Design and Construction
Version 28 (Andrew Starr, 09/07/2012 09:07 pm)
1 | 9 | Andrew Starr | h1. Guiding principles of this STM design |
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2 | 1 | Andrew Starr | |
3 | 13 | Andrew Starr | * Minimise the effects of external vibrations by minimising size and mass |
4 | 13 | Andrew Starr | * Minimise the effects of temperature variation by matching material coefficients of expansion as closely as possible |
5 | 13 | Andrew Starr | * Minimise the effects of electronic noise by locating the sensitive circuitry as close as possible to the signal source |
6 | 13 | Andrew Starr | * Maximise instrument flexibility and ease of use by doing as much of the signal processing and control as possible in the digital domain |
7 | 13 | Andrew Starr | |
8 | 13 | Andrew Starr | Also |
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10 | 13 | Andrew Starr | * Use open source tools for all development |
11 | 13 | Andrew Starr | * Source all specialised materials and components from readily-available online sources |
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13 | 9 | Andrew Starr | h1. Main building blocks of the STM |
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15 | 4 | Andrew Starr | h2. High frequency vibration isolator |
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17 | 14 | Andrew Starr | The STM relies on the precise control of the probe-sample gap. Any external vibration has the potential to disrupt this, so efforts must be made to isolate the instrument. Low-frequency vibration isolation is generally done with large pneumatic or elastomer damping systems. I'm hoping to be able to avoid these by making the instrument as compact and stiff as possible, thereby maximising its resonance frequency (and therefore its response to low frequency vibrations). To isolate from high frequencies, I have chosen the 'stacked plate' approach, as described and characterised in *"Low- and high-frequency vibration isolation for scanning probe microscopy" (A I Oliva, M Aguilar and Victor Sosa in _Measurement Science and Technology_, 9 (1998) 383-390)* |
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19 | 18 | Andrew Starr | The drawings for the isolator stack plates can be found {{dmsff(15,here.)}} The plates are each separated with 3 pieces of 5mm long, 5mm diameter pieces of viton rubber as described in the paper. I used an off-the-shelf viton o-ring from a local supplier and cut pieces off, securing with cyanoacrylate glue: |
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21 | 20 | Andrew Starr | !Base_disc_with_viton_spacers_ODE.jpg! |
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23 | 21 | Andrew Starr | The assembled stack: |
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25 | 1 | Andrew Starr | !Assembled_HF_vibration_isolator_ODE.jpg! |
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27 | 24 | Andrew Starr | I haven't yet characterised the attenuation of the isolator stack - more on this after I've done some testing. |
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29 | 25 | Andrew Starr | h2. Piezotube |
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31 | 28 | Andrew Starr | The piezotube was purchased from EBL Products Inc. The material is EBL2 (PZT-5A). The dimensions and electrode configuration are as below: |
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33 | 26 | Andrew Starr | !Piezotube_diagram.jpg! |
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35 | 25 | Andrew Starr | h2. Sample mount |
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37 | 25 | Andrew Starr | h2. Coarse positioner mechanism |
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39 | 25 | Andrew Starr | h2. Probe holder |
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41 | 4 | Andrew Starr | h2. Piezotube and coarse positioner support |
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43 | 4 | Andrew Starr | h2. Bias voltage module |
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45 | 4 | Andrew Starr | h2. Tunnelling current amplifier |
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47 | 4 | Andrew Starr | h2. Control module |