Experimental Laboratory Exercises


Emission current



Maxwell contacts

Spreading resistance

Four point
resistivity measurements

Microspark erosion


voltage contrast


Current mirror


Current Mirror

It is possible to put a current mirror inside the SEM and use it to look at the inside of the chamber. The mirror consists of an object that is charged to high voltage. For instance, we used a metal sphere on an insulating support. The sphere was charged by illuminating it with 30 kV electrons. After the sphere was charged, the energy of the electron beam was reduced to 3 kV. These electrons are reflected by the charged sphere. The electrons never touch the sphere; they are deflected by the negative charge on the sphere. The deflected electrons hit the side of the chamber somewhere and generate secondary electrons which are detected by the secondary electron detector.

The metal sphere in the middle of the image was the current mirror. The cone at the top of the image is the electron lens. The electron beam emerges from the aperture at the bottom of the cone.

This is an image looking up at the lens. The hole on the left is the aperture at the bottom of the cone. The square region on the right is the backscatter detector. The scale bar is not correct in current mirror mode. Notice that some wires are soldered to the lens and the backscatter detector. We zoomed in on one of the solder contacts.

This is a magnified view of one of the solder contacts.

An image was made using the the backscatter detector. In this case only the detector itself appears bright.

The secondary electron detector.

Part of the ring structure used to hang the wiring for the micromanipulators.

A micromanipulator. Electrical feedthroughs are visible in the background. The image clearly shows some distortions.

The charged sphere discharges slowly. This caused the images to shift and distort. After a few minutes, the sphere discharged so much that the current mirror effect was lost. To make more serious experiments with this effect it would be necessary to use a stable high voltage supply to keep the sphere at a fixed potential.