PH 451/551

Scanning Electron Microscopy

Prof. Jun JiaoOffice Hours: Wednesday, 15:00 – 17:00

Office Location: SB 2, room 376Tel. (503) 725-4228

E-mail: jiaoj@pdx.edu

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 including 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.

Required Textbook: Scanning Electron Microscopy and X-Ray Microanalysis (Third Edition) by Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C . Joy, Charles E. Lyman, Eric Lifshin, Linda Sawyer and Jo seph R. Michael.

Lectures: Thursday 15:00 – 16:50, SB2 room 155.Labs: Thursday 17:00 – 19:50, SB1, room 38 (basement ).

Instrument: Scanning electron microscope (SEM) used in this class is the ISI SS-40 SEM equipped with Oxfor d Link ISIS 300 EDX microanalysis system.

Lec. # Date Probable Topics Chapters

1 Jan. 8 Introduction & Electron Optics 1, 22 Jan. 15 Electron-Specimen Interactions 2, 33 Jan. 22 Secondary Electron and X-Rays 44 Jan. 29 Image Formation and Interpretation 45 Feb. 5 Special Topics in SEM 56 Feb. 12 Energy Dispersive X-Ray 6,77 Feb. 19 Qualitative X-Ray Analysis 8, 108 Feb. 26 Quantitative X-Ray Analysis 9, 109 Mar. 5 Specimen Preparation 11-1510 Mar. 12 Oral Presentation11 Mar. 16 Final Exam (10:10 – 12:05pm)

Lab Schedules

WEEK 1 (Jan. 8) – Introduction, Lab Tour and SEM Dem o.WEEK 2 (Jan. 15) – SEM Operation, Start-Up Procedure , Specimen Loading (Stage), Filament Saturation, Main Controls & Param eters, Shut-Down Procedure, SEM Orientations.WEEK 3 (Jan. 22) – SEM Specimen Preparations and Con tinue Practice SEM Operations.WEEK 4 (Jan. 29) – EDX Qualitative Analysis, Identif y Unknown Materials.WEEK 5 (Feb. 5) – EDX Quantitative Analysis. Materia ls MicrochemicalAnalysis.WEEK 6 (Feb. 12) – Continue Practice of Qualitative and Quantitative Analysis.WEEK 7 (Feb. 19) – Operation Exam for SEM and EDX Sy stem.WEEK 8 (Feb. 26) – Individual Project.WEEK 9 (Mar. 5) – Individual Project.WEEK 10 (Mar. 12) – Finish Project.

Laboratory Requirement: Two written lab reports are required:· Report I: “SEM Imaging.”· Report II: “EDX Qualitative and Quantitative Analyses.”

Your lab reports should include: (i) Title and purpose of the experiment. (ii) Information about the specimen and specimen preparation procedures. (iii) General description of the analytical techniques used. (iv) All data taken (including SEM images and EDX spectra). (v) Discussion of results. (vi) References. The lab reports should be submitted before the final exam.

For graduate students, a term paper focused on the application of SEM and EDX is required. The format of the paper should follow the same format as the lab report.

Oral Presentation : There will be a 10-min oral presentation for each student to report on his/her individual project using SEM and EDX techniques. Power Point presentation is required.

Grading: The final grade for this course will be based on st udents’performance in the following categories:

For Graduate Students:– Lab Report 1 15%– Lab Report 2 15%– Project Report (term paper) 20%– Project Presentation 10%– Lab Practical Exam 10%

(Sample preparation, SEM and EDX operation)– Final Exam 30%

For Undergraduate Students:– Lab Report 1 25%– Lab Report 2 25%– Project Presentation 10%– Lab Practical Exam 10%

(Sample preparation, SEM and EDX operation)– Final Exam 30%

History of SEM

• The earliest recognized work describing the concept of an SEM isthat of Knoll (1935) in Germany working in the fiel d of electronoptics ( E = hc/λ ).

• The improvement of the secondary electron detector was accomplished by Everhart and Thornley in 1960.

• The first commercial scanning electron microscope b ecame available in 1965 by Cambridge Scientific Instrumen ts.

• The SEM is one of the most versatile instruments av ailable for the examination and analysis of the microstructural char acteristics of solid objects.

• The SEM provides two outstanding improvements over the optical microscope: it extends the resolution limits and im proves the depth-of-focus resolution more dramatically (by a factor of ~300).

• The SEM is also capable of examining objects at a l arge range ofmagnifications. 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, crystallog raphic, and compositional information rapidly, efficiently, and simultaneously for the same area.

Comparison of the LM, TEM, and SEM

LM TEM SEM

Illuminationsource

Condenserlens

Specimen

Objectivelens

Projectionlens

Eye

FluorescentScreen

Condenserlens 1

Condenserlens 2

Objective lens

Signaldetector

SpecimenImage

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 µm 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, Moseley 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 ofx-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 µm wide and 1 µm deep, it is usually unnecessary to use probes of very small diameter.

