. B Signals generated in the interaction volume BackScattered electronsareproduced justbelowthe surface ofthe sample (B)andare scattered more by heavier elements than by lighter elements. The backscattered coefficient, η = (Z-1.5)/6 So, as Z increases, so does the degree of backscatter.
Back Scattered Electrons 0.5 Backscattered electrons 0.4 Electron yield 0.3 0.2 0.1 Secondary electrons 0 20 40 60 80 Atomic number (Z) Electron yield (i.e. intensity) as a function of atomic number for backscattered and Secondary electrons. Back Scattered electron have approximately the same energy as the primary electron beam and are therefore easy to detect - simply by a semiconductor placed above the sample:- Schematic of a backscattered electron detector.
BackScattered Imaging Back scattered electrons are deflected more by heavier atoms leading to a brighter contrast in BEI images the lighter the region the heavier the element present. a) b) White region Dark region Grey region a)secondary image of a cement showing surface morphology b)backscattered image of same area showing compositional inhomogeneity Three distinct regions in b), EDS analysis can then be used to find the different compositions of the these regions.
Example EDS of a Cement There were 3 distinct regions in the Backscattered Image Light region is made up predominantly of Fe. (i.e. the heaviest element) Grey region is made up predominantly of Ca. Dark region is made up predominantly of Si and Al. (i.e. the lightest elements)
Examples Images Imaged using ET Detector Imaged using InLens Detector Low kv image of platelets
Examples Images SE image of nano tubes SE Image of nano structure BE image of nano tubes
Conclusions Scanning Electron Microscopes (SEM s) are very useful tools for looking at a range of samples\materials. Surface detail, homogeneity and elemental composition can be determined in one experiment on the same sample. Newer Variable Pressure SEM s allow the imaging of non conducting samples. ESEM s, with cold stages and other peripherals allow imaging at 100% relative humidity allowing imaging of wet samples Electron Microscopy based analysis when used with other analysis techniques can assist in complete characterisation\identification of materials. Electron Microscopes provide a very powerful analysis tool in both Materials and biological fields.