Zeiss STEM Detector Deflection Element Design

Table of Contents

Roger Robbins
3/11/2009

Purpose

The Zeiss Supra 40 Scanning Electron Microscope’s Scanning Transmission Microscope sensor head was destroyed during a routine extraction event several months ago. The accident occurred because the detector head caught on the SEM stage during the extraction and was pulled off the extender rod.

We recently received a repaired sensor and realized that the accident could easily happen again due to the design of the detector housing on the end of the long copper extender rod since this operation is partially blind. This paper describes the design of a simple physical deflector cone that prevents snag type accidents.

Design Problem

The STEM detector head is a right rectangular parallelepiped shape mounted at the end of a long copper extender rod used to position the sensor under the STEM sample holder which mounts on the SEM stage. (Figure 1)

Figure 1. STEM detector head mounted at the end of a long copper extender rod. Note the rectangular face of the detector head that could catch on the stage during extraction.

The STEM detector housing has a rectangular face at the connection with the copper rod that could catch on the stage and potentially force the detector head off of the copper rod if enough force were put on the rod during extraction. Since this contact area is outside the field of view of the SEM’s video camera, the user would not know that there was a potential stage collision problem during extraction.

Design Solution

To alleviate the potential destruction due to a collision of the STEM head with the stage during extraction, the following transition piece was
designed by Roger Robbins and constructed by our shop machinist, Billy Raulston. The protection concept was to make a conical transition piece to allow the STEM head to slide over any stage obstruction without pulling the head off the rod. However, the detector connection with the copper rod is so delicate, that we decided not to remove it to affix the cone. Therefore the cone was constructed with a slot and filler piece as shown in Figure 2.
This design allows us to fit the conical piece over the rod and fasten it with set screws fitting into conical pits in the filler wedge so that the assembly is firmly clamped to the rod without having to disassemble the detector (Figure 3). In order to protect the detector from collisions with the stage in the insertion mode, we cut the circular portion of the cone shoulder flush with the bottom of the rectangular STEM head housing (Figure 4).

Figure 2. Conical transition piece design.
Figure 3. This photo shows how the transition piece fits on the copper rod at the connection with the STEM detector head.
Figure 4. This photo shows the flush cut of the conical bottom to avoid a potential forward moving snag with the stage.

If the wires are pinched between the conical transition piece and the lip of a flange, the wires will merely deflect the copper rod enough to pass through the pinch point without any change to the copper rod.

Conclusion

We now have some degree of physical protection from a snag – collision with the SEM stage during insertion and retraction of the STEM detector from the stage chamber.