Friday, August 28, 2009

How to measure the beam current density in TEM

Here I will explain for you how to measure the beam current in the TEM by a alternative method to the standard Faraday cup approach

This is a convenient method if you have a in-situ TEM-SPM sample holder which enables you to directly measure the electric current of the electron beam. Below you find a list of the things I use:
  1. Transmission electron microscope (JEOL-2000FX).
  2. in-situ TEM-SPM holder modified Nanofactory version.
  3. PXI digital multimeter card (PXI-4071) with picoamps range.
  4. Copper wire 0.25mm in diameter.
You replace the sample wire in the sample holder as the sketch below shows. Use of light weight metal wire is crucial to minimize electron back scattering. For copper it is around 30% of the electrons that are scattered and therefore will not contribute to the measurement so therefore you need to compensate for this. Besides this it should be straight forward from here on.

The highest measured beam-current was in my case 909 pA at 30kX magnification, spot size 1, condenser aperture 120 um.
Current is depending on filament power, spot-size (C1), magnification, and condenser aperture.
The CCD camera operates best in the beam current range of 20 pA (dark-screen) to 136 pA (bright-screen), while 100 pA is the normal value. This corresponds to a beam current density of 0.5 to 3.3 mA/cm2 and normal is 2.5 mA/cm2
(normal operating conditions) Below is the measurement data table.

Schematics: using the in-situ TEM probe for beam-current measurement.

Some references for you to read regarding this method

Ref. Williams, Carter
"You can measure the beam current with a picoammeter in the earth line"
"You can calibrate the Faraday cup measurement against the TEM screen exposure meter"
"The beam current is usually in the range of nanoamps to picoamps."
"If you don't have a Faraday cup, it is possible to get a approximate reading on the current by just measuring the current through a insulated line from a bulk region of the specimen and correcting for electron backscatter. Backscattering is independent of accelerating voltage and aproximately linear with atomic number up to about Z = 30. For example backscatter coeficent for Cu is about 0.3 and for Al it is about 0.15."

-I used the PXI-4071, soft-panel version at 1uA range =1pA resolution. Copper wire 0.25mm
p.79 "To measure the beam size in TEM/STEM you must form a image of the beam on the TEM viewing screen under conditions where you know, or can calibrate the magnification."
-I used the CCD-camera, it's been calibrated previously, to image the beam. I don't considerer the radial intensity distribution of the beam.

Ref. Reimer
"A fraction n of the incident electrons can leave the specimen as backscattered electrons (BSE)...n is known as the backscattering coefficient."


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