Interaction
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Dr. Di Wu, Nanjing University 9
Penetration depth: Photons
Visible light: λ ~ 500 nm, light: penetration depth ~ 50 - 300 nm Information from visible light are averaged over a few hundreds of atomic layers. layers. Ultraviolet light: severe absorption, light: only suitable to surface analysis X-ray: generally a few micrometers, dependent of ray: absorption coefficient I = I 0 exp( t ) increases with increasing atomic number. number.
Dr. Di Wu, Nanjing University 11
Penetration depth: Electrons
The smaller the atomic number, the deeper the penetration. penetration. atomic number < 20 > 40 penetration depth > 10 m < 2 m
Dr. Di Wu, Nanjing University 15
Auger electrons
vacuum The energy of Auger electrons is about 0 ~ 2 keV, absorbed easily by the materials. materials. Only Auger electrons generated within a few atomic layers beneath surface could escape out of the surface. surface. Light atoms or low energy bonds are prone to generate Auger electrons. electrons.
Dr. Di Wu, Nanjing University 6
Penetration depth: Photons
Photons are discrete quanta electromagnetic radiation. The photon radiation. identified by the wavelength, λ, energy, and frequency, ν, all of which are related the equation of is E, by
Dr. Di Wu, Nanjing University 8
Penetration depth: Photons
The penetration depth of photons depends sensitively on materials and photon energies. energies. It is not possible or instructive to give a detailed relation over the whole spectrum. spectrum. However, it is possible to give a rough description over some specific and important wavelength. wavelength.
Dr. Dห้องสมุดไป่ตู้ Wu, Nanjing University
Ec EF valence band L3 L2 L1 K
16
Characteristic X-rays Xvacuum X-ray photons have strong ability to penetrate through the sample. Therefore, Xsample. rays generated in a relatively large volume can be detected. detected. Heavy atoms have large crosscross-sections to generate characteristic X-rays. rays. Characteristic X-rays are suitable to detect heavy atoms. atoms.
Dr. Di Wu, Nanjing University 4
The criterion
Every characterization should acquire the most information on the expense of the least damage. damage. Therefore, knowledge on interaction of radiation with matters is indispensable. indispensable.
E = hν = hc / λ where h is Plank constant and c the velocity of light. light.
Dr. Di Wu, Nanjing University 7
Penetration depth: Photons
Electromagnetic spectrum spans a vast range with wavelength varying from 106 m to 10-14 m. If we want to make use of electromagnetic radiation to characterize the microstructure of materials, the wavelength of photons must be in comparison with the features we want to observe. observe. Therefore, we do not need photons with wavelength larger than 10-4 m or smaller than 10-10 m.
This affects the characterization methods using electron beams as probes because common materials are composed of elements with different atomic numbers. numbers.
Dr. Di Wu, Nanjing University
3
The dilemma
Interaction of radiation with materials under probe may cause damage to or change the microstructure of them. them. Higher resolution requires radiation with smaller wavelength, or higher energy. energy. Thus, the probability of radiation damage increases. increases.
Dr. Di Wu, Nanjing University 10
Penetration depth: Electrons
The penetration depth of electrons varies dramatically with electron energy and atomic number of materials. materials. The higher the energy, the deeper the penetration. penetration. 10 keV 30 keV ~ 0.? m ~ 2 m
Interaction of Radiations with Matters
the probe to microstructures
Dr. Di Wu, Nanjing University
1
Why ?
To characterize a microstructure it is necessary to perturb the material by interacting in some way with it.
Dr. Di Wu, Nanjing University
5
Penetration depth
Penetration depth, or mean free distance, of the incident determines the depth and volume in the sample that can be analyzed. analyzed. In most cases, the type of radiation generated in the sample is different from the incident radiation. The smaller mean free radiation. distance determines the analysis volume. volume.
13
Interaction of electrons with matters
Secondary electrons Backscattered electrons Auger electrons X-rays Diffracted electrons
Dr. Di Wu, Nanjing University
The interaction of electromagnetic radiation with crystalline solids is now understood in considerable detail so that it can be exploited to provide the necessary information.
14
Secondary electrons
Secondary electrons are used to give scanning electron microscope (SEM) images. images. Secondary electrons are low energy electrons. electrons. Thus, only those generated beneath the incident beam or in the vicinity of incident beam can escape out of the surface. surface. Therefore, the resolution of a SEM is basically determined by the incident beam area. area.
Dr. Di Wu, Nanjing University 2
Examples
One sees a matter only because the matter reflects visible light. light. X-ray and high energy electron beam could "see" crystal lattices. see" lattices.
Dr. Di Wu, Nanjing University 12
Penetration depth: Electrons
The scattering of electrons in the materials limits the space resolution. resolution.
