Cameca stage

[Gseward]

John,

A follow up to our brief discussion on stage precision (and perhaps how it relates to doing stage coordinate to image position calibrations):

I quote from the Cameca SX100 product description 17th Jan 2008 version "A large travel (50mmx80mm) automated specimen stage featuring: -Micro stepping motors (minimum 0.1um)"  and "Closed loop optically encoded specimen positioning (+/_0.5um)"

On my instrument if I go to a relatively high mag (say 30um FOV) I can move the stage with the fine-adjust rollers (such that I see a shift in the electron image) without the stage coordinates updating (as observed in the roller window). Ergo, I conclude that this is consistent with positioning precision, as defined by the motors, better than 1um (probably 0.1um) but accuracy is only guaranteed at +/-0.5um because of the encoder limitations.

From my memory of my old SX50 that I was constantly fixing, the quadrature encoders have 4 bit-states per division on the encoder grating. I don't know what the grating spacing to um conversion is, but if we assume  1um,  it is likely that all 4 states need to change for an update of the position (in um) The changing of the bit states is how Cameca immediately knows if there is a stage movement problem - if the the encoder bits do not change in the correct order, an error has occurred.

But here is where the plot thickens: If I use peaksight to define a stage scan SE image with 0.2um spacing and uncheck the 'continuous' motion button, The image is acquired. Now I can't tell if x is moving in steps, and it may be continuous, but Y cannot be moving continuously. The Y stage position duly updates 1um every 5 lines in X, and the image, although a little distorted in Y, looks similar to the beamscan equivalent (see attached). Hence, it looks like Peaksight can move the motors in steps less than 1um. I speculate wildly that perhaps during ROM based mapping an absolute position is not being used? this would mean that the stage could be moved by one step of the motor, without any positional feedback required? or, the encoder has better than 1um precision?  Perhaps someone with more insight could clarify.

Anyway, not really of any consequence, except that this implies a stage map with 0.1um pixels is perfectly valid (for SE images at least), and very useful for stage/image calibration (assuming no distortions); if only the dwell could be less than 5ms!

Gareth

[Probeman]

But here is where the plot thickens: If I use peaksight to define a stage scan SE image with 0.2um spacing and uncheck the 'continuous' motion button, The image is acquired. Now I can't tell if x is moving in steps, and it may be continuous, but Y cannot be moving continuously. The Y stage position duly updates 1um every 5 lines in X, and the image, although a little distorted in Y, looks similar to the beamscan equivalent (see attached). Hence, it looks like Peaksight can move the motors in steps less than 1um. I speculate wildly that perhaps during ROM based mapping an absolute position is not being used? this would mean that the stage could be moved by one step of the motor, without any positional feedback required? or, the encoder has better than 1um precision?  Perhaps someone with more insight could clarify.

Hi Gareth,
The words in bold are what we need to test.  More specifically, how often does the X position get updated in non-continuous mode stage scanning. If one had a fluorescent sample, one could check if the motion is continuous.

My contention is that the glass linear quadrature encoders are limited to 1 um resolution in discrete stepping mode.  In continuous mode the firmware will interpolate...

[Gseward]

But like I say, Y CANNOT be moving continuously, can it?

I can look with a fluorescent sample, with a long dwell time, but less ambiguous would be to use an oscilloscope to look at the state of the encoder bits (this I used to do regularly on my SX50, but have thus far not needed to on my SX100). Or we could simply ask Cameca!

Gareth

edit: I realize a fluorescent sample won't work with a stage scan!

[Probeman]

but like I say, Y CANNOT be moving continuously, can it?

Exactly.  As you said, it only updated every 5 scan lines in Y, that is 1 um steps.

I can look with a fluorescent sample, with a long dwell time, but less ambiguous would be to use an oscilloscope to look at the state of the encoder bits (this I used to do regularly on my SX50, but have thus far not needed to on my SX100). Or we could simply ask Cameca!

I believe that Cameca has been asked, and 1 um is what I recall they state for discrete steps.  Only continuous mode can interpolate sub micron steps.

[Probeman]

But here is where the plot thickens: If I use peaksight to define a stage scan SE image with 0.2um spacing and uncheck the 'continuous' motion button, The image is acquired. Now I can't tell if x is moving in steps, and it may be continuous, but Y cannot be moving continuously. The Y stage position duly updates 1um every 5 lines in X, and the image, although a little distorted in Y, looks similar to the beamscan equivalent (see attached). Hence, it looks like Peaksight can move the motors in steps less than 1um. I speculate wildly that perhaps during ROM based mapping an absolute position is not being used? this would mean that the stage could be moved by one step of the motor, without any positional feedback required? or, the encoder has better than 1um precision?  Perhaps someone with more insight could clarify.

Hi Gareth,
The words in bold are what we need to test.  More specifically, how often does the X position get updated in non-continuous mode stage scanning. If one had a fluorescent sample, one could check if the motion is continuous.

My contention is that the glass linear quadrature encoders are limited to 1 um resolution in discrete stepping mode.  In continuous mode the firmware will interpolate...   I think we should move to 0.1 um encoders (using UV LED encoders?).
john

[Gseward]

but like I say, Y CANNOT be moving continuously, can it?

Exactly.  As you said, it only updated every 5 scan lines in Y, that is 1 um steps.

I can look with a fluorescent sample, with a long dwell time, but less ambiguous would be to use an oscilloscope to look at the state of the encoder bits (this I used to do regularly on my SX50, but have thus far not needed to on my SX100). Or we could simply ask Cameca!

I believe that Cameca has been asked, and 1 um is what I recall they state for discrete steps.  Only continuous mode can interpolate sub micron steps.

It only UPDATES the Y position (in um) every 5 lines in X, but Y is clearly moving in steps less than 1um, otherwise the image would be complete garbage. Unless Y is also continuous but the rotation that should be apparent in the image is too small to notice.

[Probeman]

but like I say, Y CANNOT be moving continuously, can it?

Exactly.  As you said, it only updated every 5 scan lines in Y, that is 1 um steps.

I can look with a fluorescent sample, with a long dwell time, but less ambiguous would be to use an oscilloscope to look at the state of the encoder bits (this I used to do regularly on my SX50, but have thus far not needed to on my SX100). Or we could simply ask Cameca!

I believe that Cameca has been asked, and 1 um is what I recall they state for discrete steps.  Only continuous mode can interpolate sub micron steps.

It only UPDATES the Y position (in um) every 5 lines in X, but Y is clearly moving in steps less than 1um, otherwise the image would be complete garbage. Unless Y is also continuous but the rotation that should be apparent in the image is too small to notice.

Interesting that we're seeing significant smearing effects in Y in the stage scan image you posted above, though not in the beam scan image.  Do you agree?   I think your scans make my point for me.

A test on a fluorescent sample would be most interesting...

[Gseward]

I'm inclined to agree in the sense that I always believed that 1um was the limit. I think it is most likely that Peaksight uses continuous motion, despite being told to do otherwise.

Interesting that using Interpretor(sic) 'stage rmove 1 0.1' will move the stage a fraction of a micron as will 'stage rmove 1 -0.1' however both move the stage in the same direction!

[Probeman]

Here is a stage cooling system we added to our Cameca SX100 stage.



We recently had to re-solder all the power resistors on the stage board as they had de-soldered themselves from running too hot for more than 10 years!

This row of fans just plugs into a power strip and works great, but does need to be unplugged when imaging at very high magnification.

If we were really smart we'd have a device that automatically reads the beam FOV and automatically turns it off when we image above 10x or so, and then turns it back on when below 10Kx (or so).