Coarse approach

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I recently motorized the coarse approach mechanism on my STM. The coarse approach is done by first fully extending the Z-piezo (reasonably slowly) to search for the surface. If it doesn’t find the surface, it fully retracts, and a stepper motor, which drives the rear fine-pitch screw on the STM, takes a few steps to move the tip close to the sample. The distance it moves is somewhat less than the Z-piezo travel. After moving the motor, the Z-piezo fully extends again to search for the sample surface. This process repeats until the piezo finds the surface. When this happens, the Z-piezo retracts one more time, and the motor adjusts the piezo height above the sample so that, when the scanner extends one last time, it will find the sample at about the center of its travel range (just to give it some room to drift in either direction). Once the sample surface is found the Z-feedback loop switches on, and scanning begins.

Here are some images zooming in on a gold surface taken after doing a motorized coarse approach. Note the lack of craters! I’ve applied some lighting effects in Gwyddion to help make the edges of the atomic terraces more apparent. No luck resolving individual atoms on metals yet though… I believe the issue is acoustic noise. Time to build a sound-proof box…

This technique is called the “woodpecker” approach, and it prevents vibrations from the motor from crashing the tip, since only the Z-piezo brings the tip and sample into contact. Since the hard part is done by the piezo, the motor step size only needs to be smaller than the piezo range, although it’s nice to have a smaller step size to be able to center the scanner in its travel range before beginning the scan.

My scanner has about 700 nm vertical travel. Creating steps much smaller than this with a stepper motor is really no problem at all, especially since we don’t care about backlash. I used the 28byj-48 stepper motor to drive the rear fine-pitch screw on the STM. I milled a slot in the end of the screw’s plastic knob that fits over the motor shaft, while allowing the screw to move up and down. The 28byj-48 motor is very cheap and is geared down to about 2048 full steps/revolution. The fine screw moves 250 um/revolution, and the STM body’s lever reduction reduces the motion by a factor of about 20. This gives a step size of 250um/2048/20 = 6 nm.

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There are a few subtleties that came up here: when the Z-feedback is switched on, the integral term first must be initialized to the current Z-value. Otherwise, the feedback will cause a small jump in the Z-piezo when it’s switched on, which crashes the tip! Took me a while to figure out why I was still seeing craters in my scans! Another issue is thermal drift. The motor produces a substantial amount of heat, which can cause the scanner to drift out of range within minutes. The solution is simply to turn the motor off when it’s not moving, i.e. when the piezo is extending and when the scan starts. This means that we can’t use microstepping, which is I used a geared motor in the first place. I’m also running the motor on 3.3V rather than 5V to further reduce heating.

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8 thoughts on “Coarse approach

  1. hey dan! kar here, postdoc working on UHV and STM system. I and my friend are trying to construct an STM project based on your homemade STM. Hope we can do it.

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    • Hi Matthias,

      Looks great, nice job on the acoustic isolation! Since the resonant frequency of the structure is so low, I don’t think there’s any need for magnetic damping (and the way I implemented mine is really not optimal), as long as the scan head is rigid.

      I’m working on several upgrades and hopefully will post some updates soon.

      Cheers,
      Dan

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  2. Hi,

    Is the heat of the motor transferring to the scanner through the aluminium rods used to secure it or through the brass rotating end or it’s neither of them.
    In the first case, you could just secure the motor to some sort of frame that isn’t connected to the scanner head.

    Greetings
    Thomas

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    • Hi Thomas,
      The motor couples to a plastic knob on the screw, so it’s probably mostly transferred through the aluminum standoffs. I should have used plastic.
      The motor in my case does need to be mounted to the top part of the scan head, because that part can tilt.
      Cheers,
      Dan

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  3. Hi Dan,

    i`m quite impressed! It seems to work very nice and yet its very simple constructed! Are you going to use one or still three stepper motors in your upcoming design? Im kind of surprised, that the motor heating is becoming an issue, i wouldnt never expected that. I´m still planing to use three of them, so we`ll see if i can solve this problem sufficiently.
    Regarding the piezo drivers, i´m currently prefer to use some high voltage op amps in a bridge configuration (e.g. Apex PA240, Apex PA340 or Apex PA443 (dual)). This makes the whole story quite easy to handle and especially the PA443 offers very good noise characteristics.
    Due to some time demanding events at my phd thesis i wasnt able to find enough time to create a hackaday-project, but this will come.

    Greetings
    Matthias

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    • Hi Matthias,

      I’ll probably stick to 3 motors in the upcoming design, because they’re so cheap, and that design eliminates manual adjustment and tilting of the head.

      The PA240/PA340 have too much noise, but the PA443 actually looks pretty good! And the price isn’t too bad either. I hadn’t seen that one before, looks like a new product. The best thing out there is probably the PA95, but the PA443 isn’t far behind in terms of noise. Doing a quick estimate, looks like this will give about ~3 pm p-p noise at 10 kHz bandwidth for the tube I have, with 1 um Z travel. I’d say that’s good enough! I’ll probably go ahead and use this amp too, just to simplify the design.

      EDIT: whoops, I meant 3 pm, not fm.

      Cheers,
      Dan

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