Focus

When we image in SEM, we can change where the probe is focused using a Focus knob. The probe is most focused at the crossover point, or the location where the probe diameter is smallest. This plane is called the focal plane and is located a certain working distance from the pole piece.

When the image is out of focus, it appears blurry because the probe is larger than the features. The focal plane is not on the surface, and the working distance does not reflect the location of the sample.

When the image is in focus, features appear sharp because the probe is smaller than the features and can better detect edges. The working distance now tells us where the sample is located relative to the pole piece.

The image may be out of focus when the sample is above or below the focal plane because the beam continues to travel until it encounters a surface.

Aberrations in the column may cause astigmatism, or a non-circular beam cross section. Ideally, the beam should be conical with a circular cross section. If the stigmators are misaligned the image will appear stretched or smeared.

If only part of the image is blurry, these regions may be outside the depth of field. For very textured or sloped surfaces, the beam cross section may be too large in some areas even if everything else is aligned.