Filter Control: Instrumentation and Operation

1. Filter Control:Instrumentation and Operation Leah Lucas TUG Meeting October 24, 2002

2. Outline Introduction to EFTEM Instrumentation Hardware: Gatan Imaging Filter Software: Filter Control Operation Filter Control Software Alignment Tips Techniques to be covered EELS (L. Zhang 11/07/02) EFTEM (Y. Yu 11/21/02)

3. Energy-Filtering TEM (EFTEM) Expose specimen to mono-energetic electron radiation Inelastic scattering in the specimen causes poly-energetic electron beam Transmitted electrons pass through an energy filter Create energy-dispersive plane Electron path depends on kinetic energy Introduce slit aperture in energy-dispersive plane to allow energy selection Admit electrons with kinetic energy (or energy loss) within a particular interval [E1, E2] Mono-energetic-Known wavelength Poly-energetic-Obtain a distribution of energies Energy dispersive plane- energy selection in addition to scattering angle selection Analogy to conventional TEM and objective aperture –BFP (momentum dispersed as a function of scattering angle)Mono-energetic-Known wavelength Poly-energetic-Obtain a distribution of energies Energy dispersive plane- energy selection in addition to scattering angle selection Analogy to conventional TEM and objective aperture –BFP (momentum dispersed as a function of scattering angle)

4. Types of Energy Filters In-column filter “Omega” type Magnetic device Advantages: mirror symmetry reduces aberrations Disadvantages: always on/cannot remove from columnMagnetic device Advantages: mirror symmetry reduces aberrations Disadvantages: always on/cannot remove from column

5. Types of Energy Filters Post-column filter Gatan Image Filter

6. Gatan Image Filter (GIF) Entrance aperture Pre-prism focusing and alignment coils Magnetic prism Spectrum-magnifying quadrupole assembly Energy-selecting slit Quadrupole-sextupole imaging assembly Pneumatically retractable TV-rate CCD camera Slow-scan CCD camera Electrons are bent over ~90. The exact bending angle depends on its energy (greater E-loss, larger the angle). Besides bending the electron beam and creating energy dispersion, the prism also has a focusing action and at some distance behind the prism, an energy dispersed, focused image is formed of the TEM’s projector lens crossover This is the energy-loss spectrum. The energy dispersion decreases with the primary energy of the electrons and drops below 1 um/eV for >200 keV. This puts a high demands on the quality of the slit. Two quadrapoles have been added in order to magnify the energy dispersion about 5 times while maintaining focus at the slit. Post slit optics: Imaging or Spectroscopy modeElectrons are bent over ~90. The exact bending angle depends on its energy (greater E-loss, larger the angle). Besides bending the electron beam and creating energy dispersion, the prism also has a focusing action and at some distance behind the prism, an energy dispersed, focused image is formed of the TEM’s projector lens crossover This is the energy-loss spectrum. The energy dispersion decreases with the primary energy of the electrons and drops below 1 um/eV for >200 keV. This puts a high demands on the quality of the slit. Two quadrapoles have been added in order to magnify the energy dispersion about 5 times while maintaining focus at the slit. Post slit optics: Imaging or Spectroscopy mode

7. Filter Control Software GIF window Daily alignment

8. Selecting and Energy-Loss Adjust: Prism current (Energy Shift) Change absolute energy of electrons and lose focus Small hysteresis when current is varied over large range Voltage on the Drift Tube (Drift Tube Offset) Accelerate electrons and shift energy-loss spectrum by __ eV Refocus objective lens Fast response, accurate, & no hysteresis TEM primary energy/high voltage (Spectrum Offset) Max energy loss, 3 keV Only decrease primary energy

9. GIF Window Slit width Max, 70 eV Dispersion, 3.6 ?m/eV at 300 keV Mode Imaging-Achromatic aberration-, distortion-free image Spectroscopy-Energy-loss spectrum at slit When selected, Dispersion pull-down menu appears (eV/pixel) Reduction – Camera/Mag. Aperture 3.0, 2.0, 0.6 mm

10. Adjustments Window Demagnetize Remove effects of magnetic hysteresis from the post-slit GIF lenses Performed automatically when GIF turned on and switch between Spectroscopy/Imaging ACComp A & B Focus X & Focus Y - Change current through X & Y pre-prism quadrupoles SX & SY - Adjust pre-prism sextupoles Achromaticity - Adjusts one of the post-slit quadrupole lenses in order to cancel energy dispersion at final image Squareness - Correct stigmatism in x-y directions Image shift X & Y - Centers image on CCD

11. Gatan Recommended Daily Alignment Focus the spectrum at the energy-selecting slit (Change current at pre-prism quadrupoles) Focus X & Focus Y Properly focused zero-loss peak should appear as a sharp line parallel to the slit edge

12. Gatan Recommended Daily Alignment Adjust the Achromaticity Wobble Drift Tube Offset or Spectrum Offset (HV) Obtain “coincident” images Center image/spectrum on the CCD Use Image Shift X & Y

13. http://vulcan2.cwru.edu/groups/tug/index.html