Arquitetura do MEV [6]

1>

http://www4.nau.edu/microanalysis/Microprobe-SEM/Instrumentation.html

Arquitetura do MEV

2>

Elétrons detectores sinais imagem

Arquitetura do MEV

Emissão de elétrons secundários (SE):

3>

• energia entre 5 - 50 eV

• livre caminho médio - L = A / (NA r s)A - massa atômica (g/mol); NA - número de Avogadro (6.02 x 1023 át/mol);r – densidade (g/cm3); s – probabilidade de

interação.

Arquitetura do MEV

Secondary Electron Detector (SE):

4>

Standard Everhart-Thornley SE Detector

Collector Bias- 250 V to + 400 V

Photo-multiplier

Scintillator Voltage+ 10 kV to + 12 kV

Sample Chamber Vacuum; - 8x10-5 Torr or better.

Light Pipe

Collector

Requires High Vacuum

Arquitetura do MEV

SE Detector :

5>

Collector Bias- 250 V to + 400 V

Scintillator Voltage+ 10 kV to + 12 kV

PMT Light Pipe

Collector

Incident Electron Beam

Specimen

Low energy secondary electrons areattracted by the variable detector bias (collector).

Photons are guided along the light pipe to the photo-multiplier (PMT) and convertedto electrical current/signal.

The electrical current from PMT is amplified, sent to signal processor and used to form an image from true sample surface.

Backscattered electrons can also be detected.

Secondary electrons cause scintillation (conversion to photons) on a screen in front of a light guide.

Arquitetura do MEV

Tipos de elétrons secundários:

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http://www.emcourses.com/semsigs.htm

SE1 - are produced by the incident electron beam impinging upon the specimen surface, these electrons determine the basic image resolution. SE2 - are produced by the BSE as they leave the specimen surface; there may be up to four times as many type 2 electrons produced than type 1.SE3 - are produced by the BSE that strike the components of the microscope; the final lens, specimen stage, or specimen holder; a contribution of up to 40% of the secondary signal at low magnifications. The reaction volumes that are created produce secondary and backscattered electrons, as well as x-rays that relate to the component involved. SE4 - are produced by the incident electron beam impinging upon the final aperture, usually in instruments which do not have a variable aperture system.SE5 - are produced by the BSE electrons that strike the components of the microscope; the final lens, specimen stage, or specimen holder and bouncing back to irradiate an area away from that being investigated.

Arquitetura do MEV

Imagem SE:

7>Sprayed-metal; SE-detector, HV, 15 keV

Arquitetura do MEV

Emissão de elétrons retroespalhados (BSE):

8>

Arquitetura do MEV

CZ BSD backscattered Detector :

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In VP mode, backscattered electrons and secondary electrons collide with the introduced gas molecules and produce ions that dissipate the charge on the sample surface.

Higher energy electrons or back-scattered Electrons travel upwards inthe chamber towards the Backscattereddetector (BSD).

Backscattered electrons are detected by BSD, converted to electrical signal and used to form an image.

Low energy secondary electrons are detected by SE/VPSE detector.

SE, VPSE and BSE detectors

SED VPSEBSD

Universal detector system able to provide imaging solutions in

• High Vacuum (HV) • Variable Pressure (VP)• Extended Pressure (EP)

Arquitetura do MEV

10>

“Amplified signal” by head amplifier to signal processor; used to form an image.

Backscattered electrons and Secondary electrons collide with the introduced gas molecules andproduce ions that dissipate the charge on the sample surface.

Higher energy electrons (BSE) “travel upwards”!

Incident Electron Beam

Gas molecules

Specimen

Backscattered electrons

QBSD

Backscattered electrons are detected by QBSD and converted to electrical signal.

BSD backscattered Detector :

Arquitetura do MEV

CZ BSE backscattered Detector :

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Low voltage imaging of low Z materials;

To complement the true secondary electron imaging provided by VPSE detector.

Four-quadrant detector;

To provide both compositional andtopographic imaging from a single

device.

