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projects:stm:log [2025/06/15 02:56] – [Day 2025-06-14 (rahix, k8ik, hugo, q3k, ln, zoe)] rahixprojects:stm:log [2025/06/16 07:51] (current) – [Day 2025-06-15 (rahix)] rahix
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 ====== Lab Notes for the Scanning Tunneling Microscope ====== ====== Lab Notes for the Scanning Tunneling Microscope ======
  
 +==== Day 2025-06-15 (rahix) ====
 +
 +Tried reproducing the results from yesterday -> achieving tunneling was quickly done.  The goal today is to tune the control loop better.
 +
 +Achieved control for short periods of time, but the control behavior looks way too slow:
 +
 +{{:projects:stm:control-2025-06-15_17-06.png?400|}}
 +
 +Assuming that the proportional control is not doing its job right now.
 +
 +Added back an ''R8'' of 100k to shut up the I-controller, to just look at the P-controller.  It shows its familiar ringing when giving a factor using the potentiometer.  This even couples back into the probe current, as can be seen below (at this time, the tip is very far away):
 +
 +{{:projects:stm:p-ringing-2025-06-15_19-01.png?400|}}
 +
 +Frequency of the ringing is 1.47kHz.
 +
 +Added another 1µF capacitor across C7. **This gets rid of the ringing across the whole range of the ''P'' potentiometer.**
 +
 +With this, we are now able to see actual stable control over long periods of time with fast response from the control loop:
 +
 +{{:projects:stm:control-2025-06-15_20-11.png?400|}}
 +{{:projects:stm:control-2025-06-15_20-12.png?400|}}
 +
 +You can also see the piezo sweep using the I-controller until it finds the tunneling point.
 +
 +{{:projects:stm:sweep-then-find-2025-06-15_20-10.png?400}}
 +
 +What's still a bit unclear is how there can be situations like in the image above where tunneling stops again as the Z piezo moves even closer?
 +
 +Part of a very stable tunneling over a very long period today:
 +
 +{{:projects:stm:2025-06-15_20-23.png?400|}}
 +
 +During mechanical adjustment, the controller now sweeps until it finds something (left side of the picture below).  On the right, it has gone into control.
 +
 +{{:projects:stm:2025-06-15_20-28.png?400|}}
 +
 +Settings during the experiments above:
 +
 +  * ''Probenvorspannung'': Maximum
 +  * ''Sollstrom'': 0.322V (= 322pA)
 +  * ''I'': Minimum
 +  * ''P'': Maximum
 +
 +Now changing the settings somewhat in hopes of getting the control loop more tight:
 +
 +  * ''Probenvorspannung'': Maximum
 +  * ''Sollstrom'': 0.144V (= 144pA)
 +  * ''I'': 9 turns up (out of 10)
 +  * ''P'': 9 1/8 turns up (out of 10)
 +
 +You can see the faster I-response at least:
 +
 +{{:projects:stm:2025-06-15_20-50.png?400|}}
 +
 +It seems the noise has increased since yesterday.  We don't have values from before, but right now, seeing ~80mV of noise overlayed over the tunneling current signal.
 +
 +After some time, the control loop settled down quite a bit, assumingly because process disturbances decreased for some unknown reason:
 +
 +{{:projects:stm:2025-06-15_20-56.png?400|}}
 +{{:projects:stm:2025-06-15_21-00.png?400|}}
 +{{:projects:stm:2025-06-15_21-01.png?400|}}
 +
 +Managed to increase the setpoint current to 456pA:
 +
 +{{:projects:stm:2025-06-15_21-05_1.png?400|}}
 +
 +Finally tried putting the lid onto the STM for noise shielding, but I think I crashed the tip way too hard during the process as I couldn't achieve stable tunneling again afterwards...
 +
 +Should create a new tip next time and then retry more gently.  Also X/Y control is probably a good next step.
 ==== Day 2025-06-14 (rahix, k8ik, hugo, q3k, ln, zoe) ==== ==== Day 2025-06-14 (rahix, k8ik, hugo, q3k, ln, zoe) ====