Gun Alignment Issues

[Probeman]

The tungsten filament on our SX100 blew overnight and so this morning I went in to replace it. After replacing it and pumping down I decided to bring up the last column conditions just to get started with the gun alignment. The filament had failed on a metallurgical sample which we were using 20 keV, so I loaded that column condition since it was the most recent one saved.

I saw about 5 nA reading on the picoammeter, so I then proceeded to use the Align Gun button in the Cameca PeakSight softwtare. It went through its usual cycle, but at the end gave me a gun alignment failed message, which is quite unusual.

So I checked the filament saturation, increased the beam current and tried the gun align again, but each time it ended with a gun alignment failed message (except for one setting which I will get to below).

So then I'm thinking what is different about this and then I thought, OK I'll try 15 keV. And then it aligned fine!  And the alignment was actually quite good, about +/-200 to 300 for the high and around +/-30 to 50 for the low.

The only time the gun align worked at 20 keV was when I ran the saturation up to 240 or so, though the actual saturation was around 205. And when it finally worked at this high saturation, the 20 keV gun alignment was essentially the same as it was at 15 keV.

I don't usually align the gun at 20 keV (though I know I have in the past), but does anyone have any suggestions or insight into why the gun align works at 15 keV, but fails at 20 keV?

[sem-geologist]

While Gun alignment for 20kV was running, had you seen the beam XY_Hi and XY_low spots converging to the center of the screen? (that is x, y alignment coils are scanning and image is created with faraday cup readings, while for XY_hi and XY_low there are different C1 and C2 set).

We normally run our tips at 15kV with heat bellow 200, but for first alignment (or for any later realignment) we increase the heat much above 200 to get nice shape for "Gun scan high" (it should be rather oval shape with red (if colored image) intensity at middle with not-distorted edges around going smoothly through intensities at edges to 0). We align at that, and then reduce the heat below 200 to extend the tip life. To be sure that there is no changes in geometry, we run Gun scan hi again to make sure that that "dragon-like" shape of beam is at the same spot where we saw the nice oval (beam scan low is more important for good spatial resolution). If it is not then there are problems in physical tip alignment or column is much too much contaminated. We had experienced that on our SX100 the alignment is different for 20 and 15kV, and same as You we need to turn heat up for alignment. These all "how much You need to push the heat up to get a nice shape" also depends from emission current (in drop down list in GUI You can set it between 50 and 100). It affects how beam will look at set C1 and C2 positions on Faraday cup, as these condenser values for alignment are fixed, but outcome will depend from I_emission. We use normally lower I_emission to preserve the lifespan of the tungsten filament. The geometry of beam can be a bit different depending from I_e, thus keeping I_emission same for use and alignment, and increasing heat for alignment and then decreasing for using the beam is the most proper way IMHO. But I could be wrong, I had not designed that.