Scanning Electron Microscopy Notes

Introduction

  • Matthew Hunt, staff microscopist at KNI, introduces a series of three lectures on microscopy: Scanning Electron Microscopy (SEM), Gallium Focused Ion Beam (FIB), and Helium/Neon Focused Ion Beam.

SEM Example

  • Demonstration of SEM usage by scanning a beam across a 15-micron wide field of view.
  • Features include metal patterns (KNI, Caltech, circles) made of silver, gold, and titanium on silicon.
  • Secondary electron image shows topography.
  • Switching to backscattered electrons provides atomic number contrast.
  • Discussion of voltage, current, working distance, and other imaging parameters.

Resources

  • All resources, including the presentation, are available on the KNI lab wiki: lab.kni.caltech.edu.

Functionality

  • A dedicated page on the wiki helps users choose the appropriate microscope for their research at KNI.

Handouts and Books

  • Three handouts for the SEM lecture: SEM, Alignments, and Considerations for Optimizing Your SEM Images.
  • Links to handouts will be in the video description.
  • Recommended books for further study on each type of microscopy.

Analogies Between Microscope Types

  • Core concept: Drawing analogies between the three microscopy types.
  • Emission sources: Tungsten needles with extractors, suppressors, and accelerating voltages.
  • Column optics: Similar lenses and stigmata across different platforms.

Microscope Setup

  • SEM with a gallium FIB (dual beam system).
  • SEM on the main optical axis, gallium FIB offset by 52 degrees.
  • Vacuum screen showing ion getter pumps and turbo molecular pump.
  • Aperture controller and column isolation valve.
  • Internal components: Specimen on a stub, XYZ rotation tilt stage (6-inch stage on Nova 600), SEM column, secondary electron detectors (detector and through-the-lens detector), injection system needles.
  • Reverse view: Gallium column, omniprobe needle, magnetic field sensor for field cancellation.
  • NOVA 200 (2-inch stage): Includes an energy dispersive spectroscopy detector and a wavelength dispersive spectroscopy detector for X-ray analysis.

Lenses

  • Electromagnetic lenses (electron systems) vs. electrostatic lenses (ion beam systems).
  • Electromagnetic lens: Wire coil (copper) embedded in an iron shell; current generates a magnetic field that deflects off-axis electrons back towards the main axis, achieving a focus point.
  • Electrostatic lens: Utilizes an Einsel lens with positive potential and grounded plates to accelerate and decelerate the beam symmetrically.
  • Electrons spiral in a magnetic field.

Condenser Lens

  • Located in the column. Demagnifies a large spot into a smaller one using a series of lenses.

Objective Lens

  • Located at the bottom of the column.
  • Basic objective lens: Copper wire coil inside an iron core generates a magnetic field to focus the beam to a point.
  • Einsel Lens: Used at the bottom of the column to focus the beam.
  • Field-free mode: Basic objective lens configuration.
  • Immersion mode: Uses an additional coil to generate a magnetic field inside the chamber, creating a floating objective lens plane between the column and the sample, minimizing working distance for higher resolution. Not suitable for magnetic samples.

SEM Specifics

SEM Field Emission Gun

  • Source: Tungsten filament (cathode) at negative potential and anode plate at ground potential to accelerate electrons down the column.
  • Thermionic emission from the hot tungsten filament.
  • Electric field application for electron emission.
  • Zirconium oxide coating on the tip to lower the work function and increase beam brightness.
  • Grid cap at a slightly more negative potential for the first beam crossover.
  • Electrons accelerate down the column with energy proportional to the accelerating voltage.

Column Components

  • First condenser lens (usually two) to demagnify the beam.
  • Scanning and stigmata octopoles (electromagnets) to scan the beam and correct its shape.
  • Blanking plate with ammeter to measure current and prevent sample exposure.
  • Gun tilt and shift quadrupoles to steer the beam.

Key Parameters

  • Accelerating voltage, probe current, and probe convergence angle.
  • These parameters determine the probe diameter and image resolution.
  • Probe size dPdP is a function of aberrations, voltage, current, and convergence angle.

