Microscopes
1. Scanning Electron Microscope
a. the electronics in the SEM make a small probe, but stability is usually poor
b. the SEM is suitable for a variety of attachments
c. sample prep is minimal
d. 1-30 kV accelerating voltages are available
e. the resolution is approximately 20-100Å2. Microprobe
a. it may or may not have high spatial imaging
b. its column is configured to provide a high intensity, stable probe
c. it is usually equipped with several wavelength spectrometer.
d. it is intended for very accurate quantitative analysis
e. accelerating voltages of 5-30 kV are typical.3. Scanning Transmission Electron Microscope
a. high resolution is possible
b. phase identification using electron diffraction imaging
c. EDS analysis of very small areas
d. Voltages of 75-300 kV are possible
e. specimen prep is very difficult due to small specimen size
f. the operator has access to all SEM signals as wellX-ray Instrumentation
1. Wavelength Dispersive Spectrometry (WDS)
A small fraction of the x-rays leaving the specimen strike a crystal and are reflected onto the detector. The crystal will reflect only a very narrow range of wavelengths of x-rays so Bragg’s law can be applied to determine elemental content.
Since a particular crystal will diffract only a very narrow wavelength of x-rays, one crystal is useful for detecting only a few atomic numbers. Gypsum crystals will detect atomic numbered elements 11 to 14 and sodium crystals will pick up atomic numbered elements 16 to 37.
The focused x-rays are detected when they pass through a thin plastic window and enter a gas-filled cylinder containing a collector wire kept at a high positive voltage. The x-rays cause an ionization of the argon-methane gas and generate a flow of electrons to the wire. This current is measured and quantified over time. The amount of current is directly related to the energy of the original x-ray, so it is possible to determine the element from which the x-ray was emitted.
2. Energy Dispersive Spectroscopy (EDS)
A. The Detector
In EDS the detector is usually made of a single silicon crystal, 2-3 mm thick. The crystal is chosen for its purity and shape, but even so, conductivity varies greatly and it is necessary to drift lithium into the crystal to fill any vacant lattice spaces.
The atoms in this crystal are covalently bonded. Each x-ray produces a photoelectron in this valence band. A conduction band creating an electron-hole pair dissipates the energy. Each electron-hole pair produces 3.8-3.9 eV. Approximately 1,000 hole pairs are created per x-ray. The current generated by the conducted charge is proportional to the energy of the absorbed x-ray. There can be residual conductivity due to the random excitations of electrons and cryogenic temperatures help to control this.
Because of their extreme sensitivity, detectors must be protected from the interior of the column. Most standard detectors use a beryllium window to achieve this end. Lower energy x-rays (anything lighter than sodium) will not pass through this window.
Next to the crystal is a thin gold layer that aids in conduction. After this comes a space termed the dead layer in which holes appear due to incomplete Li-drifting. Some absorption occurs here, causing a “tail” on spectra peaks.
Many things can happen as the x-ray enters the detector. It may not even reach the crystal because the window, the gold layer or the dead layer may absorb it. It may be too energetic and pass through the crystal with no interaction. Or the interaction between the x-ray and the silicon atoms in the crystal may be such that the silicon atom produces an x-ray of its own which escapes from the detector and is ejected back into the column. This last situation results in the formation of an escape peak. The formation of an escape peak depends upon the angle of incidence into the detector and the energy of the parent peak. The result of the escape is an artifactual small peak 1.47 keV below the parent peak.
B. Preamplifier or Field Effect Transistor
The signal is shuttled here directly after it is collected in the crystal. The bias voltage is subtracted from the pulses. A pulsed optical feedback is used to reduce electronic noise and the result is a “dead time” in which the system cannot take in any data as it processes a signal.
C. Pulse Processor/Amplifier
Pulses from the preamp are shaped and amplified for analog to digital conversion. These pulses must be separated because the height of the pulse carries information about the energy of the x-ray. The pulse pileup rejecter may reject pulses that come into the system together.
D. Analog to Digital Converter and Multichannel Analyzer.
The pulses are converted from analog to digital signals and then sorted into channels according to height. The multichannel analyzer assembles the spectra and lets you control it through the keyboard.
