3.3. Sample Stage

The MBX microprobe has a transmitted light stage installed and can accommodate standard polished thin sections or 1" polished round sections. X, Y, and Z positioning of samples is accomplished by moving the stage with stepper motors. The stage can be moved 20 mm in the X-direction (left/right) and 50 mm in the Y-direction (in/out). Movement in the Z-direction is limited to avoid striking the optical system. Each stepper-motor step represents 1 µm of stage travel. Consequently, stage motion is reproducible to about 1 µm.

The newest Cameca microprobes (SX-50, SX-100) use optically encoded light bars to control stage (and spectrometer) movement. The direction of movement does not matter, since the positions are located using absolution positions on the encoders. The stage and spectrometers are moved using DC motors rather than AC stepper motors, and movement is very rapid and precise.

Figure 3.3a. Sample stage. The technician is holding the transmitted light assembly prior to installation.

 

Sample changing is accomplished through an airlock to avoid having to vent the entire column. The airlock cover plate is removed and a sample change unit attached. This changer is evacuated to a pressure of about 2 Torr, whereupon the airlock door can be opened and the sample inserted into a narrow forked holder (Figure 3.3b) that allows a transmitted light source to be positioned beneath them (Figure 3.3c). The transmitter light assembly consists of a right angle mirror that is positioned beneath the optical microscope and a horizontally oriented light source which is located outside of the evacuated electron column.

 

Figure 3.3b. Forked sample holder.

Figure 3.3c. Positioning the transmitter light assembly beneath the sample holder.

 

The worm gears on the stage drives and spectrometers (see below) are subject to mechanical "slop", even with very high tolerance machining. As a result, peak positions appear to shift when approached from different directions (up or down the drive screw) and stage positions are not reproducible. The MBX microprobe analytical programs use a "backlash" to compensate for this slop in which all peaks and stage positions bypassed by a certain number of steps and always approached from the same direction.