lecture 10 sem
TRANSCRIPT
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Scanning Electron Microscope
• SEM:
– Different way to produce and magnify images compared to TEM, OM
– More like a scanning probe using electron beam
– Primarily used to study the surface (or near surface) structure of bulk
specimens: morphology and chemical information
SEM images of tungsten oxide nanowires (left) and islands (right)
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Structure of an SEM • E-gun: tungsten, LaB6, FEG
• Accelerating voltage: 1-30 KV
• Beam diameter: 2-10 nm
Fig. 5.2 from
Goodhew et
al, 3ed
Diagram of the main components of SEM
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Image Production in SEM
• E-beam scans in a rectangular set of straight lines (raster)
From INCA help files, Oxford Instrument
Scanning coils and beam scanning in SEM
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Image Magnification in SEM • Image magnified without lens
• Magnification: M=L/l
• Digital images: point scanned → pixel displayed – Size of point scanned = scale bar indicated length/pixel # of scale bar
– Size of pixel display = scale bar displayed length/pixel # =1”/DPI
Schematics of the
image magnifying
process in SEM
Fig. 5.4 from
Goodhew et
al, 3ed
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Signals in SEM (1) • In principle, any radiation from specimen or any measurable change in the
specimen may be used to provide the signal forming an image.
• Major signals for imaging: secondary electrons and backscattered electrons
• Other signals:
– X-rays: chemical analysis
– Auger electrons: surface analysis
– Cathodoluminesence (CL): optical properties
– Charge collection: semiconductor properties
Fig. 5.5 from
Goodhew et
al, 3ed
Signals used in SEM
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Signals in SEM (2) • Interaction volume: the region electrons penetrate the specimen
– Various radiations are generated as a result of inelastic scattering
– Amount and type of secondary radiations alter with the penetration.
• Regions of different signals detected (sampling volume) – Radiation must escape from the specimen to be detected.
– Depend on the radiations and the specimen (mean free path)
Fig. 5.6 from
Goodhew et al, 3ed
Interaction volume and regions for different signals
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Signals in SEM (3)
Sampling volume:
• X-rays: ~ interaction volume
• BSEs: depth ~ a fraction of a micron
– A type (originate near the incident
beam): high spatial resolution with
crystallographic information.
– B type (undergo multiple
scattering): worse resolution.
• SEs: (closest to surface)
– Mainly from a region little larger
than the diameter of the incident
beam.
– Best spatial resolution
Fig. 5.7 from Goodhew et al, 3ed
(a) Generation of secondary electrons
and (b) their distribution.
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Signals in SEM (4) • Secondary electron coefficient (δ)
– Not dependent on atomic number of the specimen
– Dependent of accelerating voltage (maximum between 1 and 5 keV)
• Backscattered electron coefficient (η) – Strongly dependent on the atomic number of the specimen
– Almost independent of accelerating voltage
• Charging effect for nonconductive specimen.
Fig. 5.8 from Goodhew et al, 3ed
(a) Effect of atomic number on yields of SE and BSE
and (b) effect of accelerating voltage on total yield
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Detecting secondary electrons • Everhart-Thornley detector: scintillator-photomultiplier system
SEs strike a scintillator (phosphor) emit light through light guide, light
transmitted into photomultiplier converts photons into pulses of electrons
Fig. 5.9 from Goodhew et al, 3ed
Schematic of the Everhart-Thornley secondary
electron detector system
• Scintillator
High bias (+10 KeV) to
accelerate the SEs to excite
phosphor
• Grid (collector)
Several hundred volts
Prevents HV of scintillator
affecting the incident beam
Improves collection
efficiency
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Detecting backscattered electrons
Fig. 5.10 from Goodhew et al, 3ed
(a) Large area Robinson type Scintillator
detector. (b) Solid-state silicon detector.
• Scintillator detectors (Robinson type)
– Rapid response time
– Bulky restrict the working distance
• Solid-state detectors – High-energy BSEs excite e-h pairs in
semiconductor separated by bias
produce current be amplified.