Voltage

  • Most important parameter; controls electron energy.
  • Typical SEM voltage ranges from 0.1 kV to 30 kV.
  • Higher voltage results in deeper penetration and scattering (interaction volume).
  • Interaction volume is smaller at lower voltages.
  • Higher voltage theoretically provides higher resolution due to smaller wavelength, but aberrations limit the resolution.
  • Spherical and chromatic aberrations are worse at lower voltages.
    • Spherical aberrations: Imperfections in the lens that affect electrons more at lower voltages.
    • Chromatic aberrations: Energy spread of electrons from the tip; smaller energy spread at higher voltage.
  • For conductive specimens, higher voltage gives sharper images.
  • For non-conductive specimens, lower voltage may be better to avoid charging artifacts.
  • Lower voltage increases secondary electron yield and can help achieve charge equilibrium.

Current

  • Controls spot size; lower current yields a smaller probe diameter for better resolution but less signal.
  • Balance resolution with signal strength.
  • Use contrast value to determine optimal current: high contrast (95-98%) means you may be able to lower the current; low contrast (50-70%) indicates you might need more current.
  • Controlled through apertures and condenser lenses.

Apertures

  • Vary the physical size to control electron flow.
  • smaller apertures allow fewer, more on-axis electrons through, improving resolution.
  • Larger apertures let more electrons through over a wider range, worsening resolution.

Condenser Lens

  • Changes the crossover point inside the column.
  • Unitless metric called "spot" controls current.
  • Stronger condenser lens forces electrons to cross over higher in the column, reducing current.
  • Relationships between condenser lens strength, probe diameter, resolution, and signal.

Interaction Volume and Signals

Secondary Electrons

  • Primary electron interacts inelastically with sample electrons, kicking them out.
  • Low energy (typically <50 eV) and emitted from the top 5-10 nm of the surface.
  • Show topography of the image.
  • Everhart Thornley detector with positive bias attracts and counts electrons.
  • Through-the-lens detector (positive bias) captures secondary electrons inside the column.

Backscattered Electrons

  • Electrons elastically scattered by atoms in the specimen.
  • Lose less energy and emerge with 50-100% of their original energy (e.g., 15-30 keV from a 30 keV beam).
  • Emerge from deeper in the specimen, providing atomic number contrast.
  • Show atomic number contrast because elastic scattering is a function of atomic number (gold scatters more than silicon).
  • Backscatter images have good Z contrast with gold, titanium, and silver on silicon.

Characteristic X-rays

  • Primary electron knocks out an inner shell electron, and an outer shell electron drops to a lower energy level, emitting an X-ray photon.
  • Each element emits characteristic X-rays based on its energy transitions.
  • X-rays are emitted from the whole interaction volume, but characteristic X-rays come from a spike-shaped region.
  • X-rays on the line of sight with the detector are counted and used to generate a spectrum showing which elements are in the specimen.
  • Can be used to perform mapping, i.e., create silicon, gold, silver, and titanium maps to indicate where they come from.

Detector Guidelines

  • Secondary electrons: High signal, low current needed for high resolution.
  • Backscattered electrons: More current needed due to rare elastic events, lower resolution.
  • X-rays: Much more current needed because only a small fraction of X-rays are counted by the detector.

Magnification of Signals

  • Secondary electron image: Shows topography and imperfections.
  • Backscatter image: Flatter image with better Z contrast.
  • EDS maps: Show elemental contrast.

Voltage Selection for Different Signals

  • 30 kV provides the best resolution for conductive samples.
  • Lower voltages are useful for non-conductive samples.
  • Voltage should be approximately two times higher than the highest energy X-ray you want to detect.

Energy and Wavelength Dispersive Spectroscopy.

Energy Dispersive Spectroscopy (EDS)

  • All X-rays and energies go to the detector at once.
  • Elements can therefore be analyzed simultaneously.

Wavelength Dispersive Spectroscopy (WDS)

  • Uses an analytical crystal.
  • Sweeps an energy range.
  • Only X-rays that satisfy the Bragg condition and diffract to hit the detector are counted.
  • Slower than EDS, but has a higher resolution spectrum leading to better quantification.

