GEOLOGY
Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Subject Geology
Paper No and Title Crystallography and Mineralogy Module No and Title Scanning Electron Microscopy (SEM)
Module Tag Min Vb
Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad
Vice-Chancellor Jamia Millia Islamia Delhi
Prof. Devesh K Sinha Department of Geology University of Delhi Delhi
Prof. P. P. Chakraborty Department of Geology University of Delhi Delhi
Paper Coordinator Content Writer Reviewer Prof. Naresh C. Pant
Department of Geology University of Delhi Delhi
Prof. Naresh C. Pant Department of Geology University of Delhi Delhi
Prof. Santosh Kumar Department of Geology Kumaun University Nainital
GEOLOGY
Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM) Table of Content
1. Learning outcomes
2. Introduction and Historical Perspective 3. Beam Matter Interaction
4. Resolution and Volume of Interaction 5. Scanning Electron Microscope
6. Summary
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM) 1. Learning outcomes
After studying this module, you shall be able to:
To develop a basic understanding of effects of interaction of a beam with matter.
To know about resolution of electron beam and to understand back scattering of electrons and secondary electron generation
To understand application of SEM in characterization including chemical characterization of geological material.
2. Introduction and historical perspective
Geology represents one of the “observational” or “historical” sciences such as Astronomy where it is not normally possible to repeat the observed phenomenon experimentally and these have to be understood and explained on the basis of laws of science of other physical sciences such as physics and chemistry. Even these physical sciences extensively use basis of mathematics. In view of this limitation, good characterization becomes a pre-requisite for arriving at scientifically valid inferences.
Physical and chemical characterization of geological material at various scales is a common technique in this respect. Preliminary physical characterization of the heterogeneities of the geological material is generally restricted to the result of light with the object. This is in form of human eye, hand lens and petrological microscope examination. The resolution of an image depends on the wavelength of the investigating probe and since the wavelength of visible light varies from 700-400 nm, objects smaller than 200nm cannot be resolved by light (see section on resolution below). Besides, the light-matter interaction is unable to provide information on the chemical characterization of object. Bulk rock characterization is commonly carried out to describe the attributes of a geological material such as magma composition being represented by a solidified rock, effective bulk composition for metamorphic reaction by analyzing a part of the transformed rock, soil composition in relation to the underlying bedrock to establish its authigenic nature or trace element composition in different type of rocks to infer their tectonic settings. The rocks are essentially chemically heterogeneous and the bulk chemical characterization is actually an
GEOLOGY
Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
artificial homogenization. For example, the phenocrysts and matrix are not only chemically distinct from each other in magmatic rocks but often also represent different periods of formation of these rocks. Thus, a complex evolutionary process of formation of a magmatic rock may often be inferred in an oversimplified manner if based on bulk characterization attributes. Scanning Electron Microscope is a device, which obviates these problems and provides an instrument capable not only of providing high-resolution microscopic images but also of their chemical traits. This module introduces basic theory of the scanning electron microscope, discusses resolution and conditions suitable for good imaging and introduces the chemical characterization using energy dispersive spectrometry.
It is known for nearly a century that electrons can be accelerated in vacuum, regulated to a fine and controllable spot and made to interact with atoms and electrons of the object to decipher the nature of that object. Earliest such investigations consisted of examination of thin objects i.e. objects, which allowed through passage of electrons and these types of microscopes, are currently known as Transmission Electron Microscopes (TEM). Von Ardenne in 1938 applied scanning coils to a TEM and he is credited to have produced first Scanning Electron Microscope (SEM). The term Scanning refers to observing a signal on a Cathode Ray Tube (CRT) in such a way that the indirect signal is in synchronization with the rastering rate of the electron beam on the object. However, this today is known as Scanning Transmission Electron Microscope (STEM). A TEM or STEM utilizes high voltages (several hundred kilovolts or more) and are capable of producing atom or molecule level images of thin specimens. The conventional SEM of today can be linked to the discovery of secondary electrons by Zworykin and Hillier (1942) which were produced as a result of the interaction of a primary electron beam with protons, neutrons and electrons of the object. The next major improvement to SEM came about by change of electrostatic components by electromagnetic ones, improved signal processing and correction of inherent astigmatism by Smith (1956). A much- improved SE detector by Everhart and Thornley (1960) followed this and it is named after them. Today an SEM looks like as shown below.
