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CAFM

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Benjamin Voelcker posted on Tue, Oct 18 2011 11:09 AM

I am new to CAFM as i was reading through the manual it said that i need to check the signal using a 10 M-ohm resistor? Is this step mandatory? Do you have to set up a bias reference of some-sort before you perform a I/V scan on a material?

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Answered (Verified) replied on Tue, Oct 18 2011 11:18 AM
Verified by Benjamin Voelcker

Benjamin,

That check is not mandatory but a useful step when troubleshooting your system. Not sure what you exactly refer to by "bias reference". One of the biggest variables in CAFM is the tip, or more precisely the change in conductivity of the tip. There has bee quite some discussion on this board in the past on that matter that a search should reveal.

Stefan

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Answered (Verified) replied on Tue, Oct 18 2011 11:18 AM
Verified by Benjamin Voelcker

Benjamin,

That check is not mandatory but a useful step when troubleshooting your system. Not sure what you exactly refer to by "bias reference". One of the biggest variables in CAFM is the tip, or more precisely the change in conductivity of the tip. There has bee quite some discussion on this board in the past on that matter that a search should reveal.

Stefan

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Thank you for the suggestion. I am very new to this method. I will have to research some more. If there are any key terms people know of that has been helpful for them. It seems like a trail and error just getting that connection right for the samples.

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Also what method is the best for attaching a test sample to the metal disk? I am currently using an Innova system and we used double sided tape does this insulate the material from being conductive?

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replied on Tue, Oct 18 2011 3:16 PM

If possible I use silver epoxi or similar, indium works too to achieve proper conductivity from sample to metal puck. If already mounted with tape you may try to paint some silver over the edges.

The Innova has a slight offset in the Bias voltage so 0V may not be 0V. You may want to measure that in order to be able to correct later if desired.

Good luck,

 

Stefan

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Hi Benjamin,

I believe the purpose of 10 MOhm resistor is to restrict the current flowing through the tip-sample gap. The breakdown of the gap however depends not on the electric current but on the field in the gap and can take place at very low voltages. If you do calculations of the current densities through the gap that will most probably show the effect of this “high” protection resistance negligibly small. What you might like to do at your measurements is to reduce the upper limit for the current at your power supply, and that’s where your external resistance could help.

The danger of the high current density is the modification of the tip and/or sample. Following basics of metrology, you might like to confirm the reproducibility of your measurements.
Please refer to the permalinks on this subject at MIAWiki:
http://confocal-manawatu.pbworks.com/w/page/46002654/Electrical%20Measurements%20Reproducibility%20AFM

Hope that helped.

Cheers,
Dmitry
MIAWiki, Wiki for Mass Collaboration

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replied on Wed, Oct 19 2011 4:33 PM

Hi Dmitry,

The 10 MOhm resistor is as I pointed out for testing purposes as it creates a clearly defined environment, i.e. a 10MOhm gap. The resistor is not intended to be used in conjunction with a real sample measurement.

Best, Stefan

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Oops! Embarrassed

Thank you Stefan!!

Benjamin, sorry for irrelevant comment.

Kind regards,

Dmitry

 

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No matter how hard i try i cannot get the probe to read the linear line on the 10M ohm. I even tried reengaging it. I scanned over the spot in contact mode a few times. It still shows a harmonic signal as the sample voltage goes from -5v to 5v. Any suggestions Stefan?

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replied on Fri, Oct 21 2011 11:31 AM

Hi Benjamin,

I have no idea what you mean by "harmonic signal".

 

I am not sure what instrument you have and when it got installed and what tip you are trying but is there a chance you can ask the person who got trained during install to help you out? If that is not a choice you may want to contact Bruker AFM tech support. They may be able to setup a short remote working session with you on your instrument.

In the meantime: Get a regular through-hole 1-10MOhm resisitor (not a surface mount type). Carefully wedge that resistor in between the metallic tip mount and the sample puck, i.e. make sure that there is a connection from the sample puck to one leg of the resistor and from the other leg to the conductive part of the tip holder. You will not need an AFM tip for that! What you have now is a tip-sample gap defined by whatever resistor you place in-between. Now you can sweep your voltage, select to appropriate range on your I/V converter, and see if you get the proper response for the resistor. If that works, you have shown that the electronics and software on your system are working. The unknown going forward  are now  i) your sample and ii) your tip.

Keep us posted,

Stefan

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Thank you for post and interest Stefan. I have moved on to a sample. The tip I am using seems to be a standard for CAFM it is the SCM-PIC. I seem to be getting some sort of imagining now. If you have any tips let me know!

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I also guess i never specified my device either. I am using the Innova. 

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