Fabricating structures at the nano level can be broken down into two main methods; top down and bottom up construction.
Top down fabrication can be likened to sculpting from a block of stone. A piece of the base material is gradually eroded until the desired shape is achieved. That is, you start at the top of the blank piece and work your way down removing material from where it is not required. Nanotechnology techniques for top down fabrication vary but can be split into mechanical and chemical fabrication techniques.
The most top down fabrication technique is nanolithography. In this process, required material is protected by a mask and the exposed material is etched away. Depending upon the level of resolution required for features in the final product, etching of the base material can be done chemically using acids or mechanically using ultraviolet light, x-rays or electron beams. This is the technique applied to the manufacture of computer chips.
Bottom up fabrication can be likened to building a brick house. Instead of placing bricks one at a time to produce a house, bottom up fabrication techniques place atoms or molecules one at a time to build the desired nanostructure. Such processes are time consuming and so self assembly techniques are employed where the atoms arrange themselves as required.
Self assembling nanomachines are regularly mentioned by science fiction writers but significant obstacles including the laws of physics will need to be overcome or circumvented before this becomes a reality. Other areas involving bottom up fabrication are already quite successful. Manufacturing quantum dots by self-assembly quantum dots has rendered the top down lithographic approach to semiconductor quantum dot fabrication virtually obsolete.
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