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1、Interaction of Radiations with Mattersthe probe to microstructures1Why ?n nThe interaction of electromagnetic radiation with crystalline solids is now understood in considerable detail so that it can be exploited to provide the necessary information.To characterize a microstructure it is necessary t
2、o perturb the material by interacting in some way with it.2Examplesn nOne sees a matter only because the matter reflects visible light.n n X-ray and high energy electron beam could “see” crystal lattices.3The dilemman nInteraction of radiation with materials under probe may cause damage to or change
3、 the microstructure of them.n nHigher resolution requires radiation with smaller wavelength, or higher energy. Thus, the probability of radiation damage increases.4The criterionn nEvery characterization should acquire the most information on the expense of the least damage.n nTherefore, knowledge on
4、 interaction of radiation with matters is indispensable.5Penetration depthn nPenetration Penetration depth, depth, or or mean mean free free distance, distance, of of the the incident incident determines determines the the depth depth and and volume in the sample that can be analyzed.volume in the s
5、ample that can be analyzed.n nIn In most most cases, cases, the the type type of of radiation radiation generated generated in in the the sample sample is is different different from from the the incident incident radiation. radiation. The The smaller smaller mean mean free free distance determines
6、the analysis volume.distance determines the analysis volume.6Penetration depth: Photonsn nPhotons Photons are are discrete discrete quanta quanta of of electromagnetic electromagnetic radiation. radiation. The The photon photon is is identified identified by by the the wavelength, wavelength, , , en
7、ergy, energy, E, E, and and frequency, frequency, , , all all of of which which are are related related by by the equationthe equation where where h h is is Plank Plank constant constant and and c c the the velocity velocity of light.of light.7Penetration depth: Photonsn nElectromagnetic Electromagn
8、etic spectrum spectrum spans spans a a vast vast range range with with wavelength wavelength varying varying from from 10106 6 m m to to 1010-14-14 m. m. n nIf If we we want want to to make make use use of of electromagnetic electromagnetic radiation radiation to to characterize characterize the the
9、 microstructure microstructure of of materials, materials, the the wavelength wavelength of of photons photons must must be be in in comparison comparison with with the the features features we we want want to to observe.observe.n nTherefore, Therefore, we we do do not not need need photons photons
10、with with wavelength wavelength larger larger than than 1010-4-4 m m or or smaller smaller than than 1010-10-10 m. m.8Penetration depth: Photonsn nThe The penetration penetration depth depth of of photons photons depends depends sensitively sensitively on on materials materials and and photon photon
11、 energies. energies. It It is is not not possible possible or or instructive instructive to to give give a a detailed relation over the whole spectrum.detailed relation over the whole spectrum.n nHowever, However, it it is is possible possible to to give give a a rough rough description description
12、over over some some specific specific and and important important wavelength. wavelength. 9Penetration depth: Photonsn nVisible light: Visible light: 500 nm, 500 nm, penetration depth 50 - 300 nm penetration depth 50 - 300 nm Information Information from from visible visible light light are are aver
13、aged averaged over over a a few few hundreds of atomic layers.hundreds of atomic layers.n nUltraviolet light: severe absorption, Ultraviolet light: severe absorption, only suitable to surface analysis only suitable to surface analysisn nX-ray: X-ray: generally generally a a few few micrometers, micr
14、ometers, dependent dependent of of absorption coefficient absorption coefficient increases with increasing atomic number. increases with increasing atomic number.10Penetration depth: Electronsn nThe The penetration penetration depth depth of of electrons electrons varies varies dramatically dramatic
15、ally with with electron electron energy energy and and atomic atomic number of materials.number of materials.n nThe The higher higher the the energy, energy, the the deeper deeper the the penetration.penetration.10 keV10 keV 0.? 0.? mm30 keV30 keV 2 2 mm11Penetration depth: Electronsn nThe The small
16、er smaller the the atomic atomic number, number, the the deeper deeper the the penetration.penetration.n nThis This affects affects the the characterization characterization methods methods using using electron electron beams beams as as probes probes because because common common materials material
