To measure electron current accurately (or ion current) you need to take secondary electrons out of the measurement. This is easy to do if you have a Faraday cup. The Faraday cup traps secondary electrons which results in an accurate beam current measurement. If secondary electrons are allowed to leave the target then they either subtract from or add to the current measurement, depending on whether you are measuring electrons or ions.
It is not always possible to have a Faraday cup to measure electron or ion current in a vacuum chamber. So how can you get an accurate electron or ion current measurement without a Faraday cup? The answer is to bias the target with a battery or low noise isolated DC power supply with +90V.
The secondary electron cut off will vary depending on the sample material, angle of incidence and beam energy, but it is generally acknowledged to be approximately 50 eV.
When the sample is positively biased with a voltage of 90 to 100 volts, the secondary electrons are trapped on the surface of the sample.
As an example of how a target bias can affect the current measurement, refer to the table below.
These measurements were taken on a copper specimen. You can see that in both cases the effect of a bias resulted in a difference of approximately 2 uA, which is significant.
|Beam Voltage||Beam Type||Target current with no bias||Target current with +90 volt DC bias|
|2 kV||Electron||173 nA||2.00 uA|
|3 kV||Ion||5.2 uA||3.0 uA|
It is easy to make a bias box where you can apply +90V to the target when measuring current, and then shorting the target to ground when not measuring current. If you can’t find them elsewhere, RBD provides 45 volt batteries (RBD part number BAT-45V-213) and the clips. Two 45 volt batteries in series will give you a simple noise free 90V bias supply.
If you are in the market for a picoammeter to measure electron or ion current, The RBD Instruments 9103 USB picoammeter incorporates a +90 volt bias as an option.
Finally, here is a link to an interesting publication on scattered electrons from the University of Porto Rico – Electron Beam Specimen Interaction