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Comment by marcosdumay | Hacker News Reader

Yeah, this week I've restarted my scanning tunnel microscope that I'm failing to make work for years... The current one is a standard pair of long metal bars with the piezoelectric component on one end, with 2 screws, and a 3rd screw on the other end.

My problem is that it doesn't matter how I design the thing, either the screws offer too little precision so I can't help but to crush the tip into the sample every time, or too little travel distance so I can't help but crush the tip into the sample when adjusting the coarser screws near the tip. This is the kind of thing that looks like a non-problem on the web, because everybody just ignores this step.

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It sounds like you need to add a "stage" to help position your sample. Flexures are systems that bend to perform motion, and can do surprising things that you can't do with joined together machined pieces.

Here's an open source stage project using flexures that will likely help

https://openflexure.org/projects/blockstage/

Also, see Dan Gelbart's 18 video series about building prototypes

https://www.youtube.com/watch?v=xMP_AfiNlX4

I'm a fan of this project, but flexures are antithetical to an STM assembly. And STM needs very rigid components, the smallest vibration can interact with the height adjustment and push the tip into the sample.

But it's a great assembly for anything that doesn't have a feedback on the positioning.

Your STM is missing a proper approach mechanism. The vertical range of the piezo that is used for scanning will only be a few 100 nanometer. A screw is too coarse for that! Stick-slip mechanisms with a ramp (Besocke or Beatle-type STM https://www.researchgate.net/figure/1-Diagram-of-the-Besocke... or page 25 of https://www.bgc-jena.mpg.de/bgc-mdi/uploads/Main/MoffatDiplo...) are one solution. Even with such a mechanism, the 'approach' phase takes many minutes!

Adding another step (not a problem, just maybe a solution), a quick estimate says that if I place a few kHz signal at the sample, it will induce enough current at the tip to be detected by the preamp when the distance reaches the micrometers. That's the same range you want to stop the approximation, so it may be a nice proximity signal.

I've got to try this on my next attempt.

Sounds good!

In the future, if you'd like even more precise measurements, theoretically you could use 2 different frequencies or a reflected source, and look at the interference or superposition of the waves.

I'm by no means an expert in this, but I've heard that optical measurement (eg: laser + Michelson interferometer) could theoretically take you down to the nanometer range.

But it's easy to go overboard with this, haha.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-20-5-56...

https://iopscience.iop.org/article/10.1088/0957-0233/9/7/004

Wow, this sounds very ambitious!

Perhaps you could somehow attach the piezoelectric component or bars to a micrometer [1] which is designed for accurate and repeatable measurement?

[1] https://en.wikipedia.org/wiki/Micrometer

Yes, I could. I believe the most straight forward design would be 3 micrometers directly supporting the piezoelectric component, with no levers.

Yet, they are a bit expensive. I'm still not willing to budget all that, but I'm starting to consider it.

Digital micrometers are expensive, but you can get analog ones on AliExpress [1] or other places for around $10. Of course, the precision may not be as good as name-brand (eg: Mitutoyo) tools.

[1] https://www.aliexpress.com/wholesale?catId=0&initiative_id=S...

Yes! Thanks a lot.

Looks like the exact thing I need are micrometer heads, and some even come with nice threaded mounts.

Excellent, glad that you found this to be helpful. Good luck with your project!

One more idea, there exist worm drive micrometers which allow you to step down the linear movement per revolution even more:

https://www.global-optosigma.com/en_jp/Catalogs/pno/?from=pa...

If you have machining/fabrication skills, it might also be possible to buy a few worm gear sets and modify your micrometer to move really slowly but precisely.

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