17、s are are composed composed of of elements elements with with different different atomic atomic numbers.numbers.atomic numberatomic numberpenetration depthpenetration depth 20 10 10 mm 40 40 2 2 mm12Penetration depth: Electronsn nThe The scattering scattering of of electrons electrons in in the the
18、materials materials limits limits the the space space resolution.resolution.13Interaction of electrons with mattersSecondary electronsSecondary electronsBackscattered electronsBackscattered electronsAuger electronsAuger electronsX-raysX-raysDiffracted electronsDiffracted electrons14Secondary electro
19、nsn nSecondary Secondary electrons electrons are are used used to to give give scanning electron microscope (SEM) images.scanning electron microscope (SEM) images.n nSecondary Secondary electrons electrons are are low low energy energy electrons. electrons. Thus, Thus, only only those those generate
20、d generated beneath beneath the the incident incident beam beam or or in in the the vicinity vicinity of of incident incident beam can escape out of the surface.beam can escape out of the surface.n nTherefore, Therefore, the the resolution resolution of of a a SEM SEM is is basically basically deter
21、mined by the incident beam area.determined by the incident beam area.15Auger electronsn nThe The energy energy of of Auger Auger electrons electrons is is about about 0 0 2 2 keV, keV, absorbed absorbed easily easily by by the the materials. materials. Only Only Auger Auger electrons electrons gener
22、ated generated within within a a few few atomic atomic layers layers beneath beneath surface surface could could escape escape out out of of the surface.the surface.n nLight Light atoms atoms or or low low energy energy bonds bonds are are prone prone to to generate generate Auger electrons.Auger el
23、ectrons.vacuumEcEFvalence bandL3L2L1K16Characteristic X-raysn nX-ray X-ray photons photons have have strong strong ability ability to to penetrate penetrate through through the the sample. sample. Therefore, Therefore, X-X-rays rays generated generated in in a a relatively relatively large large vol
24、ume volume can can be detected.be detected.n nHeavy Heavy atoms atoms have have large large cross-sections cross-sections to to generate generate characteristic X-rays.characteristic X-rays.n nCharacteristic Characteristic X-rays X-rays are are suitable suitable to to detect detect heavy heavy atoms
25、.atoms.vacuumEcEFvalence bandL3L2L1Kcharacteristic X-rays17Radiation damage: Photonsn nPhotons have the least damage to materials.Photons have the least damage to materials. n nGenerally, Generally, the the damage damage from from photons photons comes comes from from heating heating effect. effect.
26、 The The amplitude amplitude of of the the damage damage depends depends on on penetration penetration depth, depth, photon photon energy and current density.energy and current density.n nExample: Example: Strong Strong laser laser can can cause cause melt melt and and evaporation evaporation in in
27、a a short short time, time, e.g. e.g. the the interaction interaction of laser with target in pulse laser deposition.of laser with target in pulse laser deposition.18Radiation damage: Electronsn nRadiolysis: Radiolysis: inelastic inelastic scattering scattering (mainly (mainly ionization) ionization
28、) breaks breaks chemical chemical bonds bonds in in materials, materials, especially especially polymers and alkali-halogen chemicals.polymers and alkali-halogen chemicals.n nKnock-on Knock-on damage: damage: direct direct knock knock out out the the atoms atoms from from lattice lattice sites sites
29、 to to form form point point defects, defects, effective effective to to all all kinds kinds of materials.of materials.n nHeating Heating effect: effect: negligible negligible to to metal metal or or other other good good heat heat conductors. conductors. It It can can be be severe severe problems p
30、roblems for for insulators. insulators. Ceramic Ceramic micropowders micropowders can can be be heated heated to to 1700 1700 0 0C C in in transmission electron microscope.transmission electron microscope.19Radiation damage: Electronsn nRadiolysis Radiolysis is is due due to to electron-electron ele
31、ctron-electron interaction. interaction. It It can can not not be be reduced reduced by by cooling cooling the sample.the sample.n nThe The higher higher the the electron electron energy, energy, the the more more the the knock-on knock-on damage. damage. The The smaller smaller the the electron electron energy, energy, the the more more the the radiolysis radiolysis and and heating heating damage. damage. It It is is not not possible possible to to wholly wholly avoid avoid electron damage.electron damage.20The End21