Choice of lens-mounted or fully retractable version

Solder ball pad; compositional image, HV, 20 keV

Solder ball pad; compositional image, HV, 2 keV

Solder ball pad; topographic image, HV, 20 keV

Arquitetura do MEV

Emissão de elétrons retroespalhados (BSE):

12>

Imagem de BSE Imagem Composta Topográfico 1

Topográfico 2 Topográfico 3 Topográfico 4

Arquitetura do MEV

Imagem BSE:

13>Polished alloy; BSD-detector, HV, 20 keV

Arquitetura do MEV

VPSE Detector :

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Solutions for imaging insulators;

Gas molecules scintillate when ionized by

secondary electrons from the specimen.

The photons emitted are detected.

The unique VPSE detector enables the

detection of SE electrons at high pressures.

True surface images &

Excellent SE imaging with VP ;

As slow secondary electrons are very effective at scintillating gas molecules,

images collected using the VPSE detector provide excellent surface detail.

Backscattered electrons do not contribute to the signal. Single Click on the movie to STOP and START. “Return” key to move to next slide.

Arquitetura do MEV

VPSE Detector :

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Incident Electron Beam

PhotonsCollision ZoneCollision Zone

PMTLight Pipe

Gas molecules

Specimen

Secondary electrons cause scintillation of gas molecules in front of a light guide.

Low energy secondary electrons are attracted by the variable detector bias.

Photons guided along the light pipe to the PMT.

Amplified signal from PMT to signal processor; used to form an image.

Secondary electrons collide with the introduced gas molecules andproduce ions that dissipate the charge on the sample surface.

Arquitetura do MEV

Imagem VPSE:

16>

Caterpillar-Egg; VPSE-detector, 15 Pa, 25 keV, -25°C

Arquitetura do MEV

STEM Detector :

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Positioning of the thin specimen close to

the objective lens improves resolution.

Designed as a cost effective compact unit;

contains both the specimen and the detector.

Consists of a diode electron detector

positioned under an electron transparent

thin specimen in a pre-aligned holder,

fits directly on the specimen stage.

SE, VPSE, and STEM detectors

VPSE SE

STEM

The collected signals are equivalent to bright

field imaging.

Arquitetura do MEV

STEM Detector :

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SE, VPSE, and STEM detectors

VPSE SE

STEM

Arquitetura do MEV

Imagem STEM:

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Polymer thin section; STEM-detector, HV, 30 keV

Arquitetura do MEV

Imagem STEM:

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Polymer thin section; STEM-detector, HV, 30 keV

Arquitetura do MEV

21

Notas de aula preparadas pelo Prof. Juno Gallego para a disciplina Microscopia Eletrônica de Varredura.® 2015. Permitida a impressão e divulgação. http://www.feis.unesp.br/#!/departamentos/engenharia-mecanica/grupos/maprotec/educacional/

Johnson, R. Environmental Scanning Electron Microscopy: An Introductionto ESEM. Philips Electron Optics, Eindhoven, 1996, pp. 1-17.

Egerton, R. F. Physical Principles of Electron Microscopy: An Introductionto TEM, SEM and AEM. Springer Science+Business Media, Inc., New York,2005, pp. 125-153.

Goldstein, J. I. et al. Scanning Electron Microscopy and X-ray Microanalysis, third edition. Kluwer Academic/Plenum Publishers, New York, 2003, pp. 21-60.

Goodhew, P. J.; Humphreys, J.; Beanland, R. Electron Microscopy andAnalysis. Taylor & Francis Inc.,New York, 2001, pp. 122-168.

Reed, S. J. B. Electron Microprobe Analysis and Scanning Electron Microscopy in Geology. Cambridge University Press, Cambridge, 2005, pp. 21-40.

Stokes, D. J. Principles and Practice of Variable Pressure EnvironmentalScanning Electron Microscopy (VP-ESEM). John Wiley & Sons Ltd, West Sussex, 2008, pp. 17-62.

Jorge Jr, A. M.; Botta, W. J. Notas de classe – Escola de Microscopia. Laboratório de Caracterização Estrutural, DEMa/UFSCar.http://www.lce.dema.ufscar.br/cursos/escola.html

Bibliografia: