Lithography is the step that allows the definition of the pattern on the substrate: thanks to a polymer resist, and using a radiations exposure, it fabricates a mask above the material to be patterned. It is a critical step to get the shapes of the microstructures, either in MEMS or microelectronics field. Of all steps used in microtechnology, lithography is probably the most used one!
Lithography defines the smallest possible size of transistors as people can see in microprocessors fabrication datas (0.6, 0.18, 0.09μm, etc.). The capacity to give a particular shape and size to a microstructure depends on the capacity to define it with lithography and transfer it to the material with etching.
As for every steps in microtechnology, there are several ways to make a lithography, and several parameters to take in account to optimize it. This page describes the principles of the most widely used ones.

Summary

Lithography consists in making a polymer based resist layer on substrate, so that the layer has the same pattern as a maskin glass and copper. The resist is made so that it is sensitive to a radiation exposure (UV, deep UV, electron beam, or even X-ray). The effect of the radiation is to locally modified resist chemistry so that the exposed areas will react differently to a solvent, that will be used for the development step. According the the nature of the resist (than can be positive or negative, see below), the solvent will remove either the exposed part of the resist or the unexposed part.
All of this may seem a bit confusing. This summary has talk of lithography in a very generalistic way. To make things simpler, let's talk in the first part about the most common use of lithography: Photolithography.
Then we will see in details the different topics concerning lithography, beginning with the mask, then the polymer resistmaterial, and the different steps of lithography: coating of the resist, exposure, and finally the development.

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