Scanning Electron Microscopy

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Scanning Electron Microscopy

• Prof. Jun Jiao
• Office Hours Wednesday, 1400 1700
• Office Location SB 2, room 370
• Tel. [phone]
• E-mail jiaoj_at_pdx.edu

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Course Description The
course is designed to introduce the theoretical
and practical concepts of scanning electron
microscopy (SEM), and to provide extensive lab
opportunities for students. Topics studied
include SEM optical principles, specimen
preparation, SEM imaging, and microchemical
analysis covering qualitative and quantitative
X-ray analyses. Lectures consider basic design of
the SEM and energy-dispersive X-ray systems and
are intended to relate operational procedures to
functions or features of these electronic
systems. Through "hands-on" SEM operation,
students will become proficient in the operation
of SEM and EDX system.
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Instrument Scanning electron microscope (SEM)
used in this class is the ISI SS-40 SEM equipped
with Oxford Link ISIS 300 EDX microanalysis
system.
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History of SEM

• The earliest recognized work describing the
  concept of an SEM is that of M. Knoll (1935) in
  Germany working in the field of electron optics.
• The improvement of the secondary electron
  detector was accomplished by Everhart and
  Thornley in 1960. The Everhart-Thornley Detector
  is a detector used in SEM. It is named after its
  designers, T Everhart and RFM Thornley. The
  Everhart-Thornley Detector has been available
  since the fifties, but remains the most
  frequently used detector in SEMs.
• The first commercial scanning electron microscope
  became available in 1965 by Cambridge Scientific
  Instruments.
• The SEM is one of the most versatile instruments
  available for the examination and analysis of the
  microstructural characteristics of solid objects.
• The SEM provides two outstanding improvements
  over the optical microscope it extends the
  resolution limits and improves the depth-of-focus
  resolution more dramatically (by a factor of
  300).
• The SEM is also capable of examining objects at a
  large range of magnifications. This feature is
  useful in forensic studies as well as other
  fields because the electron image complements the
  information available from the optical image.
• The coupling of an energy-dispersive x-ray
  detector to an SEM makes it possible to obtain
  topographic, crystallographic, and compositional
  information rapidly, efficiently, and
  simultaneously for the same area.

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Comparison of the LM, TEM, and SEM
Courtesy of James S. Young
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A Human Hair vs. Carbon Nanotubes
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Comparison of Resolution and Depth of Focus
Optical Micrograph
SEM Micrograph
SEM image shows the skeleton of a small marine
organism (the radiolarian Trochodiscus
longispinus)
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Electron Probe Microanalyzer (EPMA)

• The primary function of EPMA is to obtain
  compositional information, using characteristic
  x-ray lines, with a spatial resolution on the
  order of 1 ?m in a sample.
• Nowadays, EPMA capability can be achieved by
  installing an energy-dispersive x-ray
  spectrometer or wavelength dispersive x-ray
  spectrometer into a SEM.
• In 1913, Henry Moseley (British physicist) found
  that the frequency of emitted characteristic
  x-ray radiation is a function of the atomic
  number of the emitting element. This discovery
  led to the techniques of x-ray spectroscopy
  chemical analysis, by which the elements present
  in a specimen could be identified by the
  examination of the directly or indirectly excited
  x-ray spectra.
• Since electrons produce x-rays from a region
  often exceeding 1 ?m wide and 1 ?m deep, it is
  usually unnecessary to use probes of very small
  diameter.

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Electron Optics

• Functions of the SEM Subsystems.
• Why Learn about Electron Optics?
• Thermionic Electron Emission.
• Field Emission.
• Electron Guns (W gun, LaB6 gun, Field emission
  guns).
• Comparison of Electron Sources.
• Lenses in SEMs.
• Lens Aberrations.

