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Fig. 2.14 Scheme of the
photolithographic process
Fig. 2.15 Successive
stages of the positive
lithographic process (a-f)
As an example, consider the formation in a semiconductor of n-type zones
corresponding to the source and drain of a planar transistor (Fig.
2.15
). A negative
photoresist is applied to the layer of silicon oxide grown on a silicon substrate.
Non-transparent areas of the photomask used for illumination correspond to the
n-type areas to be formed. As a result of illumination of the photoresist its entire
surface polymerizes except for those areas being created, from which unreacted
resist is cleaned up with a suitable solvent (benzene, toluene). Then, through the
windows in the polymerized film of the resist, silicon oxide is removed with
hydrofluoric acid, after which the polymerized resist is removed by a new solvent.
In this fashion windows are formed in a layer of silicon oxide on the surface of the
integrated circuit that are used for doping silicon by phosphorus or antimony. The
process of doping involves diffusion: alloying additive is applied to the surface and