GEOLOGY
Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM) 3. Bean Matter Interaction
Numerous types of interactions occur when an electron beam impinges on matter.
For the sake of convenience, we will call the impinging beam as primary electrons.
In a generalized way, each ‘event’ of interaction actually refers to transfer of energy of the incident primary electron until such time that this energy reaches zero.
Following law of conservation, the energy of the primary electrons can only be transformed. Each of the multiple effects of this transformation is capable of providing information about the object of interaction. Though often SEM is considered to represent the surface interaction the primary electrons penetrate the surface of object and the resultant signals represent information from a 3-D volume, which is known as the interaction volume. The energy of the primary electrons is expressed in form of electron volts (eV or kilo electron volts- keV) and its numerical value is similar to that of the potential energy applied to obtain the electron beam.
Two major types of interactions result from the interaction of an electron beam with the matter. In one instance, the primary electrons may only change their path retaining most of their original energy and this type of interactions is referred as elastic scattering. The other type of interaction leads to loss of most of the energy of the primary electrons leading to a variety of object characteristic signals and this is known as inelastic scattering.
Several types of signals are produced as a result of interaction of an electron beam with the matter. These are listed below in terms of the type of particle/energy produced and shown in Figure 1;
Electrons
Secondary electrons (low energy <50 eV)
Backscattered electrons high energy)
Auger electrons
Transmitted elastic effect electrons (diffraction)
Transmitted inelastic (low energy)
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Photons
X-rays (characteristic as well as Bremmsstrahlung)
Visible light, UV, IR etc.
Electron-Hole pairs
Induced current
Variation in resistivity
Fig. 1 Possible results of interaction of an electron beam with a thin specimen of matter.
Since it is not possible to discuss in detail the entire range of scattering events, we will discuss only those effects, which are relevant to the Scanning Electron Microscope further. Any type of signal is usable to characterize the object but three conditions are necessary;
a) The signal should be distinguishable from the noise.
b) The production and recording of the signal should be of the order of scanning speed.
c) The signal should characterize one or more specific property of the object.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
4.Resolution and Volume of Interaction
At each point of the interaction, depending upon the impacting beam and the nature of material, the nature of extracted information is not only different but also comes from different segments of the volume of interaction.
The interaction volume may be controlled by either elastic or inelastic or both types of scatterings. Following Heisenberg’s uncertainty principle, it is impossible to simultaneously describe the position and energy of the atomic particles, the electron interactions and scattering events are best described in terms of probabilities.
Considering the possibility of an event within the target, its probability is given in terms of the units of area or cross section (Q) such as an element’s cross section as the effective size of an atom associated with an event. Since primary electrons of the incident electron beam has high energies (typically several thousand volts) their energies are dissipated is several events through several scatterings each with their own probabilities and different paths electrons travel between these events. Mean Free Path often denoted by gamma (Γ) is given by the following relation;
Γ = A/NoρQ
Where A is the atomic weight, is the No Avogadro number and ρ is the density.
The total mean free path for an electron is calculated as
1/ Γavg = 1/ Γa +1/ Γb + 1/ Γc …
Where a, b and c signify different events for travel of an electron through any material. It is possible to simulate these scatterings for different type of materials and for different energy electron beams following Monte Carlo simulations (on account of probabilistic nature of these scatterings). Some of these simulations are shown below illustrating the effect of atomic number (lower mean free path in higher atomic number material) and the incident electron beam energy (longer mean free paths and thus greater penetration for higher incident beam energy) on the depth of penetration of the typical primary electron beam. As is evident from below the scale the beam matter interaction is typically limited to tens to hundreds of nanometer depths.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Amphiboles in hand specimen
We will now define the resolution of the electron beam. The resolution of any probe is dependent on its wavelength. Following Broglie’s theory of propagation of particles as waves (here the incident electron beam), the wavelength can be given by;
λ = h/p
where λ is the wavelength, h Planck’s constant (6.626 x 10-34 J seconds) and p is the momentum of the particle (equivalent to the product of mass and velocity)
Therefore λ = h/mν
Velocity of an electron is dependent upon the accelerating voltage (eV) eV = ½ mν2 and ν = √2eV/m Thus, the wavelength of an electron beam can be given as λ = h/√2meV
Considering that the mass of an electron is 9.1 x 10-31 kg and e = 1.6 x 10-19 coulomb, wavelength of electrons can be computed as;
λ = 6.626 x 10-34/√2 x 9.1 x 10-31 x 1.6 x 10-19 x V = 12.25 x 10-10/√V
For an accelerating voltage of 10 keV, the wavelength of electron beam works out to be 12.25 pm (picometer) while for a 100 keV it is 3.87 pm. The maximum resolution of a probe behaving as wave is generally upto half its wavelength. Thus, details upto a few picometer can be resolved using an electron beam of appropriate energy.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Since signals result not only from the interaction area but also extend to a depth, the SEM characterization represents an interaction volume. Though the extent of this volume depends on the nature of geological material (inversely proportional to the density and directly proportional to the incident electron beam energy), the shape is characteristically like a teardrop. Different parts of this volume contribute different types of elastic and inelastic scattering (Fig. 2).
