Federica STM

Upload here all the relevant data and documentation about the large STM with XY stage and scanner embedded in the Z-motor. STM main purpose: to study nano-meso-micron-structured oxides/superconductors/heterostructures)

Here is a scheme of the electrical connections as they are at the end of the insert. Last update: 29-01-2007

The Z-motor is a piezoelectric actuated walker, based on the “slip-stick” concept. It consists of:

1. The piezo “legs”: 6 shear piezo stacks (PIC255, from PI Ceramics). Each stack is made of 4 active layers, max. applicable voltage is +/-220V, bipolar (i.e. max 440V). The max. displacement, at RT, is 2μm.
2. The slider: an exagonal hollow sapphire prism. Inside the prism it is placed the scanner tube (from EBL Products), with a macor extension piece.
3. The spring exherting the adjustable normal force on the piezo legs. At this moment the spring is made of Titanium. A ruby ball placed between the spring and the top beam plate assures that the normal force is applied in the center of the motor.
   

The prism is translated by the set of symmetrically arranged piezo “legs”; the “legs” can displace the prism up or down by sticking and slipping against the prism surface. Such a “walker” can move with steps as small as few tenths of nanometers over a distance of several mm.

Performance and reliability of the motor depend CRITICALLY on the following factors:

1. Temperature (the piezoelectric coefficient is decreases by a factor of 5 at 4K).
2. Friction: the “slip-stick” will work completely different (or won't work) if the sliding surfaces (of the “legs” and of the prism) are dirty and contaminated. Keep these surfaces clean (see after)!
3. Normal force: at low temperatures the inevitable relative changes of the termal expansions of the different parts changes the normal force. The Z-motor has been build in such a way that thermal drifts are compensated as much as possible.
   

In conclusion, our experience is that small amounts of frozen contaminants and loss of piezoelectric coefficient at low-T are the main reasons for Z-motor bad performance.
Working settings (as guidelines) are:

1. Pulse shape: parabola
2. Pulse width: 100μs
3. Amplitude: 200V (depending on the settings for the RHK DAC1 or DAC2: 1μm, bipolar allowed)
4. Retract steps: high-T=200, low-T=20
5. Approach steps: depending on performance: 2 or 10.
6. Z-HV drive: Gain=20, polarity-switch=down, EXT. (default settings are written on the front panel)

Problems with the motor performance can arise from:

1. Bad maintanance (diry surfaces, see after). Solution: keep surfaces clean (see after).
2. VTI contamination due to leaking gases which freeze on the surfaces. Solution: hunt down the leak
3. Broken connections or shorts. Solution: check for problems with the electrical connections, by checking for shorts within the piezos of the motor, within the piezos and the system ground and check the piezo capacitance (5.6 nF at RT). Be aware that shorted piezos will blow the piezo drive electronics (HV amplifiers)
4. Motor is too loose. Solution: tighten the screws of the Ti-spring (advised force: 40-60 gr. 20-30 gr is too loose.). Note that after several cooldowns the motor will become progressively more loose due to the thermal cycles.

Tips:

1. At low temperatures (below 100K) do not retract more than 20 or 50 steps, or you will have to wait a long time (even 1 hr) to get back in tunneling.
2. Zip the motor up and down a few times while cooling.
3. While cooling, the sample-tip gap will increase, so you can leave them safely very close (they won't crash).
4. While warming, the sample-tip gap will decrease and finally they will crash. Retract many steps (at least 500) when warming up.
5. If the motor “jams” at low temperatures, it can be moved again by “creatively” adjusting the parameters. Often setting the pulse width to 150 μs helps more than increasing the voltage. You can also increase the number of steps to approach from 2 to 10 or more.

Cleaning: it needs to be done only when the motor module has been opened for some reason or if air condensed on it when accidentally taking out (to air) the STM being cold (so don't do it ) Clean the sliding surfaces by rubbing gently with a cotton tip: 1.demi-water 2.isopropanol. Flush the sliding parts with dry nitrogen. Let dry well before assemblying. NEVER USE ACETON.

ALWAYS take out the STM from the cryostat after warming it up in the VTI neck and use helium overpressure to avoid air getting into the VTI.

ALWAYS CHECK THE CHECK LISTS HERE UNDER! (even if experienced and even if you've done this many times, you might forget the little thing that will screw up your measurement, your time and the one of the guy who fabricated your sample). If you find a better procedure (where “better” have to be scientifically proved) please update this page!

GENERAL RULE: USE UHV GLOVES!

1. Glue sample on sample holder with Ag-paste.
2. Make electrical connection on surface of sample with a small Ag-paste droplet.
3. Avoid excessive amounts of Ag-paste: the solvent contaminates the sample surface.
4. Switch off RHK box, if STM is connected, for SAFETY.
5. Insert the sample holder in the leaf-spring mechanism.
6. Remove sample: just remove.

1. Mount a cut tip in the tip holder (the way to do this differ depending on the tip holders).
2. Push VERY gently the top beam plate of the top piezos of the Z-motor, with two fingers, symmetrically without pushing on the spring.
3. Insert VERY gently the tip holder under the leaf springs without exerting excessive force on the piezo tube!! Use tweezers.
4. Remove: again just remove the tip holder, GENTLY with a tweezer. VERY IMPORTANT here to GENTLY push the top beam plate, or the Z-motor spring might deform. USE GLOVES when pushing the plate.

1. Set VTI-temperature to 290K (PID sensor on VTI sensor). Warm up VTI entrance with heater gun for ~10 mins, otherwise the helium gas will be too cold.
2. Lay down STM insert on the table.
3. Do SANITY check: (Z-motor) piezo connector connected? capacitance OK? Piezos NOT shorting?
4. Mount tip holder on the scanner FIRST (no force on the scanner tube!).
5. Connect COARSE Jaeger connector.
6. Switch on RHK controller.
7. Zip motor (fast-in and fast out) few times (to clean the sliding surfaces and self align the motor). Careful not to crash the tip.
8. Switch off RHK control.
9. Mount the sample.
10. Connect preamplifier, bias and SCAN Jaeger connector.
11. Switch on RHK controller and perform an approach.
12. Retract 200 steps, switch off RHK.
13. Disconnect cables, open speedy valve.
14. Pressurize VTI with helium gas, while keeping the VTI entrance warm with a heater gun.
15. Open VTI, quickly but carefully insert STM on the top of the VTI.
16. Close sliding seal, connect silicon tube to the speedy valve.
17. Open recovery valve at the 2nd valve board.
18. Keep helium flowing and keep warming up with heater gun for 20 mins.
19. Shut off speedy valve.
20. Close 2nd recovery.
21. Slowly slide STM to the bottom of the VTI.
22. Tighten sliding seal.
23. Pump/flush 4 times the VTI EVERYTIME before cooling down. (HIGH VOLTAGES OFF!)
24. OPEN MAIN RECOVERY (above the computer desk).
25. Connect cables, make an approach (VTI pressure always above RED!)
26. Cool down.

1. Set VTI-temperature at 290K (PID sensor on VTI sensor). Warm up VTI entry with heater gun.
2. Pressurize VTI with helium.
3. Disconnect cables and lift insert till top, where black marker appears.
4. When VTI-neck is warm enough (after about 10 mins), open sliding seal and remove STM insert.
5. Close VTI.
6. OPEN RECOVERY.