Dr. Di Wu, Nanjing University
Penetration depth: Photons
Visible light: λ ~ 500 nm, light: penetration depth ~ 50 - 300 nm Information from visible light are averaged over a few hundreds of atomic layers. layers. Ultraviolet light: severe absorption, light: only suitable to surface analysis X-ray: generally a few micrometers, dependent of ray: absorption coefficient I = I 0 exp( t ) increases with increasing atomic number. number.
Dr. Di Wu, Nanjing University 11
Penetration depth: Electrons
The smaller the atomic number, the deeper the penetration. penetration. atomic number < 20 > 40 penetration depth > 10 m < 2 m
Dr. Di Wu, Nanjing University 15
Auger electrons
vacuum The energy of Auger electrons is about 0 ~ 2 keV, absorbed easily by the materials. materials. Only Auger electrons generated within a few atomic layers beneath surface could escape out of the surface. surface. Light atoms or low energy bonds are prone to generate Auger electrons. electrons.
Dr. Di Wu, Nanjing University 6
Penetration depth: Photons
Photons are discrete quanta electromagnetic radiation. The photon radiation. identified by the wavelength, λ, energy, and frequency, ν, all of which are related the equation of is E, by
Dr. Di Wu, Nanjing University 8
Penetration depth: Photons
The penetration depth of photons depends sensitively on materials and photon energies. energies. It is not possible or instructive to give a detailed relation over the whole spectrum. spectrum. However, it is possible to give a rough description over some specific and important wavelength. wavelength.
Dr. Dห้องสมุดไป่ตู้ Wu, Nanjing University
Ec EF valence band L3 L2 L1 K
16
Characteristic X-rays Xvacuum X-ray photons have strong ability to penetrate through the sample. Therefore, Xsample. rays generated in a relatively large volume can be detected. detected. Heavy atoms have large crosscross-sections to generate characteristic X-rays. rays. Characteristic X-rays are suitable to detect heavy atoms. atoms.
Dr. Di Wu, Nanjing University 4
The criterion
Every characterization should acquire the most information on the expense of the least damage. damage. Therefore, knowledge on interaction of radiation with matters is indispensable. indispensable.
E = hν = hc / λ where h is Plank constant and c the velocity of light. light.
Dr. Di Wu, Nanjing University 7
Penetration depth: Photons
Electromagnetic spectrum spans a vast range with wavelength varying from 106 m to 10-14 m. If we want to make use of electromagnetic radiation to characterize the microstructure of materials, the wavelength of photons must be in comparison with the features we want to observe. observe. Therefore, we do not need photons with wavelength larger than 10-4 m or smaller than 10-10 m.
This affects the characterization methods using electron beams as probes because common materials are composed of elements with different atomic numbers. numbers.
Dr. Di Wu, Nanjing University
3
The dilemma
Interaction of radiation with materials under probe may cause damage to or change the microstructure of them. them. Higher resolution requires radiation with smaller wavelength, or higher energy. energy. Thus, the probability of radiation damage increases. increases.
Dr. Di Wu, Nanjing University 10
Penetration depth: Electrons
The penetration depth of electrons varies dramatically with electron energy and atomic number of materials. materials. The higher the energy, the deeper the penetration. penetration. 10 keV 30 keV ~ 0.? m ~ 2 m
Interaction of Radiations with Matters
the probe to microstructures
Dr. Di Wu, Nanjing University
1
Why ?
To characterize a microstructure it is necessary to perturb the material by interacting in some way with it.
Dr. Di Wu, Nanjing University
5
Penetration depth
Penetration depth, or mean free distance, of the incident determines the depth and volume in the sample that can be analyzed. analyzed. In most cases, the type of radiation generated in the sample is different from the incident radiation. The smaller mean free radiation. distance determines the analysis volume. volume.
13
Interaction of electrons with matters
Secondary electrons Backscattered electrons Auger electrons X-rays Diffracted electrons
Dr. Di Wu, Nanjing University
The interaction of electromagnetic radiation with crystalline solids is now understood in considerable detail so that it can be exploited to provide the necessary information.
14
Secondary electrons
Secondary electrons are used to give scanning electron microscope (SEM) images. images. Secondary electrons are low energy electrons. electrons. Thus, only those generated beneath the incident beam or in the vicinity of incident beam can escape out of the surface. surface. Therefore, the resolution of a SEM is basically determined by the incident beam area. area.
Dr. Di Wu, Nanjing University 2
Examples
One sees a matter only because the matter reflects visible light. light. X-ray and high energy electron beam could "see" crystal lattices. see" lattices.
Dr. Di Wu, Nanjing University 12
Penetration depth: Electrons
The scattering of electrons in the materials limits the space resolution. resolution.
Dr. Di Wu, Nanjing University