Electron Optics

1. Functions of the SEM Subsystems.2. Why Learn about Electron Optics?3. Thermionic Electron Emission.4. Field Emission.5. Electron Guns (W gun, LaB 6 gun, Field

emission guns).6. Comparison of Electron Sources.7. Lenses in SEMs.8. Lens Aberrations.

Basic Components of the Scanning Electron Microscop e

Important Definitions

• Filament heating current: the current used to resistively heat a thermionic filament to the temperature at which it e mits electrons.

• Emission current: the flow of electrons emitted by the filament.

• Beam current: the portion of the electron current that goes throu gh the hole in the anode.

• Electron Column : consists of an electron gun and two or more elect ron lenses, operating in a vacuum.

• Electron Gun : produces a source of electrons and accelerates th ese electrons to an energy in the range of 1-40 keV. Th e 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 speci men surface with a spot size of less than 10 nm while still carrying sufficient current to form an acceptable image.

• Working Distance (WD) : The distance between the lower surface of the objective lens and the surface of the specim en 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 electron s. Secondary electrons are defined purely on the basis of their kinetic energy; that is, all electrons emitted from the specimen with an ene rgy less than 50 eV.

Important Definitions continued…

Types of Electron Guns

• Tungsten (W) Hairpin Electron Gun : The typical tungsten electron gun is a “ ΛΛΛΛ” shape wire filament about 100 µµµµm. To achieve thermionic emission, the filament is heated resistively by the filament heating current.

• Lanthanum Hexaboride (LaB 6) 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 lifet ime than tungsten, but the required vacuum conditions are more stringe nt.

• Field Emission Electron Guns : The field emission cathode is usually a wire of single-crystal tungsten fashioned into a sh arp point and spot welded to a tungsten hairpin. The significance of t he 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 k V 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 a nd leave the cathode without requiring any thermal energy to lif t them over the work function barrier.

Electron Field Emission

eFzz

eEzV F −−+=

4)(

2

φAccording to the free-electron theory, at the metal-vacuum interface:

F =0.3 V/Å

e-

]'exp[')(2/32

FB

FAFJ

φφ

−=

MetalPhosphor Screen e-

Diameter of Anode:100mm Distance of Electrodes: 100mm

Potential: 1KV

Diameter of cathode:100mmElectrical field: 10 V/mm

12mm, 66 V/mm

50mm, 15 V/mm

2mm, 277 V/mm6mm, 127 V/mm

24mm, 32 V/mm

Electric Field Simulation+-

The Electron Gun

Courtesy of James S. Young

Filaments

Tungsten LaB6

Field Emission 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 inc rease in filament current will not increase the beam current.

Filament Current

False peak

Saturation point

0

Bea

m C

urre

nt

Lanthanum Hexaboride Filament

• Single crystal of LaB 6

• Tip is ~100 µm

• Chemically reactive when it gets hot

• Crystal is held by glassy carbon or graphite supports

• Carbon not reactive with LaB 6

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 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

Field Emission Filaments

Filament

Suppressor

First anode

Second anode

Electronbeam

Extractorvoltage

Acceleratingvoltage

Virtual source

The Electromagnetic Lens

Soft iron casing

Toppolepiece

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

Lens

Point on specimen

Multiple electron trajectories

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 cen ter, out of focus near the edges.

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 unsharppoint.

– 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 foc us is attained when different heights in the image of a r ough surface are all in focus at the same time. This mode requir es a small convergence angle so that the beam appears small ov er large height differences on the specimen. The small beam angle can be obtained by using a small objective lens apertur e, a long working distance, or both.

• High-Resolution Images : High-resolution images require a small probe size, and adequate probe current, and m inimal interference from external vibration and stray AC m agnetic field. Electron optically, the smallest probe is ob tained by selecting a short working distance. The penalty for using a very small probe is typically a very low probe curr ent. High brightness sources and short working distance.

Key Points for Imaging and Microanalysis

• High Beam Current for Image Quality and X-ray Micro analysis : While a probe current of at least 10^-12A is requir ed to produce a photographic image, the image may be so n oisy 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 decreas es. Currents of at 10^-10A are needed for x-ray detecti on by the energy-dispersive x-ray spectrometer (EDS) while th e wavelength-dispersive x-ray spectrometer (WDS) requ ires at lease 10^-8A. Often the probe diameter must be inte ntionally enlarged to obtain an adequate signal for microanal ysis.

Carbon coated magnetic nanoparticles

Field Emission SEM Images of Carbon Nanotubes

Arc-discharge of Carbon Nanotubes

SiO2 Nanowires

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.

1 µµµµm

(a)

100 nm

(b)

FESEM Images of CdS Nanowires

1 µµµµm 100 nm

(a) (b)

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