Electron beam lithography is a method for creating extremely small and precise patterns on a surface, either for etching or sketching. The principle is similar to Photolithography, but has several important differences. The major difference is that the smallest possible features are much much smaller, normally down to the order of 20 nanometers (though some newer devices allow one to push smaller). The method is different also in that the exposure is not done all at once with a mask, but rather point by point with a rastering narrow beam of electrons, normally in the 20-100 keV range. This makes it significantly slower - writing a large complex pattern can take hours. Photolithography, on the other hand, rarely takes over a minute. Electron-beam lithography is thus ideal for custom-building situations, such as in the scientific arena.

The process

First, you apply a chemical called a resist onto the surface. Then you shine an intense, focused electron beam onto selected regions of the resist (usually using a Scanning Electron Microscope). Depending on the kind of resist used, this will break up the resist (positive resist) or cause it to form stronger chemical bonds (negative resist). Then you develop the resist to make the weak parts dissolve. Now you have a pattern of resist on the surface, with some areas exposed and others not.

At this point, there are multiple things you can do.

One option is to etch away the exposed areas. For example, a SF₆ plasma destroys SiN and SiO. So, if your surface was made of SiN or SiO, you can bombard it with an SF₆ plasma to etch it away only in the exposed areas. The covered areas will have their resist depleted somewhat, but you can lay enough down that the surface remains covered. Depending on the substrate, you can use liquid chemicals instead of plasma.

Another option is to evaporate metal onto the surface. When the metal is down, you lift off the resist, taking the metal which fell on it off too. All that is left on the surface is tiny wires in the pattern of the exposed areas. In this case, it is useful to have the metal which is to be removed not be in contact with the metal that is to stay behind. To achieve this, one tries to make the resist develop an overhang, known more commonly as undercut.

Undercut can be achieved in two ways. The main way is just to let it happen on its own. Unless your surface is very thin, some secondary electrons will be generated in the surface, and some of them will be reflected back up into the resist. Consider what happens then:

After metal deposition, it has this cross-section:

and there is no metal contact. After liftoff, it has this cross-section:

If the electron reflection doesn't provide enough undercut, one can layer an insensitive resist on top of a sensitive one. The lower layer will be more cut away even though it receives slightly less electron radiation.

Resists

A common positive resist is PMMA. It comes in several concentrations and with different solvents, useful for getting different thicknesses. Other resists are NEB, EBR, ZEP, and UV. Of these, only NEB is a negative resist. All of these are toxic enough you really don't want to be in contact. Also, while the resists themselves are not at all small molecules, their solvents often are, and those solvents are also toxic. Fortunately, though, neither the resists nor their solvents are corrosive. So, nitrile gloves are adequate protection, while latex are not!