When a sample of crystalline silicon is
covered with silicon dioxide, the oxide-layer acts as a barrier to the
diffusion of impurities, so that impurities separated from the surface
of the silicon by a layer of oxide do not diffuse into the silicon
during high-temperature processing. A p-n junction can thus be formed in
a selected location on the sample by first covering the sample with a
layer of oxide [oxidation step] removing the oxide in the selected
region, and then performing a predeposition and diffusion step. The
selective removal of the oxide in the desired area is performed with
photolithography. Thus, the areas over which diffusions are effective
are defined by the oxide layer with windows cut in it, through which
diffusion can take place. The windows are produced by the
photolithographic process. This process is the means by which
microscopically small electronic circuits and devices can be produced on
silicon wafers resulting in as many as 10000 transistors on a 1 cm x 1
cm chip.
In fact photolithography or optical
lithography is a kind of lithography. The lithography technique was
first used in the late 18th century by people interested in
art. A lithograph is a less expensive picture made from a flat,
specially prepared stone or metal plate and the lithography is art of
making lithographs. Therefore, lithography for IC manufacturing is
analogous to the lithography of the art world. In this process the
exposing radiation, such as ultraviolet (UV) light in case of
photolithography, is transmitted through the clear parts of the mask.
The circuit pattern of opaque chromium blocks some of die radiation.
This type of chromium/glass mask is used with UV light. Other types of
exposing radiations are electrons, X-rays, or ions. Thus for IC
manufacturing we have following types of lithography. Photolithography
has been explained in this post. To know about the other types of
lithographic process, click on the link below.
