Specific details for STM design

The active components of the STM are the two electrodes, a sharp metallic probe, and a conductive (or semiconductive) sample, one of which is moved relative to the other in a x-y grid utilizing piezoelectric tubes, disks, or rods for precise positioning with angstrom accuracy. The STM tip (electrode 1) is typically tungsten (UHV conditions) or Pt/lr (ambient conditions), although other noble metal tips have been used. The second electrode is the conductive or semiconductive sample to be analyzed or manipulated. 

This section will address specific design considerations from the subset of 

The choice of materials can be broken down into those that are used to make mechanical and electrical connections to the STM from ambient temperature and those used at low temperature for the STM itself and connections to it. Since the sample and tip need to be cooled, then stabily held at that fixed depressed temperature, the STM has to be placed in a controlled cryogenic environment. A superinsulated Dewar is used to house the STM to eliminate vibrations due to boiling liquid nitrogen used in the heat shield of glass Dewars. The STM can be immersed directly in the cryo-fluid, enclosed in a separate vacuum can suspended in the fluid, or transferred from a EIHV environment 

The final electrical connections to the STM can be done with copper wires. A small amount of helium is used as an exchange gas to anchor the temperature of the whole assembly to the cryogenic fluid. The body of the STM can be made out of copper, which will respond quickly to temperature changes for variable temperature measurements and provide a uniform temperature environment for the tunnel junction. One has to estimate the differential thermal contraction of the component parts to make sure that a tunnel junction separation set at room temperature is sufficiently large to prevent tip crash on cooling. Other materials like Macor or Invar , which closely match the thermal expansion properties of the piezoelectric transducers, are used as well but take more time to thermally stabilize. Some references are given in [6.30-6.43] 

Only a limited amount of work on high materials has been performed in the UHV environment. However, a few references to UHV design are included [6.44-6.51]. A few commercial instruments are available either as a complete system (JEOL, for example) or as an add-on to a custom UHV chamber containing other characterization techniques and/or in situ sample preparation (cleaning, annealing, cleaving, pealing) or film deposition capabilities. 

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