Scanning Electron Microscopy

Scanning Electron Microscopy Peter Harris Scanning Electron Microscopy  What is scanning electron microscopy?  Basic fea...
Author: Briana Cannon
0 downloads 0 Views 3MB Size
Scanning Electron Microscopy Peter Harris

Scanning Electron Microscopy

 What is scanning electron microscopy?  Basic features of conventional SEM  Limitations of conventional SEM  Looking at “wet” samples in the high vacuum SEM  Cryo-SEM and Environmental SEM

EM Workshops 2013


Electron gun Electron Electron gun gun Condenser Condenser lens lens Objective Objective lens lens Specimen Specimen Projector Projector lens lens

Fluorescent Fluorescent screen screen Digital Digital camera camera


Scanning electron microscope

Transmission electron microscope Workshop: November 13

EM Workshops 2013

Electron sources for SEM

Schottky Field Emission Source

Tungsten thermionic source 

Low brightness

Energy spread ~ 1-2eV

EM Workshops 2013

High brightness

Energy spread < 0.5 eV

Interaction of high-energy electrons with specimen

EM Workshops 2013

Interaction of electrons with specimen

EM Workshops 2013

S = secondaries

B = backscattered

(A are auger)

Secondary and backscattered electrons

What are secondary electrons?

They are electrons dislodged from the specimen itself.

What are backscattered electrons?

They are primary electrons that have entered the sample and then escaped back out:

EM Workshops 2013

Detectors for SEM

Secondary electrons Backscattered electrons

Everhart Thornley or solid state detector (silicon diode). Usually positioned around the final lens or inside.

Everhart Thornley detector: Scintillator – photomultiplier system.

EM Workshops 2013

Information given by different signals

◊ Secondary electrons 

◊ Backscattered electrons 

◊ X-rays 

Topographic information

Compositional information

Spectra; Maps

Session on X-ray analysis: November 27 EM Workshops 2013

Origin of topographic contrast

Everhart Thornley detector

EM Workshops 2013

Early history of SEM

1935, M. Knoll, Germany – proposed concept of a scanning electron microscope.

1942, Zworykin et al., RCA Laboratories, USA - first working SEM.

1950s, C.W. Oatley et al., Engineering Department, Cambridge - major improvements in electron optics and system, and secondary electron detection (Everhart - Thornley detector). First backscattered detector.

1965: first commercial SEM, Cambridge Instrument Company, Stereoscan Mark 1.

EM Workshops 2013

EM Lab’s high vacuum SEM

EM Workshops 2013

Conventional SEM images

Compact disc

Polypropylene spherulites

Integrated circuit

EM Workshops 2013

Problems with conventional SEM

 Can only operate at low pressure (10-5 Torr) No wet samples  Surface must be electrically conducting Non-conducting samples have to be coated

EM Workshops 2013

Looking at “wet” samples in the high vacuum SEM

Animal and plant tissues up to 98% water. Options for examining these materials in the high vacuum SEM:

 Chemical fixation, drying

 Freeze drying

 Critical point drying

EM Workshops 2013

Critical point drying •

Evaporative drying of specimens can cause collapse of structures, mainly due to effects of surface tension.

Effect can be reduced by substitution of water with a liquid with a lower surface tension

Specimen prepared by CPD: Hyphae and spores in Stilton cheese

EM Workshops 2013

Cryo - SEM

Workshop: November 20

EM Workshops 2013

Cryo – SEM of food

Raw potato

EM Workshops 2013

Probiotic yoghurt with bacteria. Scale bar 5μm

Environmental SEM

Special pumping system allows water vapour to be introduced into chamber.

Cooled specimen to retain moisture.

New (backscattered) electron detector.

Cooled specimen

Workshop: November 20

EM Workshops 2013

Introducing water vapour into the chamber enables uncoated samples to be examined

Secondary electrons from sample strike water molecules.

EM Workshops 2013

Positively charged water molecules are attracted to negatively charged sample.

Negative charge at sample surface is neutralized..

EM Lab’s FEI Quanta 600 ESEM

Gaseous secondary electron detector – fits on objective lens. Pressures up to 20 Torr - samples typically cooled to 5oC.

EM Workshops 2013

Environmental SEM

Raspberry buds : Hiroyuki Imanishi, Plant Sciences

EM Workshops 2013

Becoming an EMLab user

Sherrie Foo

EM Workshops 2013

Booking system

EM Workshops 2013

Remaining workshops


13 November 2013 20 November 2013 27 November 2013

EM Workshops 2013


Transmission Electron Microscopy Environmental & Cryo SEM X-ray Analysis and Elemental Mapping

Electron Microscopy Laboratory



EM Workshops 2013

Suggest Documents