Current in SEM

The current source is our SEM machine. Beam current is divided into sample current (which passes out of the sample by a wire) and current generated by secondary electrons (which escape from the sample surface). The secondary electrons are how the SEM forms images.

2) electron beam is focused on sample and swept across it. 3)(beam current) -(secondary electron current) = (sample current)

Sample irradiation is determined by the beam current. However, we don't have a direct way to measure the beam current. Instead, we measure the sample current with our electrometer. In order to find the beam current, we must know ratio of the sample current to beam current. This ratio depends on the sample material.

In our case, we use aluminum as our reference material. Based on data from Dr. David Joy, about 66% of the beam current on aluminum is conducted as sample current.

A way to measure this ratio is to use a Faraday cup. This is the picture of the Faraday cup that we designed. It is made of aluminum with electric conductive tape to keep the top on. When the beam is directly into the hole virtually the entire beam current is captured, so that beam current equals sample current.

When using the SEM, the beam is focused on the aluminum part of the cup in order to measure what our electrometer produces as a current on aluminum. Then the beam is focused in the hole of the Faraday cup, as to not allow any secondary electrons to fly off of the surface. The ratio between the current on the aluminum and the current inside the cup is thus equal to the ratio of sample current to beam current for aluminum.

One controlable characteristic of the SEM is the spot size, roughly how large the beam is where it hits the surface. This can be used to vary the beam current: larger spot sizes yield larger currents. Unexpectedly, the results of our measurements depended on spot size (even though spot size was always much smaller than the Faraday cup hole). This is a graph of the relationship between the spot size and the sample-to-beam current ratio.