MINI VOGEL MOUNT

The cathode’s mount design has a significant impact on performance. The design must be simple, durable and precise. It must resist any movements of the crystal, despite the high operating temperatures, yet be easy to install and align. We feel we employ the best mount design in the industry with our Mini Vogel Mount.

 

In 1988, FEI of Hillsboro, Oregon introduced the Mini Vogel Mount (MVM) to provide the benefits of the original Vogel mount in a smaller, simpler, and more elegant design. Twin posts are rigidly fixed in a thick ceramic base, and bent towards the center in an inverted ‘V’. The posts are made of a molybdenum-rhenium alloy that maintains a high modulus of elasticity even at high temperatures. The posts are spread slightly during assembly to allow placement of small pyrolytic graphite (PG) blocks between the crystal and posts. The blocks act as resistive heaters, and help thermally isolate the hot crystal from the highly conductive posts. When the compressive force of the posts is released, the crystal is held with strength and precision. The clamping force of the posts will remain near 5,000 psi for the life of the cathode.

 

The structure of the MVM is amazingly robust, sustaining reasonable impact without deviating from geometric specifications. Because the graphite pads shield evaporation of the crystal in the direction of the clamping force, the emitter crystal can be fully utilized without degradation of the mount. Structural failure of the MVM is not a concern when the cathode is operated within the correct temperature and pressure range. Typically, the beam stability of the Mini Vogel Mount cathode exceeds the specification of the system in which it runs.

 

The use of rhenium shunts in conjunction with the MVM was inherited from FEI. Since many SEMs and their electronics were originally designed around a tungsten hairpin filament, the resistance change with temperature of the PG used in the MVM is inverted. The resistance of the 0.001” thick shunt enables the cathode to better match the temperature and resistance curve of a tungsten filament. In the shunted version, the current bypasses the outer PG block by taking the path of

least resistance through the shunt. All of the heating is generated by the inner block. The design takes advantage of the anisotropic properties of PG by orienting the poor thermal conduction plane in the direction between the shunt and the Mo/Re post.

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