Information Technology Reference
In-Depth Information
image is focused by a special lens system which reduces the given mask pattern to a
microscopic size. A modern photolithography machine handles several tens of 8-in.
semiconductor wafers per hour.
Currently, most chips are made using ultraviolet radiation with a wavelength of
0.248
μ
m. For some circuits, a lithographic technology with the wavelength of
0.193
m, serious problems put
under question further progress in photolithography. For example, at a wavelength
of less than 0.2
μ
m has been developed. However, beyond 0.2
μ
m too much light is absorbed by the photosensitive layer,
complicating and slowing down the process of transferring the pattern of the circuit
template. Such problems motivate investigators and manufacturers to seek alter-
natives to conventional lithographic technology. For example, the possibility of
replacing ultraviolet rays by X-rays has been under investigation in US scientific
laboratories for more than two decades.
One technology, called EUV (extreme ultraviolet) and supported by
several well-known companies, aims to improve the process of lithography in
chip manufacturing.
As already noted, modern equipment for printing circuits on silicon substrates
based on deep ultraviolet radiation (deep ultraviolet, DUV) uses light sources with a
wavelength of 248 nm. It is assumed that the wavelength of EUV radiation can be as
short as 13 nm, i.e., approximately 20 times shorter. The transition from the DUV to
the EUV lithography provides for more than tenfold decrease of the wavelength,
making it comparable to the size of just a few tens of atoms.
Nevertheless, in addition to purely physical problems, there are other factors in
the manufacturing process of circuits limiting miniaturization and the degree of
integration of transistors. Generally speaking, the properties of the devices created
on the same silicon wafer, as well as on different wafers, are not identical.
Deviations can occur at each stage of production. The nature of the possible
differences between the produced circuits and the frequency of occurrence of
completely defective devices may hamper further miniaturization of integrated
circuit elements. Note that miniaturization affects not only the length and the
width of the circuit but also the thickness of the crystal on which transistors and
connections are implemented through a series of levels. In modern chips, there may
be four or five such levels. Reducing the size of transistors and increasing their
density on the crystal brings about an increase in the number of levels. However, the
more layers exist in the circuit, the more thorough control of the production process
is required, since each of the levels will be affected by the levels underneath it. The
cost of improving control and creating connections between multiple layers may
deter the increase in the number of layers.
Among other things, the increasing complexity of integrated circuits necessitates
further improvement of production conditions, to which unprecedented require-
ments are already posed. A more precise mechanical control over the positioning of
the original silicon wafer is necessary. Sterile rooms (so-called clean room) in
which chips are manufactured should become even cleaner to exclude the pene-
tration of tiny dust particles that can destroy a complex circuit.
μ
