SEM part II
The electrons in each shell and subshell have specific ionization energies...are these shell and subshell ionization energies different for every element?
Yes. The ionization energy for the K shell in Si (1.84 keV) is different from the ionization energy of the K shell in Pt (78.4 keV).
What are the three basic components of the EDS?
a detector, a pulse processor and a multi-channel analyser or computer display
A detector consists of
-A collimator to ensure that only X-rays generated from where the primary electron beam interacts with the sample will be collected. -An electron trap to ensure that X-rays, but no electrons, enter the detector. -A window to isolate the detector crystal, under high vacuum, from the chamber of the microscope. Older windows were composed of Be which did not allow low-energy X-rays (< ~0.9 keV) to pass through it, but more modern windows are composed of polymers that will allow low-energy X-rays (down to ~0.1 keV) to pass. -A semiconductor crystal detector. -Electronics to detect the charge recorded by the detector, convert it to a voltage pulse and pass it to the pulse processor.
what is a backscattered electron?
-BSE result from elastic interactions between the incident electrons and the target specimen. -Backscattered (BS) electrons are high-energy electrons (>50 eV) from the primary incident beam that are ejected back out from the sample. -Produces an image which uses contrast to tell us about the average atomic number of the sample. -The escape depth of backscattered electrons can be greater than that of secondary electrons, consequently resolution of surface topographical characteristics can suffer. -They are sensitive to the atomic mass of the nuclei they scatter from.
What is an Auger electron?
-The Auger effect is the filling of a vacancy in an inner electron energy level in an atom by an electron from an outer energy level of the same atom. -When an electron is removed from a core level of an atom, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in a release of energy. -The energy can be transferred to another electron, which is ejected from the atom. This second ejected electron is called an Auger electron.
what is an inelastic event of the electron beam?
-secondary e- (continuous energy distribution by definition <50eV) -x-ray production: continuum (bremsstrahlung or braking) -Auger e- are of specific energies based on elemental composition (like characteristic x-rays) these are not quantitatively measured in our JEOL5310LV SEM -visible light fluorescence (cathodoluminescence)
Why are heavy metals such as gold or platinum used to coat specimens?
An extremely thin layer is applied (~10 nm). This coating is applied for two main reasons: (1) Non-conductive specimens are often coated to reduce surface charging that can block the path of SE and cause distortion of signal level and image form; and (2) Low atomic number (Z) specimens (e.g. biological samples) are coated to provide a surface layer that produces a higher SE yield than the specimen material.
What region of the interaction volume do each of the signals come from?
Approximately the top 15nm of the volume comprises the zone from which SE can be collected, the top 40% is the region of the interaction volume from which BSE can be collected and X rays can be collected from the entire interaction volume.
How are Auger electrons produced?
Auger electrons are produced when a sample is bombarded with electrons and a characteristic X-ray produced by inner shell ionization is reabsorbed, ejecting an electron.
What are Bremsstrahlung xrays?
Bremsstrahlung X-rays are produced by slowing down of the primary beam electrons by the electric field surrounding the nuclei of the atoms in the sample (see Bremsstrahlung animation). Note: Bremsstrahlung X-rays are also referred to as continuum or background X-rays. A primary beam electron may lose all of its energy in a single interaction event in which case it will produce one X-ray with energy Eo, but it is much more likely that the energy will be lost in a number of interactions in which small proportions of the initial energy are lost and an equivalent number of low-energy X-rays is produced. Braking x-radiation is emitted as electrons are slowed by the fields around nuclei of material. This radiation increases with sample atomic # because larger nuclei lead to more dense Coulombic fields. Bremsstrahlung, from bremsen "to brake" and Strahlung "radiation"; i.e., "braking radiation" or "deceleration radiation") is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon, thus satisfying the law of conservation of energy.
A electron beam can lose its kinetic energy in what two types of events?
Elastic and inelastic events
What is the system used to detect xrays in SEM?
Energy Dispersive Xray Spectrometer
What is the difference between an Auger electron and an xray?
If an inner shell electron is removed from an atom, an electron from a higher level will quickly make the transition downward to fill the vacancy. Sometimes this transition will be accompanied by an emitted photon whose quantum energy matches the energy gap between the upper and lower level. Since for heavy atoms this quantum energy will be in the x-ray region, it is commonly called x-ray fluorescence. This emission process for lighter atoms and outer electrons gives rise to line spectra. In other cases, the energy released by the downward transition is given to one of the outer electrons instead of to a photon, and this electron is then ejected from the atom with an energy equal to the energy lost by the electron which made the downward transition minus the binding energy of the electron that is ejected from the atom.
What are secondary electrons?