Fig. 2 The interaction volume resulting from interaction of an electron beam with matter. E refers to the energy of the electron beam required for a particular event. Ec is the critical excitation energy required for generating a family of characteristic X-rays.
5.
Scanning Electron Microscope
The scattering is either elastic (change of trajectory but kinetic energy and velocity nearly unchanged; e.g. SE) or inelastic (no change of trajectory possible but loss of energy; e.g. BSE, bremsstrahlung).
During elastic scattering probability of a large deviation (and very little loss of energy) is dependent on the following function.
Q>ɸo ∝ Z2/E2
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Where probability (Q) of an event of scattering at an angle greater than ɸo is directly related to the atomic number and inversely related to the energy of the incident electron beam. Thus, the deflected electrons have atomic number dependence. In case of geological material (compounds), one can have a hypothetical concept of average atomic number given by the following equation;
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑍 = ∑ 𝐶𝑖𝑍𝑖
Where Ci is the element weight fraction and Z is the atomic number.
The fraction of electrons which are scattered back are designated as backscattering co-efficient (represented by η) and can be upto 50% of the incident electron beam.
Its atomic number dependence is shown in Fig. 3. In the context of the geological material, backscattering of electrons is of great use as a petrological tool as backscattering coefficient (Ƞ=proportion of incident electrons to the backscattered electrons) is a function of atomic number of the matter.
Fig. 3 Relationship of secondary electron and back scattering coefficients with the atomic number. The filled ovals represent back scattered electrons. Vertical scale marks the proportion of SE or BSE electrons in reference to the incident electrons taken as 1. Note strong atomic number dependence of BSE electron.
Atomic
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
The back scattered electron (BSE) images are very useful in bringing out not only the inter-grain compositional contrast but also the intra-grain compositional variation. In the first BSE image (Fig. 4), symplectitic decomposition of garnet into orthopyroxene and plagioclase is well brought in atomic number contrasted scanning electron image. The high Fe-nature of orthopyroxene exsolution within the Clinopyroxene along well-defined crystallographic orientation is seen in the second BSE image (Fig. 5).
Fig. 4 BSE image of symplectite texture around garnet. Note irregular grain edges of garnet (towards right) breaking down to an intergrowth of orthopyroxene and plagioclase. It is commonly inferred as a sign of tectonic uplift or decompression.
The bright coloured minerals are ilmenite.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Fig. 5 BSE image of a clinopyroxene having crystallographic controlled this exsolution lamellae of orthopyroxene. The bright greytone nature of orthopyroxene is on account of significantly higher iron content of this mineral in comparison to the host clinopyroxene.
Secondary electrons are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. The excited electron moves towards the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface. Here it can escape if its energy exceed the surface work function, Ew, which defines the amount of energy needed to remove electrons from the surface of a material. During inelastic scattering, secondary electrons emerge on account of conduction band electron excitation. These are loosely held in the material and on account of high energy of the primary electrons, these are easily produced. Secondary electrons are of low energy (1-50 eV and majority equivalent to 5 volts) and thus can travel only to a limited extent in the sample. The secondary electrons generated within ~5nm of the sample surface are able to escape and on account of this provide excellent surface details of the object. Faster or higher
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
energy secondary electrons (upto ~1000 eV) are also produced mainly from outer covalent bonds and these can sometimes be distinguished from the ‘slow’ ones.
Since these degrade the secondary electron image, efforts are made to reduce the effect of faster secondary electrons. One of such method is coating a non-conducting object with gold or some other conducting material so that conduction/covalent bond ration increases and higher production of slow secondary electrons results. The secondary electron images, thus produced, are capable of providing morphological details at high magnification (Fig. 6 and 7).
