Monday, May 7, 2012

Dynamic Focus Correction

This is an image of an electroformed metal mesh. The mesh consists of a hexagonal lattice of holes and is used for supporting thin membranes. It is imaged at very large working distance, about 50 mm, and at a tilt angle of 70 degrees.  The large working distance provides a very large field of view, > 5 mm horizontally, and a large depth of field because of the small electron probe convergence angle at large working distances.

Even though the SEM has a depth of field of many several mm at this working distance, the back edge of the grid is in focus while most of the grid is grossly out of focus. Despite the electron probe having a very small convergence angle at large working distances, the height difference between the front and back edge of the grid is too large to have the beam in focus all along its surface. The diameter of the grid is ~ 20 mm, so at a tilt of 70 degrees this corresponds to a height difference of over 18 mm from one edge to the other. Because of the image rotation with objective lens excitation, the tilt direction is actually from the lower left portion of the image down to wards the upper right.  Each cell is approximately 400 ┬Ám in diameter: there are 50 cells across a 20 mm diameter.  Only the most distant 2-3 mm is in focus.

The last image shows the same grid at the same tilt and working distance with dynamic focus correction on. Dynamic focus correction allows the SEM objective lens to have a sweep dependent focal length. Since tilting is always in the Y-direction, this focal length dependence is linked to the Y-deflection amplifier.

The image shows about 14 mm of the surface in focus, corresponding to a height difference of 13 mm across the length of the sample.

The purpose of this example is to show how the depth of field can be enhanced using dynamic focus correction.

No comments:

Post a Comment