Scanning Electron Microscopy Basics and Applications Dr. Julia Deuschle Stuttgart Center for Electron Microscopy MPI for Solid State Research Room: 1E15, phone: 0711/ 689-1193 email: j.deuschle@fkf.mpg.de Outline 1. Setup and Instrumentation 2. Electron-Matter-Interactions 3. X-Ray Analysis 4. Literature: Goldstein et al.: Scanning Electron Microscopy And X-Ray Analysis, Springer Verlag 2003 Reimer und Pfefferkorn: Rasterelektronenmirkoskopie, Springer Verlag 1973
1. Setup and Instrumentation 1.1. The electron column Additional components: vacuum system (pre-pump and turbo molecular pump for chamber, ion getter pumps for column) electronic controls (high voltage, lens current etc.) software controls (software interface for the operator to control the microscope) stage (for sample mounting and positioning) 1. Setup and Instrumentation 1.2. The electron gun generation of free electrons acceleration of the electrons collimating or focusing of the beam
1.2. Electron guns 1.2.1 Tungsten Hairpin Cathode Thermoionic electron source, i.e. the emission is due to heating of the cathode The emitted electrons are focussed into a bundle by the grid cap (= beam formation) and crossover. The electrons are then accelerated towards the anode and enter the column. 1.2. Electron guns Emission of Tungsten Hairpin Cathode For a stable emission and hence a stable electron beam with constant beam current, the current through the filament must be sufficiently high, i.e. operated in saturation.
1.2. Electron guns 1.2.2 Lanthanum-Hexaboride-Cathode Thermoionic electron source LaB 6 crystal mounted on C or Re, 500 x100 µm lower work function compared to W, thus higher emission at the same temperature longer lifetimes but more expensive Degradation due to oxid formation and evaporation Failed tip 1.2. Electron guns 1.2.3 Field-Emission Guns 1. Cold field emission gun Emission is due to strong electrostatic fields, which concentrate at the tip. The W-tip apex is typically several nm wide. UHV conditions are needed to allow tunneling of electrons. 1 2 2. Schottky field emission gun A combination of heating and electrostatic fields are used to set electrons free from a small region of 10-30 nm. A ZrO 2 -coating on the W-tip further lowers the work function. An additional suppresor grid is used to bundle the electrons to a beam.
1.2. Electron guns: comparision Emitter Type Thermionic Thermionic Schottky TFE Cold FE Tip image comparison Cathode Material W LaB6 ZrO/W (100) W (310) Operating Temp (K) 2700 1800 1800 300 Cathode Radius (nm) 60,000 10,000 <1000 <100 Effective Source Radius (µm) 25 10 0.015 0.0025 Emission Current Density (A/cm2) 3 30 5300 17,000 Total Emission Current (µa) 200 80 200 5 Brightness (A/cm2.sr.kV) 1x10 4 1x10 5 1x10 7 2x10 7 Maximum Probe Current (na) 1000 1000 10 0.2 Energy Spread @ Cathode (ev) 0.59 0.4 0.31 0.26 Energy Spread @ Source Exit (ev) 1.5 2.5 1.3 2.5 0.35 0.7 0.3 0.7 Beam Noise (%) 1 1 1 5 10 Emission Current Drift (%/h) 0.1 0.2 <0.5 5 Operating Vacuum (hpa/mbar) <10-5 <10-6 <10-8 <10-10 Typical Cathode Life (h) 100 >1000 >5000 >2000 Cathode Regeneration (h) None None None 6-12 Sensitivity to External Influences Minimal Low Low High Stability Standard High Very high Low X-ray analysis EDS / WDS EDS / WDS EDS / WDS EDS Source: micro-to-nano 1. Setup and Instrumentation 1.3. Electron lenses 1. Condensor lens(es) to demagnify the beam and adjust the spot size to the experimental needs 2. Objective lens to focus the beam on the sample surface
1.3 Electron lenses 1.3.1 Electromagentic lenses Built up of a ferromagnetic mantle, which contains copper windings When entering the inhomogeneous magnetic field inside the lens the electrons are bend towards the optical axis and the beam is focused. The electron path is a spiral. 1.3 Electron lenses 1.3.2 Electro-static lenses trajectory Principle of a Einzel -lens: The combination of 3 tubes at different potentials acts as focusing device. The electrons travel towards positive potentials and are repelled by negative potentials.
1. Setup and Instrumentation 1.3. Electron lenses 1. Condensor lens(es) to demagnify the beam and adjust the spot size to the experimental needs 2. Objective lens To focus the beam on the sample surface design must also contain scanning coils, stigmator and the final aperture. 1. Setup and Instrumentation Scanning coils for adjusting the position of the beam on the sample surface in x- and y- direction Image formation the beam is moved step by step. First along on line in x-direction, then one step down in y-direction etc.
1. Setup and Instrumentation 1.3. Electron lenses 1. Condensor lens(es) to demagnify the beam and adjust the spot size to the experimental needs 2. Objective lens To focus the beam on the sample surface design must also contain scanning coils, stigmator and the final aperture. 1.4 Apertures Aperture size is important for controlling the beam and imaging parameters as follows: determines the probe current controls the image depth of focus minimizies the effects of aberrations The beam is characterized by 1. acceleration voltage 2. probe current 3. convergence angle 4. Spot size
1.4 Apertures Aperture size determines the probe current in limiting the amount of electrons passing through 1.4 Apertures Aperture size controls the image depth of focus Source: ammrf
1.4 Apertures Aperture size d s = 0.5C s 3 minimizes the effects of aberrations Spherical aberration d s and aperture diffraction d d cause the point spot to blur to an enlarged spot of size d. d d = 0.61 / 1. Setup and Instrumentation 1.3. Electron lenses 1. Condensor lens(es) to demagnify the beam and adjust the spot size to the experimental needs 2. Objective lens To focus the beam on the sample surface design must also contain scanning coils, stigmator and the final aperture.