Fig. 6 SE image of a marine microfossil shell.
Fig. 7 High magnification image of the opening (top-center of Fig. 6) of the shell.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
The average number of SE produced per primary electron is called the secondary- electron yield, δ, and is typically in the range 0.1 to 1.0 (varying between different materials; Fig. 4). For a given sample material, δ decreases with increase in incident energy E0 since the probability of inelastic scattering of a primary electron within the escape depth decreases. SE yield depends on the angle of tilt of the specimen relative to the primary-electron beam, φ. The value is lowest for perpendicular incidence (φ = 0) and increases with increasing angle between the primary beam and the surface-normal. This effect may make cracks appear bright in SE images. Only part of the SE signal, the SE1 component, comes from the sample surface. Other components arise from SE produced by backscattered electrons as they exit the specimen (SE2) and when BSE strike the walls of the specimen chamber (SE3).
Higher SE yield results from a low angle impact of the electron beam as the vertically impacted produces least number of SE electrons. Since higher incident energy electron beam produces higher BSE, SE2 and SE3 will also increase with high incident beam energy. Thus, as a thumb rule, a lower energy primary beam produces a better SE image.
The primary X-rays can knock out an electron from the inner shell of an atom in the object creating an electron hole. This hole is filled by another electron from the outer shell and the difference of energy is released as photons in bandwidth of X-rays.
Occasionally the energy of these photons is sufficient to knock out another electron and this electron (removed not by the primary beam) is known as Auger electron.
Inelastic scattering also produces long-wavelength radiation in the visible, IR and UV spectrum including the effect known as cathodoluminscence. It is of great significance in detecting growth related zoning in several minerals such as quartz, calcite and zircon (Fig. 8 and 9).
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Fig. 8 & 9 Cathodoluminescence images of zircon showing growth zoning.
The most important result of the electron beam-matter interaction in context of chemical characterization of matter is in the generation of characteristic X-rays as well as continuum X-ray radiation from a volume of matter. The X-rays produced are in the wave length range of 0.1 to 100 Å out of which longer wave length (λ~50 to 100 Å) are known as ‘soft’ x-rays and the shorter wave length are called as ‘hard’
X-rays as λ is inversely proportional to the energy of the electromagnetic spectrum as illustrated below.
The relationship between the energy of any part of electromagnetic spectrum can be expressed in form of following equation;
E = hν
Where h is the Planck’s constant and ν is the frequency which for electromagnetic radiations can be related to wavelength by
ν = c/λ as discussed earlier where c = speed of light and λ is the wavelength in m.
After substituting, we get
E = hc/λ
And after substituting h = 6.6260 x 10-34 Joule-second and c = 2.9978 x 108 m/sec we get
E (in joules) = 1.98636 x 10-25/λ
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
And in units of nanometers and electron volts (1m = 109 nm and 1 eV = 1.6021 x 10-
19 Joule) above equation is reduced to Duane-Hunt equation of relationship between energy and wavelength
E (in eV) = 1239.84/λ (in nm)
Characteristic X-rays can either be measured as energies or can be detected in terms of their wavelengths.
A small fraction of the incident electron beam leads to ejection of inner-shell electrons in from their orbitals, thus ionizing it for a small time period before the vacancy is filled by an outer-shell electron. The chemical characterization is generally based on inner shell electron transition.
Besides the characteristic X-rays there is also a continuum of X-rays generated during interaction of an electron beam with matter which results from interaction in form of de-acceleration of the primary electrons on account of the coulombic field of the nucleus and for this reason it is also known as bremsstrahlung or ‘breaking radiation’. This continuum is not uniform throughout the range of wavelength and contributes to the characteristic X-rays. Thus, its measurement is required for precise quantification.
The relationship between the wavelength of the characteristic X-ray (λ) and atomic number (Z) was empirically established by Moseley in 1914, which can be expressed as;
λ = K/(Z-σ)2
Where K and σ are constants for that characteristic X-ray. The latter (σ) is known as shielding constant and is approximately 1 for K lines and 7.4 for more shielded L- lines. This forms the basis of chemical characterization.
If chemical characterization is done using energy of the characteristic radiation rather than its wavelength, it is known as Energy Dispersive Spectrometry or EDS.
