Scanning Tunnelling Microscope

h1. Guiding principles of this STM design

* Minimise the effects of external vibrations by minimising size and mass

* Minimise the effects of temperature variation by matching material coefficients of expansion as closely as possible

* Minimise the effects of electronic noise by locating the sensitive circuitry as close as possible to the signal source

* Maximise instrument flexibility and ease of use by doing as much of the signal processing and control as possible in the digital domain

Also

* Use open source tools for all development

* Source all specialised materials and components from readily-available online sources

h1. Main building blocks of the STM

h2. High frequency vibration isolator

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)*

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:

!Base_disc_with_viton_spacers_ODE.jpg!

The assembled stack:

!Assembled_HF_vibration_isolator_ODE.jpg!

I haven't yet characterised the attenuation of the isolator stack - more on this after I've done some testing.

h2. Piezotube

The piezotube was purchased from EBL Products Inc. The material is EBL #2 (PZT-5A). The dimensions and electrode configuration are as below:

!Piezotube_diagram.jpg!

h2. Sample mount

h2. Coarse positioner mechanism

h2. Probe holder

h2. Piezotube and coarse positioner support

h2. Bias voltage module

h2. Tunnelling current amplifier

h2. Control module