Arquitetura do MEV [6]

Transcription

1 [6] 1>

2 Elétrons detectores sinais imagem 2>

3 Emissão de elétrons secundários (SE): 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 át/mol); r densidade (g/cm 3 ); s probabilidade de interação. 3>

4 Secondary Electron Detector (SE): Sample Chamber Vacuum; - 8x10-5 Torr or better. Collector Photo-multiplier Light Pipe Scintillator Voltage + 10 kv to + 12 kv Collector Bias V to V Requires High Vacuum Standard Everhart-Thornley SE Detector 4>

5 SE Detector : Low energy secondary electrons are attracted by the variable detector bias (collector). Collector Incident Electron Beam Secondary electrons cause scintillation (conversion to photons) on a screen in front of a light guide. PMT Photons are guided along the light pipe to the photo-multiplier (PMT) and converted to electrical current/signal. Light Pipe Scintillator Voltage + 10 kv to + 12 kv Collector Bias V to V The electrical current from PMT is amplified, sent to signal processor and used to form an image from true sample surface. Specimen Backscattered electrons can also be detected. 5>

6 Tipos de elétrons secundários: 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. 6>

7 Imagem SE: Sprayed-metal; SE-detector, HV, 15 kev 7>

8 Emissão de elétrons retroespalhados (BSE): 8>

9 CZ BSD backscattered Detector : Universal detector system able to provide imaging solutions in High Vacuum (HV) Variable Pressure (VP) Extended Pressure (EP) Low energy secondary electrons are detected by SE/VPSE detector. Higher energy electrons or backscattered Electrons travel upwards in the chamber towards the Backscattered detector (BSD). Backscattered electrons are detected by BSD, converted to electrical signal and used to form an image. SED BSD SE, VPSE and BSE detectors VPSE 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. 9>

10 BSD backscattered Detector : Backscattered electrons and Secondary electrons collide with Incident Electron Beam the introduced gas molecules and produce ions that dissipate the charge on the sample surface. QBSD Higher energy electrons (BSE) travel upwards! Backscattered electrons Backscattered electrons are detected by QBSD and converted to electrical signal. Gas molecules Amplified signal by head amplifier to signal processor; used to form an image. Specimen 10>

11 CZ BSE backscattered Detector : Low voltage imaging of low Z materials; To complement the true secondary electron imaging provided by VPSE detector. Solder ball pad; compositional image, HV, 20 kev Four-quadrant detector; To provide both compositional and topographic imaging from a single device. Solder ball pad; compositional image, HV, 2 kev Choice of lens-mounted or fully retractable version Solder ball pad; topographic image, HV, 20 kev 11>

12 Emissão de elétrons retroespalhados (BSE): Imagem de BSE Imagem Composta Topográfico 1 Topográfico 2 Topográfico 3 Topográfico 4 12>

13 Imagem BSE: Polished alloy; BSD-detector, HV, 20 kev 13>

14 VPSE Detector : Arquitetura do MEV The unique VPSE detector enables the detection of SE electrons at high pressures. Solutions for imaging insulators; Gas molecules scintillate when ionized by secondary electrons from the specimen. The photons emitted are detected. 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. 14>

15 VPSE Detector : Arquitetura do MEV Low energy secondary electrons are attracted by the variable detector bias. Incident Electron Beam Secondary electrons collide with the introduced gas molecules and produce ions that dissipate the charge on the sample surface. Secondary electrons cause scintillation of gas molecules in front of a light guide. Collision Zone Light Pipe Photons PMT Photons guided along the light pipe to the PMT. Amplified signal from PMT to signal processor; used to form an image. Specimen Gas molecules 15>

16 Imagem VPSE: Caterpillar-Egg; VPSE-detector, 15 Pa, 25 kev, -25 C 16>

17 STEM Detector : Designed as a cost effective compact unit; contains both the specimen and the detector. Positioning of the thin specimen close to the objective lens improves resolution. Consists of a diode electron detector positioned under an electron transparent thin specimen in a pre-aligned holder, fits directly on the specimen stage. VPSE STEM SE The collected signals are equivalent to bright field imaging. SE, VPSE, and STEM detectors 17>

18 STEM Detector : VPSE SE STEM SE, VPSE, and STEM detectors 18>

19 Imagem STEM: Polymer thin section; STEM-detector, HV, 30 kev 19>

20 Imagem STEM: Polymer thin section; STEM-detector, HV, 30 kev 20>

21 Bibliografia: Johnson, R. Environmental Scanning Electron Microscopy: An Introduction to ESEM. Philips Electron Optics, Eindhoven, 1996, pp Egerton, R. F. Physical Principles of Electron Microscopy: An Introduction to TEM, SEM and AEM. Springer Science+Business Media, Inc., New York, 2005, pp Goldstein, J. I. et al. Scanning Electron Microscopy and X-ray Microanalysis, third edition. Kluwer Academic/Plenum Publishers, New York, 2003, pp Goodhew, P. J.; Humphreys, J.; Beanland, R. Electron Microscopy and Analysis. Taylor & Francis Inc.,New York, 2001, pp Reed, S. J. B. Electron Microprobe Analysis and Scanning Electron Microscopy in Geology. Cambridge University Press, Cambridge, 2005, pp Stokes, D. J. Principles and Practice of Variable Pressure Environmental Scanning Electron Microscopy (VP-ESEM). John Wiley & Sons Ltd, West Sussex, 2008, pp Jorge Jr, A. M.; Botta, W. J. Notas de classe Escola de Microscopia. Laboratório de Caracterização Estrutural, DEMa/UFSCar. Notas de aula preparadas pelo Prof. Juno Gallego para a disciplina Microscopia Eletrônica de Varredura Permitida a impressão e divulgação. 21