– Slow response time
– Small size
• Through-the-lens detectors
(inlens): for high resolution SEM
– Scintillator detector placed within the
lens
– Good collection efficiency
– Very short working distance
– Restrictions on size and movement of
the sample.
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Optics of SEM
Fig. 5.11-12 from Goodhew et al, 3ed
Ray diagram of a two-lens SEM Electron beam scanning by two
sets of coils
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Performance of SEM (1) • Specimen pixel size, P: the point scanned by the
e-beam
– P = ~100 m/M
• Electron probe size:
• Depth of field: range with probe size ≤ 2P
• Ultimate resolution:
– the smallest probe which can provide an
adequate signal from the specimen
Fig. 5.11 from Goodhew et al, 3ed
Ray diagram of a two-lens SEM
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— Example: Depth of Field
Fig. 5.14 from Goodhew et al, 3ed
Aluminum powder images taken with (a) an optical microscope and (b) an SEM
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Performance of SEM (2) • Minimum usable beam current
• High-performance microscopes
Fig. 5.16 from Goodhew et al, 3ed
Minimum probe size for a given
level of signal contrast as a
function of frame scan time
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Topographic Images
• Using SE or BSE signals: small sampling volume
• Tilt effects: http://www.matter.org.uk/tem/electron_scattering.htm
– =0/sec
– Specimen tilted 20-40 towards the detector to enhance signals.
From “Invitation to the SEM World”, JEOL
Analogy between OM and SEM:
(top) SEM and (bottom) OM;
(left) diffuse and (right) direct
illumination
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Secondary Electron Signals vs. Topography
Fig. 4.13, from Leng.
SE signals vs. surface topography
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— Examples: Topographic Images
SEM images of the same area using (a) SE signal, (b) four segments
of BSE signal, and (c) one segment BSE signal
Fig. 5.19 from Goodhew et al, 3ed
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Topographic and Compositional Images
• Using BSE signal
• Effect of multi-element
backscattered detector
Schematic of principles of BSE images
From Instruction manual for MP-44120 (BEIW), JEOL
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— Examples: BSE images
Fig. 5.21 from Goodhew et al, 3ed
BSE images of a polished silver soldered joint: (a) Topographic
and (b) Compositional image
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Other Information Obtained in SEM • Crystallographic information
– Channeling contrast
– Diffraction patterns
Top: Channeling contrast in a BSE image; Right:
(a) EBSD diagram (b) An EBSD pattern from Ge.
Fig. 5.22-23 from Goodhew et al, 3ed
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Reading assignments
• The use of other signals in SEM – Charge collection mode
– Cathodoluminescence
– Other signals
• Image acquisition, processing and storage
• Specimen preparation for SEM
• Other types of SEM – Low voltage SEM: reduce charging effects
– Environmental SEM (ESEM): operate at higher pressure
for bio- or other volatile specimens; also reduce charging
effects
• Additional resources posted on MOODLE.
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Practical Tips
• Following information is collected from JEOL
documents:
– “A Guide to Scanning Microscope Observation”
– “Scanning Electron Microscope A to Z”
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Effects of Accelerating Voltage
5KV vs 25 KV (x36,000)
30KV vs 5 KV (x2,500)
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Effects of Probe Current and Size
(a) 1 nA (b) 0.1 nA (c) 10 pA 10KV, x5,400
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Edge effects
5KV 5KV? (should be >5KV) X720, tilted 50°
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Effects of Tilting
Tilted 0° Tilted 45°
5KV, x1,100
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Charging Effect
• Reduce charging:
– Coating
– Low voltage
– Low vacuum SEM (LVSEM) or environmental SEM
(ESEM)
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Effects of Astigmatism
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Effects of Aperture Alignment
• Misalignment of beam center with aperture
center results in poor image quality.
Misaligned Aligned
25KV, x21,000