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Basic Components of the Scanning Electron
Microscope
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Important Definitions

• Filament heating current the current used to
  resistively heat a thermionic filament to the
  temperature at which it emits electrons.
• Emission current the flow of electrons emitted
  by the filament.
• Beam current the portion of the electron
  current that goes through the hole in the anode.
• Electron Column consists of an electron gun and
  two or more electron lenses, operating in a
  vacuum.
• Electron Gun produces a source of electrons and
  accelerates these electrons to an energy in the
  range of 1-40 keV. The beam diameter produced
  directly by the conventional electron gun is too
  large to generate a sharp image at high
  magnification.
• Electron lenses are used to reduce the diameter
  of this source of electrons and place a small,
  focused electron beam on the specimen. Most SEMs
  can generate an electron beam at the specimen
  surface with a spot size of less than 10 nm while
  still carrying sufficient current to form an
  acceptable image.

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Important Definitions continued

• Working Distance (WD) The distance between the
  lower surface of the objective lens and the
  surface of the specimen is called the working
  distance.
• Depth-of-Focus The capability of focusing
  features at different depths within the same
  image.
• Secondary Electron are electrons of the specimen
  ejected during inelastic scattering of the
  energetic beam electrons. Secondary electrons are
  defined purely on the basis of their kinetic
  energy that is, all electrons emitted from the
  specimen with an energy less than 50 eV.

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Types of Electron Guns

• Tungsten (W) Hairpin Electron Gun The typical
  tungsten electron gun is a ? shape wire
  filament about 100 ?m. To achieve thermionic
  emission, the filament is heated resistively by
  the filament heating current.
• Lanthanum Hexaboride (LaB6) Electron Gun is a
  thermionic emission gun. It is the most common
  high-brightness source. This source offers about
  5-10 times more brightness and a longer lifetime
  than tungsten, but the required vacuum conditions
  are more stringent.
• Field Emission Electron Guns The field emission
  cathode is usually a wire of single-crystal
  tungsten fashioned into a sharp point and spot
  welded to a tungsten hairpin. The significance of
  the small tip radius, about 100 nm or less, is
  that an electric field can be concentrated to an
  extreme level. If the tip is held at negative 3-5
  kV relative to the anode, the applied electric
  field at the tip is so strong that the potential
  barrier for electrons becomes narrow in width.
  This narrow barrier allows electrons to tunnel
  directly through the barrier and leave the
  cathode without requiring any thermal energy to
  lift them over the work function barrier.

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The Electron Gun
Courtesy of James S. Young
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Filaments
Courtesy of James S. Young
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Beam Current Saturation (Tungsten)

• A constant flow of electrons into the column
  (beam current) is needed for proper operation.
• As the filament heating current is increased, so
  is the beam current to a point called saturation,
  where any further increase in filament current
  will not increase the beam current.

Saturation point
Beam Current
False peak
0
Filament Current
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Lanthanum Hexaboride Filament

• Single crystal of LaB6
• Tip is 100µm
• Chemically reactive when it gets hot
• Crystal is held by glassy carbon or graphite
  supports
• Carbon not reactive with LaB6

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Comparison of Lanthanum Hexaboride and Tungsten
Hairpin Filaments

• Tungsten Hairpin
• stable beam current
• short life
• large tip
• large area (probe diameter)
• LaB6
• stable beam current
• longer life
• smaller tip
• smaller area (probe diameter)

• high work function
• low brightness
• lower vacuum
• low resolution
• 2700 K
• lower work function
• higher brightness
• higher vacuum
• higher resolution
• 1800 K

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Field Emission Filaments

• Cold Field Emitter (CFE)
• single crystal of tungsten
• operate at room temperature
• very bright
• long lasting
• require very high vacuum (lt 10-10 Torr)
• contaminates easily
• require frequent flashing (sudden heating)
• poor current stability
• Thermal Field Emitter
• like a cold field emitter, but heated to 1800 K
• does not contaminate easily, no flashing
• larger energy spread than CFE

• The Schottky Field Emitter
• single crystal of tungsten coated with zirconium
  oxide (ZrO)
• heated to 1800 K
• ZrO lowers the work function
• larger emitting area than CFE
• larger virtual source size
• small energy spread
• high current density
• good current stability
• does not easily contaminate no flashing
• long life

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Field Emission Filaments

• A very fine wire of single- crystal tungsten
  fashioned to a sharp point
• Tip is 100nm or less
• Local electric field forms at tip, which
  decreases the energy (work function) needed by an
  electron to escape the cathode.
• Three types of FE cathodes

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Field Emission Filaments
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The Electromagnetic Lens
Soft iron casing
Top polepiece
Polepiece gap (brass)
Copper wire windings
Bottom polepiece
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The Electromagnetic Lens

• The electrons move through the lens in a helical
  path, a spiral, not a straight line.
• One effect is that the image in an SEM will
  appear to rotate if you vary the accelerating
  voltage or the working distance.