EDS is available as an option with most of the modern scanning electron microscopes. The early detectors of EDS required cooling commonly with liquid nitrogen. However, solid-state detectors can now resolve ~130 eV and these are used
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
in current EDS devices. EDS patterns obtained can be interpreted in terms of the mineralogy and other details as shown in Fig. 11.
Kolihan
Chlorite
retrogressed
Biotite
Relict grt
Fig. 11 BSE image of garnet schist from Khetri Copper Belt. Note strong hydrothermal alteration indicated by the development of chlorite around biotite.
Presence of retrograded garnet is also detected by the EDS spectra.
6.
Summary
Requirement of detailed characterization is higher in those sciences, which are dominantly of observational nature. Geology is an observational science wherein the operative processes not only have long ceased but may not have any known present day precedence and the effects may be very poorly preserved. Added to this is the element of time, which may be of the order of billions of years. This necessitates a thorough characterization of the geological material. The objects are characterized on different scales such as outcrop, structural attributes, bulk physical and chemical properties and the physical and chemical nature of the heterogeneities.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Microscopy is one of the primary method of study of geological objects. However, the optical microscope is limited by the wavelength of the visible light besides not being able to provide the information on the chemistry of the studied material. A regulated beam of electrons provides means for higher resolution study of objects and a device using this as the probe is known as electron microscope. If signals arising out of the real time rastering interaction of the electron beam of this electron beam can be seen on a CRT than the device is known as Scanning Electron Microscope (SEM). Sensu stricto SEM currently is limited to the surface interaction of the primary electron beam and for thin specimen, where the primary high-energy beam passes through the sample is known as Transmission Electron Microscope (TEM) or Scanning Transmission Electron Microscope (STEM).
Conventional SEM can achieve resolutions of upto a few picometers. However, often the primary object is to achieve quality imaging. Beam matter interaction mainly involves interaction of primary electron beam with the atoms of the objects.
On account of uncertainty associated with position and energy of the electrons of the object, the beam-matter interaction is best described in form of probability of the event. The event is known as scattering and two main types of scattering events are possible. Elastic scattering refers to the event when there is very minor loss of energy of the primary electrons and often their path is changed significantly and inelastic scattering when the primary electrons a large part of their energy.
Elastic scattering is proportional to the atomic number with higher atomic number producing higher proportion of electrons, which are known as back-scattered electrons.
The fraction of back scattered electron of the primary electron beam is known as back- scattering coefficient. Inelastic scattering produces secondary electrons (SE) which are low-energy electrons. SE can be produced in multiple ways but the low energy SE from near the surface produce excellent morphological images of the object. Inelastic scattering also generates several other signals the most significant for characterization amongst them is the characteristic X-rays. A qualitative chemical estimation of the composition of the object can be obtained by analyzing these and in an SEM; this is commonly achieved through Energy Dispersive Spectrometry.
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
Frequently Asked Questions-
Q1. What is the difference between an electron microscope and a scanning electron microscope?
Q2. Which factors define the resolution of an electron beam?
Q3. Discus the relative depths from where secondary electrons and back scattered electrons mainly result?
Q4. Derive the relationship between energy and wavelengths of an electromagnetic radiation?
Multiple Choice Questions-
1. Wavelength of visible light varies between : a) 200-300 nm
b) 400-700 nm c) 400-700 pm d) None of the above Ans: b
2. "Bremsstrahlung" results from a) Elastic scattering b) Inner shell transitions c) Inelastic scattering
d) Characteristic Peak intensity Ans: c
3. Back scattering electrons are a) High energy electrons b) Low-energy electrons c) Photons
d) Auger electrons Ans: a
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Paper: Crystallography and Mineralogy Module: Scanning Electron Microscopy (SEM)
4. Major difference between sample preparation for SEM and STEM is a) In requirement of surface coating
b) In nature of sample c) In polishing of sample d) In thickness of sample Ans: d
5. The basis of quantitative chemical characterization in SEM is a) Duane-Hunt equation
b) Mosley’s law c) Elastic scattering d) None of the above Ans: b
Suggested Readings:
1. Goldstein, J., Newbury, D. E., Joy, D. C., Lyman, C. E., Echlin, P., Lifshin, E., Sawyer, L., & Michael, J. R. (2003). Scanning Electron Microscopy and X-ray Microanalysis, 3rd Edn. Springer Science & Business Media. ISBN: 1461502152, 9781461502159.