Energy Dispersive Spectroscopy
The detector
The detector is based on a semiconductor device, usually a crystal of silicon although intrinsic germanium detectors have been used on TEMs. 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.
The 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. There is also a windowless configuration that removes the absorption issue.
- 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.
The operating principle is the same for all types of detector; the energy of the incoming X-ray is dissipated by the creation of a series of electron-hole pairs in the semiconductor crystal. A high bias voltage is applied across the crystal and this causes electrons and holes to move to electrodes on opposite sides of the crystal, producing a charge signal which is passed to the pulse processor. The size of the signal is proportional to the energy of the incoming X-ray. For a silicon detector, ~3.8 eV is used to generate each electron-hole pair (~2.9 eV for Ge). So for an incoming Ni Kα X-ray of energy 7.477 keV, 1968 electron-hole pairs will be produced, and for an Al Kα X-ray of 1.487 keV, 391 electron-hole pairs will be generated.
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 (~ -70°C) and employ thermoelectric (Peltier) cooling which is a significant saving in time and money.
The energy of the incoming X-ray, in this case Ca Kα, generates electron-hole pairs in a silicon crystal detector. A bias voltage across the detector causes movement of electrons and holes to opposite sides of the crystal, generating a charge signal.
Si(Li) detector crystals are about 3 mm thick. X-rays produced in SEMs may have energies up to ~30 keV, and these will be efficiently processed by the Si(Li) crystal. Higher energy X-rays, e.g., 100-400 keV as produced in TEMs, will pass through the Si(Li) crystal and its efficiency declines at energies above ~25 keV. Intrinsic Ge detectors maintain their efficiency to process X-rays with energies in excess of 100 keV which is why they were preferred for some TEM detectors.