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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