Working Distance

  • The distance from the objective lens to focal plane.
  • Practical: Focus the image and tell the microscope where the sample is located.
  • Theoretical: Convergence angle of the beam changes with the working distance.
    • Short working distance: Steep convergence angle and smaller probe size.
    • Long working distance: Shallow convergence angle and poorer resolution from larger spot size.
  • There is an inverse relationship between resolution and depth of field.
    • Long working distance: Good depth of field, poorer resolution.
    • Short working distance: Poor depth of field, good resolution.
  • Working distance is a balance between depth of field and resolution.
  • In practice, the working distance is often geometrically confined by the experimental apparatus i.e. gallium focused ion beam or EDS.

Eccentric Height

  • A height in the chamber where, at any tilt angle, the microscope will be imaging the same thing i.e. the same spot.
  • It is important to work at eccentric height because it provides a geometrically reproducible spot in the chamber.

Imaging Modes

  • Field-free mode: Uses the single objective lens.
  • Immersion lens: Smaller effective working distance, but can't get electrons out to Everhart Thornley detector.
  • Weak immersion/EDS/EDX Mode: Balance between the other two for purposes of trapping electrons such that they do not affect the X-ray energy spectrum.

Scanning Filters

Frame Integration/ averaging can be used to compensate for geometrical or voltage based artifacts by scanning the same frame a large number of times for short periods of time.

Alignments

The basic alignment workflow is to: Adjust he source tilt and get the beam concentric down the column before using focus to tell us if we have an astigmatism or lens alignment problem.

Astigmatism

Correcting for ellipticity of the beam; Stigmata (electromagnets) can be used to make to circular.

Lens Alignment

Beam must be centered on the objective lens to ensure that the image remains stationary when moving the focus.

Detector Bias

  • Positive bias on Everhart Thornley detector: Attracts electrons.
  • Positive bias on TLD: Attracts a mixture of secondary and backscattered electrons.
  • Negative bias on TLD: Suppresses secondary electrons and only lets backscattered electrons through, enhancing Z contrast.
  • Zero volt bias on TLD: Allows some electrons to go up and some to go back down to the sample, achieving charge equilibrium.

Measurement and Calibration

There are NIST measurement calibration standards and calibration must take place at the specific image settings used to acquire the original image to be measured.

Sample Preparation

  • Nonconductive samples can be imaged at lower voltages, but can also be coated in 5nm of amorphous carbon.
  • Specimens can be put in an oxygen and argon plasma cleaner to remove carbon.

Applications and Techniques

E-beam lithography

Electron beam lithography can be used to pattern devices, though electrons generally have to penetrate about 80 microns into silicon.

Multi-step Patterns.

Multistep patterns can be constructed by going from CAD to titanium liftoff with alignment markers to backscatter electron images used for their z contrast.

Environmental SEM

Unique in that it can operate at higher pressures in the chamber by using a differential pumping arm which prevents gasses traveling up the column and oxidizing the tip. Useful for two things:

  • Imaging biological specimens without drying them out by maintaining their morphology with a higher pressure (1-27 millibar for the environmental SEM versus ~E-5 millibar for a high vacuum system).
  • Imaging highly non conductive specimens at higher pressure with water vapor.

Electrical Probe Station

  • A four point probe station that fits over the stage of SEM. Allows the user to do measurements on the devices based on whatever kind of function generator is hooked up to the system.
  • Has a hot and cold stage (can heat the stage t up to 240240 C and use liquid nitrogen to cool down the stage to −185-185 C.

Atomic Force Microscopy

AFM, as opposed to SEM, allows the user to quantitatively measure Z topography as well as lots of other properties using a stylus. Specifically this particular SEM has a peak force tapping mode which allows the user to generate a force curve for tap and therefore extract some force curve or elastic modulus properties.

Transmission Electron Microscopy

TEM allows the user to get very high resolution by the utilization of higher energy (between 200 KeV and 300 KeV electrons). Can be used to do diffraction studies, image quantum dots, and perform grain boundary studies.

Gallium FIB and helium FIB (discussed in lectures 2 and 3) can be used to make these samples by using a tungsten needle to take a thin sample of the bulk material and welding it to the grid with platinum before thinning it electron transparency in TEM by taking off > 100 nm layers of the bulk material. It can also be used for deposition purposes etc.