Transcript Document 7163320

Scanning Electron
Microscopy
Prof. Jun Jiao
Office Hours: Wednesday, 14:00 – 17:00
Office Location: SB 2, room 370
Tel. (503) 725-4228
E-mail: [email protected]
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.
Instrument: Scanning electron microscope
(SEM) used in this class is the ISI SS-40
SEM equipped with Oxford Link ISIS 300
EDX microanalysis system.
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.
Comparison of the LM, TEM, and SEM
TEM
LM
SEM
Illumination
source
Condenser
lens 1
Condenser
lens
Specimen
Condenser
lens 2
Objective lens
Objective
lens
Signal
detector
Projection
lens
Specimen
Image
Eye
Fluorescent
Screen
Courtesy of James S. Young
A Human Hair vs. Carbon Nanotubes
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)
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.
Electron Optics
1.
2.
3.
4.
5.
6.
7.
8.
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.
Basic Components of the Scanning Electron Microscope
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.
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.
Types of Electron Guns
•
Tungsten (W) Hairpin Electron Gun: The typical tungsten electron gun
is a “ Λ” 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.
The Electron Gun
Courtesy of James S. Young
Filaments
Tungsten
Field Emission
LaB6
Field Emission
Courtesy of James S. Young
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
•
0
False peak
Filament Current
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
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
Field Emission Filaments
•
•
Cold Field Emitter (CFE)
•
– single crystal of tungsten
– operate at room temperature
– very bright
– long lasting
– require very high vacuum (< 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
Field Emission Filaments
•
A very fine wire of singlecrystal 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
Field Emission Filaments
Suppressor
Virtual
source
Filament
Extractor
voltage
First anode
Accelerating
voltage
Second anode
Electron
beam
The Electromagnetic Lens
Soft iron casing
Top
polepiece
Polepiece gap
(brass)
Copper wire
windings
Bottom
polepiece
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.
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
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.
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.
Lens Aberrations
• Pin cushion and Barrel distortions:
– Spherical aberration at the final imaging lens.
Pin cushion distortion
Barrel distortion
Lens Aberrations
Pin Cushion Distortion or Chromatic Aberration?
3kV
10kV
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.
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.
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.
Carbon coated magnetic
nanoparticles
Field Emission SEM Images of Carbon Nanotubes
Arc-discharge of Carbon Nanotubes
SiO2 Nanowires
(b)
(a)
1 m
100 nm
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.
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.
SiO2 Nanowires with Sharp Tips
Position Controlled Growth of Carbon Nanotubes