Secondary electrons are emitted by atoms near the surface of a sample material. The electrons of these atoms become excited and have sufficient energy to escape the sample surface. Secondary electron imaging, being more surface sensitive, has greater resolution. Incident beam electrons create SE's while entering the sample and BSE's create SE's while leaving the sample.
Electron penetration trajectories increase with increasing accelerating voltages (10,15,30 kv)
TRUE
Heavier elements appear brighter than lighter elements in a backscattered electron image
TRUE. Heavier elements backscatter more efficiently and therefore appear brighter than lighter elements in a backscattered electron image
How are xrays detected in the SEM?
The X-rays are detected by an Energy Dispersive detector which displays the signal as a spectrum, or histogram, of intensity (number of X-rays or X-ray count rate) versus X-ray energy. The energies of the Characteristic X-rays allow the elements making up the sample to be identified, while the intensities of the Characteristic X-ray peaks allow the concentrations of the elements to be quantified.
The depth and diameter of the electron beam's interaction volume depends on what two things?
The acceleration voltage (kV) and the density (mean atomic number) of the sample. The interaction volume will be larger for a larger accelerating voltage, but smaller for samples with a higher mean atomic number
What does the pulse processor do?
The charge generated in the detector crystal is converted to a voltage pulse and passed to a pulse processor that removes noise from the signal, discriminates the energies of the incoming X-rays and discriminates between X-rays that arrive in the detector almost simultaneously.
What is the detector made of?
The detector is based on a semiconductor device, usually a crystal of silicon. The first detector developed for commercial systems in the late 1960s was the lithium-drifted silicon or Si(Li) detector, but it is now giving way to the silicon-drift detector or SDD.
What type of xrays result from the electron beam interaction with the sample?
The electrons from the primary beam penetrate the sample and interact with the atoms from which it is made. Two types of X-rays result from these interactions: Bremsstrahlung X-rays, which means 'braking radiation' (and are also referred to as Continuum or background X-rays), and Characteristic X-rays.
Why is a vacuum important in the SEM
The environment within the column is an extremely important part of the electron microscope. Without sufficient vacuum in the SEM, the electron beam cannot be generated nor controlled. If oxygen or other molecules are present, the life of the filament will be shortened dramatically. This concept is similar to when air is allowed into a light bulb. The filament in the light bulb burns out. Molecules in the column will act as specimens. When these molecules are hit by the 1o electrons the beam will be scattered. A basic requirement for the general operation of the SEM is the control and operation of the vacuum system. When changing samples, the beam must be shut off and the filament isolated from atmospheric pressure by valves. A vacuum is obtain by removing as many gas molecules as possible from the column. The higher the vacuum the fewer molecules present.
What is the Li portion of the detector?
The intrinsic region of each detector is Li doped and is the region that counts x-rays by converting their energy into electron current. Normal P (boron doped) or N (phosphorus doped) type silicon is not a good x-ray detector because of excessive e- and hole current even in the absence of an x-ray event.
What does the multi-channel analyser do?
The output from the pulse processor is passed to a multi-channel analyser or computer display. The energy range of the spectrum, e.g., 0-30 keV for SEMs is divided into a number of channels, e.g., 1024, 2048 or 4096 channels, with energy widths of 5, 10 or 20 eV per channel. The number of X-rays with the relevant energy is assigned to each channel, and the result displayed as a histogram of intensity (number of X-ray counts) versus Energy. In the example below, channel 1 has an energy range from 5500-5520 eV and 498 X-ray counts; channel 2 has an energy range from 5520-5540 eV and has 477 counts, and so on.
Why must the detector be cooled?
To minimize electronic noise, the detector must be cooled. Si(Li) detectors are cooled to liquid nitrogen temperatures and are attached to dewars that require regular filling. SDD can operate at higher temperatures (~ -70oC) and employ thermoelectric (Peltier) cooling which is a significant saving in time and money
How are xrays produced by the SEM?
When the incident beam passes through the sample creating secondary electrons, it leaves thousands of the sample atoms with holes in the electron shells where the secondary electrons used to be. If these "holes" are in inner shells, the atoms are not in a stable state. To stabilize the atoms, electrons from outer shells will drop into the inner shells, however, because the outer shells are at a higher energy state, to do this the atom must lose some energy. It does does this in the form of X-rays.
What is the energy of the xrays produced in an SEM
X-rays produced in SEMs may have energies up to ~30 keV, and these will be efficiently processed by the Si(Li) crystal
What is an elastic event of the electron beam?
backscattered electrons are beam electrons that scatter at angles up to 90 degrees from initial beam trajectory (by definition >50electronV)
