Omicron STM-1

[P.C. Snijders]

Hi,

I have an Omicron room temperature STM-1 (horizontal tip, sample biased, tip grounded).

When I use the auto approach, the tip always ends up as if it is crashed (fully retracted "in the red", and the scope tells me the current is maximum).

Going one (!) coarse step backward, the tip can not see the surface anymore (fully extended "in the yellow" and the scope does not see a tunneling current).

So the tip never ends up in tunneling contact with a moderately extended piezo. It's either crashed or can't find the surface.

After going one (largest) coarse step backwards, I can manually approaching by taking smaller coarse steps one by one, and checking (going "forward") whether I can find the surface.

Most of the times this results in the exact same thing, even with the smallest coarse step size that my dial allows me to use: the tip never finds the surface, and at some point after one smallest coarse step size it's crashed.

I had a very useful extensive discussion with Omicron.

Based on that I tested boards. No luck.

I exchanged the scala controller with another controller from colleagues. No luck.

I checked the voltages going to the z-piezo - they are okay (+ 138 and -127 V). I checked the piezo+cable capacitance: 3 or 4 nF. That's normal.

During a very slow auto approach I checked whether the voltage to the piezo changes as it should be (whether ramps). It did.

In vacuum everything seems normal, outside of vacuum everything seems normal, but it does not work.

The only thing I can not exchange is the pre-amp cable as Omicron does not have one available to loan me.

The symptoms are consistent with the piezo not changing length despite the correct piezo voltages being sent to them. However, the measured capacitance is correct.

According to the manual, "the auto approach runs 50-200 nm per step". I presume that means that a coarse step is 50-200 nm. This makes me thing that the change in length of the piezo will be about 500 nm max. That means it's optically not possible to see this (if a microscope could be set up close enough to have a look).

I would welcome any helpful suggestions as to what could be the cause, or how to find out whether the tip actually moves upon application of piezo voltages.

THanks,

Paul

[Petra Reinke]

Paul,

is there any indication that the feedback loop actually "thinks" it is in contact already? Meaning it does detect a current from somewhere which is not due to tunneling but a shortcircuit?

best, Petra

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Petra Reinke

Assoc. Professor

Department of Materials Science and Engineering

Director Engineering Physics Graduate Program

University of Virginia

phone: 434 924 7203

email: petra....@virginia.edu 

http://www.virginia.edu/ms/faculty/reinke.html

[Chen Wang]

Hi all,

I've been struggling with a problem that sounds eerily familiar to the one described in the original post. The only difference is that in our case, this issue will occasionally go away and we can get a scan.... making troubles even more difficult to diagnose.

A few questions for the OP:

• When you refer to piezo voltages, are you talking about those that are sent from the piezo driver card or are you somehow measuring inside the chamber?
• Can you explain how you are checking the piezo-cable capacitance?
• Were you able to resolve this issue at all?

Petra mentioned that there could be a short circuit that misreads as tunneling. The only way I could think to test this was to try to approach different areas of the sample. In many difference x,y positions, the same problem occurred. To test z, I tried approaching the nut atop the ceramic front plate, and again, same problem. Can you think of anything else?

Thanks in advance to any help people can offer.

-- Chen

[Yc Xie]

Hi,

Have you resolved your problem?

Did you check the tunnelling current when the tip is crashed on the surface using a oscilloscope? Does the tunnelling current go to the maximum when the tip is crashed? 

I met problem with tip auto approach before on the Omicron VT STM. Our tip is always crashed on the sample surface,i.e., no tunnelling current detected. The problem is with the IVC (current voltage converter in the UHV). I have a photo of the IVC.

It seems that all the voltage from the controller and the piezo are ok. The problem may be inside the UHV chamber.

[P.C. Snijders]

Yes, it was solved. There were multiple problems, but not the IVC.

I sent a summary to the group Jan. 11.

Paul

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[Repairman]

Hi. I realize it's been a few years since this post, but I saw your picture and wanted to reach out to you.

My team is working with an Omicron VT STM. We have an IVC similar (if not the exact same model) that's similar to the one that's shown in your image. We are having mechanical issues with our IVC, and we were looking to build a replacement. I was wondering if you might know what components were used in the IVC, or if you might know of a place that sells the IVCs? Our STM manual shows a block diagram of the IVC and how it's arranged, but it doesn't show what components were used. 

[Y Xie]

Hi, This is quite long time ago. As far as I can remember, the IVC was broken for our STM and we bought a new one from Omicron. My PhD supervisor replaced the broken one with the new one, and then everything went to normal.

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[Repairman]

Thank you for your reply. Unfortunately, my coworker, the one who uses the STM, contacted Omicron about the issue. They said that they don't sell it anymore, so either we buy one from somewhere else (which comes with its own risks), or we build our replacement, which would be a lot cheaper than buying a new one. That's why I'm trying to explore options. We have two IVCs, and one of them broke, though we did attempt to fix it (simple mechanical error, accidentally pulled a pin off due to excessive force). The other one is in use. We could open up the 'broken' one and see what parts are being used for it, but if that approach doesn't go anywhere, then we would be down one IVC.