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The Electromagnetic Lens

• The focal length of the lens can be adjusted
  changing the amount of DC current running through
  the coils.

Point of crossover
Multiple electron trajectories
Lens
Point on specimen
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Lens Aberrations

• Chromatic aberration
• Electrons of different energies focus at
  different focal points.
• More energetic electrons (shorter wavelength)
  have longer focal lengths.
• Results in larger focal points.
• Caused by low kV, variations in lens current,
  large aperture angle, and in TEM thick specimens.
• Can be seen at low magnifications, sharp in the
  center, out of focus near the edges.

Chromatic aberration of a single lens causes
different wavelengths of electron to have
differing focal lengths.
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Lens Aberrations

• Spherical aberration
• Electrons near the edge of the lens are bent more
  than those near the center.
• Because the magnetic field between the lens
  polepieces is not uniform.
• Results in unsharp point and image distortion.
• Can be corrected with small aperture.

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Lens Aberrations

• Pin cushion and Barrel distortions
• Spherical aberration at the final imaging lens.

Pin cushion distortion
Barrel distortion
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Lens Aberrations
Pin Cushion Distortion or Chromatic Aberration?
3kV
10kV
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Lens Aberrations

• Astigmatism
• Strength of lens is asymmetrical it is stronger
  in one plane than another.
• Caused by machining errors, non-homogeneous
  polepiece iron, asymmetrical windings, dirty
  apertures.
• Results in out-of-focus stretched image.
• Corrected with stigmator coils.

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Key Points for Imaging and Microanalysis

• High Depth-of-Focus Images This is the SEM
  capability most often used in routine microscopy.
  High depth of focus is attained when different
  heights in the image of a rough surface are all
  in focus at the same time. This mode requires a
  small convergence angle so that the beam appears
  small over large height differences on the
  specimen. The small beam angle can be obtained by
  using a small objective lens aperture, a long
  working distance, or both.
• High-Resolution Images High-resolution images
  require a small probe size, and adequate probe
  current, and minimal interference from external
  vibration and stray AC magnetic field. Electron
  optically, the high resolution is obtained by
  selecting the smallest probe and a short working
  distance. The penalty for using a very small
  probe is typically a very low probe current. High
  brightness sources, small probe size and short
  working distance are ideal conditions for high
  resolution imaging.

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Key Points for Imaging and Microanalysis

• High Beam Current for Image Quality and X-ray
  Microanalysis While a probe current of at
  least 10-12A is required to produce a
  photographic image, the image may be so noisy
  that image detail is lost. By increasing probe
  size (weakening the first condenser lens), the
  probe contains more current and image quality
  improves. But, the resolution decreases. Currents
  of at 10-10A are needed for x-ray detection by
  the energy-dispersive x-ray spectrometer (EDS)
  while the wavelength-dispersive x-ray
  spectrometer (WDS) requires at lease 10-8A.
  Often the probe diameter must be intentionally
  enlarged to obtain an adequate signal for
  microanalysis.

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Carbon coated magnetic nanoparticles
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Field Emission SEM Images of Carbon Nanotubes
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Arc-discharge of Carbon Nanotubes
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SiO2 Nanowires
(a)
1 ?m
The length and diameter of the SiO2 nanowires
varied, ranging from a few microns to tens of
microns. The diameter ranges from 50 nm to 800 nm.
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FESEM Images of CdS Nanowires
(a)
(b)
1 ?m
100 nm

• CdS/SiO2 Composite Nanowires were observed as
  curved wires terminated with a Au nanoparticle.

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SiO2 Nanowires with Sharp Tips
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Position Controlled Growth of Carbon Nanotubes