Gun shift

Purpose: Shift the electron beam sideways so that it comes down along the optical axis.
Importance: ESSENTIAL for minimizing movements between different spot sizes and for having the beam correctly along the optical axis for all spot sizes.
Method: Minimize spot displacement when spot size (that is focal length of C1 lens) is changed.

Procedure
The gun shift alignment consists of two steps :

• A first step in which spot 9 is centered with the beam deflection coils.
• A second step in which spot 3 is centered with the gun deflection coils.

The two steps are repeated until the change in gun shift is very small. In the gun shift procedure the spot-size dependent gun shift values for spots 3 and 9 are reset to zero (for proper alignment the spot-size dependent gun shift should therefore be done after the gun shift procedure).

Description
Gun alignment consists of two parts: gun tilt and gun shift. The gun tilt alignment makes sure that the electron beam enters the microscope (that is, the C1 - spot size - lens) parallel to the optical axis, while gun shift ensures that the beam goes through the center of the C1 lens.

When spot size 6 is used (or any spot size above that), the C1 lens is strong. Under these conditions all aberrations of the microscope system above it are demagnified by the lens (C1 works in the opposite way of the magnification system; instead of magnifying the image, it makes the image of the source - the electron gun - smaller). Thus the demagnification of the misalignment of the gun shift is much smaller for spot 9 than for spot 3, so spot 9 gives the reference ('no' gun shift) and spot 3 defines the gun shift itself.

These diagrams show schematically how the beam position would change as a function of spot number when spots 3 and 9 are used for gun shift alignment (left) and spots 1 and 11 (right). For spots 3 and 9 the overall shift is smaller. The remaining deviations are corrected by the spot-size dependent gun shift.

Page last modified on 11/19/01