1.5 Stigmator Astigmatism of lenses is due to machining inaccuracies, material or winding imperfections When the probe cross-section that hits the surface is not circular, steaking of the image features appears and needs to be corrected Spot shape Octupole stigmator corrects asymmetric probe shape into a circular probe Examples for astigmatic images and corrections 1.5 Stigmator
1. Setup and Instrumentation 1.6. Electron detectors 1. Setup and Instrumentation 1.6.1 Everhart-Thornley detector usable for SE and BSE detection depending on the bias of the grid 1.6.2 BSE (semiconductor) detector BSE hitting the detector create electronhole pairs, which are seperated. The current is proportinal to the energy of the incident electron. Source: wikipedia
1. Setup and Instrumentation View inside the SEM chamber BSE-detector: angular disc inserted between polepiece and samlpe SE-detector: positioned at the side of the chamber/sample Collector grid is biased for effiecient collection of electrons 2. Electron-Matter-Interactions Signal generation upon beam-solid-interaction Signal used for spectroscopy, i.e. chemical analysis of the material Signal used for imaging Source: ammrf
2.1 The Interaction Volume Vizualizing the interaction volume PMMA sample irradiated for a certain time with a 20 kv beam, which has been chemically etched for successively longer time increments. Depending on the energy deposition in the material, the molecular bonds weaken, resulting in faster etching rates than the surrounding material. 2.1 The Interaction Volume 10 kv 20 kv 30 kv Simulation of the interaction volume The interaction volume is much lager than the beam diameter. It depends on acceleration voltage The higher the acceleration voltage the bigger the interaction volume will be sample material The higher the amount of high atomic number elements inside the sample the smaller the interaction volume will be. C, Z=6 Fe, Z=26 U, Z=91
2.1 The Interaction Volume Vizualizing the interaction volume as a function of atomic number N 2 Z=7 Ar Z=18 Gases glow upon entering of the electron beam The possibility for backscattering of PE increases with increasing atomic number. Thus, the yield of BSE can be used for z-contrast imaging, i.e. material contrast. 2.2 Signals used for imaging Primary electrons (PE): Incident (= beam) electrons hitting the sample Secondary electrons (SE): Electrons that are created from inelastic scattering processes with beam electrons. They possess low energies and can only escape from shallow depths Backscattered electrons (BSE): Incident electrons, that undergo elastic scattering and escape from the surface again.
2.2 Signals used for imaging BSE chamber SE 3 to detector SE 1 SE 2 escape depth BSE range surface envelope 2.3 Examples How does the signal type affect an image? acquired using SE 3D appearance due to edge effect Useful imaging mode for topography acquired using BSE inclusions with different composition become more visible useful imaging mode for z-contrast J. Deuschle Übungen Materialcharakterisierungs- und Testmethoden Sommersemester 2010
2.3 Examples How does the signal type affect an image? acquired using SE Surface topography is clearly visible acquired using BSE different compositions can be seen J. Deuschle Übungen Materialcharakterisierungs- und Testmethoden Sommersemester 2010 2.3 Examples How does geometry affect an image? beam incident angle = 0 Tilted: beam incident angle 80 SE are emitted only from very shallow depth increasing the beam angle will raise the SE yield Surface topography leads to changes in beam incident angle, thus the different SE yields will give contrast
2.3 Examples Topological contrast and edge effect Cleavage surface 3. X-Ray Analysis Combining SE- and BSE- signal provides complementary information a more complete picture can be achieved acquired using SE acquired using BSE For a full characterization of the material, spectroscopy methods, using x-ray signals, can be used to add chemical information.
3. X-Ray Analysis 3.1. Creation of x-rays under electron beam irradiation Characteristic x-rays are generated upon transitions between subshells. The energy difference is characteristic for each element, so detection of the energy of the x-ray photon gives information about the elements present in the sample. 3. X-Ray Analysis 3.2. Energy Dispersive Spectroscopy (EDS) Capturing of a photon in the detection system creates a current pulse, which is analyzed. The height of the pulse is proportional to the energy of the incoming photon. EDX: a complete spectrum is obtained from one measurement. fast, but poor energy resolution mostly used for qualitative analysis in scanning mode elemental distribution maps can be acquired
3. X-Ray Analysis 3.3. Wavelength Dispersive Spectroscopy (WDS) The x-ray photons emitted from the sample undergo a diffraction process, which separates one specific wavelength, for which the Bragg condition is met. The proper photons are focused on the detector and captured for analysis. WDX: Only one element can be measured at a time Slow and complicated, but with 10 x better resolution than EDX mostly used for quantitative analysis or trace element identification 3. X-Ray Analysis Quantification of a WDX-spectrum The intensities (counts per second) measured on the sample of unknown composition can be compared to intensities from a standard with known composition. Thus, the weight fractions of elements in the sample can be calculated.